Guidelines for the lesson “Classification of steels by weldability” using modular technology

What carbon content ensures good weldability - Metalworker's Handbook

Weldability is the ability of a metal to form high-quality welded joints that meet the operational requirements for them.
The possibilities and conditions for the formation of a high-quality welded joint are determined by many factors, the most important of which are:

• characteristics and properties of welded metals;
• selection of electrode and filler metal;
• welding modes;
• heating temperature, etc.

Weldability is significantly affected by the chemical composition of the steel, in particular the content of carbon and alloying elements. The effects of individual elements manifest themselves in different ways - especially when combined with carbon.

Among the main characteristics of steel weldability, it is worth highlighting the tendency to crack formation and the mechanical properties of the welded joint. They can be determined by welding control samples.

Formula for determining the weldability of steel

If the chemical composition of the steel is known, its weldability can be determined by its equivalent carbon content. To do this, use the formula:

With eq. = C + Mn/20 + Ni/15 + (Cr + Mo + V)/10.

The numbers in this formula are constant values, and the symbols of each chemical element indicate its maximum inclusion in a certain grade of steel, expressed as a percentage.

The equivalent carbon content obtained from this formula is an indication of the weldability of steels, which can be divided into four groups:

• well weldable (SEQ does not exceed 0.25%);
• satisfactorily weldable (Seq = 0.25% - 0.35%);
• limited weldability (Seq = 0.35 – 0.45%);
• poorly weldable (SEQ exceeds 0.45%).

The good weldability of low-carbon steels can be judged by a strong welded connection with the base metal without cracks and a decrease in ductility in the heat-affected zone.

The weldability of alloy steels is assessed by the possibility of obtaining joints that are resistant to cracking and hardened structures, as well as by a decrease in strength, corrosion, and so on.

A characteristic of the weldability of thermally strengthened steels is a tendency to reduce strength in the heat-affected zone at a temperature of 400-720º C, depending on the tempering temperature of the steel during its production at the plant. Thus, the production of a strong welded structure is possible only if a detailed study and consideration of the weldability of steel is taken into account.

Weldability of low carbon steels

Low-carbon alloys lend themselves well to welding. The following points can be noted:

1. In such alloys the carbon concentration is less than 0.25%. This indicator is characteristic of alloys that have increased flexibility and relatively low hardness of the surface layer. In addition, the value of fragility is reduced. Therefore, low-carbon steels are often used to create sheet blanks. By adding small amounts of alloying elements, corrosion resistance can be improved.
2. To improve the basic characteristics, various alloyed elements can be added to the composition, but in small quantities. Examples include manganese and nickel, as well as titanium.

Low carbon steel

As a rule, such metals do not need to be heated before processing, and after the procedure, hardening or tempering is performed only if necessary.

Classification of steels by weldability

To make it easier to determine the weldability of metals, steel grades were divided into 4 groups of weldability of parts. To present each classification, as well as its features, a weldability table has been created:

C (equivalent) = C + (Mn/6) + ((Cr + Mo +V)/5 + (Ni + Cu)/15),

where C is carbon, and the other letter elements are the concentration of alloying components. All values ​​are in percentages.

Classification of steel according to the degree of its weldability

Steel is represented by various groups of grades, each with its own physical and chemical properties. As a result, metal products have different weldability rates. Depending on this parameter, iron-carbon alloys are divided into four categories.

1. Good When welding, a high-quality seam is obtained. The metal does not require preheating to carry out the work, and the work itself is carried out in the usual way and using all known technologies.
2. Satisfactory To create a high-quality welded joint, steel products must be prepared, that is, heated.
3. Limited Before welding, metal products are first heated, and after joining they are also subjected to heat treatment.
4. Bad This type of steel is characterized by the fact that during welding (after it) cracks form on the surface, and “hardening” structures can also appear, reducing the strength and reliability of the joint, making it brittle.

What is included in the concept of metallurgical weldability of metals?

