With the development of iron and steel, petrochemical industry, ship industry, electric power industry and automobile industry, all kinds of low alloy high strength steel, medium high alloy steel, super strong steel and various alloy materials are widely used. There are many new problems in welding production, among which cold cracks are common and serious. Because of the cold crack, many accidents have been caused in the welding structure at home and abroad, which cause great property loss and threaten people’s life safety.
In recent years, there are more researches on cold cracks in steel, petrochemical industry, ship and electric power, but less in automobile industry. With the development of automobile industry, welding technology is widely used in gear assembly and other auto parts, and the harm of cold crack is increasing. Therefore, the paper studies the cause of cold crack, taking the gear assembly of automobile transmission as an example. Taking the gear assembly of automobile transmission produced by our factory as an example, this paper puts forward various solutions from reducing the content of diffusible hydrogen and improving the structure.
1.Definition and classification of welding cold crack
When the welding joint is cooled to a lower temperature, the welding crack is called cold crack. The cold cracks are mainly distributed under the weld bead, the root of the weld, the toe of the weld and the surface of the weld. The fracture condition is that the fracture occurs when ε ≥ δ min, that is, the ductility δ min at the local position of the welded joint is less than the effect of the strain ε.
Welding cold cracks include quenching embrittlement crack, delayed crack and low plastic embrittlement crack. The quenching embrittlement crack refers to the hardening structure of steel with high hardening tendency under the action of welding thermal cycle and the crack under the action of stress. Delayed crack refers to the welding cold crack that occurs after the welding joint cools to room temperature for a certain time (several hours, days, or even 10 days) . The influencing factors of delayed crack are hardening structure, hydrogen and stress. Low plastic embrittlement crack refers to the crack caused by the low plasticity of the base metal or the weld metal itself. Under the action of welding thermal stress and restraint stress, the strain is greater than its ductility. The low plastic embrittlement crack has nothing to do with the effect of hydrogen, and has nothing to do with the brittle and hard structure formed by welding.
2.Difference between cold crack and hot crack
The differences between cold cracks and hot cracks are mainly manifested in the generating conditions, generating positions and directions, appearance characteristics and metallographic structure. See Table 1 for the differences. There are also some special cases, such as cold cracks, some of which can be extended to hours to weeks or even longer, some of which are along the grain boundary.
3.Causes of welding cold cracks
The heating temperature near the crack will be very high when the high strength steel is hardened, which will cause the austenite grain to grow seriously, and the coarse austenite will be transformed into the coarse martensite under the rapid cooling. Martensite is a brittle and hard structure which is easy to fracture. Therefore, when there is martensite in the welded joint, the crack is easy to form and expand.
The main reason of cold crack is the improper welding technology and the matching between base metal and welding material. The former is mainly due to the presence of free water and crystal water at the welding site, which is reflected in the high humidity of the base metal, welding material and environment. During the welding process, the water and organic matter are decomposed by the arc to produce H, and then hydrogen embrittlement occurs. The latter is mainly due to the inconsistent contents of B, V, Mo, Co, C and other elements in the base metal and the welding material, resulting in the difference of hardening sensitivity between the two materials, and then cold cracks. In addition, factors such as welding speed, ambient temperature, slag inclusion and insufficient welding will also affect the generation of cold cracks.
4 Solutions to prevent cold cracks
According to the influencing factors of cold crack, this paper analyzes the gear components of dct300 automobile transmission produced by our company, mainly from the aspects of material composition and carbon equivalent, high effective hardening layer depth of gear, insufficient allowance of hard turning gear 1, low preheating temperature, etc., and finds out the scheme to prevent cold crack.