Control Of Welding Cold Crack Of 30CrMnSi Steel Joints

A vital component is designed by 30 CrMnSi steel, however, there is a series of problems in the welding process. First of all, a large number of cold cracklings which have made the weld defect rate structure cannot meet the requirements of the plan, resulted a lot of the cost of waste, therefore it is extremely necessary to control the welding cold cracklings of the structures. To solve the problem, using infrared device to preheat welding and heat immediately after or during welding to reduce the hardness structure and the cold cracking susceptibility.In order to approaching the actual production conditions, the 1.8mm thick 30 CrMnSi steel plate have used in the welding experiment which is the butt welding. CO2 gas shielded arc welding(CO2 welding) and Tungsten Inert Gas arc Welding(TIG welding) are used respectively on study the influence of the preheated and heating immediately after or during welding on the hardness structure. The effect of multiple repair welding on microstructure and properties of joints also have been studied, and analyzes the effect of the heating during welding on the joints.The results show that with a large tensile stress the micro-crack tip formed cold cracking eventually. Smaller CO2 welding heat input helps to reduce the effective hardness structure and reduce the tendency of cold cracking. Using infrared device during welding could reduce the content of the brittle plate martensite at completely quenched zone of joints. When the distance between infrared device and torch is 250 mm, the maximum hardness of the completely quenched zone is 511.1HV which is reduced 164.1HV at the ordinary welding condition. And the tensile strength is 950 MPa, about 91.7% of the base material. At this distance, infrared heating during welding could reduce the number of the hardness structure at completely quenched zone effectively, result reduce the cold cracking susceptibility in a certain extent.In the case of TIG welding conditions, using infrared device with the distance between infrared radiation device and torch from 50 mm to 150 mm could reduce the content of hardness structure effectively, but when the distance is more than 150 mm, cause of the heating happened after the martensite transformation the position of the organization experienced "tempering" which resulted martensite grain growth and the cold cracking susceptibility improved. When the preheating temperature is 150 ℃, the maximum hardness of completely quenched zone is 470 HV which is smaller than 560 HV at the no preheating condition. Heating immediately after welding with 6min is obvious to reduce the cold cracking susceptibility. Due to the tempering effect is more obvious that the cold cracking sensitivity increases when the time is increased.TIG welding is used at repair welding. The repair welding with three different programs were studied and the effects of the repair welding time are analyzed. With increasing of repair welding time, the three different repairing welding forms of the tensile strength joints were shown a downward tendency. the U-groove program have the biggest effect on the joints, and the influence of the other two programs are smaller to the joints. Using infrared heating immediately after welding could reduce content of hardness structure in completely quench zone effectively to improve anti-cold cracking performance of the joints.