There are quite a large number of parameters that determine the basic properties of the metal. Among them, the weldability indicator is distinguished. Today, steel welding is carried out extremely often. This method of joining metals and other materials is characterized by high efficiency, so the weld can withstand heavy loads. If the indicator is poor, it is difficult, in some cases even impossible, to carry out such work. All metals are divided into several groups, which we will discuss in more detail later.

Weldability of steels

Basic criteria establishing weldability

When assessing the weldability of steels, attention is always paid to the chemical composition of the metal. Some chemical elements can increase or decrease this indicator

Carbon is considered the most important element, which determines strength and ductility, degree of hardenability and fusibility. Conducted studies indicate that when the concentration of this element is up to 0.25%, the degree of workability does not decrease. An increase in the amount of carbon in the composition leads to the formation of hardening structures and the appearance of cracks.

Concept of weldability

Other features that relate to the issue under consideration include the following points:

Almost all metals contain harmful impurities that can reduce or increase weldability. Phosphorus is considered a harmful substance; when the concentration increases, cold brittleness appears. Sulfur causes hot cracks and red brittleness. Silicon is present in almost all steels; at a concentration of 0.3%, the degree of machinability does not decrease. However, if you increase it to 1%, refractory oxides may appear, which reduce the indicator in question. The welding process is not difficult if the amount of manganese is no more than 1%. Already at 1.5% there is a possibility of the appearance of a hardening structure and serious deformation cracks in the structure. The main alloying element is chromium. It is added to the composition to increase corrosion resistance. At a concentration of about 3.5%, the weldability indicator remains practically unchanged, but in alloyed compositions it is 12%. When heated, chromium leads to the appearance of carbide, which significantly reduces corrosion resistance and complicates the process of joining materials. Nickel is also the main alloying element, the concentration of which reaches 35%. This substance can increase ductility and strength. Nickel causes an improvement in the basic properties of the material. Molybdenum is included in the composition in small quantities. It helps increase strength by reducing the grain structure. However, when exposed to high temperatures, the substance begins to burn out, causing cracks and other defects to appear. Copper is often added to the composition as an alloying element. Its concentration is about 1%, due to which the corrosion resistance slightly increases

An important feature is that copper does not impair welding processing.

Weldability criteria

Depending on the characteristics of the structure and chemical composition of the material, all alloys are divided into several groups. Only by taking this classification into account can you choose the most suitable alloy.

Classification of steels by weldability

Alloys that do not form cracks when heated have good machinability. According to this characteristic, four main groups are distinguished:

1. Good weldability means that the steel remains strong and reliable after heat treatment. In this case, the created seam can withstand significant mechanical stress.
2. A satisfactory degree allows processing without preheating. Due to this, the process is significantly accelerated and costs are reduced.
3. Limited weldability steels are difficult to process; welding can only be carried out using special equipment. That is why the cost of the process itself increases.
4. Poor weldability does not allow the processing in question, since cracks may appear after the weld is completed. That is why such materials cannot be used to obtain critical elements.

Classification of steels by weldability

Each group is characterized by its own specific characteristics that need to be taken into account. Steel 20 belongs to the first group, while the common steel 45 has low weldability.

Effect of carbon content on steel weldability

In many ways, it is carbon that determines the main performance characteristics of the alloy. Too high a concentration of such a chemical leads to increased hardness and strength, but also brittleness. In addition, the degree of weldability is reduced several times. Other features include the following:

1. If the carbon composition does not exceed 0.25%, then the indicator under consideration remains at a fairly high level.
2. Too much carbon in the composition leads to the fact that the metal, after thermal exposure, begins to change its structure, due to which cracks appear.

It is worth considering that the chemical-thermal procedure being carried out can lead to a decrease in compliance with the connection method in question. That is why the improvement of the alloy is carried out after creating the structure by processing the seam.

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 exhibits a mixed granular austenitic 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˚C/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 HV), but brittle needle-like structure.

