Clever Geek Handbook

Steel weldability: classification. Steel weldability groups

Steel is the main structural material. It is an iron-carbon alloy containing various impurities. All 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

Assessment of weldability of steel is carried out according to the value of the main indicator - carbon equivalent C equiv . This is a conditional coefficient taking into account the degree of influence of the carbon content and the main alloying elements on the characteristics of the seam.

The weldability of steels is influenced by the following factors:

  1. Carbon content.
  2. The presence of harmful impurities.
  3. The degree of alloying.
  4. Type of microstructure.
  5. Environmental conditions.
  6. The thickness of the metal.

The most informative parameter is the chemical composition.

The distribution of steels in weldability groups

Given all of the above factors, the weldability of steel has various characteristics.

Classification of steel by weldability.

  • Good (at C equiv ≥0.25%): for low carbon steel parts; It does not depend on the thickness of the product, weather conditions, the availability of preparatory work.
  • Satisfactory (0.25% ≤ equiv ≤0.35%): there are restrictions to the environmental conditions and the diameter of the welded structure (air temperature up to -5, in calm weather, thickness up to 20 mm).
  • Limited (0.35% ≤ equiv ≤0.45%): previous heating is necessary to form a high-quality seam. It contributes to “smooth” austenitic transformations, the formation of stable structures (ferritic-pearlitic, bainitic).
  • Bad ( equiv ≥0.45%): the formation of a mechanically stable welded joint is impossible without previous temperature preparation of the metal edges, as well as subsequent heat treatment of the welded structure. For the formation of the desired microstructure, additional heating and smooth cooling are necessary.

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

Heat treatment

Depending on the weldability group of steels and the corresponding technological features, the characteristics of the welded joint can be adjusted using sequential temperature influences. There are 4 main methods of heat treatment: hardening, tempering, annealing and normalization.

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

Heat treatment of metal structures during the preparatory work is carried out:

  • annealing - to relieve stress inside the metal, to ensure its softness and suppleness;
  • previous heating to minimize temperature differences.

Rational management of temperature effects allows you to:

  • prepare the part for work (remove all internal stresses by grinding the grains);
  • reduce temperature differences on cold metal;
  • improve the quality of the welded object by thermal correction of the microstructure.

Correction of properties by temperature differences can be local or general. Edging is heated using gas or electric arc equipment. To heat the entire part and smooth cooling, special furnaces are used.

The effect of microstructure on properties

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

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

The welding joint must have sufficient hardness, strength, quality indicators of ductility, so the martensitic characteristics of the weld are not acceptable. Low-carbon alloys have a ferritic, ferrite-pearlite, ferrite-austenitic structure. Medium carbon and medium alloyed steel - pearlitic. High carbon and high alloyed - martensitic or troostitic, which is important to bring to a ferritic-austenitic appearance.

Mild Steel Welding

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 seam porosity. Sulfur and phosphorus can concentrate at the edges of the grains, increasing the fragility of the structure. Welding is most simplified, however, it requires an individual approach.

Carbon steel of ordinary quality is divided into three groups: A, B and C. Welding is carried out with metal of group B.

The weldability of steel grades BCt1 - BCt4, 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 the brands: G - high manganese content; kp, ps, cn - "boiling", "semi-calm", "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 high-quality weld, it is necessary to saturate the weld pool with carbon C and manganese 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 work in special conditions). Rod welding OMM-5 and UONI 13/45 is performed under the influence of direct current. Work with the help of electrodes TsM-7, OMA-2, SM-11 is carried out by a current of any characteristic.
  2. Gas welding. Most often undesirable, but possible. It is carried out using filler wire Sv-08, Sv-08A, Sv-08GA, Sv-08GS. Thin low-carbon metal (d 8mm) is welded in the left way, thick (d 8mm) - in the right. Deficiencies in weld properties can be removed by normalization or annealing.

Welding of low carbon steels is performed without additional heating. There are no restrictions for simple form details. Volumetric and lattice structures are important to protect from wind. It is desirable to weld complex objects in a workshop at a temperature of at least 5 ° C.

Thus, for grades BCt1 - BCt4, steel 10 - steel 20 - weldability is good, almost without restrictions, requiring a standard individual selection of the welding method, type of electrode and current characteristics.

Mid-carbon and high-carbon structural steels

The saturation of the alloy with carbon reduces its ability to form good compounds. During the temperature effects of an arc or a gas flame, sulfur accumulates along the edges of the grains, leading to red breaking, phosphorus to cold breaking. Most often, materials alloyed with manganese are welded.

This includes structural steels of ordinary quality VSt4, VSt5 (GOST 380-94), high-quality 25, 25G, 30, 30G, 35, 35G, 40, 45G (GOST 1050-88) of various metallurgical production.

The essence of the work is to reduce the amount of carbon in the weld pool, saturate the metal in it with silicon and manganese, and ensure optimal technology. It is important to prevent excessive carbon loss, which can lead to destabilization of the mechanical properties.

