Technological process of gas welding of medium-carbon steels. Welding carbon steels. Welding medium carbon steels

To improve the properties and characteristics of steels, various additives are introduced into their composition. By changing the crystal lattice of the material, additives affect not only the strength or corrosion resistance of the material, but also the ability to weld. For some alloys, welding is very easy, but there are materials that require a special approach.

One of the most common additives in steel production is, of course, carbon. According to GOST 380-2005, depending on its quantity in the steel composition, the latter can be:

• low-carbon, with a carbon content of no more than 0.25% by volume;
• medium-carbon, containing carbon in the amount of 0.25% -0.6%;
• high-carbon, which contain from 0.6% to 2.07% carbon by volume of the material.

Welding carbon steels is characterized by a number of features that make it possible to obtain a high-quality, uniform weld.

When connecting parts made of carbon steel, they are positioned so that the seam is “in weight”. To do this, the parts are securely fixed on the welding table using assembly devices - clamps, brackets, vices.

At the beginning and end of the seam, special strips are installed from the same material as the parts being welded. The beginning and end of the welding process occurs on these strips. Thus, the seam along its entire length is uniform, has stable properties and has precisely specified characteristics.

Having secured the parts and expansion bars in the desired position, tack the metal along the length of the seam. It is preferable to tack on the reverse side of the seam.

If the thickness of the parts being welded is large and it is planned to carry out multi-layer welding in several passes, tack welding can be done from the front side of the seam.

When multilayer welding, each previous layer is inspected for cracks and lack of penetration. If they are detected, the weld metal is cut off, the edges are cut, and the process is repeated.

The main requirement when welding is that the strength of the metal of the weld and the heat-affected area should not be inferior to the strength of the metal of the parts.

Low carbon

Low-carbon steel, which contains, in addition to carbon, alloying additives, is welded, as a rule, using any of the welding technologies.

The work does not require a highly qualified welder. Such materials are among the easily weldable steels. Therefore, conventional arc welding can be successfully used here.

Features of welding low-carbon steels are a reduced carbon content in the weld metal and an increased amount of alloying additives, so some strengthening of the weld metal in relation to the metal of the parts is possible.

Another problem that should be taken into account is the increased fragility of the seam when performing multi-layer welding.

To make connections on low-carbon steels, electrodes with rutile and calcium-fluoroisrutile coatings are used. Professional welders use electrodes coated with a little iron powder. Of the electrodes produced by industry, the following brands are suitable for welding: UONI-13/85, TsL-14, TsL-18-63.

Low carbon steels are easy to weld. In this case, you can even do without the use of flux, and gas is consumed in a small volume.

To obtain a high-quality joint with a strength no less than that of the base metal, silicon-manganese welding wire is used. Upon completion of work with the seam, the flame is not extinguished or removed from the joint of the parts, but is smoothly deflected, allowing the seam to cool.

If you remove the flame immediately, then without flux the weld material, being heated, will oxidize. To give the seam better strength properties, the weld metal is usually forged and heat treated.

Medium carbon

Due to the large amount of carbon, joining such parts is complicated. In the results of the work, this is expressed in the fact that the metal of the part and the welded joint can be of different strengths. In addition, near the edges of the seam, cracks and pockets with pronounced fragility of the material can form.

To avoid these disadvantages, electrodes are used whose material contains a low amount of carbon.

With an increase in current required to heat the parts being connected, penetration of the base metal is possible. To eliminate such cases, the edges of the parts to be joined are cut.

Another measure to improve the quality of the connection is preheating and constant heating of the parts during the process. When welding steels with a semi-automatic machine, to improve the quality of the seam, it is better to move the electrode not across, but along the joint of the parts and use a short arc. Electrodes of the brands UONI-13/55, UONI-13/65, OZS-2, K-5a are used for work.

When using acetylene for welding medium-carbon steels, a burner flame is achieved such that the gas flow rate is 75-100 dm³/h. For products with a thickness of 3 millimeters or more, general heating up to 250-300 °C or local heating up to 600-650 °C is used.

After welding, the seam is forged and subjected to heat treatment. To weld metal products with an amount of carbon close in content to high-carbon steels, a special flux is used.

High carbon

Steels with a high carbon content are very difficult to weld. Other alternative methods are used to connect parts made of such materials.

Welding of high-carbon steels that are resistant to corrosion is carried out only during repair work.

In this case, pre-heating of the seam area to 250-300 °C and subsequent heat treatment of the seam are used. It is absolutely not allowed to carry out welding work with high-carbon steels at air temperatures below 5 °C or when there is a welding work drafts

If all conditions are met, welding of high-carbon steels is carried out using the same techniques as medium-carbon steels.

Gas welding with acetylene is allowed. The power of the burner flame should ensure gas consumption in the range of 75-90 dm³/h per 1 millimeter of seam thickness.

To prevent oxidation, fluxes are used whose compositions are similar to those used in welding medium-carbon steels. After gas welding The seam is forged and then tempered.

Austenitic

Austenitic steels are materials that contain the high-temperature phase of iron - austenite. They are included, for example, in the group of chromium-nickel steels, which can work in various aggressive environments ah and at very high temperatures.

The main feature when welding corrosion-resistant steel is the need to ensure resistance to intercrystalline corrosion in the heat-affected zone.

The problem is that even with preheating of the steel, chromium carbides fall out of the crystal lattice along the heating boundaries. As a result of a decrease in the amount of this element in the material, upon reheating, corrosion cracking appears at the boundaries.

In practice, it may be necessary to create structures using austenitic steels with chromium-nickel alloying additives that will operate at high temperatures. To weld such structures, it is necessary to select materials in which the carbon content is as low as possible.

If it is necessary for the percentage of carbon to be higher, and at the same time, steel structures fulfill their purpose in aggressive environments and high temperatures, you need to choose an alloying additive that is similar in properties to carbon.

Titanium, zirconium, tantalum, vanadium, and tungsten can be used as such an additive. These elements bind carbon, which is released from the steel during subsequent heating, and prevent the depletion of heat-affected areas during the welding process.

Stainless steel

Most often, stainless steels used in industry obtain their anti-corrosion properties through the introduction of alloying additives - chromium and nickel.

When welding chrome-plated parts, it must be taken into account that at high temperatures (more than 500 °C), oxidation of the joint of parts is possible.

To avoid this, use or TIG welding (TIG). This technology involves the implementation of welding operations without air access directly to the welding zone. Accordingly, the absence of oxygen, the presence of which is mandatory in the air, eliminates the prerequisites for oxidation of the material.

Limiting the access of air is carried out by introducing argon, an inert gas into the welding zone, which, being heavier than air, displaces it. Sometimes this method is called steel welding with argon. In fact, steel is either simply welded together with an arc, or using filler material.

Tig welding requires special equipment. The work is carried out with non-consumable tungsten electrodes, the requirements for which are determined by GOST 10052-75.

The second problem is this. Stainless steels have a high coefficient of thermal expansion, and when welding sheet steel, when the joint is long in comparison with the linear dimensions of the part, the weld may bend during the cooling process.

The problem is solved by setting gaps between the sheets and using tacks to fix the parts in the desired position.

Instrumental

Tool steel is one of the hard, mechanically resistant materials. It is used to make metalworking and carpentry tools and parts of equipment for various industries.

The working parts of tools - drills, cutters, the purpose of which is to influence materials for the purpose of processing them, obviously must be stronger and harder than the materials being processed. Such properties are achieved by including a large amount of carbon and alloying additives - nickel, chromium, molybdenum.

Welding of tool steel is used in the repair of equipment and tools. In this case, high demands are placed on the welding seams: the joints must be homogeneous with the rest of the material, and their strength must not differ in order to avoid stress concentrations during operation.

To ensure compliance with such requirements, it is necessary to use special electrodes. In most cases, this may be UONI-13/NZH/20ZH13.

When welding special carbon steels, the use of which is narrowly focused, electrodes designed for specific grades are used.

With the correct determination of the characteristics of the material, type of welding and modes, when using electrodes of the appropriate brands, the welds will have high strength and corrosion resistance.

Depending on the chemical composition, steel can be carbon or alloy. Carbon steel is divided into low-carbon (carbon content up to 0.25%), medium-carbon (carbon content from 0.25 to 0.6%) and high-carbon (carbon content from 0.6 to 2.07o). Steel, which in addition to carbon contains alloying components (chromium, nickel, tungsten, vanadium, etc.), is called alloyed. Alloy steels are: low-alloy (the total content of alloying components, except carbon, is less than 2.5%); medium alloyed (total content of alloying components, except carbon, from 2.5 to 10%), highly alloyed (total content of alloying components, except carbon, more than 10%).