The welding joint must have sufficient hardness, strength, and qualitative indicators of ductility, therefore the martensitic characteristics of the weld are not acceptable. Low-carbon alloys have a ferritic, ferrite-pearlite, ferrite-austenitic structure. Medium carbon and medium alloy steels – pearlitic

High-carbon and highly alloyed - martensitic or troostitic, which is important to lead to a ferritic-austenitic form

Weldability of hardened steel

A common heat treatment can be called hardening. It involves exposure to high temperatures, which can change the structure of the material. After cooling, the structure is restructured, due to which the structure is strengthened and the hardness of the surface layer increases. Other features include the following:

1. Hardening involves increasing the carbon concentration in the surface layer. That is why the degree of weldability is significantly reduced.
2. The workpiece is heated in order to simplify the work being done. A gas heating pad or other heat source can be used for this.

Hardened steel is difficult to process. In addition, if tempering has not been carried out previously, there may be an excess of stress in the structure, which leads to the appearance of cracks.

Re-processing seams may not improve their strength.

In conclusion, we note that metals from various groups have good weldability. An example is some stainless steel, which even after exposure to heat is corrosion resistant. That is why for welding work it is recommended to choose a material that is characterized by good machinability.

Source

Welding pipes made of steel 20 and 09g2s

The alloys have a low carbon content. They do not harden, are not prone to overheating, and are resistant to cracking. For manual or automatic fastening of structural lightly alloyed substances 09G2S and 20, the equipment UONI-13.55, E42, ANO-21, OZS-12, MR-3 LUX is suitable. If the technology is followed, the proper selection of raw materials and equipment, a strong welded joint section is obtained.

Technological instructions are disclosed in more detail in STO 00220368-011-2007. The document contains requirements for materials and methods of metal processing of dissimilar compounds made of carbon, high- and low-alloy steels and alloys.

Selection of equipment and electrodes

There are the following types of welding machines:

1. AC transformers.
2. Rectifiers.
3. Inverters.

Transformers have the following advantages:

1. Low cost.
2. Simple device.
3. Reliability.
4. Durability.

They are used in cases where high demands are not placed on the quality of the connection, since the arc burns worse with alternating current and the seam turns out to be uneven.

They are heavy, more expensive and lose more power, but provide high quality connections. Metal losses are reduced, since on direct current it is less splashed.

Inverters are the most practical.

Their advantages:

• small size and weight (about 3 kg);
• high open circuit voltage - 90 V versus 50 V for the transformer;
• additional functions that make it easier to ignite and maintain the arc.

According to the welding method, the machines are divided into types:

1. Manual. Use coated consumables that melt.
2. Semi-automatic and automatic machines. A refractory electrode made of tungsten or graphite is used. The weld is protected from oxidation by supplying gas (argon, carbon dioxide, etc.) or using flux.

Features of welding low-alloy steels

Welding of structural steels 15HSND, 15GS, 14G2, 14G2AF, 16G2AF

For welding low-alloy steel grades 15HSND, 15GS, 14G2, 14G2AF, 16G2AF, etc. Manual arc welding with E50A or E44A electrodes is well suited. But the highest quality welded joints are obtained when welding with UONI-13/55 and DSK-50 electrodes. But, the best results are obtained when welding with direct current with reverse polarity. At the same time, welding must be carried out at low currents, 40-50 A per millimeter of electrode diameter.

Automatic arc welding of these steel grades is performed using Sv-08GA or Sv-10GA welding wire under AN-348-A or OSTS-45 fluxes.

Metal structures made from steels 15HSND, 15GS, 14G2, 14G2AF, 16 G2AF can be welded at an ambient temperature of at least -10°C. If the ambient temperature is in the range from -10°C to -25°C, then preheating is necessary during welding. The heating width of the welding zone is 100-120 mm on both sides of the seam. Preheating temperature 100-150°C. At ambient temperatures lower than -25°C, welding of the above steels is not permissible.

