Research On Surface Modification Of Commercial Pure Iron Processed By Surface Self Nano-crystallization

Stainless steel has been widely used in various fields as it has good corrosion resistance,heat resistance,low temperature strength and mechanical properties.However,its price is more expensive than ordinary steel. In actual use,the material failure usually occurs in the material surface, such as corrosion and wear. Therefore,in a number of occasions that only the surface need to be Stainless Steel,manufacturing a good mechanical properties and corrosion resistance surface structure can save a lot of alloy than using stainless steel,which greatly reduce production costs. Based on this theoretical,in the paper Cr,Ni elements were diffused into pure iron surface with surface self-nanocrystallization(SSNC) , So that the surface layer composition of pure iron neared to the level of stainless steel,the interior remained as pure iron. This would not only meet the needs of the stainless steel environment,but also save a lot of cost and resources,It had great significance on the practical application.Nano-grain size surface layer was made in the surface of pure iron by high-energy shot peening(HESP).Because a large volume fraction of grain boundaries in nanostructure may act as fast atomic diffusion channels, and large number of non equilibrium defects with high stored energy may reduce activation energy, which may increase atomic diffusion coefficient while diffusion. After samples SSNCed, the nano-layer of pure iron surface was characterized, the thermal stability of the surface layer and the relationship between the depth of affected layer and processing duration were studied. Then, single Ni and Cr-Ni were diffused to pure iron surface on the 600℃~ 850℃temperature range to make pure iron surface alloying. Optical microscopy, scanning electron microscopy and X-ray diffraction method were employed to characterize the microstructure feature of the surface layer before and after diffusion treatment, the composition distribution after the diffusion treatment was detected by energy disperse spectrum. The hardness and the corrosion resistance of the alloy layer were studied as well.The test results show that with treatment duration increasing, the thickness of pure iron surface deformation increased,while HESP time reached 6min, the sample surface grain was refined to nanometer scale ,the deformation layer thickness reached about 135μm,the grain size increased with depth increasing from surface to inside;while HESP time was over 6min, the deformation layer thickness no longer significantly increased. The HESP microstructure of pure iron had relatively good thermal stability while annealing at the temperature below 600℃.The surface layer nano-grains and deformation layer grains had grown in varying degrees after Ni or Cr-Ni diffusion treatment at 600℃~ 850℃,which affected the positive role of nanocrystals to atomic diffusion;the diffusion effect was well at middle temperature,a 10μm alloy layer was formed. After samples SSNCed and diffused,the surface hardness had greatly improved, and in the 40μm range declined rapidly until reached to matrix hardness. The corrosion resistance of pure iron sample surface after HESP decreased compared with the original sample,and after the diffusion treatment,the corrosion resistance increased to higher than the original sample.