Super-Carburization of Low Alloy Steel in a Vacuum Furnace with a Low Pressure Acetylene Atmosphere

• Main Authors: Lee-Der Liu, 呂理得
• Other Authors: Fan-Shiong Chen
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/86008619787628702919

博士 === 大同大學 === 材料工程研究所 === 91 === The super-carburization process can provide outstanding performance in the surface hardening of high-alloyed steel. Because of large amount of massive carbides accumulated in the steel surface by repeated process of carburization and precipitation, the hardness and wear-resistance of the work will be significantly improved. However, the fatigue strength of the high-alloyed steel works is reduced due to the excessive high core hardness after hardening. This study aimed at the super carburization process on low-alloyed steel fulfilled with a two-chamber vacuum furnace, which equipped with automatic process control system and oxygen sensor. The high repeatability of the emf readings of the YSZ oxygen sensor could just function well as in the endothermic gas, which could successfully response the carbon potential of the atmosphere when acetylene was used as a carburizing medium in a vacuum furnace First, three commercial steels, JIS SCM 415, JIS SNCM 220 and AISI 1008 were carburized in vacuum furnace at temperature ranging between 900℃ and 1050℃ in an acetylene atmosphere to characterize for the hardness distribution and the microstructure of the carburized layers of each steel. Experimental results show that, under the same treatment conditions, the SNCM 220 steel exhibited the best performance in both the ECD (Effective Case Depth) and TCD (Total Case Depth) of the carburized layers. However, the carbon concentration in the carburized surface of the SCM 415 steel was higher than that for the SNCM 220 steel. The 1008 steel was the worst among the steels investigated. Furthermore, The carbon potential of the atmosphere for the SCM 415 steel should be controlled lower than that for the SNCM 220 steel if the same carbon concentration in the work surface is an essential requirement. The plain carbon steel AISI 1008 required the highest carbon potential among the three steels. Secondly, this article studies the super-carburizing treatment on selected low-alloyed steel, JIS SCM 415, to obtain high surface hardness and to retain the core toughness. The experiment was conducted in vacuum furnace with 0.6 kPa acetylene gas at 950℃, 30 minutes for each carburizing cycle. The carburized specimens obtained from one to five carburizing cycles were evaluated on their wear-resistance by a Plint wear tester. The results show that the degree of spheroidization and the amount of massive carbides increase progressively as the treating cycles. The formation of massive carbides saturated at the four-cycle of carburization and precipitation. The results of wear test manifested that the wear-resistance of super-carburized specimens were dominated by the degree of spheroidization of carbides, i.e., the higher the roundness of the carbides the better the wear-resistance it performed. Deep-hole carburization of steel has been done in a two-chamber vacuum furnace under a low-pressure acetylene atmosphere in this article. The experimental results showed that uniform carburized layer could be obtained in the deep holes both the blind hole with aspect ratio beyond 30 and through hole with 400-mm circular stepped channel by virtue of the forced-convection gas flow method. This innovative method has been applied for patent by Tatung Company.