Welding Continuous Cooling Transformation of High Strength and Weathering Steels for Railway Vehicles

Taking high strength and weathering steel for railway vehicles with Cr mass fraction of 3.7% and 4.5% as the research object, experiments of continuous cooling transformation in the welding heat affected zone were carried out on the DIL805A thermal expansive apparatus. Laws of microstructure transformation and hardness change of tested steels under different cooling rates were studied. The results show that the ferrite, pearlite, bainite and martensitic transform occurs in the No.1 tested steel with Cr mass fraction of 3.7%, whereas only bainite and martensite transformation occurs in the No.2 tested steel with Cr mass fraction of 4.5%. When mass fraction of Cr increases from 3.7% to 4.5%, the phase transition point of the tested steel significantly shifts to the right, the ferrite and pearlite transformation are disappeared and the starting transition temperature of martensite decreases;the hardness of No.2 tested steel is larger at the same cooling rate, and a wave valley occurs due to the influence of V precipitation at a cooling rate of 10℃/s. The No.1 tested steel can obtain a better comprehensive structure when the cooling rate is less than or equal to 0.5℃/s, but the No.2 tested steel needs heat treatment after welding to achieve desired microstructure.