Fe-Ni-Cr-Mo 燒結硬化型合金鋼製程及機械性質之改善

Abstract

金屬射出成形(Metal Injection Molding, MIM)因適合製作形狀複雜之零件且成本低，故與傳統鋼鐵鑄鍛加工產品相比，有較大的競爭優勢。而其中又以燒結硬化型粉末冶金合金鋼最具有競爭力，此合金鋼不需後續熱處理製程，即可達到高硬度、高強度之機械性質。故本研究希望以燒結硬化型Fe-Ni-Cr-Mo成分為基礎，經過燒結、深冷處理與回火等製程後，獲得最佳的機械性質。 結果顯示，燒結後無深冷及回火的試片，其硬度可達約52HRC，抗拉強度約1900MPa，延展性約3%。經過液態氮深冷處理後，硬度可上升至55HRC，抗拉強度可增加至2000MPa以上，但其延展性下降至約1%。而若施以回火處理，可增加其延展性至約5%，而回火性質以180℃及200℃為回火溫度時，機械性質最佳，標準差亦最小。 本研究發現碳含量介於0.5wt%與0.6wt%之間時，可得最佳之抗拉強度，約2000MPa，密度約7.5g/cm3，過多的碳含量會使晶界析出碳化物，破裂面會轉換成劈裂面，機械性質也因此下降。研究結果亦顯示，冷卻速度較慢時，即使試片碳含量低，其晶界上也會析出碳化物，以3℃/min降溫時，碳含量為0.6wt%時晶界即析出碳化物，而以25℃/min降溫時，碳含量需高於0.8wt%才會析出碳化物。

Metal injection molding (MIM) process is superior in making parts with complicated shapes compared to the press-and-sinter process. When sinter-hardening steels, which can attain high hardness and high strength without quenching treatment after sintering, are used along with the MIM process, the process and products will become even more competitive. The objective of this study was thus to find suitable processing parameters and alloy compositions to improve the mechanical properties of the current MIM products. To optimize the alloying, the effects of adding Ni, Cr and Mo on the mechanical properties were investigated. Different sintering temperatures and different tempering conditions were also studied. The effect of cryogenic treatment was employed to further improve the mechanical properties by transferring the retaining austenite to martensite. The results show that parts after sintering can attain 52HRC hardness, 1900MPa tensile strength, and 3% elongation. After cryogenic treatment using liquid nitrogen, the hardness and tensile strength were further increased to 55HRC and 2000MPa, but the elongation was decreased to about 1%. After tempering, the elongation was increased to 5% and the standard deviations of mechanical properties were also improved. The optimum tempering conditions were 180℃ or 200℃, for 2 hours. The results also show that the optiunm carbon content between 0.5 and 0.6wt%. The tensile strength thus attained was about 2000MPa. When the carbon content was excessive, and the cooling rate was slow, such as at a rate of 3℃/min, carbides would precipitate at the grain boundaries and brittle-type of fractures were observed. Thus, the mechanical properties were poor. But when a higher cooling rate of 25℃/min was used, carbide precipitation did not occur unless the carbon content was greater than 0.8wt%. These results show that using the composition of Fe-8Ni-0.8Cr-0.8Mo-0.5C, good mechanical properties of 2000 MPa, 55HRc and 5 % elongation, can be attained using a cooling rate of 25℃/min and tempering at 180℃ for 2 Hrs. No need of quenching is required.