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

  1. Initial heating of the edges to 100-200 ° C for a width of up to 150 mm. Only grades BCt4 and steel 25 are welded without additional heating. For medium-carbon, with satisfactory weldability, a full normalization is performed before starting work. For high-carbon, preliminary annealing is required.
  2. Arc welding is carried out by coated calcined electrodes, ranging in size from 3 to 6 mm (OZS-2, UONI-13/55, ANO-7), under direct current. work in the environment of flux or protective gases (CO 2 , argon) is possible.
  3. Gas welding is carried out by a carburizing flame, in the left-handed manner, 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 in order 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 with almost no restrictions, at an external temperature of at least 5 ° C. At lower temperatures, initial heating and high-quality heat treatment are required.

Welding low alloy steels

Alloy steels are steels that during smelting are saturated with various metals in order to obtain the desired properties. Almost all of them have a positive effect on hardness and strength. Chrome and nickel are part of heat-resistant and stainless alloys. Vanadium and silicon give elasticity, are used as a material for the manufacture of springs and springs. Molybdenum, manganese, titanium increase wear resistance, tungsten - red resistance. At the same time, positively affecting the properties of parts, they impair the weldability of steel. In addition, they increase the degree of hardening and the formation of martensitic structures, internal stresses and the risks of cracking in the joints.

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

Low-alloy low-carbon 2GS, 14G2, 15G, 20G (GOST 4543-71), 15KHSND, 16G2AF (GOST 19281-89) are weldable. Under standard conditions, they do not require additional heating and heat treatment upon completion of the processes. However, some restrictions still exist:

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

Possible ways:

  • Arc welding with a direct current of 40 to 50 A, electrodes E55, E50A, E44A.
  • Automatic submerged arc welding using filler wire Sv-08GA, Sv-10GA.

The weldability of steel 09G2S, 10G2S1 is also good, the requirements and possible methods of implementation are the same as for alloys 12GS, 14G2, 15G, 20G, 15KHSND, 16G2AF. An important characteristic of 09G2S, 10G2S1 alloys is the absence of the need for edge preparation for parts with a diameter of up to 4 cm.

Welding medium alloy steels

Middle alloyed steels 20KhGSA, 25KHGSA, 35KHGSA (GOST 4543-71) produce more significant resistance to the formation of unstressed joints. They belong to the group with satisfactory weldability. They require preheating to temperatures of 150-200 ° C, performing multilayer seams, hardening and tempering at the end of welding work. Execution Options:

  • The current strength and diameter of the electrode when welding with an electric arc is selected strictly depending on the thickness of the metal, taking into account the fact that thinner edges are more hardened during work. So with a product diameter of 2-3 mm, the current value should be in the range of 50-90 A. With a thickness of the edges of 7-10 mm, the DC power of reverse polarity increases to 200 A using 4-6 mm electrodes. We use rods with cellulose or calcium fluoride protective coatings (Sv-18KhGSA, Sv-18KhMA).
  • When working in a protective CO2 gas environment, it is necessary to use the wire Sv-08G2S, Sv-10G2, Sv-10GSMT, Sv-08Kh3G2SM with a diameter of up to 2 mm.

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

Heat resistant and high strength steels

Welding work with heat-resistant iron-carbon alloys 12MX, 12X1M1F, 25X2M1F, 15X5VF must be carried out with preliminary heating to temperatures of 300-450 ° C, with final hardening and high tempering.

  • Arc welding in a cascade way of forming a multilayer weld using calcined coated electrodes UONII 13 / 45MX, TML-3, TsL-30-63, TsL-39.
  • Gas welding with an acetylene feed of 100 dm 3 / mm using filler materials Sv-08KhMFA, Sv-18KhMA. The pipe connection is carried out with the previous gas heating of the entire joint.

When welding medium alloyed high-strength materials 14Kh2GM, 14Kh2GMRB it is important to follow the same rules as for heat-resistant steels, taking into account some nuances:

  • Thorough trimming and tacking.
  • High temperature annealing of the electrode (up to 450 ° C).
  • Preheating up 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 heat-resistant alloys, some of them: 09164, 1512, 1013, 08175, 081882, 031615, 1517149. Weldability of steels (GOST 5632-72) belongs to the 4th group.

Weldability characteristics of high carbon high alloy steel:

  1. It is necessary to reduce the current strength 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. Reducing the content of phosphorus, lead, sulfur, antimony, increasing the numerical presence of molybdenum, vanadium, tungsten through the use of special coated rods.
  4. The need to form a mixed weld microstructure (austenite + ferrite). This ensures plasticity of the deposited metal and minimization of internal stresses.
  5. Mandatory edge heating 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 12Kh18N9 austenitic steel: UONII 13 / NZh, OZL-7, OZL-14 with coatings Sv-06Kh19N9T, Sv-02Kh19N9; for martensitic steel 20X17H2: UONII 10X17T, AN-V-10 with a coating of Sv-08X17T; for austenitic-ferritic steel 12X21H5T: TsL-33 coated with Sv-08X11V2MF.
  7. In gas welding, the supply of acetylene should correspond to a value of 70-75 dm 3 / mm, the filler wire used is Sv-02Kh19N9T, Sv-08Kh19N10B.
  8. Submerged work using NZh-8 is possible.

Steel weldability 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 high-quality compounds can be adjusted using a well-thought-out technological approach, special equipment and work conditions.

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