Processing,microstructure And Mechanical Properties Of Gradient Nanostructured 18CrNiMo7-6 Gear Steel

How to simultaneously improve the strength and ductility of a material is a long lasting challenge in materials science and technology.Homogeneous nanostructured materials are in general of high strength but low ductility.Generation of gradient nanostructures by structural architecturing at multiple scales can suppress the occurrence of strain localization during plastic deformation,leading to a good combination of strength and ductility.The techniques like surface mechanical attrition treatment(SMAT),surface mechanical grinding treatment(SMGT)have been reported to be able to produce surface gradient nanostructures in various metals.However,they are hardly applicable in industrial manufacture due to their low processing efficiency.In the present work,a new technique,rotationally accelerated shot peening(RASP),was applied to achieve surface nanocrystallization of 18CrNiMo7-6 steel.The steel was treated by RASP for samples with two different initial microstructures that were prepared by normalizing and high temperature tempering,and quenching and low temperature tempering,respectively.The grain size\grain orientation and microcracks were characterized by means of optical microscopy(OM),scanning electron microscopy(SEM),electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM).The mechanical properties of the samples before and after RASP processing were measured with micro-hardness tester and tensile testing machine.The main results are summarized as follows:(1)RASP processing can successfully produce a gradient structure with the structural scales changing from nanometers to microsmeters in the surface layer of the normalized and high temperature tempered sample,but hardly produce a similar layer of gradient structure in the quenched and low temperature tempered sample.(2)For the normalized and high temperature tempered sample,both the velocity and time length of RASP can obviously change the microstructural evolution.The RASP velocity has a greater influence on the extent of surface nanocrystallization and the thickness of deformed layer.However,the formation of surface microcracks is more sensitive to the the length of RASP time.(3)After RASP processing,the yield strength of the normalized and high temperature tempered sample is greatly improved up to 550 MPa,which is an increase of by 69% as compared with the yield strength of the sample before RASP.There is a gradient in microhardness from the surface to the center of the sample,and the maximum value of microhardness at the top surface reaches HV260.(4)The tensile strength of the quenched and low temperature tempered samples after RASP is slightly improved.The microhardness tests reveal that hardening and softening occur at different depths in the cross-section of the sample.Specifically,the top surface and center region of the sample are hardened while the subsurface region was softened,resulting a “W”-like profile of cross-sectional microhardness distribution.(5)The characterization of microstructures at different depths before and after RASP treatments show that the softening occurring in the subsurface is attributed to dynamic recovery,while the hardening in the top surface and the center region is caused by different mechanisms.For the former it is resulted from dislocation strengthening and gran refinement;for the latter it is mainly due to dislocation strengthening.