Steel weldability: classification. Weldability groups of steels

2024 Author: Howard Calhoun | [email protected]. Last modified: 2024-01-02 13:48

Steel is the main structural material. It is an iron-carbon alloy containing various impurities. All components included in 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 that determine the weldability of steel

Evaluation of the weldability of steel is made by the value of the main indicator - the carbon equivalent of С_{equiv. This is a conditional coefficient that takes into account the degree of influence of the carbon content and the main alloying elements on the characteristics of the weld.}

The following factors affect the weldability of steels:

1. Carbon content.
2. Presence of harmful impurities.
3. Degree of doping.
4. Microstructure view.
5. Environmental conditions.
6. Metal thickness.

The most informative parameter is the chemical composition.

Distribution of steels by weldability groups

Subject toall these factors, the weldability of steel has different characteristics.

Classification of steels by weldability.

• Good (with 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 restrictions on 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%≦C_{eq≦0.45%): Pre-heating is required to form a quality seam. It promotes "smooth" austenitic transformations, the formation of stable structures (ferritic-pearlitic, bainitic).}
• Bad (С_{eq≧0, 45%): 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. Additional heating and smooth cooling are required to form the desired microstructure.}

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

Heat treatment

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

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

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

• annealing - to relieve stress inside the metal, ensuring its softness and pliability;
• preheated to minimize temperature differences.

Rational management of temperature influences allows:

• prepare the part for work (relieve all internal stresses by grinding 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. Edge heating is carried out using gas or electric arc equipment. Special furnaces are used to heat the entire part and smoothly cool it.

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. So, when heated to a temperature of 727 ˚C, it is a mixed granular austenitic structure. The cooling method determines the transformation options:

1. Inside the oven (speed 1˚C/min) - pearlite structures are formed with a hardness of about 200 HB (Brinell hardness).
2. Onair (10˚С/min) – sorbitol (ferrite-pearlite grains), hardness 300 HB.
3. Oil (100˚C/min) – troostite (ferrite-cementite microstructure), 400 HB.
4. Water (1000˚C/min) – martensite: hard (600 HB) but brittle acicular structure.

The welding joint must have sufficient hardness, strength, plasticity quality indicators, so the martensitic characteristics of the seam are not acceptable. Low-carbon alloys have a ferritic, ferrite-pearlitic, ferrite-austenitic structure. Medium carbon and medium alloy steels - pearlitic. High-carbon and high-alloyed - martensitic or troostite, which is important to bring to a ferritic-austenitic form.

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 be concentrated at the edges of the grains, increasing the fragility of the structure. Welding is the most simplified, however, requires an individual approach.

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

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 grades: G - high content of manganese; kp, ps, cn - “boiling”, “semi-calm”, “calm”respectively.

Low-carbon 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 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 operation in special conditions). Welding with OMM-5 and UONI 13/45 rods is carried out under the action of direct current. Work with the help of electrodes TsM-7, OMA-2, SM-11 is carried out with 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 way. Deficiencies in the properties of the seam can be removed by normalizing or annealing.

Welding of low-carbon steels is performed without additional heating. For details of a simple form, there are no restrictions. It is important to protect volumetric and lattice structures from the wind. It is desirable to weld complex objects in a workshop at a temperature not lower than 5˚С.

Thus, for grades VST1 - VST4, steel 10 - steel 20 - weldability is good, practicallywithout restrictions, requiring a standard individual selection of the welding method, electrode type and current characteristics.

Medium and high carbon structural steels

Saturation of the alloy with carbon reduces its ability to form good compounds. In the process of thermal effects of an arc or a gas flame, sulfur accumulates along the edges of the grains, leading to red brittleness, phosphorus to cold brittleness. 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. At the same time, it is important to prevent excessive carbon losses, which can lead to destabilization of mechanical properties.

Features of welding with medium and high carbon steels:

1. Initial edge heating up to 100-200˚С for width up to 150 mm. Only grades Vst4 and steel 25 are welded without additional heating. For medium-carbon steels with satisfactory weldability, full normalization is carried out before starting work. Pre-annealing is required for high-carbon steels.
2. Arc welding is carried out with coated calcined electrodes, ranging in size from 3 to 6 mm (OZS-2, UONI-13/55, ANO-7), under direct current. possible to work influx or shielding gases (CO_{2, argon).}
3. Gas welding is carried out with a carburizing flame, left hand method, with preheating to a temperature of 200˚C, with a uniform low power supply of acetylene.
4. Mandatory heat treatment of parts: quenching and tempering or separate tempering to minimize internal stresses, prevent cracking, soften hardened martensitic and troostite structures.
5. Contact spot welding is performed without limitation.