Welding low-alloy steels 09G2S, 10G2S1, 10G2S1D

Assessment of the weldability of steel grades such as 09G2S, 10G2S1, 10G2S1D, etc. can be given a good one (see the table of weldability of steels), and this is due to the fact that they are not subject to hardening, are not prone to overheating and are resistant to the formation of hot and cold cracks in the weld and heat-affected zone. Welding of low-alloy structural steels of these grades can be performed either by manual or automatic arc welding.

For manual welding, electrodes of the E50A and E55A brands are well suited. For automatic welding, welding wire of the Sv-08GA, Sv-10GA or Sv-10G2 brands is used. To protect the welding zone, fluxes AN-348-A or OSTS-45 are used.

Welding of sheets made of steels 09G2S, 10G2S1, 10G2S1D, with a thickness of less than 40 mm, is carried out without cutting the edges. And, subject to the technology and welding conditions, the mechanical properties of the weld are almost as good as the mechanical properties of the base metal. The uniform strength of the weld is due to the transition of alloying elements from the electrode wire into the metal of the weld.

Welding chromium-silicon-manganese low-alloy steels 25KhGSA, 30KhGSA, 35KhGSA

Welding of low-alloy steels 25KhGSA, 30KhGSA, 35KhGSA, etc. complicated by the fact that they are prone to the formation of cracks during welding and the appearance of hardening structures. And the smaller the thickness of the welded edges, the higher the risk of the formation of hardening zones and the appearance of cracks in the weld metal and, especially, the heat-affected zone.

The tendency of these steels to weld defects is due to the increased carbon content in their composition (0.25% or more). Welding of these steels can be performed with welding wire Sv-08 or Sv08A, as well as electrodes of these brands.

For particularly important welds, it is recommended to use Sv-18KhGSA or Sv-18KhMA electrodes with the following types of protective coating: TsL-18-63, TsK18M, UONI-13/65, UONI-13/85, UONI-13/NZh.

When welding low-alloy chromium-silicon-manganese steels, depending on the thickness of the metal being welded, the following welding modes are recommended:

When welding metals with a thickness of more than 10 mm, multilayer welding is performed with short time intervals between subsequent layers. In the case when parts of different thicknesses are welded, the welding current is selected according to the thicker thickness and a larger arc zone is directed to it.

After welding, it is recommended to carry out heat treatment to eliminate hardening structures. To do this, the product is heated to a temperature of 650-680? C, maintained at this temperature for a time, depending on the thickness of the metal (1 hour per 25 mm thickness) and cooled in air or in water.

Welding of low-alloy structural steels in shielding gases is carried out according to modes for automatic or semi-automatic submerged arc welding. In the case of welding in a carbon dioxide environment, welding wire of the Sv-08G2S or Sv-10G2 brand with a diameter of 1.2-2 mm is used.

In the case of using electroslag welding, choose welding wire of the Sv-10G2 brand, which is suitable for any thickness of the parts being welded. AN-8 flux is used as protection. With this welding method, welding can be carried out at any temperature.

Additional

Determination of steel sensitivity to cold cracking

Cold cracks form after welding due to tensile residual stresses. Their strength depends on the rigidity of the resulting structure and the thickness of the seam. Its value can be determined by the stiffness intensity coefficient - K. It characterizes the applied force, which opens the gap by 1 mm, which is also 1 mm wide in the welded joint. It is calculated like this:

where Kq is a constant, which is considered to be equal to 69, S is the thickness of the steel sheet (in mm). It is important to note that the ratio is only valid if the sheet thickness does not exceed 150 mm.

How steel can be susceptible to cold cracking can be determined by the parametric equation:

where Рш is the “embrittlement” coefficient (this is the name for the process when a metal goes from a viscous state to a brittle one), H is the amount of diffusion hydrogen, K is the hardness intensity coefficient.

