Welding of medium carbon steels. Welding carbon steels - how to properly perform the welding process? Welding technology for medium alloy steels

» 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 he who determines the welding mechanical properties 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 concentration 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 the method manual arc welding coated electrodes. Applying 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 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 for processing.

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 provides such steels equal strength 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.

The weld deposited using the multilayer welding method is different increased level of fragility.

Connections have high resistance against MCC due to 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 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 performing welding in the lower spatial position.

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

• ANO-5.
• OMM-5.
• TsM-7.

The following grades of welding materials are used for welding:

• AN-7.
• ANO-1.
• VSP-1.
• WCC-2.
• DSK-50.
• K-5A.
• KPZ-32R.
• MR-1.
• RBU-5.
• SM-5.
• UP-1/55.
• UP-2/55.
• E-138/45N.
• ERS-1.
• ERS-2.

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

3. Electroslag welding 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 carried out with 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 performed with electrode wire with a diameter of 3-5 mm; semi-automatic - 1.2-2 mm. Welding is in progress DC reverse polarity. The welding mode varies significantly.

6. The most optimal way is welding cored wires. The current strength ranges from 200 to 600 A. Welding is recommended to be carried out in the lower position.

7. For welding in protective gases 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 by an 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 - or. 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 several difficulties:

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

However, these problems are quite easily solved by implementation of the following 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 needs to be cleaned from dirt, rust, oil, scale and other contaminants that 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, you should select the right welding mode. 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 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 using electrodes with the main 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.

Special coating of welding materials SSSI 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;
• must be produced crater filling, otherwise the risk of crack formation increases;
• It is recommended to heat treat the seam.

2. Gas welding carbon steels in thin sheet format performed left way with wire, also used normal welding flame . 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 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. Technology submerged arc welding 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 this method: high performance; 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 method argon arc welding 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. 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 accompanying heating up 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 make annealing, which must be carried out until the product has cooled to a temperature of 20°C.
• An important condition is the inadmissibility of welding in drafts and at temperatures environment 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 narrow rollers, with cooling of each deposited layer.
• The contractor should also follow the rules provided for joining medium-carbon alloys.

This is a demonstration sample (spring, files, bearing and food-grade stainless steel are welded together). 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.

Video

Features of welding high-carbon steels

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

Cool structures made of high carbon steels need slowly, in air, to normalize the structure.

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 wide application received electrodes of type E46T with rutile coating. 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 must 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 fixing 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 dm3/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 conditions 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 structures and 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 - 20%, the electrode is moved along the seam with longitudinal vibrations. When manually welding galvanized steel top scores 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.

Welding carbon steels has a number of features and certain difficulties, which are due precisely to the fact that carbon is the main alloying element in them.

1 Main features of welding carbon steel

Carbon steels include steels with a carbon content from 0.1 to 2.07%. Alloys in which this element is contained in an amount of 0.6-2.07% are called high-carbon, 0.25-0.6% - medium-carbon, less than 0.25% - low-carbon. The welding technology for each of these groups of alloy steels is different. At the same time there is general recommendations, which should be adhered to when welding products made of alloys that include carbon as the main alloying element. We'll talk about them.

Butt welds, connected semi-automatically using flux-cored wires and in a protective atmosphere, coated electrodes (manually), as well as using gas welding, are in most cases welded by weight. If automatic equipment is used, it is necessary to use methods that, firstly, guarantee sufficient penetration of the root of the seam, and secondly, eliminate the possibility of burn-throughs.

For different methods welding has its own standards that describe the requirements for the parameters of seams and the process of preparing the edges of the parts being joined. In order to securely fix the components included in them together, it is recommended to assemble welded structures using special tacks or assembly devices.

Tacks are usually used in a semi-automatic process in carbon dioxide or when using coated electrodes for alloy carbon steels. The thickness of the metal determines the length of these tacks, and their cross-sectional area is usually about 2.5–3 centimeters (up to a third of the cross-sectional area of ​​the resulting weld). It is advisable to apply them on the side that is opposite to the single-pass main seam. In cases where we are talking about multi-pass seams, tacks are applied with reverse side in relation to the very first layer.