Based on their microstructure, steels are classified into pearlitic, martensitic, austenitic, ferritic and carbide classes.

According to the production method, steel can be:

a) ordinary quality (carbon content up to 0.6%), boiling, semi-calm and calm. Boiling steel is produced by incomplete deoxidation of the metal with silicon; it contains up to 0.05% silicon. Calm steel has a uniform, dense structure and contains at least 0.12% silicon. Semi-quiet steel occupies an intermediate position between boiling and calm steels and contains 0.05-0.12% silicon;

b) high-quality - carbon or alloyed, in which the content of sulfur and phosphorus should not exceed 0.04% of each element;

c) high-quality - carbon or alloy, in which the content of sulfur and phosphorus should not exceed 0.030 and 0.035%, respectively. Such steel also has increased purity for non-metallic inclusions and is designated by the letter A, placed after the brand designation.

According to their intended purpose, steels can be used for construction, engineering (structural), tool steels, and steels with special physical properties.

Structures made of medium carbon steel can be welded well provided the rules set out in Chap. 13, as well as the following additional instructions. In butt, corner and T-joints, when assembling the elements being connected, the gaps provided by GOST should be maintained between the edges so that welding transverse shrinkage occurs more freely and does not cause crystallization cracks. In addition, starting with a steel thickness of 5 mm or more, edges are cut in butt joints, and welding is carried out in several layers. The welding current is reduced. Welding is carried out with electrodes with a diameter of no more than 4-5 mm using direct current of reverse polarity, which ensures less melting of the edges of the base metal and, consequently, a smaller proportion of it and a lower C content in the weld metal. Electrodes E42A, E46A or E50A are used for welding. The steel rods of the electrodes contain little carbon, so when they are melted and mixed with a small amount of medium-carbon base metal, there will be no more than 0.1-0.15% carbon in the weld. In this case, the weld metal is alloyed with Mn and Si due to the molten coating and thus turns out to be equal in strength to the base metal. Welding of metal with a thickness of more than 15 mm is carried out in a “slide”, “cascade” or “blocks” for slower cooling. Preliminary and accompanying heating is used (periodic heating before welding of the next “cascade” or “block” to a temperature of 120-250 ° C). Structures made of steel grades VSt4ps, VSt4sp and steel 25 with a thickness of no more than 15 mm and without rigid components are usually welded without heating. In other cases, preliminary and auxiliary heating and even subsequent heat treatment are required. The arc is lit only at the site of the future seam. There should be no unwelded craters and sharp transitions from the base to the deposited metal, undercuts and intersections of seams. Creating craters on the base metal is prohibited. An annealing roller is applied to the last layer of the multi-layer seam.

Welding medium-carbon steel grades VSt5, 30, 35 and 40, containing carbon 0.28-0.37% and 0.27-0.45%, is more difficult, since with increasing carbon content the weldability of the steel deteriorates.

Medium-carbon steel of the VSt5ps and VSt5sp grades used for reinforced concrete reinforcement is welded using the bath method and conventional extended seams when connected to overlays (16.1). For welding, the ends of the connected rods must be prepared: for welding in the lower position, cut off with a cutter or saw, and for vertical welding, cut. In addition, they must be cleaned at the joints to a length that exceeds the weld or joint by 10-15 mm. Welding is performed with electrodes E42A, E46A and E50A for extended bead seams. At air temperatures down to minus 30 °C, it is necessary to increase the welding current by 1% for every 3 °C drop in temperature from 0 °C. In addition, you should use preheating of the joined rods to 200--250 °C for a length of 90--150 mm from the joint and reduce the cooling rate after welding by wrapping the joints with asbestos, and in the case of bath welding, do not remove the forming elements until the joint has cooled to 100 °C and below.

At lower ambient temperatures (from -30 to -50°C), you should be guided by a specially developed welding technology, which provides for preliminary and simultaneous heating and subsequent heat treatment of reinforcement joints or welding in special hothouses.

Welding of other structures made of medium carbon steel grades VSt5, 30, 35 and 40 should be carried out in compliance with the same additional instructions. Rail track joints are usually welded using bath welding with preheating and subsequent slow cooling, similar to reinforcement joints. When welding other structures made of these steels, preliminary and auxiliary heating, as well as subsequent heat treatment, should be used.

Welding high-carbon steels of grades VStb, 45, 50 and 60 and cast carbon steels with a carbon content of up to 0.7% is even more difficult. These steels are mainly used in castings and tool making. Their welding is possible only with preliminary and concomitant heating to a temperature of 350-400 ° C and subsequent heat treatment in heating furnaces. When welding, the rules specified for medium carbon steel must be followed. Good results are achieved when welding with narrow beads and in small areas with cooling of each layer. After welding is completed, heat treatment is required.

Carbon structural steels include steels containing 0.1 - 0.7% carbon, which is the main alloying element in steels of this group and determines their mechanical properties. An increase in carbon content complicates welding technology and obtaining high-quality welded joints. In welding production, depending on the carbon content, carbon structural steels are conventionally divided into three groups: low-, medium- and high-carbon. The welding technology for steels of these groups is different.

Most welded structures are currently made from low-carbon steels containing up to 0.25% carbon. Low-carbon steels are well-welded metals with almost all types and methods of fusion welding.

The welding technology for these steels is selected from the conditions of compliance with a set of requirements, ensuring, first of all, the equal strength of the welded joint with the base metal and the absence of defects in the welded joint. The welded joint must be resistant to transition to a brittle state, and the deformation of the structure must be within limits that do not affect its performance. The weld metal when welding low-carbon steel differs slightly in composition from the base metal - the carbon content decreases and the manganese and silicon content increases. However, ensuring equal strength during arc welding does not cause difficulties. This is achieved by increasing the cooling rate and alloying with manganese and silicon through the welding materials. The effect of cooling rate is significantly manifested when welding single-layer seams, as well as in the last layers of a multi-layer seam. The mechanical properties of the metal in the heat-affected zone undergo some changes compared to the properties of the base metal - for all types of arc welding, this is a slight strengthening of the metal in the overheating zone. When welding aging (for example, boiling and semi-quiet) low-carbon steels in the recrystallization area of ​​the heat-affected zone, a decrease in the impact toughness of the metal is possible. The metal of the heat-affected zone becomes embrittled more intensively during multilayer welding compared to single-layer welding. Welded structures made of mild steel are sometimes subjected to heat treatment. However, for structures with single-layer fillet welds and multilayer welds applied intermittently, all types of heat treatment, except hardening, lead to a decrease in strength and an increase in the ductility of the weld metal. Seams made by all types and methods of fusion welding have quite satisfactory resistance to the formation of crystallization cracks due to the low carbon content. However, when welding steel with an upper limit of carbon content, crystallization cracks can appear, primarily in fillet welds, the first layer of multi-layer butt welds, single-sided welds with full edge penetration and the first layer of butt welds welded with a mandatory gap.

Manual welding with coated electrodes has become widespread in the manufacture of structures made of low-carbon steels. Depending on the requirements for the welded structure and the strength characteristics of the steel being welded, the type of electrode is selected. In recent years, electrodes of the E46T type with rutile coating have become widely used. For particularly critical structures, electrodes with calcium fluoride and calcium fluorine-rutile coatings of type E42A are used, which provide increased resistance of the weld metal against crystallization cracks and higher plastic properties. High-performance electrodes with iron powder coating and electrodes for deep penetration welding are also used. The type and polarity of the current are selected depending on the characteristics of the electrode coating.

Despite the good weldability of low-carbon steels, sometimes special technological measures should be taken to prevent the formation of hardening structures in the heat-affected zone. Therefore, when welding the first layer of a multilayer weld and fillet welds on thick metal, it is recommended to preheat it to 120-150°C, which ensures the resistance of the metal against the appearance of crystallization cracks. To reduce the cooling rate, before correcting defective areas, it is necessary to perform local heating to 150°C, which will prevent a decrease in the plastic properties of the deposited metal.

Low-carbon steels can be gas welded without much difficulty using a normal flame and, as a rule, without flux. The flame power with the left method is selected based on the consumption of 100--130 dm3/h of acetylene per 1 mm of metal thickness, and with the right method - 120--150 dm3/h. Highly qualified welders work with a high-power flame - 150-200 dm 3 / h of acetylene, using filler wire of a larger diameter than in conventional welding. To obtain a connection of equal strength with the base metal when welding critical structures, silicon-manganese welding wire should be used. The end of the wire should be immersed in a bath of molten metal. During the welding process, the welding flame must not be diverted from the pool of molten metal, as this can lead to oxidation of the weld metal with oxygen. To compact and increase the ductility of the deposited metal, forging and subsequent heat treatment are carried out.

