Effect of Annealing on Microstructure and Hardness of Ultra-high Strength Low Alloy Bainitic Steel

Three different annealing processes regarding forge cooling and isothermal softening were designed for ultra-high strength low alloy bainite steel produced by a domestic steel plant. The microstructure of the samples under different annealing processes was observed by scanning electron microscope,and the hardness was measured by a Brinell hardness tester. The results show that the microstructure of the materials under air cooling +sub-thermal spheroidization annealing is composed of tempered tortolite,where the carbides are intermittent or patchy,and hardness is 270 HB. The microstructure under air cooling +isothermal spheroidization annealing is still composed of tempered tortolite,where the carbides are thick strips,and hardness is 290 HB. The microstructure under furnace cooling +sub-thermal spheroidization annealing is composed of tempered sostenite,where the M-A islands are decomposed,the carbides are spheroidized and aggregated,and the ferrite is recovered. Hardness of steel with the later process is significantly reduced to 245 HB,which is appropriate for cutting performance.