| The High-carbon high-chromium steels have high hardness and excellent wear resistance due to the large amount of carbides distributed in the matrix,and have been widely used in tribological applications as the engineering components.In order to further improve their surface wear resistance,they are usually subjected to nitriding treatment.In recent decades,although the effects of nitriding process parameters on the microstructure and wear properties of the high-carbon high-chromium steels have been extensively studied,there is a lack of understanding of the microstructure evolution of the nitrided layer during nitriding and the potential internal factors affecting the wear resistance as well as the wear damage mechanism of nitrided high carbon and chromium steels.In this paper,the high-carbon and high-chromium steel Cr12 W was selected as the test material,and the nitrided layers with different microstructure and hardness were obtained by changing the tempering temperatures of the samples prior to nitriding,so as to fully reflect the wear properties of the nitrided Cr12 W steel.The results showed that the hardness of the nitrided layer gradually decreased with the increase of tempering temperature,but the depth of the nitrided layer gradually increased.Under the normal load of 75 N,when the counterbody material was GCr15 steel ball,the wear resistance of nitrided sample was about40 times higher than that of unnitrided sample,indicating that nitriding treatment effectively improved the wear resistance of material,but paradoxically,the wear rate of nitrided samples with different surface hardness was similar.Although the surface hardness of nitrided sample was close to that of GCr15 steel ball,its wear resistance was much better than that of GCr15 steel ball.When the counterbody material was Si C ball,the wear mechanism of nitrided sample was different,the wear resistance of nitrided sample decreased gradually with the decrease of surface hardness.Subsequently,the microstructure evolution of the nitrided layer during nitriding and the wear damage mechanism of the nitrided samples fretted against the GCr15 steel balls were further studied to clarify the main internal factors that affect the wear resistance of the nitrided sample.During nitriding,the carbides in nitrided layer were observed to be gradually transformed into nitrides,and the carbon atoms released by these converted carbides precipitated along the grain boundary to form the white network precipitate(cementite).The coarse nitrides formed by the transformation of primary carbides were the key factor for the excellent wear resistance of the nitrided samples.The coarse nitrides with high hardness and excellent wear resistance made the steel ball lose its cutting ability and be rapidly worn,and the wear damage of the nitrided layer was mainly caused by the abrasive removal of the matrix material.As the normal load increased to 40 N and 80 N,the higher contact pressure led to the formation of central smooth zone,which effectively reduced the wear rate of the friction pair.And the area of the central smooth zone increased with the increase of normal load,leading to a more more effective protection for the friction pair.Thus,when the normal load increased from 40 N to 80 N,the wear volume of the nitrided sample hardly increased.The effect of different counterbody materials on the wear properties of high-carbon high-chromium steel before and after nitriding was compared and analyzed to better understand the the wear behavior of carbides/nitrides.The results indicated that the unnitrided sample fretted against the Si C balls exhibited excellent wear resistance compared to that fretted against the GCr15 steel balls,which was mainly caused by the excellent wear resistance of the Si C ball.The low wear rate of the Si C ball resulted in a small contact area between the two friction surfaces,the generated high temperature caused the matrix material on the worn surface of the unnitrided sample to be softened.Thus,the strong combination of the oxide layer and the softened matrix material effectively protected the friction pair,reducing the wear rate of the sample.The long rod-shaped primary carbide acted as the barrier to effectively prevent the softened matrix material and oxide layer from being removed by shear stress,thereby improving the wear resistance of the material.However,when the counterbody material was GCr15 steel ball,both of them were quickly worn,and the carbides had a limited effect in improving the wear resistance of the sample.In the loading cases of 10 N and 30 N,the nitrided samples fretted against the GCr15 steel balls also had a larger wear volume than those fretted against the Si C balls.Since both the nitrided samples and the Si C balls had excellent wear resistance,leading to that the main wear mechanism between the friction interface under lower load of 10 N and 30 N was the oxidation mechanism,so the wear rate and friction coefficient of the nitrided samples were low.When the normal load was increased to 60 N,the wear volume of the nitrided sample hardly increased,but the high normal load further improved the cutting ability of the Si C ball,resulting in a rapid increase in the wear volume of the nitrided sample.Therefore,in this loading case,the wear volume of the nitrided sample fretted against the Si C ball was larger.A nanocrystalline layer was prepared on the surface of the high-carbon and high-chromium steel by ultrasonic surface rolling technology.The results showed that surface ultrasonic rolling can not only refine the coarse grains,forming a nanocrystalline layer on the surface of the material,but also effectively refine the carbides,making the coarse primary carbides be sufficiently fractured and dissolved,thereby changing the microstructure of the surface material.However,due to the massive dissolution of carbides,especially the disappearance of the primary carbides,the wear volume of the treated sample was similar to that of the untreated sample.Finally,the USRP-treated and un-treated samples were subjected to gas nitriding to study the effect of surface nanocrystallization prior to nitriding on the microstructure and wear properties of the nitrided high-carbon high-chromium steel.The surface nanocrystallization pretreatment can not only increase the depth and hardness of the nitrided layer,but also eliminate the white network precipitation in the diffusion layer,leading to that the cementite precipitation was very small and evenly distributed in the nitrided layer.However,due to the disappearance of coarse nitrides,the wear volume of the nitrided sample with USRP-pretreatment was obvious larger and its wear resistance was obviously reduced,especially under the normal load of 60 N.This confirmed that the coarse nitrides transformed from the primary carbides played a vital role in the wear properties of the nitrided layer,which effectively improved the comprehensive wear resistance of the nitrided layer. |
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