The difference between medium-carbon steels and low-carbon steels mainly lies in the different carbon content. Medium carbon steels contain 0.26 - 0.45% carbon. The increased carbon content creates additional difficulties when welding structures made from these steels. These include low resistance to crystallization cracks, the possibility of formation of low-plasticity hardening structures and cracks in the heat-affected zone, and the difficulty of ensuring equal strength of the weld metal with the base metal. Increasing the resistance of the weld metal against crystallization cracks is achieved by reducing the amount of carbon in the weld metal by using electrode rods and filler wire with a reduced carbon content, as well as reducing the proportion of the base metal in the weld metal, which is achieved by welding with edge preparation in modes that ensure minimal penetration of the base metal and the maximum value of the weld shape coefficient. This is also facilitated by electrodes with a high deposition rate. To overcome the difficulties that arise when welding products made of medium-carbon steels, preliminary and concomitant heating, modification of the weld metal and double-arc welding in separate pools are performed. Manual welding of medium-carbon steels is carried out with calcium fluoride-coated electrodes of the UONI-13/55 and UONI-13/45 grades, which provide sufficient strength and high resistance of the weld metal against the formation of crystallization cracks. If high ductility requirements are imposed on the welded joint, it is necessary to subject it to subsequent heat treatment. When welding, the application of wide beads should be avoided; welding is performed with a short arc and small beads. Transverse movements of the electrode must be replaced with longitudinal ones, craters must be welded or placed on technological plates, since cracks can form in them.

Gas welding of medium-carbon steels is carried out using a normal or slightly carburizing flame with a power of 75-100 dm3/h of acetylene per 1 mm of metal thickness only in the left way, which reduces overheating of the metal. For products with a thickness of over 3 mm, general heating up to 250-350°C or local heating up to 600-650°C is recommended. For steels with carbon content at the upper limit, it is advisable to use special fluxes. To improve the properties of the metal, forging and heat treatment are used.

High-carbon steels include steels with a carbon content in the range of 0.46-0.75%. These steels are generally not suitable for the manufacture of welded structures. However, the need for welding arises during repair work. Welding is carried out with preliminary, and sometimes with accompanying heating and subsequent heat treatment. At temperatures below 5°C and in drafts, welding cannot be performed. The remaining technological methods are the same as for welding medium-carbon steels. Gas welding of high-carbon steels is carried out with a normal or slightly carburizing flame with a power of 75 - 90 dm3/h of acetylene per 1 mm of metal thickness, heated to 250 - 300 ° C. The left-hand welding method is used, which allows to reduce the overheating time and the time the metal of the weld pool remains in the molten state. Fluxes of the same composition as for medium-carbon steels are used. After welding, the seam is forged, followed by normalization or tempering.

In recent years, heat-strengthened carbon steels have found application. High-strength steels make it possible to reduce the thickness of products. The welding modes and techniques for heat-strengthened steels are the same as for conventional carbon steel of the same composition. Welding materials are selected taking into account ensuring equal strength of the weld metal with the base metal. The main difficulty in welding is the softening of the area of ​​the heat-affected zone that is heated to 400 - 700 °C. Therefore, for heat-strengthened steel, low-power welding modes are recommended, as well as welding methods with minimal heat removal into the base metal.

Steels with protective coatings are also used. Galvanized steel is most widely used in the manufacture of various designs of sanitary pipelines. When welding galvanized steel, if zinc gets into the weld pool, conditions are created for the appearance of pores and cracks. Therefore, the zinc coating must be removed from the edges being welded. Considering that traces of zinc remain on the edges, additional measures should be taken to prevent the formation of defects: compared to welding conventional steel, the gap is increased by 1.5 times, and the welding speed is reduced by 10 g-20%, the electrode is moved along the seam with longitudinal vibrations . When manually welding galvanized steel, the best results are obtained when working with rutile-coated electrodes, which ensure a minimum silicon content in the weld metal. But other electrodes can also be used. Due to the fact that zinc fumes are extremely toxic, welding of galvanized steel can be done in the presence of strong local ventilation. After completing the welding work, it is necessary to apply a protective layer to the surface of the seam and restore it in the area of ​​the heat-affected zone.

Carbon steel is an alloy of iron and carbon with minor amounts of silicon, manganese, phosphorus and sulfur. In carbon steel, unlike stainless steel, there are no alloying elements (molybdenum, chromium, manganese, nickel, tungsten). The properties of carbon steel vary greatly depending on a slight change in carbon content. As the carbon content increases, the hardness and strength of the steel increase, while the toughness and ductility decrease. With a carbon content of more than 2.14%, the alloy is called cast iron.

Classification of carbon steels

• low-carbon (with carbon content up to 0.25%)
• medium carbon (with a carbon content of 0.25 - 0.6%)
• high-carbon (with a carbon content of 0.6 - 2.0%)

Steel is classified according to production method:

1. Ordinary quality (carbon up to 0.6%) boiling, semi-calm, calm

There are 3 groups of ordinary quality steels:

• Group A. Supplied according to mechanical properties without regulation of steel composition. These steels are usually used in products without subsequent pressure treatment and welding. The larger the number of the conditional number, the higher the strength and the lower the ductility of the steel.
• Group B. Comes with a guarantee of the chemical composition. The higher the reference number, the higher the carbon content. Subsequently they can be processed by forging, stamping, or exposure to temperature without preserving the initial structure and mechanical properties.
• Group B. Can be welded. Supplied with a guarantee of composition and properties. This group of steels has mechanical properties in accordance with the numbers in group A, and the chemical composition - in accordance with the numbers in group B, with correction according to the deoxidation method.

2. High-quality with sulfur content up to 0.030% and phosphorus up to 0.035%. Steel has increased purity and is designated by the letter A after the steel grade

According to the intended purpose, steel can be:

• construction
• mechanical engineering (structural)
• instrumental
• steels with special physical properties

Such steels weld well. To correctly select electrodes of the desired type and brand, the following requirements must be taken into account:

• Equally strong welding connection to the base metal
• Defect-free weld
• Optimal chemical composition of weld metal
• Stability of welded joints under vibration and shock loads, high and low temperatures

For welding low-carbon steels, electrodes of the brands OMM-5, SM - 5, TsM - 7, KPZ-32R, OMA - 2, UONI - 13/45, SM - 11 are used

Welding carbon steels

Carbon increases the ability of steel to be hardened. Steel with a carbon content (0.25–0.55%) is subject to quenching and tempering, which significantly increases its hardness and wear resistance. These qualities of steel are used in the production of mechanism parts, axle shafts, gears, housings, sprockets and other parts that require increased wear resistance. Often, welding becomes the only technology for the manufacture and repair of machine parts, frames of production equipment, etc.

Problems of welding carbon steels and methods for solving them

However, welding carbon steels is difficult for the following reason: the carbon contained in such steels contributes to the formation of crystallization hot cracks and low-plasticity hardening formations and cracks in heat-affected zones during welding. The metal of the seam itself differs in properties from the base metal, and carbon reduces the resistance of seams to cracking, increasing the negative effects of sulfur and phosphorus.

The critical carbon content in a weld depends on:

• unit design
• seam shapes
• content of various elements in the seam
• preheating the seam area

Accordingly, methods for increasing resistance against the formation of hot cracks are aimed at:

• Limiting elements that promote cracking
• Reduction of tensile stresses in the seam
• Formation of the optimal weld shape with the most homogeneous chemical composition

In addition, an increased carbon content contributes to the formation of low-plasticity structures, which, under the influence of various stresses, are prone to the formation of cold cracks and destruction. To prevent this, methods are used to eliminate factors that contribute to the occurrence of such conditions.

Requirements for welding technology of carbon steels

When making welded joints on steels with a high carbon content, the following conditions must be observed to ensure the resistance of the welds to cracking:

• Use welding electrodes and wire with low carbon content
• Use welding modes and technological measures that limit the drift of carbon from the base metal into the weld (edging, increased overhang, use of filler wire, etc.)
• Introduce elements that promote the formation of refractory or rounded sulfide formations (manganese, calcium, etc.) in the weld.
• Use a certain order of sutures, reduce the rigidity of the nodes. Use other modes and methods to reduce stress in the weld seam
• Select the desired weld shapes and reduce its chemical heterogeneity
• Minimize the content of diffusible hydrogen (use low-hydrogen electrodes, drying shielding gases, cleaning edges and wires, calcining electrodes, wires, fluxes)
• Ensure slow cooling of the weld seam (use multi-layer, double-arc or multi-arc welding, surfacing of an annealing bead, use exothermic mixtures, etc.)

