Research On Carburized Layer And Heat Treatment Of Cone And Bit Leg Of Tri-cone Bit

The bit leg and cone are main part of the tri-cone bit. Depending on conditions, the steels should be carburized to obtain a high surface hardness, good contact fatigue strength and enough toughness. Reasonable carburized layer carbon content and heat treatment process are necessary condition for good performance of hardened steel.In this paper, 15 CrNiMo and 15MnNi4 Mo steels were used as bit leg and cone test steels. Microscope,SEM, EMPA, XRD, Rockwell hardness and other instruments were used to study The influence of highest carbon content of carburized layer, quenching temperature and mode on microstructure and mechanical properties of carburized layer and core. Determine the best carbon content range of carburized layer and the heat treatment process.The results show that the highest carbon content of 15 CrNiMo steel carburized should be controlled at 1.00 to 1.05(wt.%). Within this range, carburized layers were composed of acicular martensite, fine particles carbide and austenite. Its hardness and wear resistance significantly better. When the carburized layers’ maximum carbon content less than 1.00(wt.%), there were no significant diffusion layer carbide, and it’s wear greatly reduced. When the carburized layers’ maximum carbon content higher than 1.05(wt.%), massive or semi-mesh carbide were produced and it’s hardness and wear reduced. 15 CrNiMo steel’s quenching temperature should be controlled at 810 ℃after carburization, the surface hardness, wear resistance and toughness of 61.0HRC, 21.5HRC and 97 J respectively. There were more ferrite and lack of strength in the core at 800 ℃. The microstructure of carburized layer was coarse and the was poor relatively at 820 ℃. Secondary quenching process can refine the grain, the carburized layer and center with a good organization, but there is a complex process and higher costs.The highest carbon content of 15MnNi4 Mo steel carburized should be controlled at 0.85 to 0.90(wt.%). Within this range, carburized layers were composed of acicular martensite, fine particles carbide and austenite. Its hardness and wear resistance significantly better. When the carburized layers’ maximum carbon content less than 0.85(wt.%), there were more austenite, and it’s wear reduced. When the carburized layers’ maximum carbon content higher than 0.90(wt.%), massive or semi-mesh carbide were produced and it’s hardness and wear reduced. 15MnNi4 Mo steel’s quenching temperature should be controlled at 805 ℃ after carburization, the surface hardness, wear resistance and toughness of 59.4HRC, 37.5HRC and 86 J respectively. There were more ferrite and lack of strength in the core at 800 ℃. The microstructure of carburized layer was coarse and the was poor relatively at 810 ℃. Secondary hardening process can also optimize 15 Mn Ni4Mo steel nitrided layer and center performance to some extent.