| The hardenability plays an important role in production and application of gear steels.A narrow hardenability band is one of the most important characteristics of high-quality gear steels.However,nowadays the hardenability band of most gear steels can be controlled only within 8 HRC,which is evidently broader than the requirements of 4 HRC for high-quality products.Therefore,it is significant to study the hardenability and its effect on microstructure and properties of carburized gear steels.Four types of carburized gear steels with different hardenability were investigated by various experimental techniques in this work.The variation of Jominy curves of experimental steels and its influence factors were studied and a modified model of hardness distribution function was proposed for hardenability prediciton.Meanwhile,the prediction model of microstructure was optimized by using the critical cooling rate at 500 ℃ instead of that at 700 ℃,and the effect of hardenability on heat treatment distortion was also studied.Furthermore,the rotating bending fatigue and rolling contact fatigue properties for gear steels were discussed.Applicability of three current hardenability prediction models is evaluated according to the results of Jominy tests of 20CrMoH and 20CrNiMoH steels.The prediction model of hardness distribution function is modified,so that the standard error of predicted values in the distance of 5 mm(J5),9 mm(J9)and 15 mm(J15)from quenched end is less than 2 HRC compared with measured values.It is also shown that effect of grain size of austenite on hardenability cannot be ignored when the grain size is more than grade No.7.Furthermore,the variation range of grain size should be controlled within 1 grade,if the hardenability band of narrower than 4 HRC is expected.The temperature field model of end-quench specimen is established by using ABAQUS finite element software,and it can be used to calculate microstructure and hardness in the distance of 5 mm(J5),9 mm(J9)and 15 mm(J15)from quench end combined with Maynier model.Moreover,the values of microstructure and hardness calculated by instant cooling rate at 500 ℃ instead of that at 700 ℃ are in better agreement with measured values.The microstructure and hardness of 20CrNiMoH steel in the distance of 9 mm(J9)from quench end are calculated by the above model,and the results show that the variation range of the fraction of martensite should not exceed 16.7%if the hardenability band is expected to be narrower than 4 HRC.The results of the Navy-C ring specimens show that heat treatment distortion of gear steels is mainly related to the fraction of martensite in the core,which is increased with the increase of hardenability.For the 20CrNiMoH steel,it is mainly bainite structure in the core of the C-ring specimens,which have small heat treatment distortion when the hardness value in the distance of 9 mm(J9)from quench end in Jominy test is below 32 HRC.The fraction of martensite is increased in the core of the C-ring specimens when the hardness value in the distance of 9 mm(J9)from quench end exceeds 32 HRC.The heat treatment distortion is increased slowly in the original stage and rapidly in the subsequent stage with increase of the hardness value in the distance of 9 mm(J9)from quenched end,due to the differences in phase transformation.There is also a positive correlation between heat treatment distortion and hardness value in the distance of 9 mm(J9)from quench end of different types of gear steels.The rotating bending fatigue properties of gear steels are related to the hardness,the size and state of inclusions,and the grain size of austenite in carburized layer.The fatigue fracture is mainly initiated from inclusions located in carburized layer.The depth of spalling in rolling contact fatigue specimens were near the location where the shear stress calculated by Hertz theory is maximume.It is concluded that the fatigue properties can be improved by grain refinement in carburized layer as well as high cleanliness for carburized gear steels. |
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