Technological features of welding carbon steels

Some features of preparation and welding of parts made of carbon steels:

When welding carbon steel, the base metal is cleaned of rust, dirt, scale, oil and other contaminants, which are sources of hydrogen and can form pores and cracks in the weld. The edges and adjacent areas of metal up to 10 mm wide are cleaned. This ensures a smooth transition to the base metal of the structure and the strength of the weld under various loads.

• Assembling parts for welding. Edge cutting

When assembling parts for welding, a gap must be maintained, depending on the thickness of the parts. The gap width is 1-2 mm larger than when assembling elements of well-welded steels. Edge cutting should be carried out with a metal thickness of 4 mm or more, which helps reduce the transfer of carbon into the seam. Since there is a high tendency to hardening, small cross-section tacks should be abandoned or local preheating should be used before tacks.

• The welding mode should provide the least penetration of the base metal and optimal cooling speed. The correct choice of welding mode can be confirmed by the results of measuring the hardness of the weld metal. In optimal mode, it should not exceed 350 HV.
• Critical components are welded in two or more passes. The weld to the base metal should have a smooth approach. Frequent arc breaks, craters on the base metal and burns are not allowed.
• Critical structures made of carbon steels, as well as units with a rigid contour, etc. are welded with preheating. Heating is carried out in the temperature range of 100–400 °C, and the higher the heating temperature, the higher the carbon content and the thickness of the parts being welded.
• Cooling of welded joints after finishing carbon steel welding should be slow. For this purpose, the welded unit is covered with a special heat-insulating material, moved to a special thermostat, or used after welding heating.

Welding consumables for welding carbon steels

• For welding steels with a carbon content of up to 0.4%, welding electrodes can be used that are suitable for welding low-alloy steels with minor restrictions. For manual welding, electrodes with a basic type of coating are used, which ensure a minimum hydrogen content in the weld weld. Electrodes of the brands UONI-13/45, UONI-13/55, etc. are used.
• Mechanized welding of carbon steel in shielding gas involves the use of wire grades Sv-08G2S, Sv-09G2STs or similar, as well as a gas mixture of carbon dioxide and oxygen (with the latter content up to 30%) or carbon dioxide. It is allowed to use oxidizing argon gas mixtures (70-75% Ar+20-25% CO2+5% O2). The most optimal wire thickness is 1.2 mm.
• If carbon steel has undergone heat treatment or alloyed, then the Sv-08G2S electrode wire will not provide the necessary mechanical properties. In these cases, complex-alloyed wires of the brands Sv-08GSMT, Sv-08KhGSMA, Sv-08Kh3G2SM, etc. are used for welding.
• Automatic submerged arc welding of carbon steel is carried out using Sv-08A, Sv-08AA, Sv-08GA wires when used together with AN-348A, OSTS-45 fluxes. It is recommended to use fluxes AN-43 and AN-47, which have good technological properties and resistance to cracking.
• Welding materials (wire, electrodes) must comply with the requirements of standards and technical specifications. Electrodes with significant coating defects must not be used. The wire must be free of dirt and rust; fluxes and electrodes must be calcined before use at temperatures recommended in the accompanying technical documentation. Only welding carbon dioxide should be used for welding. Food-grade carbon dioxide can only be used after additional drying.

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Welding low-carbon steels – Osvarke.Net

Low-carbon steels are steels with a low carbon content of up to 0.25%. Low-alloy steels are steels containing up to 4% alloying elements, excluding carbon.

The good weldability of low-carbon and low-alloy structural steels is the main reason for their widespread use in the production of welded structures.

Chemical composition and properties of steels

In carbon structural steels, carbon is the main alloying element. The mechanical properties of steels depend on the amount of this element. Low-carbon steels are divided into steels of ordinary quality and high-quality ones.

Ordinary quality steel

Depending on the degree of deoxidation, ordinary quality steel is divided into:

• boiling - kp;
• semi-calm - ps;
• calm - sp.

Boiling steel

Steels of this group contain no more than 0.07% silicon (Si). Steel is produced by incomplete deoxidation of steel with manganese. A distinctive feature of boiling steel is the uneven distribution of sulfur and phosphorus throughout the thickness of the rolled product. If an area with sulfur accumulation enters the welding zone, it can lead to the appearance of crystallization cracks in the weld and the heat-affected zone. When exposed to low temperatures, such steel can become brittle. Having succumbed to welding, such steels can age in the heat-affected zone.

Calm steel

Mild steels contain at least 0.12% silicon (Si). Calm steels are obtained by deoxidizing steel with manganese, silicon, and aluminum. They are distinguished by a more uniform distribution of sulfur and phosphorus in them. Calm steels respond less to heat and are less prone to aging.

Semi-quiet steel

Semi-quiet steels have average characteristics between calm and boiling steels.

Carbon steels of ordinary quality are produced in three groups. Group A steels are not used for welding; they are supplied according to their mechanical properties. The letter “A” is not used in the designation of steel, for example “St2”.

Steels of groups B and C are supplied according to their chemical properties, chemical and mechanical, respectively. The letter of the group is placed at the beginning of the steel designation, for example BSt2, VSt3.

Semi-quiet steel grades 3 and 5 can be supplied with a higher manganese content. In such steels, the letter G is placed after the grade designation (for example, BSt3Gps).

For the manufacture of critical structures, ordinary steels of group B should be used. The manufacture of welding structures from low-carbon steels of ordinary quality does not require the use of heat treatment.

Quality steels

Low-carbon quality steels are supplied with normal (grades 10, 15 and 20) and increased (grades 15G and 20G) manganese content. High-quality steels contain a reduced amount of sulfur. For the manufacture of welding structures from steels of this group, hot-rolled steels are used, less often heat-treated steels. To increase the strength of the structure, welding of these steels can be carried out with subsequent heat treatment.

Low alloy steels

If special chemical elements are introduced into carbon steel that are not initially present in it, then such steel is called alloyed steel. Manganese and silicon are considered alloying components if their content exceeds 0.7% and 0.4%, respectively. Therefore, VSt3Gps, VSt5Gps, 15G and 20G steels are considered both low-carbon and low-alloy structural steels.

Alloying elements are capable of forming compounds with iron, carbon and other elements. This helps improve the mechanical properties of steels and reduces the cold brittleness limit. As a result, it becomes possible to reduce the weight of the structure.

Alloying a metal with manganese increases impact strength and resistance to cold brittleness. Welding joints made from manganese steels are characterized by higher strength under alternating impact loads. The resistance of steel against atmospheric and sea corrosion can be increased by alloying with copper (0.3-0.4%). Most low-alloy steels for the production of welding structures are used in the hot-rolled state. The mechanical properties of alloy steels can be improved by heat treatment, therefore some grades of steel for welded structures are used after heat treatment.

Weldability of low-carbon and low-alloy steels

Low-carbon and low-alloy structural steels have good weldability. Their welding technology must ensure equal mechanical properties of the weld and the base metal (not lower than the lower limit of the properties of the base metal). In some cases, due to the operating conditions of the structure, a reduction in some mechanical properties of the seam is allowed. The seam must be free of cracks, lack of penetration, pores, undercuts and other defects. The shape and geometric dimensions of the seam must correspond to the required ones. The welded joint may be subject to Additional requirements, which are related to the operating conditions of the structure. Without exception, all welds must be durable and reliable, and the technology must ensure the productivity and economy of the process.

The mechanical properties of a welded joint are influenced by its structure. The structure of the metal during welding depends on the chemical composition of the material, welding conditions and heat treatment.

Preparation and assembly of parts for welding

Preparation and assembly for welding is carried out depending on the type of welding joint, welding method and metal thickness. To maintain the gap between the edges and the correct position of the parts, specially created assembly fixtures or universal fixtures (suitable for many simple parts) are used. Assembly can be performed using tacks, the dimensions of which depend on the thickness of the metal being welded. The tack can be 20-120 mm long, and the distance between them is 500-800 mm. The cross-section of the tack is equal to approximately a third of the seam, but not more than 25-30 mm2. Tack welding can be done by manual arc welding or mechanized gas shielded welding. Before proceeding to welding the structure, the tacks are cleaned, inspected, and if any defects are present, they are cut out or removed by other methods. During welding, the tacks are completely remelted due to the possible occurrence of cracks in them as a result of rapid heat removal. Before electroslag welding, the parts are placed with a gap that gradually increases towards the end of the weld. Fixing the parts to maintain their relative position is done using staples. The staples should be at a distance of 500-1000 mm. They must be removed as the suture is applied.

