| 420 stainless steel has high hardness,strength and wear resistance,and is widely used in the fields of knives,turbine blades and surgical medical instruments.However,poor processing ability is a difficult point limiting its application.Metal injection molding（MIM）can quickly manufacture 420 stainless steel parts with complex geometries and reduce the machining process.But 420 stainless steel sintered by conventional solid phase sintering can not achieve high density.To solve this problem,MIM420,420+0.3C and 420Nb samples were prepared by adding graphite or pre-alloyed 420Nb.The effects of C and Nb on the densification and pore-grain boundary separation behavior of 420 stainless steel were studied by comparing the relationship between C and Nb on the density,grain size,pore state and grain boundary growth behavior of 420 stainless steel,and the―inflection point of sintering‖has been studied.Then,the porous 420 stainless steel with different porosity were prepared by the method of injection molding combined with pore-forming agent,the density and mechanical properties of the porous 420 stainless steel were tested,and the effect of the pore on the microstructure and mechanical properties of the porous 420 stainless steel were studied.The results show that adding C at 1330°C and 1350°C can reduce the oxide on the surface of the powder,promote the formation and growth of the sintered neck,and accelerate the densification process.In the middle and later stages of sintering,C will accelerate the diffusion rate of Fe atoms,leading to the separation of pores and grain boundaries,making the grain boundary pores into intragranular pores,thereby inhibiting grain boundary diffusion and reducing the densification rate.When sintering at 1370°C,liquid phase are generated,which will quickly densify the material.The density of 420+0.3C sintering at 1370°C for 4h will reach 98.32%.At the same time,C will speed up the movement of grain boundaries and promote grain growth,the grain size of sample sintered at 1370°C for 4h is 367μm.Nb in 420Nb combined C atoms to form Nb C,which hinders the densification of sintering at low temperature and early sintering stage.Nb C particles in the matrix hinders the movement of grain boundaries and retards the pore-grain boundary separation behavior,which promote sintering at later stage of sintering.The relative density of sample sintered at 1370°C for 4h reaches 97.24%.Then,Nb C can suppress the growth of grains in the material,and the grain size of sample sintered at 1370°C for4h is only 60μm.The grain boundary area（SV）and pore area(SVP)have a good linear relationship,indicating that the interaction between the pore and the grain boundary is related to the chemical composition and temperature rather than porosity.Through theoretical calculations,when the ratio of grain size to pore size is 28,the contact probability between pores and grain boundaries is only 10%.At this time,the sintering rate is very slow and the grain growth is signifcant,indicating that sintering has reached a inflection point,which is in good agreement with the experimental results.The relative density of porous 420 stainless steel prepared by MIM is 62%～92%,and the density decreases with the increase of PMMA content.After debinding,PMMA escapes from the green part to form open pores.The open porosity is 5%to36%,which increases with the increase of PMMA content.The tensile strength of 420porous stainless steel is 77MPa～491MPa,and the yield strength is 34MPa～407MPa,which decreases with the increase of PMMA content.This is because the existence of pores reduces the area of the material which is subjected to external loads,and stress concentration easily takes place at the tip of the pores,resulting in an inferior properties.The research shows that the fracture mode of porous 420 stainless steel is brittle fracture,and the porous 420 stainless steel containing 50%PMMA sintered at1200°C for 1 hour is 51.41%and 51.71%lower than sample containing 10%PMMA in tensile and yield strength respectively. |
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