Before starting welding, the tacks must be thoroughly cleaned and visually inspected. If cracks are found during such an inspection, they must be removed. Another point is that it is necessary to achieve complete melting of the tacks used. Otherwise due to increased speed As heat dissipates, cracks may appear on them, which impair weldability and make the entire welding process poor in quality.

Carbon alloys demonstrate high efficiency when applying multiple seams and when welding products on both sides. Multilayer welding is recommended for parts with large thickness, as well as for structures operating in critical conditions. If, after the process, undercuts, cracks, pores, lack of penetration and other defects are found in the seams, you should:

• mechanically remove metal in a “dangerous” place;
• clean the defect area;
• weld the cleaned area.

When using the electroslag welding method, the products must be mounted with a certain gap, which should have a slight expansion towards the end. The relative position of the elements of the structure to be welded is fixed using staples (the distance between them is from 50 to 100 centimeters). In addition, during the electroslag process and during automatic arc welding, strips are mounted on the seam (at the beginning and at the end), which facilitate the procedure and provide the specified parameters of the seam.

2 How is welding of low-carbon steels performed?

The weldability of such steels is considered relatively simple among professionals if any methods and types of joining parts by melting are used. A specific welding technology is assigned taking into account the fact that there should be no significant defects in the welded joint at the end of the procedure.

It is worth noting that when welding alloyed alloys with low carbon content, the base metal has a number of differences from the weld metal:

• in the metal of the compound the proportion of silicon and manganese increases, but carbon becomes less;
• there is a change mechanical characteristics heat-affected metal (electric and usually lead to insignificant strengthening of the material in the overheated area);
• there is a possibility that the metal near the weld will reduce its impact strength (this is observed when welding non-aging alloys);
• During a multilayer welding process, the weld metal can quickly become embrittled.

All these differences do not have a significant impact on the quality of the weld obtained by fusion welding.

Also, no difficulties arise when gas welding steels alloyed with a small amount of carbon (up to 0.25%). Moreover, as a rule, flux is not used in gas operations. With the right method of such welding, from 120 to 150 cubic decimeters of acetylene per hour are consumed per millimeter of the thickness of the welded product, with the left method - from 100 to 130. It is also possible to use a more powerful flame (consumption - up to 200 cubic decimeters). But then it is necessary to take a larger cross-section of filler wire.

Excellent weldability of products made from low-carbon alloy steels is also observed when coated electrodes are used. Optimal welding results are provided by rods with rutile (E46T) and calcium-fluoroisrutile (E42A) layers. Coated welding rods with added iron powder are also popular among professional welders.

Electroslag welding of products made of low-carbon steels is carried out using fluxes AN-22, FC-1, AN-8, FC-7, AN-8M. The wire is selected taking into account the composition of the alloy. So, for example, St3 is welded using wire Sv-08Gs, Sv-10G2, SV-08GA, and boiling steel grades - Sv-08A.

3 Subtleties of welding medium-carbon steels

The weldability of these alloys is not as good as low-carbon alloy steels, since they contain large amounts of carbon. The following difficulties are noted when welding medium-carbon materials: lack of equal strength of the base metal and the weld metal; high risk of the formation of large cracks and hardening non-plastic structures in the area near the weld; low resistance to crystallization defects.

However, all these problems when welding medium-carbon alloys are not so difficult to resolve. You can use welding rods with an increased deposition rate, surfacing wire and special electrodes for carbon steel with a low carbon content. In this case, manual arc welding proceeds without difficulty. It is also recommended to increase the weldability of parts by:

• implementation of a separate (several baths) two-arc welding process;
• changes in the structure of the weld metal (use of special edge cutting modes that ensure the lowest degree of penetration of the base metal);
• heating (both concomitant and preliminary) of the workpieces to be joined.

Electric arc welding of structures made of medium-carbon alloy steels is in most cases carried out using UONI rods (13/45 and 13/55). They have a special coating (calcium fluoride), which guarantees an increase in the resistance of the weld metal to cracks (crystallization) and excellent strength of the resulting weld.