For automatic welding methods, lead-in and exit bars should be installed. With automatic welding, it is difficult to ensure high-quality penetration of the weld root and prevent metal burns. For this purpose, remaining and removable linings and flux pads are used. You can also weld the root of the seam using manual arc welding or semi-automatic welding in shielding gases, and the rest of the seam is performed using automatic methods.

Welding by manual and mechanized methods is performed by weight.

The edges of welding parts are thoroughly cleaned of slag, rust, oil and other contaminants to prevent the formation of defects. Critical structures are welded mainly on both sides. The method of filling the groove edges when welding thick-walled structures depends on its thickness and the heat treatment of the metal before welding. Lack of penetration, cracks, pores and other defects identified after welding are removed with a mechanical tool, air-arc or plasma cutting, and then welded back. When welding low-carbon steels, the properties and chemical composition of the welded joint largely depend on the materials used and welding modes.

Manual arc welding of low-carbon steels

To obtain a high-quality connection using manual arc welding, it is necessary to choose the right welding electrodes, set the modes and apply the correct welding technique. The disadvantage of manual welding is the high dependence on the experience and qualifications of the welder, despite the good weldability of the steels in question.

Welding electrodes should be selected based on the type of steel being welded and the purpose of the structure. To do this, you can use the electrode catalog, where the passport data of many brands of electrodes is stored.

When choosing an electrode, you should pay attention to the recommended conditions for the type and polarity of the current, spatial position, current strength, etc. The passport for the electrodes may indicate the typical composition of the deposited metal and the mechanical properties of the connection made by these electrodes.

In most cases, welding of low-carbon steels is carried out without measures aimed at preventing the formation of hardening structures. But still, when welding thick-walled fillet welds and the first layer of a multilayer weld, preheating the parts to a temperature of 150-200 ° C is used to prevent the formation of cracks.

When welding non-heat-strengthened steels, a good effect is achieved using cascade and slide welding methods, which does not allow the weld metal to cool quickly. Preheating to 150-200° C gives the same effect.

For welding heat-strengthened steels, it is recommended to make long seams along cooled previous seams in order to avoid softening of the heat-affected zone. You should also choose modes with low heat input. Correction of defects during multilayer welding should be done with large-section seams, at least 100 mm long, or the steel should be preheated to 150-200 ° C.

Gas shielded arc welding of low carbon steels

Welding of low-carbon and low-alloy steels is carried out using carbon dioxide or its mixtures as a shielding gas. You can use mixtures of carbon dioxide + argon or oxygen up to 30%. For critical structures, welding can be performed using argon or helium.

In some cases, carbon and graphite electrode welding is used for welding on-board connections with a thickness of 0.2-2.0 mm (for example, capacitor housings, canisters, etc.). Since welding is performed without the use of a filler rod, the content of manganese and silicon in the weld is low, resulting in a loss of joint strength that is 30-50% lower than that of the base metal.

Carbon dioxide welding is performed using welding wire. For automatic and semi-automatic welding in different spatial positions, wire with a diameter of up to 1.2 mm is used. For the lower position, use a 1.2-3.0 mm wire.

As can be seen from the table, Sv-08G2S wire can be used for welding all steels.

Submerged arc welding of low carbon steels

A high-quality welded joint with equal strength of the seam and the base metal is achieved through the correct selection of fluxes, wires, welding modes and techniques. It is recommended to perform automatic submerged arc welding of low-carbon steels with wire with a diameter of 3 to 5 mm, semi-automatic submerged arc welding with a diameter of 1.2-2 mm. For welding low-carbon steels, AN-348-A and OSTS-45 fluxes are used. Low-carbon welding wire of the Sv-08 and Sv-08A grades, and for critical structures you can use Sv-08GA wire. This set of welding consumables makes it possible to obtain welds with mechanical properties equal to or exceeding those of the base metal.

For welding low-alloy steels, it is recommended to use welding wire Sv-08GA, Sv-10GA, Sv-10G2 and others containing manganese. Fluxes are the same as for low-carbon steels. Such materials make it possible to obtain the necessary mechanical properties and resistance of the metal from the formation of pores and cracks. When welding without bevel, increasing the proportion of base metal in the weld metal can increase the carbon content. This increases the strength properties, but reduces the plastic properties of the connection.

Welding modes for low-carbon and low-alloy steels differ slightly and depend on the welding technique, type of joint and seam. When welding single-layer fillet welds, fillet and butt welds of thick steel grade VSt3 in modes with low heat input, hardening structures can form in the heat-affected zone and ductility may decrease. To prevent this, the cross-section of the seam should be increased or double-arc welding should be used.

To prevent weld destruction in the heat-affected zone when welding low-alloy steels, modes with low heat input should be used, and for welding non-heat-strengthened steels, modes with increased heat input should be used. In the second case, to ensure the plastic properties of the seam and adjacent zone are no worse than the base metal, it is necessary to use double-arc welding or preheating to 150-200 ° C.

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Welding carbon steels: high, low, medium, alloy, stainless, electrodes, technology, submerged arc

Home page » About welding » How to weld correctly » Welding carbon steels

Carbon steel is an alloy of iron and carbon with a small content of useful impurities: silicon and manganese, harmful impurities: phosphorus and sulfur. The carbon concentration in steels of this type is 0.1-2.07%. Carbon acts as the main alloying element. It is this that determines the welding and mechanical properties of this class of alloys.

Depending on the carbon content, the following groups of carbon steels are distinguished:

• less than 0.25% - low carbon;
• 0.25-0.6% - medium carbon;
• 0.6-2.07% - high carbon.

Welding low carbon steels

Due to the low carbon concentrate, this type has the following properties:

• high elasticity and plasticity;
• significant impact strength;
• Can be processed well by welding.

Low-carbon steels are widely used in construction and in the production of parts using cold stamping.

Welding technology for low carbon steels

Low carbon steels are best welded. Their connection can be carried out using manual arc welding using coated electrodes. When using this method, it is important to choose the right brand of electrodes, which will ensure a uniform structure of the deposited metal. Welding must be done quickly and accurately. Before starting work, you need to prepare the parts to be connected.

Gas welding is carried out without the use of additional fluxes. Metal wires with a low carbon content are used as filler material. This will help prevent pores from forming.

Gas welding in an argon environment is used to process critical structures.

After welding, the finished structure must be subjected to heat treatment through a normalization operation: the product should be heated to a temperature of approximately 400°C; stand and cool in air. This procedure helps ensure that the steel structure becomes uniform.

Features of welding low-carbon steels

Good weldability of such steels ensures equal strength of the weld with the base metal, as well as the absence of defects.

The weld metal has a reduced carbon content, the proportion of silicon and manganese is increased.

During manual arc welding, the heat-affected area is overheated, which contributes to its slight strengthening.

A weld deposited using multilayer welding is characterized by an increased level of fragility.

The compounds are highly resistant to MCC due to their low carbon concentration.

Types of welding of low-carbon steels

1. The first method for joining low-carbon steels is manual arc welding with coated electrodes. To select the optimal type and brand of consumables, the following requirements must be taken into account:

• weld seam without defects: pores, undercuts, uncooked areas;
• equal strength connection with the main product;
• optimal chemical composition of the weld metal;
• stability of seams under shock and vibration loads, as well as high and low temperatures.

The performer receives the lowest level of stress and deformation when welding in a lower spatial position.

The following types of electrodes are used for welding ordinary structures:

Welding electrodes ANO-6

• ANO-3.
• ANO-4.
• ANO-5.
• ANO-6.
• OZS-3.
• OMM-5.
• TsM-7.

The following grades of welding materials are used for welding critical structures:

2. Gas welding is carried out in a protective environment of argon, without the use of flux, using metal wire as a filler material.

3. Electroslag welding is carried out using fluxes. Wire and plate electrodes are selected taking into account the composition of the base alloy.

4. Automatic and semi-automatic welding is carried out in a protective environment; pure argon or helium is used, carbon dioxide is often used. CO2 must be of high quality. If the combination of oxygen and carbon is oversaturated with hydrogen or nitrogen, this will lead to pore formation.

5. Automatic submerged arc welding is performed with electrode wire with a diameter of 3-5 mm; semi-automatic - 1.2-2 mm. Welding is performed with direct current of reverse polarity. The welding mode varies significantly.