The value of Psh is found by solving the Bes-Sio equation:

The results of repeated studies helped to establish the threshold value at which the sensitivity of steel to the formation of cold cracks manifests itself. This happens if the Pw value exceeds 0.286.

Classification of steels according to physical, chemical and technological characteristics

According to the physical properties in the classification (EN 10027 standard), steel groups are distinguished:

– with special physical properties (electrical conductivity, coefficient of linear expansion, etc.); – with special magnetic properties (magnetic permeability).

Classification of steels according to mechanical properties:

– strength (for example, Rm 500 N/mm2, 500 ≤ Rm 700 N/mm2, Rm ≥ 700 N/mm2); – yield strength (for example, Re = 235, 275...or Re 360, Re 380 N/mm2); – relative elongation (for example, δ≥15, 25 or 35%); – impact strength (for example, impact work of 27, 40 or 60 J at +20, 0, -20, -40, -60°C); – other characteristics.

According to chemical characteristics, steels are classified into:

– resistant to chemical corrosion (at normal temperatures – stainless steels; at high temperatures – heat-resistant steels); – resistant to electrochemical corrosion (steels for operation at normal, elevated or high temperatures, resistant to MCC).

Technological classification characteristics:

– method of producing steel (boiling, semi-calm, calm steel); – thermal and thermomechanical treatment (hardenability, annealing, normalization, hardening and tempering, cold hardening, cold rolling, hot forming, etc.); – the ability of steels to be processed by pressure (for example, stamping), cutting, casting, etc.; – weldability (according to the Sekv criterion, the content of the ferrite phase in austenitic steels, etc.).

Classification of steels by purpose:

When classifying steels by purpose, one group may contain steels of different alloying systems and different quality classes.

Non-alloy steels are classified according to purpose into the following groups:

– structural general purpose; – general purpose construction; – for pressure vessels; – pipe; – mechanical engineering; – shipbuilding; – automatic (with a high content of P and S); – reinforcement; – rail; – cold- and hot-rolled for cold working; – instrumental; – electrical engineering.

Alloy steels are classified according to their intended purpose into:

– construction; – mechanical engineering; – shipbuilding; – for pressure vessels; – for pipelines; – for nuclear reactors; – for cryogenic equipment; – for bearings; – stainless steels; – heat-resistant steels; – heat-resistant; – heat-resistant; – instrumental; – high-speed; – with special physical properties.

Stainless steel

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

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

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

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

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

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

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

What is the weldability of materials?

Physical weldability of metals is the property of materials to form a monolithic joint, i.e. their ability to mutually crystallize with the formation of solid solutions, chemical compounds and fine mixtures of phase components (eutectics). These processes occur at the boundary of the base and deposited metal and characterize weldability from the point of view of the possibility of forming a metallic bond and the fundamental possibility of obtaining permanent welded joints.

Technological weldability of metals is a technological characteristic of a metal that determines its response to the effects of welding and the ability to form a permanent welded joint with specified performance properties at the lowest cost. That is, it reflects the technological response of the material to the thermal, force and metallurgical effects of welding.

The weldability of a metal depends on its chemical and physical properties, the type of crystal lattice, the degree of alloying, the presence of impurities and a number of other factors.

The weldability of steels is assessed according to the following indicators:

• the tendency of the weld metal to form hot and cold cracks;
• tendency to change the structure in the heat-affected zone and to form hardening structures;
• physical and mechanical qualities of the welded joint (strength, ductility, impact strength, etc.);
• compliance of the special properties of the welded joint with the requirements of the technical specifications for the design (corrosion resistance, heat resistance, heat resistance, resistance to brittle fracture at low temperatures, etc.).

Simply put, the difference between materials with good and poor weldability is that more complex welding technology is required to join the latter.