The technology of arc welding of medium-carbon products provides the following features:

• due to the risk of cracks forming, it is advisable to weld craters, as well as perform longitudinal movements of the electrode instead of transverse ones;
• narrow rollers should be applied using a short electric arc;
• It is recommended to perform heat treatment of the seam after welding (especially when it is technical specifications must have increased ductility).

Gas joining of alloyed medium-carbon alloys is carried out using a slightly carburizing or standard flame. In this case, only the left method is used, and the flame power varies from 75 to 100 cubic decimeters per hour. After welding, you can perform heat treatment or forging of the metal. These operations will significantly improve the properties of steel. If parts whose thickness exceeds three millimeters are welded, gas welding technology requires heating them to approximately 650 (local heating) or up to 350 (general heating) degrees.

Separately, we will say that it is possible to weld medium-carbon structures even at low temperatures (-30 degrees or less). In such situations, a special welding technology is used, which requires mandatory heat treatment of the products after welding and constant heating of the metal (first it is preheated to the temperatures indicated above, and then heated throughout the entire operation). If the stated requirements are met, the quality of the seam will be impeccable.

4 Is it possible to weld high-carbon alloys?

The high carbon content in such steels makes them unsuitable for the production of welded structures. But often when carrying out repair activities there is a need for welding high-carbon alloys. In these cases, they are welded using methods that are used for steels with medium carbon content. The only condition is that welding of high-carbon products is not carried out in drafts and when the ambient temperature is less than five degrees Celsius.

Welding of steels with a high (up to 0.75 percent) carbon content using the gas method is carried out using a carburized (slightly) or normal flame, with a capacity of no more than 90 cubic meters of acetylene per hour. In this case, the metal is heated to 300 degrees ( required condition to obtain a quality connection). Welding of high-carbon alloys is performed using the left-hand method. This makes it possible to reduce the time the metal remains in the melt state and the time it overheats.

Carbon structural (machinery or construction) steels are those that contain up to approximately 2% carbon. First you need to know that the steel is filled:

• up to 0.25% are called low carbon;
• from 0.26% to 0.6 - medium carbon;
• from 0.6 to 2% - high-carbon.

And all of them do not have alloying elements. Anything higher than this content is called cast iron. Carbon determines the strength characteristics and directly affects the weldability of steels.

Composition, purpose and use

These materials are widely used in the national economy. Starting from the manufacture of simple nails to high-strength and especially critical structures.

The conversation here will be about working with steels saturated with medium amounts of carbon. These are materials where its share ranges from 0.25% to 0.45%. This percentage is the main difference from low-carbon steels. It imparts hardness to steel, but makes weldability worse. Used in shipbuilding and mechanical engineering. Since all carbon steels are also classified by quality, there are also manganese additives from 0.7% to 1%. In industry, medium-carbon steel is used in a normalized state, this is when the rolled product undergoes a certain heat treatment before the welding process. In welded-cast and welded-forged structures, 35 steel or 40 steel are usually used.

Characteristics of Medium Carbon Steel

An unpleasant feature of these materials is the appearance of hardening structures in the weld, near the weld and in the heat-affected zone (HAZ). These “bad” structures almost guarantee dangerous conditions for the “brittleness” of the connection. This means that when choosing a steel grade, the manufacturer not only focuses on its strength characteristics, but also on how the welded joint will “behave” during preparation, during the manufacturing process, and what the mechanical properties of the joint will be after welding and during operation of the product.

Sometimes destruction occurs due to the fact that strong residual stresses appear in the connection and the ductility of the metal is greatly reduced. This is precisely the result of the wrong choice of material, welding method and welding technology.

Concept of weldability

Here you need to understand the “ability” of the material to withstand high-temperature conditions during a certain welding process without the appearance of sections of metal with “low plasticity” in the connection, which “provoke” the occurrence of cracks, or the fact that the connections are destroyed during operation. Simply put, this is the ability of metal parts to be joined by thermal action, without deteriorating the mechanical properties of the welded product.

