Study On Preparation,microstructure And Properties Of Nickel-free High Nitrogen Austenitic Stainless Steel Foam

As people pay more attention to health,nickel-free high-nitrogen austenitic stainless steel without nickel allergic reaction is gradually used in medical implants,wearable devices and medical devices.Among them,Fe Cr N austenitic stainless steel has better mechanical properties,corrosion resistance and biocompatibility than Fe Cr Mn N austenitic stainless steel because it does not contain manganese,but it requires a longer solution nitriding time,which results in the limit of its application.The foam structure can not only solve the stress shielding effect caused by the mismatch of mechanical properties between medical implants and human bone,but also hope to greatly shorten the solution nitriding time of Fe Cr stainless steel.In this paper,the foam structure is applied to the solid solution nitridation of Fe Cr stainless steel to obtain a practical nickel-free high nitrogen austenitic stainless steel foam block.First of all,using 430 stainless steel powder as the raw material,stainless steel foams with different porosity were prepared by space holder-leaching technology and high vacuum sintering;after that,430 stainless steel foams with different porosity were subjected to nitriding under different process conditions;finally,stainless steel foams with different Cr contents were obtained by ball milling and subjected to solution nitriding and solution treatment.XRD,SEM,TEM,XPS,nitrogen analysis,porosity test,compression test and electrochemical test were applied to analyze the structure,microstructure and properties of stainless steel foams,revealing the influence of pores,nitriding process,Cr content and heat treatment on the structure and properties of stainless steel foams.The main research results of this article are as follows:?1?430 stainless steel foam with uniform pores,predictable porosity,and excellent mechanical properties is prepared by using space holders with different volume fractions.Carbamides is employed as space holders to creat 430 stainless steel foams with porosities from18.33%to 72.24%.And it was found that the carbamide volume fraction?_c is in the function of the green specimen density?_r and sintering volume shrinkage?.Using?_r and?ascan accurately predict the porosity corresponding to carbamide volume fraction of 0?1 of stainless steel foam.The grain size and compressive yield strength of stainless steel foam decrease with the increase of porosity,and the compressive yield strength is higher than the predicted value of Gibson-Ashby model when the porosity is low,but it conforms to the model=.When the porosity is high,the experimental value of yield strength is lower than that of the Gibson-Ashby model.?2?The microstructure obtained by solution nitriding of 430 stainless steel foam is martensite,and some residual austenite can be remained by increasing the nitriding pressure.The structure gained by solution nitriding is lath-shaped high-nitrogen martensite,the thickness of the lath is about 0.5?m,and its squareness is higher than that of high-carbon martensite.The martensite structure greatly increases the strength of the stainless steel foams,but the plasticity decreases;the structure obtained by nitriding furnace cooling is Cr₂N+ferrite pearlite,in which Cr₂N has a hexagonal structure,and the layer thickness is about 30 nm.The foam structure transforms the nitriding process into multiple plates nitriding,so that the nitrogen content and microstructures of the stainless steel foams with porosities of 18.33?72.24%after nitriding are uniform along the axial direction,but increasing the porosity can improve the nitrogen content of stainless steel foams after nitriding.The porosity of 56.23%can make the stainless steel foams reach the maximum nitrogen content after 5 h of nitriding,which is the best porosity that can take into account the nitriding efficiency,mechanical properties and application.The nitriding pressure of 3.046×10⁵ Pa leads to a more stable residual austenite microstructure in the stainless steel foam,and the plasticity is also improved.?3?When the Cr content of stainless steel foam is increased to 25 wt.%,the single austenite microstructure can be found after solution nitriding,and the solution treatment can greatly improve its mechanical properties and corrosion resistance.The adding in Cr content increases the maximum solubility of nitrogen in the stainless steel foam,and improves the stability of supercooled austenite,and increases the austenite content in the stainless steel foam,and obtains a single austenite microstructure at a Cr content of 25 wt.%.The increase in nitrogen content enhances the strength of austenite,so that the mechanical properties do not show a significant decrease while the austenite content increases.Dissolved nitrogen also improves the pitting corrosion resistance of stainless steel foam.The solution treatment refines the austenite grains to 5?10?m,which promotes the yield strength and elastic modulus by 45.63%and 59.32%,and the self-corrosion potential is upgraded by 0.3327 V,and the pitting corrosion resistance is also greatly improved.