Carbon has the greatest influence on the weldability of steels. Weldability deteriorates with increasing carbon content, as well as a number of other elements. For the manufacture of welded structures, structural low-carbon, low-alloy and alloy steels are mainly used. The main difficulties in welding alloy steels are their tendency to form hardening structures, hot and cold cracks, as well as deterioration of mechanical properties - primarily a decrease in ductility in the welded joint zone. The higher the carbon content in the steel, the more pronounced these disadvantages are, and the more difficult it is to ensure the required properties of the joint.

Approximate quantitative indicators of the weldability of steels are the equivalent carbon content, determined by the formula:

where the content of carbon and alloying elements is expressed as a percentage. Depending on the equivalent carbon content, structural steels are divided into 4 groups, which are characterized by satisfactory, limited or poor weldability.

Group 1: good weldability, SEQ ≤ 0.25%, weldability without the use of special techniques (st.2; st.3; 10G2; 09G2; 10G2S).

Group 2: satisfactory weldability, Sekv -0.25 - 0.35 - strict adherence to welding conditions, the use of special filler materials, in some cases - preliminary and concomitant heating to 100 - 1500 C, heat treatment are required (steels 15GS, 15 KhM, 10HSND , 14HGS, 15HSND, 15HGSA, 18G29).

Group 3: limited weldability, SEq - 0.35 - 0.45, requires heating to 100 - 2000 C and tempering after welding. Before welding, the parts are subjected to heat treatment (steels 12N1MF, 20KhMFL, 15Kh1M1FL, 30KhGS, 35G2, 30KhM, 10GN2MFA, 15Kh2NMFA).

Group 4: poor weldability, SEq > 0.45. High tendency to cold cracks in the weld and heat-affected zone. When welding, preheating to 250 - 4000C and subsequent heat treatment are required (steels 45Kh, 45G, 40G2, 40KhS, 40KhMFA, 35KhGSA, 30KhI3A, 40KhN2MA, 36Kh2R2MFA).

There is still no generally accepted method for determining the weldability of metals. In most cases, the techniques are based on welding special samples in which harsh conditions for the weld are created. However, there are also calculation methods that relate the maximum hardness and type of structure of the heat-affected zone of a given steel to the need to heat the part before welding, the design of the joint and the thickness of the metal. Calculation methods make it possible to theoretically calculate welding modes that ensure obtaining a given hardness and structure.

Weldability of steels - what affects it?

Steel is the main structural material, which is an alloy of iron with carbon and various impurities. All elements that make up steel products influence its characteristics (in particular, the weldability of steels).

Basic criteria establishing weldability

The main indicator of weldability is the carbon equivalent, which is designated as Seq. This conditional coefficient takes into account the level of influence of carbon and alloying components on the properties of the weld.

Factors affecting the weldability of steels:

• Metal sample thickness
• Volume of harmful impurities
• Environmental conditions
• Carbon capacity
• Alloy level
• Microstructure

The main parameter for information is the chemical composition of the material.

How do alloying impurities affect weldability?

Influence of the main alloying elements on the weldability of steel

• Phosphorus and sulfur are harmful impurities. of these chemical elements for low-carbon steels 0.4-0.5%.

• Carbon is an important component in the composition of alloys, which determines such indicators as hardenability, ductility, strength, and other properties of the material. carbon within 0.25% does not affect the quality of welding. The presence of more than 0.25% of this chemical. element contributes to the formation of hardening joints, heat-affected zones, and cracks form.

• Copper. copper as an impurity is no more than 0.3%, as an additive for low-alloy steels - within 0.15-0.50%, as an alloying component - no more than one percent. Copper improves the corrosion resistance of the metal without compromising the quality of welding.

• Manganese. manganese up to one percent does not complicate the welding process. If manganese is 1.8-2.5%, then the formation of hardening structures, cracks, and heat-affected zones cannot be ruled out.

• Silicon. This chemical element is present in the metal as an impurity - 0.30 percent. This amount of silicon does not affect the quality of the metal connection. If silicon is present in the range of 0.8-1.5%, it acts as an alloying component. In this case, there is a possibility of the formation of refractory oxides, which impair the quality of the metal connection.

