Research on Deformation/Nitriding Hybrid Treatment Process and Structure and Properties of Nitrided Layer of Austenitic Stainless Steel
|School||Dalian Maritime University|
|Course||Vehicle Operation Engineering|
|Keywords||Shot peening Plasma Nitriding Strain-induced Martensite High-temperature-short-time Plasma Nitriding|
Austenitic stainless steel (ASS) possesses good deformation property, welding property and corrosion resistance property, which has been widely applied in the field of chemical, medical, food and aviation industries. The annual production of ASS accounts for two thirds of the world total production of stainless steel. However, due to the extremely low hardness (Hv<250), the wear resistance property of ASS is very poor, which leads to early failure of mass components and parts even under low load according to the serious wear. And because ASS can not be strengthened by heat treatment like common steel materials, lots of researchers study on the surface strengthening of ASS in recent years. How to enhance the hardness of ASS without loss of its corrosion resistance has been the hot research point in the research of surface strengthening of ASS. The current research status is as follows:deformation strengthening treatment can not achieve satisfied strengthening effect towards ASS and furthermore deformation could lead to unstable corrosion resistance property of ASS; the efficiency of ordinary plasma nitriding process of ASS is very low and the nitrided layer is very thin; recently the hybrid treatment of deformation pre-treatment and nitriding is used to enhance the nitriding efficiency of ASS, however according to the restriction of the surface mechanical attrition treatment and equal-channel angle pressing equipments, these kind of hybrid treatments could not applied to the components and parts with complex shapes in industry manufacturing.This paper researched on the strengthening treatment process towards ASS which possesses actual industrial application value. The research contents can be classified as two parts:The first part studied the hybrid treatments by combining of ordinary industrial shot peening and plasma nitriding to enhance the nitriding efficiency of ASS with retaining or even improving corrosion resistance ability, which takes advantage of the composite effects of physics effect (fine grains and phase transformation) and chemical effect (chemical contents modification) in the hybrid treatments; Second part studied high-temperature-short-time plasma nitriding treatment to enhance the nitriding efficiency of ASS and achieve nitrided layer with good wear resistance and corrosion resistance properties, and also studied the nitriding thermodynamics and nitrides precipitation kinetics of ASS.The main conclusions are as follows:1. The hybrid treatments, which combines ordinary industrial shot peening and plasma nitriding, effectively decreases the nitriding diffusion activation energy of ASS, and achieves a twice thicker nitrided layer than ordinary nitriding treatment under the same treatment parameters. When the nitriding temperature bellows500℃, the nitrided layer achieved by hybrid treatments on the surface of ASS is mainly composed of phase S, which possesses good corrosion resistance property and wear resistance property. By changing the nitriding parameters, such as temperature and duration etc, the nitriding micro-mechanism and nitriding kinetics of hybrid nitriding treatments are studied, which provides theoretical basis for the industrial application of deformation/nitriding hybrid treatments.2. Industrial shot peening treatment leads to formation of strain-induced martensite in ASS, which preferentially forms at the positions of single-direction shear band, intersection of multi-direction shear bands and intersection of shear band and grain boundary. During the plasma nitriding, the entrance of nitrogen atoms into the surface layer promotes the transformation from strain-induced martensite to expanded austenite, and the evolution process is as bellow:strain-induced martensite→expanded martensite→expanded austenite.3. A nitrided layer mainly composed of phase S can be achieved on the surface of ASS by high-temperature-short-time plasma nitriding treatment, and the thickness of nitrided layer gets up to9.0μm when nitriding duration is up to1h, which evidently shortens the nitriding time compared to ordinary nitriding treatment (4-5h).