.440 C Stainless Steel Metal Injection Molding Process

Metal Injection Molding(MIM or PIM)is a new near-net forming technology for fabricating high-quality powder metallurgy compacts.It is much superior over ancient precise foundry or press/sintering technology and has wide application prospects. However,there are still two difficulties:deformation and size precision,sintering densification and property.In this work the processing for producing 440C stainless steel parts by metal injection molding technique were investigated.The solvent and thermal debinding were studied,and the effect of sintering temperature, sintering time,and sintering atmosphere on density,mechanic performance and corrosion resistant performance were analyzed systematically.The factors sintering deformation of injection molding 440C stainless steel were discussed,and measures controlling the sintering deformation have been proposed.By studying the process of solvent debinding,it is found that debinding rate increased with debinding temperature and time,but decreased with the increasing of powder loading.The ideal solvent debinding process is following:After immersed in a bath with methylene dichloride for 4 hours at 37℃.The optimal thermal debinding process was made by TGA of feedstock:from room temperature to 200℃,the temperature increased with heating rate of 2℃/min held at that temperature for 1 hour;and from 200℃to 385℃by low heating rate of 2℃/min held at that temperature for 1 hour;next,from 385℃to 425℃,with rate of 2℃/min and remained at 425℃for 1 hour.The following is from 425℃to 600℃with the rate of 3℃/min and remained at 600℃for 30min to make sure that the binder was removed completely.Then the temperature increased with 5℃/min to 950℃to form sintering neck to keep the shape.The content of carbon were respectively 1.05%and 1.08%after debinding under H₂ and Ar atmosphere.Effect of sintering temperature and holding time on the mechanical properties and microstructure of 440C stainless steel is investigated.The appropriate sintering temperature range for injection molding of 440C stainless steel is between 1230℃and 1240℃under the vacuum atmosphere.Specimens sintered at 1230℃for 90 min have the density of 7.34 g/cm³,sintered at 1240℃for 30 min have the density of 7.56 g/cm³.Compared with vacuum sintering at 1260℃,the sintering at the same temperature in argon is better for the control of dimensional accuracy and performance,but has a lower density.The ideal heat treatment process for 440C stainless steel is following:specimens were quenched at 1050℃,then tempered at 150℃for 2 hours.The microstructure of injection molding 440C stainless steel was consisted of martensite,residual austenite,carbide. After heat treatment,specimens sintered at 1230℃for 90 min have the tensile strength of 795.4 MPa,the hardness of 53.8 HRC,and sintered at 1240℃for30 min have the tensile strength of 876.3 MPa, the hardness of 57.7 HRC.In NaCl solution,pitting mainly exist in injection molding 440 C stainless steel specimens.The corrosion resistance increase with increasing of sintering temperature,it is mainly own to increasing of density.And specimens sintered under Ar atmosphere have better corrosion resistance than under vacuum atmosphere.Carbide precipitation is one of the reasons for decreasing of 440 C stainless steel corrosion resistance after tempering.Deformation mechanism of injection molding of 440C stainless steel compacts is investigated and the results indicate the deformation of as -sintered specimens is caused by density gradient,fluctuations in the carbon content and other factors.Some methods,such as choosing appropriate sintering temperature,avoiding placing debinding specimens in the air for a longer period of time,are presented to prevent the as-sintered compacts from deformation.Sintering shrinkage of stainless steel was investigated.It is found that the shrinkage increased with sintering temperature and hold time. Shrinkage ratio in the longitude is larger than in the width and the thickness.And increase the powder loading can decrease the shrinkage and make the specimens shrink more uniformly.Therefore,increasing of powder loading is benefit for the size precision.