• Nickel, like chromium, is present in low-carbon steels, its content is up to 0.3%. In low-alloy metals, nickel can be about 5%, in high-alloy metals - about 35 percent. The chemical component increases the ductility and strength characteristics of the metal, and improves the quality of welded joints.

• Chromium. The amount of this component in low-carbon steels is limited to 0.3 percent, its content in low-alloy metals can be in the range of 0.7-3.5%, alloyed - 12-18 percent, high-alloyed - approximately 35%. At the time of welding, chromium promotes the formation of carbides, which significantly impair the corrosion resistance of the metal. Chromium promotes the formation of refractory oxides, which negatively affect the quality of welding.

• Molybdenum. The presence of this chemical element in the metal is limited to 0.8 percent. This amount of molybdenum has a positive effect on the strength characteristics of the alloy, but during the welding process the element burns out, as a result of which cracks form in the deposited area of ​​the product.

• Vanadium. This element in alloy steels can range from 0.2 to 0.8 percent. Vanadium helps to increase the plasticity and viscosity of the metal, improves its structure, and increases the hardenability index.

• Niobium, titanium. These chemical components are contained in heat-resistant, corrosion-resistant metals, their concentration is no more than one percent. Niobium and titanium reduce the sensitivity of the metal alloy to intergranular corrosion.

Bottom line

The weldability of steel is considered a comparative indicator, depending on the chemical composition, physical characteristics, and microstructure of the material. At the same time, the ability to create high-quality welded joints can be adjusted thanks to a thoughtful technological approach, meeting the requirements for welding, and the availability of modern special equipment.

Weldability groups

Taking into account all the above criteria, weldability can be divided into groups with different properties.

Classification of metals by weldability:

Good - the Sek coefficient is at least 0.25% - for products made of low-carbon steels, regardless of weather conditions, product thickness, and preliminary preparation.

• Satisfactory - the Sek coefficient is in the range of 0.25-0.35%. Limitations: on the diameter of the welded product, environmental conditions. The thickness of the material is allowed no more than 2 cm, the air temperature should not be lower than minus 5 degrees, calm weather.
• Limited – Sekv coefficient in the range of 0.350-0.45%. To form a high-quality welded joint, preheating of the material is required. This procedure is necessary for a “smooth” austenitic transformation and the creation of stable structures (bainite, ferrite-pearlite).
• Bad – the Sek coefficient is about 45% (steel 45). In this case, it is impossible to ensure the stability of the welding joint without preheating the metal edges and heat treating the finished structure. To create the required microstructure, it is necessary to additionally carry out heating and cooling.

Depending on the category and technological parameters, the properties of welded joints can be adjusted by successive temperature effects. Heat treatment can be carried out in several ways: tempering, hardening, normalization, annealing. The most popular are hardening and tempering. Such procedures increase the hardness and, accordingly, the strength of the welded joint, prevent the formation of cracks in the material, and relieve stress. The tempering rate will depend on the desired characteristics of the material.

Methods for calculating carbon equivalent

The properties of steel generally depend on the presence of other metals in the alloy of iron and carbon. Knowing their content, using an empirical formula it is not difficult to calculate the value of the so-called carbon equivalent (Ce). This value allows you to determine what results to expect from welding metal products.

In Russia, to evaluate the welding characteristics of rolled products used to create structures, they use the formula approved by GOST GOST 27772-88:

In Europe, the following relationship is used for calculations:

In Japan, this method for determining carbon equivalent is:

where C, P, Cr, Mn, Cu, V, Si, Ni, Mo are the mass fractions (in %) of carbon, phosphorus, chromium, manganese, copper, vanadium, silicon, nickel, molybdenum.

Steel is considered not prone to cracking if the carbon equivalent value “C” is less than 0.45%. Otherwise, when there is already a possibility of their occurrence, the parts requiring connection must be heated before welding.