Weldability of steel: classification. Weldability welded steels

Steel is the main structural material. It is an iron-carbon alloy with various impurities. All the components that make up its composition affect the properties of the ingot. One of the technological characteristics of metals is the ability to form high-quality welded joints.

Factors determining the weldability of steel

Evaluation of weldability of steel is based on the value of the main indicator - the carbon equivalent of C _eq . This is a conditional coefficient that takes into account the degree of influence of the content of carbon and basic alloying elements on the characteristics of the joint.

The weldability of steels is influenced by the following factors:

1. Carbon content.
2. The presence of harmful impurities.
3. Degree of doping.
4. Type of microstructure.
5. Environmental conditions.
6. Thickness of metal.

The most informative parameter is the chemical composition.

Distribution of steels by weldability groups

With all these factors in mind, the weldability of steel has different characteristics.

Classification of steels for weldability.

• Good (at a value of C _eq ≥0.25%): for low-carbon steel parts; Does not depend on the thickness of the product, weather conditions, availability of preparatory work.
• Satisfactory (0,25% ≤С _eq ≤0,35%): there are limitations to the environmental conditions and the diameter of the welded structure (air temperature to -5, in windless weather, thickness up to 20 mm).
• Restricted (0,35% ≤С _eq ≤0,45%): for the formation of a quality weld, the previous heating is necessary. It promotes "smooth" austenitic transformations, the formation of stable structures (ferritic-pearlitic, bainitic).
• Bad (C _eq ≥0.45%): the formation of a mechanically stable welded joint is impossible without the previous temperature preparation of the metal edges, as well as subsequent heat treatment of the welded structure. To create the necessary microstructure, additional heating and smooth cooling are necessary.

Weld weldability groups make it easy to navigate the technological features of welding specific iron-carbon alloys.

Heat treatment

Depending on the weldability of the steels and the corresponding technological features, the characteristics of the welded joint can be corrected by successive temperature influences. There are 4 main ways of heat treatment: hardening, tempering, annealing and normalization.

The most common are quenching and tempering for hardness and simultaneous strength of the weld seam, stress relief, prevention of cracking. The degree of tempering depends on the material and the desired properties.

Heat treatment of metal structures during preparatory works is carried out:

• Annealing - to relieve stress within the metal, ensure its softness and compliance;
• Previous heating in order to minimize the temperature difference.

Rational management of temperature effects allows:

• Prepare the workpiece (remove all internal stresses by grinding the grains);
• Reduce temperature drops on cold metal;
• Improve the quality of the welded object by thermal correction of the microstructure.

The correction of properties by temperature differences can be local or general. The edges are heated by means of gas or electric arc equipment. To heat the entire part and smooth cooling, special furnaces are used.

Influence of microstructure on properties

The essence of heat treatment processes is based on structural transformations inside the ingot and their effect on the solidified metal. Thus, when heated to a temperature of 727 ˚C, it is a mixed granular austenite structure. The cooling method determines the transformation options:

1. Inside the furnace (speed 1? C / min) - pearlite structures with a hardness of about 200 HB (Brinell hardness) are formed.
2. In air (10˚С / min) - sorbitol (ferrite-pearlite grains), hardness 300 НВ.
3. Oil (100 ° C / min) - troostite (ferrite-cementite microstructure), 400 HB.
4. Water (1000˚С / min) - martensite: solid (600 НВ), but a fragile needle structure.

Welding must have sufficient hardness, strength, qualitative indicators of plasticity, so the martensitic characteristics of the weld are not acceptable. Low-carbon alloys have a ferritic, ferrite-pearlite, ferrite-austenite structure. Medium-carbon and medium-alloy steels are pearlitic. High-carbon and high-alloy - martensitic or troostite, which is important to lead to ferrite-austenitic appearance.

Welding of low-carbon steels

The weldability of carbon steels is determined by the amount of carbon and impurities. They are able to burn out, turning into gaseous forms and giving a low-quality suture of porosity. Sulfur and phosphorus can concentrate along the edges of grains, increasing the fragility of the structure. Welding is most simplified, however, requires an individual approach.

Carbon steel of normal quality is divided into three groups: A, B and B. Welding works are carried out with Group B metal.

Weldability of steel grades VSt1 - VSt4, in accordance with GOST 380-94, is characterized by the absence of restrictions and additional requirements. Welding of parts with a diameter of up to 40 mm occurs without heating. Possible indicators in brands: D - increased manganese content; Kp, ps, cn - "boiling", "half-quiet", "calm", respectively.

