Annealing Process Optimization of High Frequency Longitudinal Resistance Welded Low-Carbon Ferritic Stainless Steel Pipe

With the development of economy and technology, the application of ferritic stainless steel is becoming increasingly wider. 12Cr ferritic stainless steel has low carbon equivalent and good weldability, and it can not only be applied to a variety of conditions, but also reduce the production cost. High frequency longitudinal resistance welding is an advanced welding technology with high quality and efficiency. In this work, low-carbon ferritic stainless steel pipe has been joined successfully by high frequency longitudinal resistance welding. Microstructure characteristics and mechanical properties of the stainless steel pipe joint after annealing at different temperatures for 3 min were investigated by OM, SEM, TEM and mechanical testing. In the process of high frequency longitudinal resistance welding, the weld zone was heated quickly to a high austenization temperature which led to a coarse grain structure in this zone assisted by high pressure. The weld zone presented martenite and ferrite microstructure with irregular grain. As a result, the hardness of the weld zone reached 315 HV and the impact energy dropped to near zero. After annealing at 950 degrees C for 3 min, the decomposition of martensite was the main reason the decrease of hardness (260 HV) in weld zone. The microstructure of weld zone was composed of ferrite and bainite, resulting in the increase of impact energy from 0 to 23 J.