Measures required when preparing this steel for welding:

• use only regulated base materials, for example: mild steel;
• welding methods will be used only those that guarantee the weld the required characteristics (welding with coated electrodes, submerged arc welding, in shielding gases);
• competently design welded structures (exclude contrasting transitions from one section to another, avoid “crowding” of seams in a small area of ​​the product, whenever possible give priority to butt joints);
• special attention to assembly quality (minimize gaps and displacements, avoid tension in structures);
• try to use heat treatment, it relieves unnecessary internal stress.

Welding process and types

As mentioned above, a significant carbon content complicates the welding process. To overcome the above difficulties and make the weld metal resistant to cracks in any fusion welding, it is necessary to reduce the level of carbon in the weld metal. To do this, use welding materials with a low carbon content and reduce the amount of base metal in the joint. Simply, the edges are given the appropriate cutting shape.

It is advisable to provide preheating to a temperature of 250-3000 C. Due to this, it is possible to almost eliminate the occurrence of hardening structures in the HAZ (heat-affected zone).

Mechanized and

It is necessary to use such modes in which the penetration of the base metal would be minimal and the weld shape coefficient would be maximum. Increase share electrode metal in the seam. In semi-automatic operation, this is achieved by using small diameter wire and minimal current. In this case, it is better to work with direct current of straight polarity.

Alloying is also a good idea. To achieve this, it is enough to use wire with a reduced content of sulfur and phosphorus, with the addition of silicon and manganese. In automatic welding, alloying occurs due to flux.

Manual

For this welding, electrodes with a basic coating are used. They provide alloying, the seam becomes resistant to cracks. But to avoid brittle hardening structures in the HAZ, slow cooling of the product is desirable. To do this, reduce the welding speed, preheat and use two or more extended arcs. The higher the carbon content, the higher the heating temperature during welding (accompanying heating) should be. But all the same, when, with all the listed methods, it is completely impossible to impart the required ductility to the joint, hardening and tempering is used.

Electroslag method

This is a special welding method that uses a slag bath to heat the melting zone. Heating is carried out by electric current. Here, thanks to the wide ability to change the shape coefficient of the bath and slow cooling, the conditions are created for creating a high-quality connection. By feeding the wire at a speed not exceeding a critical value, high resistance against crystalline cracks is ensured.

Problems here may arise if the carbon content exceeds 0.33%. Then you need to use wire with manganese and silicon.

Carbon dioxide welding

The technology of this type is in many ways similar to manual arc welding or submerged arc welding. It is also based on reducing the percentage of base metal in the weld and ensuring favorable penetration. But it is rarely used in mass production.

It is important to remember that with any method of welding medium carbon steel, the most important point in the preparation and process is to impart the necessary ductility to the joint. And the way to ensure this plasticity is already selected based on specific situation at which welding will take place.

Visual inspection of welded joints

Control of welded joints is an integral part of the entire technological welding process.

Visual inspection is one of the many methods to which all welded joints are subjected without exception. And not only. Work on visual inspection begins already at the stage of acceptance of basic and welding materials in welding production. But in this article only visual control will be considered. But first you need to understand the problems that it solves and what it is aimed at.

Defects in welded joints

Defects in welding production are defined as non-compliance with the norms and rules according to which the joint is made.

These “jambs” that arose during the welding process are divided into internal and external. External ones are revealed by visual inspection of the connection. Looking ahead, it should be clarified that visual control itself does not exist as a separate method. It always goes in conjunction with measurement. In production, that’s what they call it – visual and measuring control. Well, in order to start measuring, it is necessary to visually identify defects, record them, and during measurement determine whether the identified inconsistencies are acceptable or not and how they will affect the operation of the product. Defects should be identified already at the stage of preparation for welding. Since they directly affect the quality of the final welding product.

Defects during preparation for welding, the reasons for their occurrence and their impact on the quality of the connection

Inconsistencies in preparation and assembly lead to subsequent welding defects. For example: incorrect bevel angle of the edges, large or, on the contrary, small bluntness, axial displacement, mismatch of joining planes, increased gap and the geometry of the seam is unacceptably violated!

Unprocessed and uncleaned edges, damp surfaces or unheated electrodes, delaminations, incorrectly selected welding modes and pores, fistulas and lack of fusion along the seam are guaranteed!