Low-carbon high-quality steel is represented by grades with the designation of hundredths of carbon, indicating the degree of deoxidation and manganese content (GOST 1050-88): steel 10 (also 10kp, 10ps, 10g), 15 (also 15kp, 15ps, 15g), 20 (also 20kp, 20ps, 20G).

To ensure a quality weld, it is necessary to carry out the process of saturation of the welding bath with carbon C and Mangan Mn.

Welding methods:

1. Manual arc using special, initially calcined electrodes, with a diameter of 2 to 5 mm. Types: E38 (for medium strength), E42, E46 (for good strength up to 420 MPa), E42A, E46A (for high strength of complex structures and their operation under special conditions). Welding with OMM-5 and SSSI 13/45 rods is performed under the action of a direct current. Work with the help of electrodes ЦМ-7, ОМА-2, СМ-11 are conducted by a current of any characteristic.
2. Gas welding. Most often it is undesirable, but possible. It is carried out using the filler wire Sv-08, Sv-08A, Sv-08HA, Sv-08GS. A thin low-carbon metal (d 8mm) is welded in the left way, thick (d 8mm) - right. Disadvantages of the properties of the seam can be removed by normalization or annealing.

Welding of low-carbon steels is performed without additional heating. There are no restrictions for details of a simple form. Volumetric and latticework is important to protect from wind. Complex objects should be welded in a shop at a temperature of at least 5 ° C.

Thus, for grades ВСт1 - ВСт4, steel 10 - steel 20 - weldability is good, practically without limitations, requiring a standard individual selection of the welding method, electrode type and current characteristics.

Medium-carbon and high-carbon structural steels

The saturation of the alloy with carbon reduces its ability to form good compounds. In the process of thermal effects of the arc or gas flame, sulfur accumulates along the edges of the grains, leading to redness, and phosphorus to cold shortness. Most often, materials are doped with manganese.

This includes structural steel of ordinary quality VS4, VST5 (GOST 380-94), qualitative 25, 25G, 30, 30G, 35, 35G, 40, 45G (GOST 1050-88) for different metallurgical production.

The essence of the work is to reduce the amount of carbon in the weld pool, saturate the metal in it with silicium and manganese, and ensure the optimal technology. It is important not to allow excessive loss of carbon, which can lead to destabilization of mechanical properties.

Features of welding operations with steels of medium and high carbon content:

1. Primary heating of edges up to 100-200 ° C for a width of up to 150 mm. Only grades ВСт4 and steel 25 are welded without additional heating. For medium-carbon, having satisfactory weldability, a full normalization is performed before the start of the work. For high-carbon preparatory annealing is necessary.
2. Arc welding is carried out with coated calcined electrodes, size from 3 to 6 mm (OZS-2, UONI-13/55, ANO-7), under constant current. It is possible to work in a medium of flux or protective gases (CO ₂ , argon).
3. Gas welding is carried out by a carburizing flame, left-hand method, with the previous heating to a temperature of 200 ° C, with a uniform low power supply of acetylene.
4. Mandatory heat treatment of parts: hardening and tempering or separate tempering to minimize internal stresses, prevent cracking, soften hardened martensitic and troostite structures.
5. Contact spot welding is performed without restriction.

Thus, medium- and high-carbon structural steels are welded practically unrestricted, at an ambient temperature of at least 5 ° C. At lower temperatures, initial heating and high-quality heat treatment are mandatory.

Welding of low-alloy steels

Alloyed steels are steels that are saturated with various metals during melting in order to obtain the specified properties. Virtually all of them have a positive effect on hardness and strength. Chromium and nickel are a part of heat-resistant and stainless alloys. Vanadium and silicon give elasticity, they are used as a material for making springs and springs. Molybdenum, manganese, titanium increase wear resistance, tungsten - reddishness. At the same time, positively affecting the properties of parts, they impair the weldability of steel. In addition, the degree of hardening and formation of martensitic structures is increased, internal stresses and risks of crack formation in the joints.

The weldability of alloy steels is also determined by their chemical composition.

Low-alloyed low-carbon 2GS, 14G2, 15G, 20G (guest 4543-71), 15HSND, 16G2AF (GOST 19281-89) are well welded. Under standard conditions, no additional heating and heat treatment is required upon completion of the processes. However, there are some limitations:

• A narrow range of permissible thermal conditions.
• Work should be carried out at a temperature of at least -10 ° C (in conditions of lower atmospheric temperatures, but not below -25 ° C, preheat to 200 ° C).

Possible ways:

• Electric arc welding with a direct current power of 40 to 50 A, electrodes E55, E50A, E44A.
• Automatic arc welding with flux using the filler wire Sv-08HA, Sv-10GA.