6. The most optimal method is welding with flux-cored wires. The current strength ranges from 200 to 600 A. Welding is recommended to be carried out in the lower position.
7. For gas shielded welding, carbon dioxide is used, as well as mixtures of inert gas with oxygen or CO2.

Connecting products less than 2 mm thick. carried out in an atmosphere of inert gases with a tungsten electrode.

To increase arc stability, improve weld formation and reduce the sensitivity of the deposited metal to porosity, mixtures of gases should be used.

Welding in a carbon dioxide atmosphere is intended for working with alloys with a thickness of more than 0.8 mm. and less than 2.0 mm. In the first case, a consumable electrode is used, in the second - graphite or carbon. The type of current is constant, the polarity is reversed. It should be noted that this method is characterized by an increased level of spattering.

Welding medium carbon steels

Medium carbon steels are used in cases where high mechanical properties are required. These alloys can be forged.

They are also used for parts produced by cold plastic deformation; are characterized as calm, which allows them to be used in mechanical engineering.

Welding technology for medium carbon steels

These alloys are not welded as well as low carbon steels. This is due to several difficulties:

• lack of equal strength of the base and deposited metals;
• high level the risk of the formation of large cracks and non-ductile structures in the heat-affected zone;
• low resistance to the formation of crystallization defects.

However, these problems can be resolved quite easily by following these recommendations:

• the use of electrodes and wire with a low carbon content;
• welding rods must have an increased deposition rate;
• to ensure the lowest degree of penetration of the base metal, the edges should be cut, the optimal welding mode should be set, and filler wire should be used;
• preliminary and accompanying heating of workpieces.

The carbon steel welding technology, when following the above recommendations, does not reveal any problems or difficulties.

Features of welding medium-carbon steels

Before welding, the product must be cleaned of dirt, rust, oil, scale and other contaminants, which are a source of hydrogen and can contribute to the formation of pores and cracks in the seam. The edges and adjacent areas with a width of no more than 10 mm are subject to cleaning. This guarantees the strength of the connection under various types of loads.

Assembling parts for welding requires maintaining a gap, the width of which depends on the thickness of the product and should be 1-2 mm. more than when working with well-welded materials.

If the thickness of a medium carbon steel product exceeds 4 mm, edge cutting must be performed.

For the least penetration of the base metal and the optimal level of cooling, the welding mode should be selected correctly. The correctness of the choice can be confirmed by measuring the hardness of the deposited metal. In optimal mode, it should not be higher than 350 HV.

Responsible nodes are connected in two or more passes. Frequent arc breaks, burns (burning) of the base metal and crater formation on it are not allowed.

Welding of critical structures is carried out with preheating from 100 to 400°C. The higher the carbon content and thickness of the parts, the higher the temperature should be.

Cooling should be slow, the product should be placed in a thermostat or covered with heat-insulating material.

Types of welding of medium carbon steels

Welding of medium-carbon steels can be carried out in several ways, which we will discuss below.

1. Manual arc welding is performed with electrodes with a basic type of coating, ensuring a low hydrogen content in the deposited metal. Most often, performers use the following electrodes for welding carbon steels:

• ANO-7.
• ANO-8.
• ANO-9.
• OZS-2.
• UONI-13/45.
• UONI-13/55.
• UONI-13/65.

The special coating of UONI welding materials guarantees an increase in the joint’s resistance to cracking and also ensures the strength of the seam.

The following nuances should be taken into account:

• instead of transverse movements, longitudinal ones must be performed;
• it is necessary to weld the craters, otherwise the risk of crack formation increases;
• It is recommended to heat treat the seam.

2. Gas welding of thin-sheet format carbon steels is performed using the left-hand method using wire, and a normal welding flame is also used. The average acetylene consumption is 120-150 l/h per 1 mm. thickness of the alloy being welded. In order to reduce the risk of crystallization cracks, welding materials with a carbon content of no more than 0.2-0.3% should be used.

Thick-walled products should be joined using the right-hand gas welding method, which is characterized by higher productivity. Acetylene calculation is also 120-150 l/h. To avoid overheating of the working area, the flow rate must be reduced.

Gas welding of carbon steels also includes the following features:

• reduction of oxidation in the weld pool is achieved by using a flame with a slight excess of acetylene;
• the use of fluxes has a positive effect on the process;
• To avoid brittleness in the heat-affected zone, cooling is slowed down by preheating to 200-250°C or subsequent tempering at a temperature of 600-650°C.

After welding, the product can be heat treated or forged. These operations significantly improve the properties.

The technology of gas welding of carbon steels has been developed to obtain joints with the necessary mechanical properties. Therefore, it is important for the performer to take into account these specific features.

3. The technology of submerged arc welding of carbon steels involves the use of welding wire and fused fluxes: AN-348-A and OSTS-45. Welding is carried out at low current values. This allows you to “saturate” the deposited metal with the required level of silicon and manganese. These elements intensively transfer from the flux to the weld metal.

Advantages of this method: high productivity; the deposited metal is reliably protected from interaction with air, which ensures high quality connections; the efficiency of the process is achieved due to low spattering and due to the reduction of metal losses due to waste; arc stability guarantees a fine-flaky weld surface.

4. Performers often use the argon arc welding method with a non-consumable electrode. The main difficulty when welding medium-carbon steels using this method is that it is difficult to avoid the formation of pores due to slight deoxidation of the base metal. To solve this problem, it is necessary to reduce the proportion of base metal in the deposit. To do this, it is necessary to correctly select the modes for welding carbon steel with argon. Welding is carried out with direct current of straight polarity.

The voltage value is set depending on the thickness of the structure for single-pass welding and based on the height of the bead, which is 2.0-2.5 mm for multi-pass welding. Approximate current indicators can be determined as follows: 30-35 A per 1 mm. tungsten rod.

Welding high carbon steels

Demonstration welding of steel from springs with Zeller 655 electrode

The need for high-carbon steels arises when carrying out repair work, in the production of springs, cutting, drilling, woodworking and other tools, high-strength wire, as well as in those products that must have high wear resistance and strength.

Welding technology for high carbon steels

Welding is possible, as a rule, with preliminary and concomitant heating to 150-400°C, as well as subsequent heat treatment. This is due to the tendency of this type of alloy to become brittle, sensitive to hot and cold cracks, and chemical heterogeneity of the weld.

For your information! Exceptions are possible if you use specialized electrodes for dissimilar steels. See photo and caption below.

• After heating, it is necessary to carry out annealing, which must be carried out until the product cools down to a temperature of 20°C.
• An important condition is the inadmissibility of welding in drafts and at ambient temperatures below 5°C.
• To increase the strength of the connection, it is necessary to create smooth transitions from one to another metal being welded.
• Good results are achieved when welding with narrow beads, with cooling of each deposited layer.
• The contractor should also follow the rules provided for joining medium-carbon alloys.

This demonstration sample (spring, files, bearing and food grade stainless steel). If you do not pay attention to the quality of the seams, the welds were not made by professional welders, the photo confirms that welding of “non-weldable” steels is quite possible.

Features of welding high-carbon steels

The working surface must be cleaned of various types of contaminants: rust, scale, mechanical irregularities and dirt. The presence of contaminants can lead to the formation of pores.

Structures made of high-carbon steels need to be cooled slowly, in air, to normalize the structure.

Preheating critical products to 400°C allows one to achieve the required strength.

Types of welding of high-carbon steels

1. The best option The welding process is carried out using manual arc welding using coated electrodes. Working with high-carbon steels has a large number of specific characteristics. Therefore, welding of high-carbon steel is carried out with specially designed electrodes, for example, NR-70. Welding is carried out with direct current of reverse polarity.

2. Submerged arc welding is also used to join this type of alloys. It is quite difficult to evenly coat the work area with flux manually. Therefore, in most cases, automatic technology is used. The molten flux forms a dense shell and prevents the influence of harmful atmospheric factors on the weld pool. For submerged arc welding, transformers are used that produce alternating current. These devices allow you to create a stable arc. The main advantage of this method is the small loss of metal due to small spattering.

It is important to note that the gas welding method is not recommended. The process is characterized by the burning of a large amount of carbon, resulting in the formation of hardening structures that negatively affect the quality of the weld.

However, if ordinary structures are welded, then the use of this method is possible. The connection is made on a normal or low flame, the power of which does not exceed 90 m3 of acetylene per hour. The product must be heated to 300°C. Welding is carried out using the left-hand method, which makes it possible to reduce the time the metal is in the molten state and the duration of its overheating.

Welding stainless steel and carbon steel

Welding corrosion-resistant and carbon steels is a prime example of joining dissimilar materials.

