Study On Surface Grain Refinement Of Pure Iron By Cyclic Nitriding-quenching Process

Grain refinement is one of the effective methods to enhance the strength and toughness of metallic materials.The ultra-refinement of surface grains of materials has always been a research hotspot in the surface strengthening technology field.In this study,in order to obtain ultra-refinement grains on the surface of industrial pure iron,the cyclic nitriding-quenching process was designed based on the cyclic quenching grain refinement technology.Via optimizing the nitriding and quenching process parameters,the ultra-refinement and even nanocrystallization of the surface grains was achieved.In addition,a variety of characterization methods were used to reveal the grain refinement process,and the microstructure evolution of the nitrided layer and the mechanism of during the cyclic nitriding-quenching process were discussed.Then,the hardness,tensile and abrasion tests were used to characterize the property of the refined grain layer.Firstly,the influence of different cyclic nitriding-quenching process on grain refinement was studied.The process of combination of nitriding and cyclic quenching under low-pressure and high temperature was carried out at 750-850℃.The results show that cyclic quenching after nitriding could significantly refine the surface grains.After nitriding and quenching at 750℃ for 3 times,the surface grains are refined from the original ferrite grain size of 30-50μm to 2-3μm.Increasing the nitriding and quenching temperature to 800-850 ℃,the degree of grain refinement decreased,which indicated that the nitrogen content and quenching temperature had a huge impact on the grain refinement process.Therefore,an optimized process of cyclic low-temperature nitriding+high-temperature quenching was proposed to further refine the grain size of the nitrided layer to 1-2μm.According to the rules obtained from the previous process exploration,a cyclic nitriding-quenching process under high nitriding potential nitriding was proposed.The optimum grain refinement effect was achieved by cyclic nitriding-quenching process of atmospheric nitriding at 550℃ for 2h and then quenching at 750℃.The mechanism of grain ultra-refinement was dicussed.The nitrided layer is composed of α’ and retained austenite γ after nitriding and quenching,which is completely decomposed into α+γ’tempered structure in the next nitriding stage,then the tempered structure transformed finer austenite grains in the subsequent quenching process.Furthermore,nanoscale α and γ’ grain were found in the TEM characterization of the nitriding stage of 4 cycles,which were transformed into γ grains within 10 nm and α’ grains around 70 nm in the subsequent quenching process,indicating that cyclic nitriding-quenching process could realize the nanocrystallization of the nitrided layer.