Effect Of Nitrogen Content On Phase Transition Of 18Cr18Mn Stainless Steel During Solidification

Traditionally,the addition of nitrogen in steel will lead to the production of non-metallic compounds,which will reduce the comprehensive properties of steel.Therefore,the nitrogen content in steel should be controlled within a certain range.However,nitrogen is not entirely devoid of merit in steel.Nitrogen can interact with other alloying elements in the steel in high-nitrogen steel,thus improving the mechanical properties,corrosion resistance and biocompatibility of the material,which makes high-nitrogen austenitic stainless steel become one of the new structural materials widely used in recent years.At present,The main common nitriding technology of stainless steel consists of solid nitriding and liquid nitriding,but the two are respectively in the effect of nitrogen increasing and industrial large-scale production has certain deficiencies,this study combined the advantages of the two,put forward a new nitriding process of melt holding in mushy zone.Its melting characteristics are: in the solid-liquid two-phase coexistence zone for "semi-solid" insulation,so that the reaction is fully carried out,nitrogen fully into the austenitic phase,and finally obtain high nitrogen austenitic stainless steel.Through experimental research and theoreti cal analysis,the following conclusions are drawn in this paper:(1)In this study,18Cr18 Mn alloy was fabricated with utilizing a vacuum/high pressure induction melting furnace by nitriding process of melt holding in mushy zone.The results show that the nitriding process of melt holding in mushy zone can effectively increase the nitrogen content in steel and reduce the porosity.Under the nitrogen pressure of 0.1MPa,with the increase of the holding time from 0min to 20 min,the nitrogen content increased f rom 0.161% to 0.384%,and the porosity decreased from 3.69% to 3.25%.(2)Observing the microstructure of alloy was found that the alloy microstructure is mainly composed of ferrite,austenite and the sigma phase,and with the increase of the content of nitrogen,austenite volume fraction increased gradually,ferrite and precipitated phase volume fraction decreases,ferrite microstructure morphology also from mesh,sheet into island and short rodlike.When the nitrogen content reaches 0.279%,the structure at room temperature is a mixture of ferrite and austenite.There is no sigma phase.So the high nitrogen content inhibits the nucleation of the sigma phase.(3)The nitrogen increase mechanism of melt holding in mushy zone was studied,which can be explained from two aspects: on the one hand,holding in the mushy zone promoted the transformation of ferrite to austenite,and the increase of austenite content further improved the solubility of nitrogen;On the other hand,the increase of nitrogen content in the liquid phase will promote the transformation of the liquid phase to austenite,increase the austenite content in the microstructure,and finally increase the nitrogen content in the alloy.(4)The effect of nitrogen content on the peritectic reaction pro cess of the alloy was analyzed,results showed that increasing nitrogen content could promote the peritectic transformation.The content of nitrogen affects the volume fraction of ferrite to austenite and peritectic reaction in the solidification process.(5)The effect of nitrogen content on mechanical properties of stainless steel was investigated.The results showed that the yield strength of stainless steel first decreased,then increased and then decreased slightly with the increase of nitrogen content;The elongation of stainless steel decreases first and then increases with the increase of nitrogen content.At the same time,with the increase of nitrogen content,the fracture mode of stainless steel changed from brittle intergranular fracture and transgranular fracture to ductile fracture,and the plasticity of the material increased with the increase of nitrogen content.