Preliminary and concomitant heating of products to a temperature of approximately 600°C will make it possible to obtain a seam with a more uniform structure. After work, you need to carry out heat treatment, this will help avoid the formation of cracks. For welding stainless steel and low-carbon steels, two methods are used in practice, which involve the use of welding rods:

• high-alloy steel electrodes or nickel-based electrodes fill the weld seam;
• The edges of the low-carbon steel product are welded with alloy electrodes, then the clad layer, the stainless steel edges are welded with special electrodes for stainless steel.

Welding of stainless and carbon steels can also be carried out using the argon arc method. However, this technology is used extremely rarely and only for working with particularly critical structures.

The contractor can also make a connection using semi-automatic welding using a metal electrode in a protective environment of inert gases.

Welding carbon and alloy steels

Welding and surfacing of carbon and low-alloy steels is carried out using electrodes of types E42 and E46.

Welding of carbon steels and alloy steels using the electric arc method is performed with electrode materials that provide the necessary mechanical characteristics and heat resistance of the weld metal:

Electrodes TsL-39

The main problem is hardening of the heat-affected zone to prevent the formation of cold cracks. To solve this problem you need:

• to slow down cooling, you need to heat the products to a temperature of 100-300°C;
• instead of single-layer welding, use multi-layer welding, in which welding is performed on a small section over the uncooled previous layer;
• calcinate electrodes and fluxes;
• the connection is made with direct current of reverse polarity;
• To increase ductility, products should be tempered to 300°C immediately after welding.

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§ 75. Welding of low-alloy steels

Alloy steels are divided into low-alloy (alloying elements in total less than 2.5%), medium-alloy (from 2.5 to 10%) and high-alloy (more than 10%). Low-alloy steels are divided into low-alloy low-carbon steels, low-alloy heat-resistant steels and low-alloy non-carbon steels.

The mechanical properties and chemical composition of some grades of low-alloy steels are given in table. 33.

33. Mechanical properties of low-alloy low-carbon steels with a given chemical composition

The carbon content in low-alloy low-carbon structural steels does not exceed 0.22%. Depending on the alloying, steels are divided into manganese (14G, 14G2), silicon-manganese (09G2S, 10G2S1, 14GS, 17GS, etc.), chromium-silicon-manganese (14KhGS, etc.), manganese-nitrogen-vanadium (14G2AF, 18G2AF, 18G2AFps, etc.), manganese-oniobium (10G2B), chromium-silicon-nickel-copper (10HSND, 15HSND), etc.

Low-alloy low-carbon steels are used in transport engineering, shipbuilding, hydraulic engineering, pipe production, etc. Low-alloy steels are supplied in accordance with GOST 19281 - 73 and 19282 - 73 and special technical specifications.

Low-alloy heat-resistant steels must have increased strength at high operating temperatures. Heat-resistant steel is most widely used in the manufacture of steam power plants. To increase heat resistance, molybdenum (M), tungsten (B) and vanadium (F) are introduced into their composition, and to ensure heat resistance - chromium (X), which forms a dense protective film on the metal surface.

Low-alloy, medium-carbon (more than 0.22% carbon) structural steels are used in mechanical engineering, usually in a heat-treated state. The welding technology for low-alloy medium-carbon steels is similar to the technology for welding medium-alloy steels.

Features of welding low-alloy steels. Low-alloy steels are more difficult to weld than low-carbon structural steels. Low alloy steel is more sensitive to thermal influences during welding. Depending on the grade of low-alloy steel, during welding, hardening structures or overheating may form in the heat-affected zone of the welded joint.

The structure of the heat-affected metal depends on its chemical composition, the cooling rate and the length of time the metal remains at the appropriate temperatures at which the microstructure and grain size change. If austenite is obtained in hypoeutectoid steel by heating (Fig. 100), and then the steel is cooled at different rates, then the critical points of the steel decrease.

Rice. 100. Diagram of isothermal (at constant temperature) decomposition of low-carbon steel austenite: A - beginning of decomposition, B - end of decomposition, A1 - critical point of steel, Mn and Mk - beginning and end of the transformation of austenite into martensite; 1, 2, 3 and 4 - cooling rates with the formation of various structures

At a low cooling rate, a pearlite structure (a mechanical mixture of ferrite and cementite) is obtained. At a high cooling rate, austenite disintegrates into component structures at relatively low temperatures and structures are formed - sorbitol, troostite, bainite, and at a very high cooling rate - martensite. The most fragile structure is martensitic, therefore, during cooling, the transformation of austenite into martensite should not be allowed when welding low-alloy steels.

The cooling rate of steel, especially thick steel, during welding always significantly exceeds the usual cooling rate of the metal in air, as a result of which martensite may form when welding alloy steels.

To prevent the formation of a hardening martensitic structure during welding, it is necessary to apply measures that slow down the cooling of the heat-affected zone - heating the product and using multilayer welding.

In some cases, depending on the operating conditions of the products, overheating is allowed, i.e., the enlargement of grains in the metal of the heat-affected zone of welded joints made of low-alloy steels.

At high operating temperatures of products, in order to increase creep resistance (deformation of a product at high temperatures over time), it is necessary to have a coarse-grained structure in the welded joint. But metal with very coarse grains has reduced ductility and therefore the grain size is allowed to a certain limit.

When operating products at low temperatures, creep is eliminated and a fine-grained metal structure is required, providing increased strength and ductility.

When welding low-alloy steels, coated electrodes and other welding materials are selected so that the content of carbon, sulfur, phosphorus and other harmful elements in them is lower compared to materials for welding low-carbon structural steels. This makes it possible to increase the resistance of the weld metal against crystallization cracks, since low-alloy steels are significantly prone to their formation.

Low alloy steel welding technology. Low-alloy low-carbon steels 09G2, 09G2S, 10HSND, 10G2S1 and 10G2B are not hardened during welding and are not prone to overheating. Welding of these steels is carried out under any thermal conditions, similar to the welding conditions of low-carbon steel.

To ensure equal strength of the connection, manual welding is performed with E50A type electrodes. The hardness and strength of the heat-affected zone are practically no different from the base metal.

When welding with flux-cored wire and shielding gas, welding materials are selected so as to ensure the strength properties of the weld metal at the level of strength achieved by electrodes of the E50A type.

Low-alloy low-carbon steels 12GS, 14G, 14G2, 14KhGS, 15KhSND, 15G2F, 15G2SF, 15G2AF during welding can form hardening microstructures and overheating of the weld metal and heat-affected zone. The number of hardening structures decreases sharply if welding is performed with a relatively high heat input, which is necessary to reduce the cooling rate of the welded joint. However, a decrease in the metal cooling rate during welding leads to grain coarsening (overheating) of the weld metal and heat-affected metal due to the increased carbon content in these steels. This is especially true for steels 15ХСНД, 14ХГС. Steels 15G2F, 15G2SF and 15G2AF are less prone to overheating in the heat-affected zone, since they are alloyed with vanadium and nitrogen. Therefore, welding of most of these steels is limited to narrower limits of thermal conditions than welding of low-carbon steel.

The welding mode must be selected so that there is no large number of hardening microstructures and strong overheating of the metal. Then it is possible to weld steel of any thickness without restrictions at an ambient temperature of at least - 10°C. At lower temperatures, preheating to 120 - 150°C is required. At temperatures below - 25°C, welding of products made from hardening steels is prohibited. To prevent large overheating, welding of 15KhSND and 14KhGS steels should be carried out at reduced heat input (at lower current values ​​with electrodes of smaller diameter) compared to welding of low-carbon steel.

To ensure equal strength of the base metal and the welded joint when welding these steels, it is necessary to use electrodes of the E50A or E55 type.

The technology for welding low-alloy medium-carbon steels 17GS, 18G2AF, 35ХМ and others is similar to the technology for welding mediums of non-alloy steels.

The most consumed metal in the world is steel; in fact, steel is not a metal, but an alloy of iron and carbon. At the moment, the total amount of steel produced in the world exceeds one and a half billion tons per year. Steels are divided into carbon and alloyed; alloyed steels are distinguished by the fact that during the production process various elements are added to the steel (for example, nickel to increase corrosion resistance, manganese to increase strength characteristics and so on), giving it special properties. Carbon steels are most often used for welding, there are low-carbon steels containing less than 0.3% carbon, they lend themselves well to any welding, medium-carbon steels with a content of 0.3 to 0.6% are less amenable to the welding process, but stronger, but less ductile, high-carbon steels are the strongest, but have low relative elongation and are the least amenable to the welding process. They differ in carbon content, and, consequently, in chemical and physical properties.

