Welding characteristics of commonly used metal materials

The weldability of metal materials refers to the ability of metal materials to obtain excellent welding joints under conditions such as certain welding processes, including welding methods, welding materials, welding specifications and welding structural forms. A metal, if it can be used in more common and If a simple welding process obtains excellent welding joints, it is considered that this metal has good welding performance. The weldability of metal materials is generally divided into two aspects: process weldability and use weldability.

1. Welding of carbon steel

(1) Welding of low carbon steel

Low carbon steel has low carbon content and low manganese and silicon content. Under normal circumstances, it will not cause serious structural hardening or quenching structure due to welding. This kind of steel has excellent plasticity and impact toughness, and the plasticity and toughness of its welded joints are also good. Extremely good. Preheating and postheating are generally not required during welding, and special process measures are not required to obtain welded joints with satisfactory quality. Therefore, low carbon steel has excellent welding performance and has the best welding performance among all steels. Types of steel.

(2) Welding of medium carbon steel

Medium carbon steel has a higher carbon content, and its weldability is worse than that of low carbon steel. When the CE is close to 0.25%, the weldability is good. As the carbon content increases, its hardening tendency increases, and under the influence of heat It is easy to produce martensite structure with low plasticity in the area. When the weldment is very rigid or the welding material and process parameters are improperly selected, cold cracks are prone to occur. When multi-layer welding is welding the first layer of weld, the base metal is fused to the weld. The large proportion of carbon in it increases the carbon, sulfur and phosphorus content, making it easy to produce thermal cracks. In addition, when the carbon content is high, the pore sensitivity also increases.

(3) Welding of high carbon steel

High-carbon steel with a CE greater than 0.6% has high hardenability. It is easy to produce hard and brittle high-carbon martensite. It is easy to produce cracks in the weld and heat-affected zone, making it difficult to weld. Therefore, this type of steel is generally not used to manufacture welded structures. , and used to manufacture high-hardness or wear-resistant components or parts, most of their welding is to repair damaged parts. These parts should be annealed before welding to reduce welding cracks, and then heat treated again after welding. .

2. Welding of low alloy high strength steel

The carbon content of low-alloy high-strength steel generally does not exceed 0.20%, and the total alloying elements generally does not exceed 5%. It is precisely because low-alloy high-strength steel contains a certain amount of alloying elements that its welding performance is somewhat different from that of carbon steel. Welding characteristics are shown in:

(1) Welding cracks in welded joints

Cold cracked low-alloy high-strength steel contains C.Mn.V.Nb and other elements that strengthen the steel, so it is easy to be hardened during welding. These hardened structures are very sensitive. Therefore, if the rigidity is large or the restraining stress is high, if Improper welding process can easily produce cold cracks. Moreover, such cracks have a certain delay, which is extremely harmful. For Mn-Mo-Nb and Mn-Mo-V low-alloy high-strength steels, such as 07MnCrMoVR, due to Nb.V .Mo is an element that promotes strong sensitivity to reheat cracks. Therefore, during post-weld heat treatment of this type of steel, care should be taken to avoid the sensitive temperature area of reheat cracks to prevent the occurrence of reheat cracks.

(2) Embrittlement and softening of welded joints

Strain aging embrittlement Welded joints need to undergo various cold processes (blanking shearing, barrel rolling, etc.) before welding. The steel will produce plastic deformation. If the area is further heated to 200 to 450C, strain aging will occur. .Strain aging embrittlement will reduce the plasticity of the steel and increase the brittle transition temperature, resulting in brittle fracture of the equipment. Post-weld heat treatment can eliminate this strain aging of the welded structure and restore the toughness.

The heat-affected zone of the welded joint is softened. Due to the action of welding heat, the outside of the heat-affected zone (HAZ) of low-carbon quenched and tempered steel is heated above the tempering temperature, especially the area near Ac1, which will produce a softening zone with reduced strength. The structure of the HAZ zone Softening increases with the increase of welding line energy and preheating temperature, but generally the tensile strength of the softened zone is still higher than the lower limit of the standard value of the base material. Therefore, the softening problem of the heat-affected zone of this type of steel only needs to be properly processed. , without affecting the performance of its joints.

3. Welding of stainless steel

Stainless steel can be divided into four categories according to its different steel structures, namely austenitic stainless steel, ferritic stainless steel, martensitic stainless steel. Austenitic-ferritic duplex stainless steel. The following mainly analyzes austenitic stainless steel and two-way stainless steel. welding characteristics.

(1) Welding of austenitic stainless steel

Austenitic stainless steel is easier to weld than other stainless steels. It will not undergo phase transformation at any temperature and is not sensitive to hydrogen embrittlement. The austenitic stainless steel joint also has good plasticity and toughness in the welded state. The main problems in welding are: Welding hot cracking, embrittlement, intergranular corrosion and stress corrosion, etc. In addition, due to poor thermal conductivity and large linear expansion coefficient, welding stress and deformation are large. When welding, small welding heat input should be used as much as possible, and preheating should not be performed , and reduce the interlayer temperature. The interlayer temperature should be controlled below 60C, and the weld joints should be staggered. To reduce heat input, the welding speed should not be increased excessively, but the welding current should be adapted to the reduction.

(2) Welding of austenitic ferritic two-way stainless steel

Austenitic ferritic bidirectional stainless steel is a duplex stainless steel composed of two phases, austenite and ferrite. It combines the advantages of austenitic steel and ferritic steel, so it has high strength, good corrosion resistance and The characteristics of easy welding. Currently, there are mainly three types of duplex stainless steel: Cr18, Cr21 and Cr25. The main characteristics of welding this type of steel are: lower thermal tendency compared with austenitic stainless steel; compared with pure ferritic stainless steel It has a low embrittlement tendency after welding, and the degree of ferrite coarsening in the welding heat-affected zone is also low, so the weldability is better. Since this type of steel has good welding properties, preheating and postheating are not required during welding.