The Carburization Quenching Distortion Rule And Mechanism Study Of A New Kind Of Micro-distortion And High Strength High Toughness Bainitic Steel

This dissertation studied a new micro-distortion, high strength and high toughness bainitic steel GDL-1 's carburization and quenching distortion, we find out the rule and the mechanism of carburization and quenching distortion of GDL-1 steel.The GDL-1 steel is developed by Professor Liang Yilong. The GDL-1 steel's mechanic performance is outstanding. Compared with 20CrMoH steel, each mechanics performance index has big superiority. δ_0.2=850 1050MPa, δ_b = 1100¹400MPa, δ=12-20%, ψ = 45 — 60%, α_k=85¹80J/cm², bending fatigue performance δ_-1 = 700MPa, K_1c =100.7¹70MPa·m^1/2 , The crystal grain size level is 8 9.The GDL-1 steel test specimens were carburized, after the carburization the specimens were cooled to room temperature with different cooling rate. By measure the different cooling rate's distortion, the carburization layer's depth and the microhardness, and with temperature field simulation, residual stress measuring, we have studied the GDL-1 steel's carburization and quenching distortion characteristics and the distortion rule.The GDL-1 steel's carbon content is 0.2%, mainly adds Mn and Si and a little other alloy elements. Si enhanced the body strength and hardness, the GDL-1 's high temperature strength is higher than common steel. Mn enhanced the hardenability greatly. The GDL-1 steel with small cooling speed can gain ideal hardness than traditional steel, and it's outstanding organization and mechanics performance, could realize the GDL-1 steel's micro distortion. After carburization and cooling by air, the distortion is very little, and it is able to attain HRC59-64 of skin hardness, HRC35-45 of heart hardness. Carbon concentration is ideal, carburization speed is big and carburization layer'depth is thick. Carburization layer's microstructure is composed of tiny martensite and few dispersered tiny residual austenite, the transitional layer is composed of acicular or plank bainite and martensite organization, the heart department is composed of plank bainite and grain bainite.The test specimen's distortion was directly affected by different cooling speed of different cooling medium. Along with quenching cooling rate reducing, the GDL-1 steel's quenching distortion reducing. The thermal stress was the primary factor affects the distortion during quenching. Plastic deformation under high temperature is the main origin of distortion. The temperature field modeling and residual stress measuring indicates, the test specimen is cooled by air, the temperature difference between inside and outside is small, along with the cooling speed increasing, the temperature difference between inside and outside increasing, thus causes the distortion to increase.