Low carbon steel belongs to a large group of structural steels. The carbon content in it is no more than 0.3%; due to such a low percentage, it has the following properties:

• High plasticity and elasticity;
• Well suited to the welding process;
• High impact strength.

This brand is widely used in construction due to the fact that it is very easy to weld, since there is very little carbon in its structure, which has a bad effect on the welding process, since brittle structures and porosities can form in the metal seam, which then lead to failure. Also, due to its high softness, parts are made from it using cold stamping.

Welding carbon steels

Absolutely all grades of steel can be welded. However, each type of metal has its own technology for welding. The welding technology for carbon steels must meet the requirements, which include:

• Uniform distribution of seam strength along the entire length;
• The absence of welding defects, the seams should not have various cracks, pores, grooves, and so on;
• The dimensions and geometric shape of the seam must be made in accordance with the standards prescribed in the relevant GOST 5264-80;
• Vibration stability of the welded structure;
• The use of electrodes with low hydrogen and carbon content, which can have a negative impact on the quality of the seam;
• The structure must be strong and rigid.

Thus, the technology must be as efficient as possible, that is, give the highest process performance while ensuring high strength and reliability.

The mechanical properties of the weld metal and welded joint completely depend on the microstructure, which is the chemical composition, and is also determined by the welding mode and heat treatment, which is carried out both before and after welding.

Low carbon steel: welding technology

As mentioned above, low-carbon steels lend themselves best to the welding process. They can be welded using gas welding in an oxy-acetylene flame without additional fluxes. Metal wires are used as an additive. Hydrogen, which can form pores, can negatively affect the welding process. To prevent this problem, it is recommended to carry out the welding process with a filler metal containing a small amount of carbon.

After the welding process, the structure must be thermally treated to improve the mechanical properties - ductility and strength will be the same. Heat treatment of welded structures is carried out by a normalization operation, which consists of heating the product to a certain temperature, approximately 400 degrees, holding and further cooling in air. As a result, the structure is equalized, carbon in the form of cementite in the metal diffuses into the grains, due to which the structure becomes uniform.

Gas welding is carried out in the presence of argon, which creates a neutral environment. Structures that are welded in an argon environment have a more important purpose.

Welding of low-carbon steels can be done manually; arc welding of such material requires the right choice electrode. When choosing an electrode, it is necessary to take into account the following factors, which will ensure a uniform weld structure without defects. Before carrying out the welding process, it is necessary to calcinate the electrodes in order to prepare them for further work, remove hydrogen. Welding low-carbon iron alloys must be precise and fast, and the metal parts must be prepared before starting the process.

Welding medium carbon

The welding procedure for steel parts with a medium carbon content, from 0.3% to 0.55%, is more difficult compared to low carbon, since a higher amount of carbon can negatively affect the weld. Carbon reduces the cold brittleness limit - that is, destruction at low temperatures, increases strength and hardness, but reduces the ductility of the weld.

Electrodes with low carbon content are used for welding, which ensure a strong connection.

Welding high carbon steels

Steels with a high percentage of carbon content, from 0.6% to 0.85%, are very difficult to weld. Gas welding cannot be used in this case, since in the process carbon burns out in large quantities and hardening structures are formed, which deteriorate the quality of the weld. It is best to use arc welding in this case.

Requirements

When welding carbon steels, to achieve maximum parameters, the following requirements must be met:

• Welding electrodes and wires must have a low percentage of carbon to avoid unnecessary defects;
• It is necessary to ensure that carbon from the metal does not transfer into the weld under the influence of high temperature; for this, wire is used for welding steels with an average carbon content and higher, for example Forte E71T-1, Bars-71. These types are ideal for welding steels with a carbon content above 0.3%;
• When carrying out the welding process, fluxes should be added, which contribute to the formation of refractory formations;
• Reduce chemical heterogeneity of the seam by subsequent heat treatment;
• Reduce the hydrogen content by calcining the electrodes, using electrodes with low hydrogen content, etc.

Peculiarities

The following features of welding carbon steels should also be noted:

• Before carrying out this operation, it is necessary to thoroughly clean the material being welded from rust, mechanical irregularities, dirt, and scale. These contaminants contribute to the formation of cracks in the weld;
• Welding structures made of carbon steels need to be cooled slowly, in air, so that the structure normalizes;
• When carrying out the welding process, critical parts require preheating, up to approximately 400 degrees; with the help of heating, the required strength of the seam will be ensured; also in this case, welding can be carried out in several approaches.

Thus, the welding process of carbon steels depends mainly on their carbon content. Therefore, it is necessary to consider what content and choose the right one technological scheme to get a high-quality, durable product that can last a long time.

Medium carbon steel structures can be welded well if the welding rules and the following additional guidelines are strictly followed. In butt, corner and T-joints, when assembling the elements being connected, the gaps provided by GOST should be maintained between the edges so that welding transverse shrinkage occurs more freely and does not cause crystallization cracks. In addition, starting with a steel thickness of 5 mm or more, edges are cut in butt joints, and welding is carried out in several layers. The welding current is reduced.

Welding high carbon steel

Welding high-carbon steels of grades VSt6, 45, 50 and 60 and cast carbon steels with a carbon content of up to 0.7% is even more difficult. These steels are mainly used in castings and tool making. Their welding is possible only with preliminary and concomitant heating to a temperature of 350-400 ° C and subsequent heat treatment in heating furnaces. When welding, the rules for medium carbon steel must be followed; we will discuss this process below.

Welding technologies for high carbon steels

Good results are achieved when welding narrow beads and small areas with cooling of each layer. After welding is completed, heat treatment is required.

Welding medium carbon steel

Welding medium-carbon steel grades VSt5, 30, 35 and 40, containing carbon 0.28-0.37% and 0.27-0.45%, is more difficult, since the weldability of steel deteriorates with increasing carbon content.

Medium carbon steel of the VSt5ps and VSt5sp grades used for reinforced concrete reinforcement is welded using the bath method and conventional extended seams when connecting to the linings (Fig. 16.1). For welding, the ends of the connected rods must be prepared: for welding in the lower position, cut off with a cutter or saw, and for vertical welding, cut. In addition, they must be cleaned at the joints to a length that exceeds the weld or joint by 10-15 mm. Welding is performed with electrodes E42A, E46A and E50A for extended bead seams. At air temperatures down to minus 30 ° C, it is necessary to increase the force

Rice. 16.1. Welding joints of reinforced concrete reinforcement: a - bathroom; 1 - horizontal; 2 - vertical; b - suture

welding current by 1% when the temperature decreases from 0°C for every 3°C. In addition, you should use preheating of the joined rods to 200-250 °C for a length of 90-150 mm from the joint and reduce the cooling rate after welding by wrapping the joints with asbestos, and in the case of bath welding, do not remove the forming elements until the joint is cooled to 100 °C and below.

At lower ambient temperatures (from -30 to -50 °C), you should be guided by a specially developed welding technology, which provides for preliminary and concomitant heating and subsequent heat treatment of reinforcement joints or welding in special hothouses.

Welding of other structures made of medium carbon steel grades VSt5, 30, 35 and 40 should be carried out in compliance with the same additional instructions. Rail track joints are usually welded using bath welding with preheating and subsequent slow cooling, similar to reinforcement joints. When welding other structures made of these steels, preliminary and auxiliary heating, as well as subsequent heat treatment, should be used.

Electrodes

Welding is carried out with electrodes with a diameter of no more than 4-5 mm using direct current of reverse polarity, which ensures less melting of the edges of the base metal and, consequently, a smaller proportion of it and a lower C content in the weld metal. Electrodes E42A, E46A or E50A are used for welding. The steel electrode rods contain little carbon, so when they are melted and mixed with a small amount of medium-carbon base metal, there will be no more than 0.1-0.15% carbon in the weld.

In this case, the weld metal is alloyed with Mn and Si due to the molten coating and thus turns out to be equal in strength to the base metal. Welding of metal with a thickness of more than 15 mm is carried out in a “slide”, “cascade” or “blocks” for slower cooling. Preliminary and accompanying heating is used (periodic heating before welding of the next “cascade” or “block” to a temperature of 120-250 ° C). Structures made of steel grades VSt4ps, VSt4sp and steel 25 with a thickness of no more than 15 mm and without rigid components are usually welded without heating. In other cases, preliminary and auxiliary heating and even subsequent heat treatment are required. The arc is lit only at the site of the future seam. There should be no unwelded craters and sharp transitions from the base to the deposited metal, undercuts and intersections of seams. Creating craters on the base metal is prohibited. An annealing roller is applied to the last layer of the multi-layer seam.