Dissolution And Precipitation Of Nitrogen During Zone Melting Of 304 Stainless Steel

304 stainless steel has good corrosion resistance and hot working performance,but low hardness and wear resistance restrict its application range.Nitrogen alloying improves the hardness and corrosion resistance of 304 stainless steel.Current stainless steel main nitrogen alloying of solid-phase nitriding and liquid nitriding two kinds,but solid phase nitriding nitriding time is long,and poor effect of nitriding shortcomings,liquid nitriding is mainly by adding nitrogen as a nitrogen source,easy to introduce another kind of impurity,limit the choice of steel grade,and for the sample of the specifications of the demand is higher.In this paper,the regional melting process under nitrogen atmosphere is adopted,and nitrogen alloying treatment is carried out on 304 stainless steel by adjusting nitrogen pressure and zone melting speed,and samples with different nitrogen contents are obtained.In theory,continuous nitrogen alloying of rod-like materials can be realized,which has advantages of short nitriding time and stable effect.In this paper,the effect of nitrogen pressure and zone melting speed on nitrogen content of 304 stainless steel was studied.Combined with the effect of nitrogen on pores morphology and microstructure of stainless steel,the dissolution and precipitation behavior of nitrogen in zone melting 304 stainless steel and its microstructure were studied.The results showed that:(1)The actual nitrogen content in 304 stainless steel is lower than the theoretical value,mainly because the regional melting speed is faster,and the nitrogen does not fully diffuse into the melting zone,resulting in the actual nitrogen content is lower than the equilibrium solubility.In addition,the formation of pores during solidification is also one of the reasons that the nitrogen content in matrix is lower than the theoretical value.(2)Nitrogen pressure has a significant effect on pores morphology.With the increase of nitrogen pressure,the porosity decreases gradually,the average pores size decreases,and the pores diameter tends to be uniform.With the increase of nitrogen pressure,the ferrite phase area gradually decreases and disappears,thus reducing the amount of nitrogen precipitation.This is the direct reason why the porosity decreases with the increase of nitrogen pressure.(3)Due to the poor solubility of nitrogen in solid and liquid phase,nitrogen bubbles will nucleate,grow up and escape from the melting zone during solidification,which is also an important reason for the lower nitrogen content than the theoretical value.Due to the opposite dissolution trend of nitrogen with temperature change in pure iron and 304 stainless steel,the pores morphology,actual pores rate and theoretical pores rate in the two materials were compared,and the escape coefficient of nitrogen in 304 stainless steel and pure iron was determined to be 0.9 and 0.6 respectively.(4)With the increase of nitrogen content,the austenite volume fraction in 304 stainless steel increases gradually,while the ferrite volume fraction decreases gradually.Meanwhile,the austenite grain size tends to grow.The tropism of ferrite and austenite in nitrogen 304 stainless steel is the coexistence of K-S and N-W relationship.