Weldability of steel 09G2S, 10G2S1 is also good, the requirements and possible ways of performing the same as for alloys 12GS, 14G2, 15G, 20G, 15HSND, 16G2AF. An important characteristic of alloys 09G2S, 10G2S1 is the absence of the need for preparation of edges for parts with a diameter of up to 4 cm.

Welding of medium alloy steels

Medium-alloy steels 20ХГСА, 25ХГСА, 35ХГСА (гость 4543-71) produce more significant resistance to the formation of unstressed seams. They belong to a group with satisfactory weldability. They require preheating to temperatures of 150-200 ° C, multilayer joints, quenching and tempering at the end of welding operations. Implementation options:

• The current strength and electrode diameter when welding with an electric arc are chosen strictly depending on the thickness of the metal, taking into account that the thinner edges are harder to harden during work. So, with a product diameter of 2-3 mm, the current value should be within the range of 50-90 A. With a thickness of 7-10 mm, the DC strength of the reverse polarity increases to 200 A with the use of 4-6 mm electrodes. Used rods with cellulose or fluoride-calcium protective coatings (Sv-18HGSA, Sv-18XMA).
• When working in a CO ₂ shielding gas, it is necessary to use a wire of S-08G2S, S-10G2, S-10GSMT, S-08X3G2SM with a diameter of up to 2 mm.

Often for these materials, an argon-arc method or submerged arc welding is used.

Heat-resistant and high-strength steels

Welding works with heat-resistant iron-carbon alloys 12МХ, 12Х1М1Ф, 25Х2М1Ф, 15Х5ВФ it is necessary to conduct with preheating to temperatures of 300-450 ° С, with final hardening and high tempering.

• Electric arc welding by cascade way of multilayered seam design, using calcined coated electrodes УОНИИ 13 / 45МХ, ТМЛ-3, ЦЛ-30-63, ЦЛ-39.
• Gas welding with the supply of acetylene 100 dm ³ / mm with the use of additive materials Sv-08HMFA, Sv-18KhMA. The pipe joint is performed with the previous gas heating of the entire joint.

When welding medium-doped high-strength materials 14H2GM, 14X2GMRB it is important to follow the same rules as for heat-resistant steels, taking into account some of the nuances:

• Careful trimming of edges and use of potholders.
• High-temperature calcination of the electrode (up to 450 ° C).
• Preheating to 150 ° C for parts more than 2 cm thick.
• Slow cooling of the seam.

High-alloy steels

The use of special technology is necessary when welding high-alloy steels. These include a huge range of stainless, heat-resistant and high-temperature alloys, some of them: 09H16N4B, 15H12VNMF, 10X13SU, 08H17N5MZ, 08H18G8N2T, 03H16N15MZB, 15H17G14A9. Weldability of steels (GOST 5632-72) refers to the 4th group.

Weldability characteristics of high-carbon high-alloy steel:

1. It is necessary to reduce the current by an average of 10-20% due to their low thermal conductivity.
2. Welding should be carried out with a gap, electrodes up to 2 mm in size.
3. Reduction in the content of phosphorus, lead, sulfur, antimony, increase in the numerical presence of molybdenum, vanadium, tungsten by using special coated rods.
4. The need to form a mixed microstructure of the seam (austenite + ferrite). This ensures the ductility of the weld metal and the minimization of internal stresses.
5. Mandatory heating of the edges on the eve of welding. The temperature is selected in the range from 100 to 300 ° C, depending on the microstructure of the structures.
6. The choice of coated electrodes in arc welding is determined by the type of grains, properties and working conditions of the parts: for austenitic steel 12Х18Н9: УНИНИИ 13 / НЖ, ОЗЛ-7, ОЗЛ-14 with coatings СН-06Х19Н9Т, S-02Х19Н9; For martensitic steel 20Х17Н2: УОННИИ 10Х17Т, АН-В-10 with coating Sv-08Х17Т; For austenitic-ferritic steel 12Х21Н5Т: ЦЛ-33 with coating Sv-08Х11В2МФ.
7. In gas welding, the acetylene supply should correspond to a value of 70-75 dm ³ / mm, the filler wire used is Sv-02Kh19N9T, Sv-08Kh19N10B.
8. Possible work under the flux with the use of NZH-8.

Weldability of steel is a relative parameter. It depends on the chemical composition of the metal, its microstructure and physical properties. At the same time, the ability to form qualitative connections can be corrected with the help of a thoughtful technological approach, special equipment and working conditions.