Thus, medium- and high-carbon structural steels are welded practically without restrictions, at an external temperature of at least 5˚С. At lower temperatures, initial preheating and high-quality heat treatment are mandatory.

Welding of low alloy steels

Alloy steels are steels that are saturated with various metals during melting in order to obtain 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 hardness. At the same time, positively affecting the properties of parts, they worsen the weldability of steel. In addition, the degree of hardening and the formation of martensitic structures, internal stresses and the risk of cracking in the seams increase.

The weldability of alloy steels is also determined by theirchemical composition.

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

• Narrow range of allowable thermal conditions.
• Work should be carried out at a temperature not lower than -10˚С (in conditions of lower atmospheric temperatures, but not lower than -25˚С, apply preheating up to 200˚С).

Possible ways:

• Electric arc welding with direct current 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 alloys 09G2S, 10G2S1 is the absence of the need to prepare edges for parts with a diameter of up to 4 cm.

Welding of medium alloy steels

Medium-alloyed steels 20KhGSA, 25KhGSA, 35KhGSA (GOST 4543-71) produce more significant resistance to the formation of loose seams. They belong to the group with satisfactory weldability. They require preheating to temperatures of 150-200˚С, multilayer welds, hardening and tempering upon completion of welding. Options:

• Current and electrode diameter when welding with an electric arcis chosen 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 an edge thickness of 7-10 mm, the direct current of reverse polarity increases to 200 A using electrodes 4-6 mm. Rods with cellulose or calcium fluoride protective coatings (Sv-18KhGSA, Sv-18KhMA) are used.
• When working in a protective gas environment CO_{2 it is necessary to use wire Sv-08G2S, Sv-10G2, Sv-10GSMT, Sv-08Kh3G2SM with a diameter of up to 2 mm.}

The argon arc method or submerged arc welding are often used for these materials.

Heat resistant and high strength steels

Welding with heat-resistant iron-carbon alloys 12MX, 12X1M1F, 25X2M1F, 15X5VF must be carried out with preheating to temperatures of 300-450˚С, with final hardening and high tempering.

• Electric arc welding in a cascade way to design a multilayer seam, using calcined coated electrodes UONII 13 / 45MH, TML-3, TsL-30-63, TsL-39.
• Gas welding with acetylene supply 100 dm3/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 cleaningedges and use of tacks.
• High-temperature annealing of the electrode (up to 450˚C).
• Preheat up to 150˚C for parts over 2 cm thick.
• Slow seam cooling.

High alloy steels

The use of a 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: 09Kh16N4B, 15Kh12VNMF, 10Kh13SYu, 08Kh17N5MZ, 08Kh18G8N2T, 03Kh16N15MZB, 15Kh17G14A9. Weldability of steels (GOST 5632-72) belongs to the 4th group.

High carbon high alloy steel weldability characteristic:

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. Reduce the content of phosphorus, lead, sulfur, antimony, increase the abundance of molybdenum, vanadium, tungsten through the use of special coated rods.
4. The need to form a mixed weld microstructure (austenite + ferrite). This ensures the ductility of the deposited metal and the minimization of internal stresses.
5. Mandatory edge heating on the eve of welding. The temperature is selected in the range from 100 to 300˚С, 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 12X18H9: UONII 13 / NZh, OZL-7, OZL-14 with Sv-06Kh19N9T coatings,Sv-02X19H9; for martensitic steel 20Kh17N2: UONII 10Kh17T, AN-V-10 coated with Sv-08Kh17T; for austenitic-ferritic steel 12Kh21N5T: TsL-33 coated with Sv-08Kh11V2MF.
7. When gas welding, the supply of acetylene should correspond to the value of 70-75 dm3/mm, the filler wire used is Sv-02Kh19N9T, Sv-08Kh19N10B.
8. Submerged arc operations are possible using NZh-8.

The 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 high-quality joints can be adjusted with the help of a well-thought-out technological approach, special equipment and working conditions.