Effect Of Nitrogen Content On Microstructure And Mechanical Properties Of Cr10Mn9Ni0.7 Alloy

Stainless steel is one of the most common metals in daily production and life.It is widely used because of its good mechanical properties and excellent corrosion resistance.Stainless steel nitriding treatment,which use nitrogen instead of nickel,can not only guarantee the performance of stainless steel,but also save resources,so it has been widely concerned.The main common nitriding technology of stainless steel consists of liquid nitriding and solid nitriding,however,the effect of liquid nitriding is poor and the process is complex,the nitriding time of solid nitriding is long and the thickness of nitriding layer is limited.In this paper,the nitriding process was improved and a new nitriding method was put forward,the Nitriding Process of Melt Holding in Mushy Zone.This process use the characteristics of coexistence of solid-liquid two-phase in the mushy zone temperature range.In this process,we preserve heat for a period of time,so that nitrogen has a full time into the liquid phase,and spread to the austenite phase,and ultimately improve the nitrogen content in room temperature microstructure.Based on the experimental and theoretical research,the following conclusions are drawn:(1)In the process of heat preservation,the nitrogen content increased gradually with the increase of holding time,when the pressure is 0.1MPa,the nitrogen content can be increased from 0.17%to 0.23%,while the porosity decreased from 1.86%to 1.37%;when the pressure is 0.4MPa,the nitrogen content increased from 0.29%to 0.37%,the porosity decreased from 1.41%to 1.06%.The results show that the heat preservation process can effectively increase the nitrogen content in the steel and reduce the porosity of the ingot casting.(2)we explored the micro structure changes with the nitrogen content in the process of heat preservation in mushy zone.The results show that in the process of heat preservation in mushy zone,the continuous dissolution of nitrogen in molten steel leads to the isothermal transformation of ferrite in the micro structure,peritectic reacton of a part of ferrite and liquid phase makes austenite.The other part of ferrite change into austenite by solid phase transformation.When the nitrogen content in the liquid phase reaches a certain level,it will directly change into austenite in the subsequent cooling.(3)In this paper,we discuss the mechanism of nitrogen increasing in heat preservation:on the one hand,the increase of nitrogen content in the mushy zone promotes the transformation from ferrite to austenite,and the increase of austenite content increases the solubility of nitrogen in the alloy;on the other hand,the increase of nitrogen content promotes the transformation of liquid phase into austenite,which increases the content of austenite in room temperature microstructure.(4)The yield strength of austenitic stainless steel can be improved significantly with the increase of nitrogen content when the deformation of high nitrogen austenitic stainless steel is less than 2%.When the deformation is more than 5%,the deformed martensite is produced in the sample,which makes the stress of the sample with lower nitrogen content increase significantly,while the nitrogen content in the sample is higher,cold hardening is still the main strengthening factor.