Fundamental Research On Preparation Of High Nitrogen Steel Powders By High-pressure Gas Atomization And SLM Forming

High nitrogen austenitic stainless steels are a new class of advanced engineering materials by using nitrogen elements instead of nickel elements to obtain austenitize structure.They have attracted extensive attention due to the excellent mechanical properties,desirable wear and corrosion resistance,biological consistency,which render them available in many fields.Besides,the phase composition and microstructure of high nitrogen steel are directly affected by the content and distributed state of nitrogen,which can further determine its comprehensive performance.Therefore,it will provide important academic value and useful purpose to reveal the occurrence and transfer behavior of nitrogen and the nitrogen increasing mechanism for promoting the popularizing application of high nitrogen steel.The aim was to successfully prepare the different nitrogen-containing Fe Cr17Mn11Mo3Nx powders.First of all,the parameter setting and feasibility of the experiment were conducted and analyzed by establishing thermodynamic model of increasing nitrogen in molten steel combined with phase diagram calculation.Then,the influence mechanism of the process parameters on phase composition and structure characteristic of the powders were investigated,which will reveal the nitrogen increasing mechanism and optimize the preparation process of spherical powders.Based on this,nitrogen-containing powders were used to be molded using additive manufacturing.The main contents and results are as follows:Different nitrogen-containing powders were successfully prepared via melting and gas atomization process,then the effect of atomization pressure and chamber pressure on powder characteristic was investigated.It revealed that with atomization and chamber pressure increasing,the quantity of irregular particles gradually reduced and even disappeared,the particle surface tended to be smoother,the powder size became more uniform and the proportion of fine powders increased in which the atomization pressure played a dominant role in refining powders.In addition,the nitrogen content also increased,and the effect of increasing nitrogen in process of melting with Ar was not as obvious as that of melting with N₂,especially in high pressure,which was caused by different nitrogen increasing mechanism.High nitrogen powders were successfully prepared via adding Mn N with high-pressure melting(N₂)and high-pressure gas atomization(N₂)process.It indicated that the nitrogen content of powders increased with chamber pressure increasing,and it reached 0.703 wt%with a chamber pressure at 0.1 MPa and the phase composed of?-Fe and a small amount of residual?-Fe.While the nitrogen content can be 0.836 wt%,0.932wt%and 1.010 wt%at 0.3,0.5 and 0.7 MPa,respectively,with the single phase of?-Fe.The melting process played a dominant role in the increase of nitrogen,which was consisted largely of the dissolution of Mn N,while a slight or even no nitrogen increasing occurred during gas atomization process.The nitrogen content can be effectively controlled by combining gas atomization with high chamber pressure.And spherical powders with uniform and fine particle size,rapid solidification structure and high nitrogen content can be finally obtained.Based on the present analyses,the optimized preparation process to prepare high-nitrogen powders was adding Mn N with high-pressure melting(N₂)and high-pressure gas atomization(N₂),having melting pressure at0.7 MPa,top-blowing pressure at 0.9 MPa and atomization pressure at 3.0 MPa,respectively.Characterizations of the prepared powder via Ar melting and N₂ gas atomization process indicated that there only existed a micro nitrogen layer with 10 nm on the surface in form of dissolved,adsorptional and compound state,without nitrogen increasing in the inner side.While it indicated that the state of N in powders prepared via N₂ melting and N₂ gas atomization process was mainly dissolved,accompanying with a small amount of adsorption and compound forms,which will decrease and even disappear with the etch depth increasing.Selective laser melting was used to conduct the experimental investigation on formability of different powders.It showed that the porosity defect and nitrogen content reduced,the compactness was improved and the volume fraction of?-Fe increased with the energy density of laser increasing.With the parameter at P=180 W,v=700 mm·s^-1,h=50?m,D=50?m and d=40?m,the nitrogen-containing and high-nitrogen powders presented better formability.Figure 79;Table 14;Reference 249...