Mechanical Properties and Processing of High Strength Sinter-Hardening MIM Steels

• Main Authors: Chen-Chia Sun, 孫振家
• Other Authors: 黃坤祥
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/13178222877674212439

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 96 === Sinter-hardening steels that contain high hardenability elements, such as Mn, Cr, Ni, Mo and Cu, could attain good mechanical properties without quenching. In this experiment, the Ni content was adjusted in order to find the best composition. The results show that 5wt% Ni can reach a tensile strength of 2055MPa, and the elongation was 2.8%. To improve the homogenization of Ni, Fe-2Ni prealloy powder was used, but the tensile strength can only reach 1930MPa even after sintering at 1330℃. The Cr was usually added by 17-4PH stainless steel. In this study, different stainless steel powders like Nitronic60 and Nanoflex were used. The results show that the mechanical properties of using Nitronic60 are almost the same as those using 17-4PH, and the mechanical properties of using Nanoflex are worse. The cryogenic treatment can transform the retain austenite into martensite and result in higher tensile strength and hardness. The retained austenite usually become stable if the time between the end of sintering and the start of cryogenic treatment is too long. The results show that the tensile strength was 1993MPa when the waiting time was 1 hour, and it decreased slightly to 1960MPa when the waiting time was 30 days. The effect of the cryogenic treatment time is also examined, and 30 minutes is enough. The proper tempering temperature is 200℃. If the tempering temperature is higher than 240℃, temper embrittlement could happen and result in the lower elongation and impact energy. Metal injection molding process has many advantages in making small parts with complicated shapes, such as gears or ratchets, which must sustain high stresses. These parts are usually operating under large torques, so their torsional strength is another important property. However, there are few references and data available. In this study, the torsional strength was also measured by a self-designed instrument. The results show that the highest torsional strength of this alloy system is 2043MPa without tempering and the torsional angle is 196 degrees. After tempering at 240℃, the highest torsional angle can reach 580 degrees and the torsional strength is 1305MPa.