Preparation And High Temperature Oxidation Resistance Of 304 Stainless Steel Oxide Film

304 stainless steel is widely used in automotive manufacturing,power equipment,petrochemical and other traditional industrial fields due to its excellent comprehensive mechanical properties and high temperature oxidation resistance.With the advancement of science and technology,the power of machinery has been significantly improved as while as the service temperature of mechanical parts.More attentions have been paid to the high temperature oxidation resistance of heat-resistant steel.Although 304 stainless steel has excellent oxidation resistance in 800?environment,when the temperature reaches 900?,the high temperature oxidation rate will be higher,which limits its application in a wider field.Therefore,how to improve the high temperature oxidation resistance of 304 stainless steel and the relevant analysis of its oxidation mechanism are particularly important.In this paper,oxide films were prepared on the surface of 304 stainless steel by means of pre-oxidation and sol-gel methods.X-ray diffraction(XRD),oxidation kinetics,scanning electron microscopy(SEM),and related oxidation kinetic equations were used to investigate the change of high temperature cyclic oxidation resistance of 304 stainless steel oxide film at 900?for 100 h.The main research contents are as follows:(1)The effect of the pre-oxidation temperature(400?-900?)on the high temperature cyclic oxidation resistance of 304 stainless steel was studied.The results show that when the pre-oxidation temperature is 400?-500?,the oxide film formed on the surface of the stainless steel after the pre-oxidation treatment is not perfect,which has a limited effect on improvement of the high temperature oxidation resistance.When the pre-oxidation temperature is 900?,the Cr element oxidizes seriously at the grain boundaries after the pre-oxidation treatment so that cracks and other defects are easy to form at the grain boundaries during cyclic oxidation.At the same time,a large amount of the Cr element on the surface of the stainless steel is consumed,which may reduce its high temperature oxidation resistance.A complete and compact oxide film is formed on the surface of the stainless steel pre-oxidized at 700?,which effectively delays the oxygen inward diffusion,enriches Cr elements on the surface of the substrate and selectively oxidizes,thus producing Cr₂O₃ with excellent high temperature resistance.When the pre-oxidation temperature is700?,the Cr element oxidizes slowly at the grain boundary,so the stainless steel has the best resistance to high temperature cyclic oxidation.(2)The effect of SiO₂ film coating layers prepared by sol-gel method on the high temperature cyclic oxidation resistance of 304 stainless steel was studied.The research results show that the adhesion of the SiO₂ film to the stainless steel substrate is good,which promoted the selective oxidation of the stainless steel to form Cr₂O₃ and NiCr₂O₄ scales with good high temperature resistance.Furthermore,the high temperature oxidation resistance of stainless steel was significantly improved.The sample with three-layer SiO₂film has the best high temperature oxidation resistance,the oxidative weight gain and oxidative peeling of which are only 58.1%and 41.4%of those of the uncoated sample.The single-layer SiO₂ has a small thickness so that its effect on reducing the diffusion flux of oxygen is limited.Samples with six or nine layers of SiO₂ film will easily cause cracks and peeling due to the mismatch between the thermal expansion coefficient of the film and the substrate,and weaken the effect of the film on the reduction of oxygen inward diffusion rate.(3)SiO₂ and Al₂O₃ sols were prepared separately by sol-gel method.Mullite sol was prepared by mixing SiO₂ and Al₂O₃ sols with Si:Al molar ratio of 1:3.The effect of mullite film layers on the high-temperature cyclic oxidation performance of 304 stainless steel was studied and its protection mechanism was analyzed.The research results show that the mullite film sintered with nine times of mullite sol is dense and uniform.Al_1.4Si_0.3O_2.7 and Cr₂O₃ ceramic phases are mainly formed after oxidation test.Compared with that of the untreated sample,oxidation curve of the coated sample showed a slow oxidation growth rate.The three-layer mullite sample film has a complete structure,and the film has good adhesion to the stainless steel substrate.It promoted the selective oxidation of the surface of stainless steel and generated protective Cr₂O₃ and NiCr₂O₄ oxide layers,which has best resistance to high temperature oxidation.