The Oxidation Resistance Of Surface Nanostructures In 1Cr17 Stainless Steel At High Temperature

The oxidation of metals is one of the main failures for the parts used in high temperature and is related to their service life and functional reliability directly. The high temperature oxidation resistance of materials has been received high concern as the development of heat energy industry, solid oxide fuel cell, supercritical water etc. in recent years.High temperature oxidation of metallic materials depends on whether the protective and compact films on their surfaces could be formed. Surface shot blasting and surface nanocrystallization can promote the protective scale formation and the Cr diffusion from substrate to substrate/oxide interface at the process of oxidation, which can increase the content of Cr in the oxide film and the density of the oxide film. So the metals' ability of oxidation resistance is improved. But the usual method of surface shot blasting just can refine the initial coarse-grains into subgrains and has little promotion effect on the diffusion of alloying elements. Nanostructured materials have a large number of grain boundaries which are in favor of the diffusion of elements, but all the methods of nanocrystallization are obtained from coating at present, such as PVD,CVD etc. All these methods have high costs and just can be applied to small-size parts which lead to the limits of their applications.Using lCrl7 stainless steel, this paper has studied the high temperature resistance of the nanostructured surface obtained through plastic deformation induced by high energy shot peening, comparing with traditional shot blasting. The high temperature thermal balance was used to detect oxidation kinetic curves. X-ray Diffraction (XRD), Transmission Electron Microscope (TEM), Scan Electron microscope (SEM) and Energy Dispersive X-ray Detector (EDX) were also employed to analyze nanostructured surface layer, the morphology and component of surface oxide films.The experimental results showed that the peak of XRD pattern had been broaded evidently after 2 hours high energy shot peening at the shot peening speed of 55 m/s using CfMo alloy pills of 1 mm in diameter. It is estimated that the grain size of the surface layer is about 20 nm, which means high energy shot peening can make lCrl7 stainless steel surface nanocrystallized. Through the successive oxidation kinetic curves obtained in H2O+O2 atmosphere at 700 °C, it can be obtained that traditional shot blasting can improved the oxidation resistance, but not as remarkably as nanocrystallizing shot peening did. Furthermore, the compact and high Cr content oxide layers on the surface nanostructured samples were formed both in pure vapor after 120-hours exposure at 500°C and in air after 20 hours at from 500°C to 600°C. This also means nanocrystallizing shot peening can improve oxidation resistance remarkably.All the oxides are (FeCr)2C>3 type as the XRD results showed. And the longer the shot peening time, the higher the Cr content in oxide films. Therefore, nanocrystallizing shot peening can promote selective oxidation of Cr and diffusion from substrate to the substrate/oxide interface, which promoted the formation of high Cr content oxide layer and improved the ability of preventing from the diffusion of metallic atoms from substrate to surface and oxygen atoms from surface to substrate furthermore. Thus it can prevent substrate metals from further oxidation.Otherwise, the surface nanostructured grain size is still smaller than initial grain size after high temperature oxidation though they had grown in various degree. For example, the average grain diameter of the surface layer is 4.8 um after 20 hours in H2O+O2 atmosphere at 700°C, and it is still smaller than original sample which is about 25um grain diameter. It is still advantageous for alloys with positive grain boundary effect.According to the analysis of above-cited results, it can be believed in that the surface nanocrystallization induced by plastic deformation could improve the high temperature oxidation resistance of lCrl7 stainless steelthrough promoting selective oxidation and the Cr diffusion from sub-layer to upper-layer and forming a more compact and higher Cr content (FeCr)2C>3 layer. Moreover surface nanocrystallization could accelerate the formation of compact oxide layer. As a result the oxidation resistance is improved.