Effect Of Melt Holding In Mushy Zone On The Nitrogen Content Porosity And Microstructure Of 201 Stainless Steel

Nitrogen alloying of stainless steel has many benefits:firstly, nitrogen alloying can significantly improve the stainless steel strength, but does not reduce the plasticity; secondly, the nitrogen is a strong austenite stabilizing elements, can reduce or even replace the expensive nickel in stainless steel, and thus has remarkable economic benefits; in addition, nitrogen alloying can improve the corrosion resistance of stainless steels. Therefore low nickel austenite stainless steel with manganese and nitrogen instead of some nickel, can not only reduce the production cost, but also can guarantee the strength and corrosion resistance, and thus has received extensive attention. There are two problems in the production of high nitrogen austenitic stainless steel. One is the low nitrogen content; the other is formation of gas pores in the castings.This study, The effects of melt temperature and holding temperature under a nitrogen atmosphere were investigated. The solidification micro structure, size of ferrite region, diffusion of nitrogen in mushy zone were correlated with the nitrogen content and gas porosity.The results of this study have shown that the nitrogen content of the sample with melt holding in mushy zone is significantly higher than the samples with liquid nitriding, indicating melt holding in mushy zone is an effective way to increase the nitrogen content. With the increase of holding time in mushy zone, the nitrogen content increases gradually. When the nitrogen pressure is 1 atm, the nitrogen content of the sample hold in mushy zone for 15 min can reach 0.230 wt.%; when the nitrogen pressure is 4 atm, the nitrogen content of the sample hold in mushy zone for 15 min can reach a maximum value of 0.377 wt.%.A thermodynamic analysis was conducted for the diffusion of nitrogen from liquid to austenite. It was shown the chemical potential of nitrogen in liquid phase is higher than that in austenite, and the difference of the chemical potential is the main driving force for the diffusion of nitrogen from liquid to austenite.The experimental results also showed that the porosity of the sample melted in mushy zone is lower than the porosity of the sample melted in liquids temperature indicating the melt holding in mushy zone can effectively reduce the possibility of the gas pore formation. During the melt holding in mushy zone, nitrogen diffuses from the liquid phase to austenite and the precipitated nitrogen bubbles may remelted into liquid. This is equivalent to a "second increase of nitrogen content", which can reduce the porosity and promote the diffusion of nitrogen.The micro structure consists of austenite and ferrite. With the increase of nitrogen content, austenite gradually increased, gradually reducing ferrite. We can control the holding time in mushy zone, to control the amount of nitrogen content and residual iron ferrite, in order to achieve the desired properties of the practical application of the material.Melt holing in mushy zone is an effective process of increasing nitrogen, the nitrogen content increased significantly, significantly lower porosity, to provide a new way for increasing nitrogen content of austenitic stainless steel.