Increased current strength, rapid movement of the electrode along the seam and undercuts smile at us!

If the arc is abruptly broken, there will definitely be an unwelded crater at the end of the seam.

All defects create a local stress concentration, reduce the useful cross-section of the seam, weakening the structure, and in some cases even spread further along the seam. For example, cracks and microcracks. It is clear that such a design will not withstand even a minimum service life.

Correct assembly is accompanied by external inspection and measurement using special verified devices, templates and standards. And the shape and dimensions of the seams are specified by technical specifications, which stipulate the number of passes and the depth of penetration.

A word about external defects

These outdoor “welding pests” include the following::

• sagging - flow of molten metal onto the base;
• undercuts - point or oblong grooves in the base metal, running along the edges of the seam;
• unwelded craters - a depression at the end of the weld when the arc suddenly breaks;
• burns - a through hole when welding the first layer of a seam;
• arson is the result of “striking” by the electrode when the arc is excited;
• cracks - rupture of metal along a seam or adjacent metal;
• pores - a round-shaped cavity;
• splashes - frozen drops on a connection;
• fistula - a defect in the form of a funnel in the seam.

It is all these defects that visual inspection is designed to identify and record.

Visual inspection

When carrying out welding work, preparatory actions are also subject to external inspection and often measurement. The quality of the material is checked - the presence or absence of defects on the metal (burrs, dents, cleanliness of edges), the preparation of structural elements of the edges (correct cutting angle, gap, alignment), the quality and correctness of tack welding. Structures that were assembled with violations technical specifications, are rejected.

During the welding process itself, the welder (he is the natural and first quality controller of the connection), in addition to monitoring the welding mode and the stability of the arc, observes how the beads are made when making multi-layer seams. It is extremely important to control the quality of the initial pass (weld root). Because it is the first layer that “paints” the entire subsequent “picture” of the welded joint. Very often you even have to examine the root using a 4-7x magnifying glass.

With visual inspection finished products A magnifying glass is also used. First of all, all those “welding jambs” that were mentioned above are identified. Most of them are unacceptable and must be corrected. Also great attention, during visual inspection, attention is paid to the shape of the seam, the correct “pattern” of the scales and the “overall picture” of the distribution of metal in the reinforcement of the seam.

Each seam made in different “poses” has its own characteristic appearance and shape.

When inspecting particularly critical products and structures (especially in the military and space industries), the appearance of the seams is often compared with specially made standards. The geometry is controlled using templates and measuring tools.

Visual control is quite informative and is a cheap and fast control method. And with careful observation of the welding process, the occurrence of many defects can be eliminated. Visual inspection is an inexpensive procedure and very effective in the technological process.

Conditions for visual and measuring control

To conduct a high-quality VIC it is necessary to create certain conditions on any site. Whether it’s a high-tech production facility where they work in white coats and gloves, a welding shop or an installation site. They include:

• convenience of specialist approach;
• possibility of connecting local lighting 12 V;
• illumination should be at least 500 lux (500 lux);
• indoor walls, ceilings and tables should be painted in light colors;
• ensuring sufficient visibility for the specialist's eye. The surface is viewed at an angle of more than 300 to the plane and from a distance of no less than 600 mm;
• cleaning surfaces as required by regulatory documents;
• measures for safe control.

Only after a thorough visual inspection has been carried out and all inconsistencies have been corrected, the connections are subjected to other control methods, if required by the design documentation.

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. Distinctive feature boiling steel is the uneven distribution of sulfur and phosphorus throughout the thickness of the rolled product. If an area with an accumulation of sulfur 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”.

Steel 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. 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. Additional requirements may be imposed on the welded joint, which are related to the operating conditions of the structure. All without exception welding seams must be durable and reliable, and the technology must ensure productivity and cost-effectiveness of the process.

The mechanical properties of a welded joint are affected 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 welding connection, 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 gas welding, 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. Disadvantage manual welding is a great 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. Automatic welding Submerged arc welding of low-carbon steels is recommended to be performed 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 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.