Study On Effect Of Heat Treatment After Shot Blasting On High Temperature Steam Oxidation Resistance Of S30432 And TP347H Steels

With the development of large capacity power generation equipment,the working medium parameters?steam temperature and pressure?of the unit are improved continuously,which puts forward higher requirements for the performance of metal materials used in coal-fired power unit.S30432 and TP347H are two typical austenitic stainless steels,which have been widely used in the manufacture of high temperature superheaters,reheaters and various high temperature and high pressure resistant pipes fitting for generator sets.In order to improve the oxidation resistance of high temperature water vapor,the inner wall of austenitic stainless steel pipes are usually shot blasted.Heat treatment is required to eliminate the stress for the shot blasted pipes after welding and bending.Whether the heat treatment process has an impact on the high temperature oxidation resistance is of great significance to evaluate the safety of materials in service.In this paper,the effects of shot blasting on the oxidation behavior and oxidation resistance of S30432 and TP347H stainless steel under simulated conditions were investigated.The S30432 steel shot blasted samples were heat treated at different temperatures?700-800°C?to study the impact of heat treatment on oxidation behavior and oxidation resistance in high-temperatur steam atmosphere.The microstructure of deformed layers was observed by using optical microscope?OM?.The oxidation resistance of each sample at high-temperature steam was analyzed by oxidation kinetics curve.The composition,morphology and structure of oxidation products were studied by means of XRD,SEM and EDS.And the effect of shot blasting and heat treatment on oxidation behavior of S30432 and TP347H stainless steel at high-temperature steam was discussed.After shot blasting,there is a deformed layer with a depth of about 150?m on the surface of S30432 steel samples.The original austenite grains in the deformed layer are fragmented,the grain boundary area is increased,and a large number of slip bands,high-density dislocation groups,deformation twins and martensite formed by shot blasting provide more rapid diffusion channels for Cr.The effective diffusion coefficient of Cr is significantly increased,which is beneficial to the formation of Cr-enriched oxide films on the surface of the sample.After oxidation for 2000 h in high temperature steam,the oxide film thickness of S30432 steel shot blasted surface is only8.34%of the unblasted surface,and the oxidation resistance is improved significantly.After shot blasting,the oxidation product of S30432 steel changed from a continuous double-layer oxidation film to a single Cr-rich oxidation film,which is mainly composed of Cr₂O₃,?Fe,Cr?₃O₄,?Fe,Cr?₂O₃ and Fe₃O₄,which reduces the diffusion rate of oxygen and metal ions,and slows down the speed of oxidation products growth effectively.The depth of deformed layer on the surface of TP347H steel shot blasted samples is about 130?m,and obvious grain fragmentation can be seen in the deformed layer.After oxidation for 2000 h in high temperature steam,the oxide film thickness of TP347H steel shot blasted surface is 54.95%of the unblasted surface.After shot blasting,the oxidation product of TP347H steel changed from continuous double layer oxide film to double layer oxide film with uneven thickness distribution.The outer layer of the oxidation product is mainly Fe₃O₄ and Fe₂O₃,the inner layer is?Fe,Cr?₃O₄ and?Fe,Cr?₂O₃.And dark gray Cr-rich strip is locally distributed on the interface of oxide film and substrate.The Cr-rich strip hinders the diffusion of oxygen toward the substrate and reduces the oxidation rate.After heat treatment at 700-800°C,the depth and grain size of the fragmented grain layer in deformed layer of S30432 steel shot blasted samples are similiar with those of the unheated S30432 steel shot peened samples,and the microstructure of the deformed layer after oxidation for 2000 h is more or less the same as those before oxidation.After oxidation for 2000h in 620°C water vapor,the kinetic curves of oxide film thickness on S30432 steel shot blasted surfaces with different heat treatment are basically the same,and it is similar with that of S30432 steel shot blasted sample without heat treatment.The Cr-rich oxide film composed of Cr₂O₃,?Fe,Cr?₃O₄ and?Fe,Cr?₂O₃ etc.is formed on the surface of the shot blasted samples and a Fe-rich globular oxide is locally present.The composition of the oxidation product is basically the same as that of the unheated S30432 steel shot blasted samples.The oxidation behavior of S30432 steel shot blasted samples with different heat treatment are similar.