| Owing to its excellent mechanical and corrosion resistance properties,high nitrogen nickel-free austenitic stainless steel(HNASS)is widely used.Metal injection molding(MIM)is an advanced technology which can be used to prepare low cost and high performance HNASS in large quantities.However,current research reports on the preparation of HNASS using powder metallurgy or metal injection molding have not discussed the nitriding process,the distribution of nitrogen content after nitriding and the influence of nitrogen content on the decomposition of austenite.Secondly,there is also no report on the influence of online quenching and cooling atmosphere on the microstructure and properties of MIM HNASS.Hence,Using gas atomized Cr-Mn-Mo-N stainless steel powder as raw powder and microcrystalline wax(MW),high density polyethylene(HDPE),ethylene-vinyl acetate polymer(EVA)and stearic acid(SA)as binder systems,a dense HNASS was prepared by MIM and the preparation,microstructure and properties were studied and optimized:1)Sintering and nitriding.The effects of sintering temperature(1200 ? 1380 ?)and time(2 ? 4 h)on nitrogen content,distribution of nitrogen and nitrides,microstructure and properties of MIM HNASS were studied.The results show that the density increase gradually while the content of nitrogen and nitrides decrease gradually with the increase of sintering temperature;High nitrogen content can promote the precipitation of nitrides and decrease the stability of austenite during cooling.After sintering at 1200 ? for 2 h,the highest tensile yield strength and tensile strength of the sample with 1087 MPa and 1113 MPa respectively were obtained.After sintering at 1320 ? for 2 h,the microstructure with less ferrite and nitrides,the highest elongation(25%)and the best corrosion resistance can be obtained.2)Solution treatment.The effects of solution treatment temperature(850 ? 1150 ?)and time(1.5 ? 3 h)on the precipitation of nitrides,microstructure and properties were studied for MIM HNASS.The results show that at 850 and 950 ?,the nitrides are mainly long grain boundaries and internal nitrides with large size,and they strongly reduce the mechanical properties.When the solid solution treatment is at 1050 ?,fine grain boundary nitrides are mainly precipitated,which can improve the yield strength of the sample without reducing the plasticity strongly;The nitrogen content,microstructure and properties of samples sintered at different temperatures(1200 ? 1380 ?)and solution treated at 1150 ?and 1.5 h were analyzed.The results show that the single phase austenite structure can be obtained after solution treatment when sintering at 1320 ? and below.When the sintering temperature is 1320 ? and sintering time is 2 hours,the sample has the best mechanical and corrosion resistance after the solid solution treatment at 1150 ? and 1.5 h.3)On-line quenching and cooling gas optimization.The effects of on-line quenching(750 ? 1150 ?)and cooling gas optimization(750 ? 1320 ?)at different temperature on the microstructure and properties of sintered samples were studied.It is found that when quenching and switching to argon at lower temperature(750 and 950 ?),ferrite and nitrides can be find on the surface layer of samples.When the argon switching temperature is1320 ?,a lot of ferrite and precipitates(?)are formed on the surface layer of the sample.When the quenching temperature and argon switching temperature are 1150 ?,the whole sample has a single austenite,and its mechanical and corrosion resistance are the best.In this paper,the sintering nitriding,solution treatment,on-line quenching process and cooling atmosphere optimization of HNASS prepared by MIM are comprehensively studied.It not only brings a new method for the preparation of high performance engineering HNASS,but also brings a theoretical reference and a new way for industrial production of MIM HNASS with controllable microstructure,properties and low cost. |
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