| Two heavy-duty gear steels,20Cr2Ni4A and 17Cr2Ni2 MoVNb,are widely used in transmission components for high-speed trains and heavy-duty vehicles after gas carburizing followed by reheating and oil quenching treatment.Low-pressure carburizing followed by high-pressure gas quenching is a new type of carburizing heat treatment process,and it is being increasingly used to replace gas carburizing followed by reheating and oil quenching treatment due to its less pollution and higher efficiency.Currently,research work on these two heavy-duty gear steels has mainly focused on the process,microstructure and properties after traditional gas carburizing,and the adaptability of low-pressure carburizing followed by high-pressure gas quenching is still unclear for them.Especially,since the surface carbon content during the low-pressure carburizing is higher than gas carburizing,the NbC precipitates maybe dissolve into austenite more remarkably according to Ohtani’s [Nb][C] solid solubility product equation,which could cause coarse and ununiform grain sizes,low fatigue properties,and high quench distortion for the 17Cr2Ni2 MoVNbsteel.Therefore,by using 17Cr2Ni2 MoVNband 20Cr2Ni4A gear steels as research objects,this work is aimed to study the influence of low-pressure carburizing followed by high-pressure gas quenching on their microstructure and fatigue properties,and also to investigate the distortion mechanism in different gas quenching processes.Microstructures of the experimental steels under low-pressure carburizing followed by high-pressure gas quenching showed that the grain sizes in the carburized layer and the core were fine and uniform after low-pressure carburizing at 930 ℃.However,there was abnormal grain growth in the carburized layer and the core of 20Cr2Ni4A steel,which was grain refined by Al N precipitates,after low-pressure carburizing at 980 ℃.There was no significant difference in grain sizes in the carburized layer and the core of 17Cr2Ni2 MoVNbsteel,which was grain refined by NbC precipitates.The results of the extraction phase analysis method and carbon replica showed that the size and amount of NbC precipitates in the carburized layer and the core of 17Cr2Ni2 MoVNbsteel were similar.This indicated that both the carburized layer and the core of 17Cr2Ni2 MoVNbsteel were refined by the pinning effect of NbC precipitates on grain boundaries during low-pressure carburizing process,and the observed dissolution of NbC at high carbon content was quite different from the result calculated by Ohtani’s equation.According to the results of the Nb-containing gear steels with different carbon contents,a new equation of [Nb][C] solid solubility product,which is applicable for low-pressure carburizing process,was obtained as follows,log{[Nb][C]} = 1.23-5020/ T+ [C](1887/T-0.75)Comparing the effects of gas carburizing and low-pressure carburizing on the fatigue properties of 17Cr2Ni2 MoVNbsteel,it was found that the fatigue limits of the specimens after both gas carburizing and low-pressure carburizing were about 980 MPa.However,at high stress amplitude,the fatigue life of the specimens after low-pressure carburizing was longer than that of the specimens after gas carburizing.Compared with gas carburizing,low-pressure carburizing was followed by gas quenching at a higher temperature,leading to a higher volume fraction of retained austenite in the carburized layer.During the fatigue test,the blocky retained austenite in the carburized layer underwent martensitic transformation,and the volume fraction of retained austenite in the carburized layer decreased from 12.3% to 7.2% for gas carburizing specimens,and from 21.4% to 4.8% for low-pressure carburizing specimens,respectively.The transformation of retained austenite in the carburized layer increased the hardness of carburized layer,which made the fatigue limits roughly same.On the other hand,it could also prevent crack propagation,which made the fatigue life of the specimens after low-pressure carburizing longer than that of he specimens after gas carburizing specimens under high stress amplitude.At the carburizing temperatures of 930 ℃ and 980 ℃,the fatigue limits of17Cr2Ni2 MoVNbsteel were similar,being 977 MPa and 967 MPa,respectively,and those of 20Cr2Ni4A steel were quite different,being 967 MPa and 865 MPa,respectively.It showed that the fatigue limits of 20Cr2Ni4A steel had significantly decreased after lowpressure carburizing at higher temperature,and the fatigue crack initiation site was shifted from non-metallic inclusions to the matrix.It could be presumably attributed to the abnormal growth of grains in the carburized layer.At the same time,the abnormal growth of grains caused the grain boundaries to become weak areas,which lead to a change in the cracking mode.On the other hand,the grain sizes of 17Cr2Ni2 MoVNbsteel after low-pressure carburizing at 930 ℃ and 980 ℃ were similar,and thus it resulted in similar fatigue limits.The experimental results of the heat treatment distortion of C-type specimens of17Cr2Ni2 MoVNbsteel after carburizing followed by gas quenching at different pressures and interrupted quenching processes showed that the pressure had no evident effect on the quench distortion,while the interrupted quenching process at different isothermal temperatures could effectively reduce the distortion.The interrupted quenching process of gas quenching after isothermal treatment at 450 ℃ could decrease 71.1% of heat treatment distortion compared with the directly quenching process.It was attributed to the existance of about 40% bainite in the core of the isothermal process.In addition,there was about 80% bainite in the center of the thickest part of the isothermal treatment at 350 ℃.The martensitic transformation had already occurred in the thinnest part,so its distortion amount was higher than that of the isothermal treatment at 450 ℃.It showed that the distortion of carburized C-type specimens after carburizing and quenching was mainly dependent on the type of microstructure at the thinnest part,but not that at the thickest part. |
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