Effects of austempering and Q&P on the microstructure and mechanical strength of S50C medium carbon steel

• Main Authors: Kai-YiChen, 陳凱怡
• Other Authors: Li-Hui Chen
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/21294505204314169337

碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 101 === In this study, the microstructure and mechanical properties of S50C were investigated after austempering and Q＆P heat treatment, respectively. The results showed that lower bainite and a small amount of retained austenite were formed after austempering heat treatment with holding temperature 350°C which is above Ms for 60 minutes while tempered martensite, lower bainite and retained austenite were observed after austempering heat treatment with holding temperature 320, 290 and 260°C which are below Ms for 60 minutes. For application, the eyeglass specimen was used as tensile test specimen. With the holding temperature increased, the phase fraction of tempered martensite, lower bainite and retained austenite were varied, which lead to the decrease of the hardness and tensile strength of eyeglass specimen. In addition, the specimen was under austempering heat treatment for different holding time at 260°C and the tensile strength of standard tensile test specimen was examined as comparison to tensile strength without notch effect. The result indicated that the tensile strength of eyeglass specimens were about 70 % of that measured from the standard specimen, and the amount of displacement of eyeglass specimens decreased less than that measured from the standard specimen with the increase of holding time. That is, the notch effect on the tensile strength of eyeglass specimen with different holding time at 260°C was not obvious. But notch effect on the ductility with short holding time at 260°C was obvious. Tempered martensite and lower bainite and retained austenite were formed after different conditions of Q＆P heat treatment. As partitioning temperature increased, the tensile strength and hardness of S50C were decreased because the carbon in martensite diffused to adjacent austenite which caused different degree of tempering. Tensile strength and hardness of S50C decreased as partitioning time increased. Moreover, the decrease of tensile strength and hardness with holding time for 5 to 15 minutes was more obvious than 15 to 60 minutes. In this work, the optimum condition among austempering and Q＆P heat treatment was that obtained by austempering heat treatment at 260°C for 15 minutes (A26015). Tensile strength of eyeglass specimen is about 1350 MPa and the standard tensile test specimen is up to about 1930 MPa, both of them are better than those after tempering at 300°C.