高炭素マルテンサイトの低温焼戻し挙動に及ぼす合金元素の影響

Effects of alloying elements on low-temperature tempering behaviors of high-carbon martensite

丸澤 賢人*; 工藤 航平*; Zhang, Y.*; 宮本 吾郎*; 古原 忠*; 諸岡 聡  

Marusawa, Kenji*; Kudo, Kohei*; Zhang, Y.*; Miyamoto, Goro*; Furuhara, Tadashi*; Morooka, Satoshi

The objective of this study is to clarify microstructure evolution during low-temperature tempering of high carbon martensite with addition of various third alloying elements, by particularly focusing the early-stage reactions. Fe-0.8mass%C (Base alloy) and Fe-0.8mass%C-2at%M (M alloy: M = Al, Si, Mn, Cr) were used. The as-quenched specimens were either continuously heated to 873 K at various rates of 5-20 K/min, or isothermally held at various temperatures of 333-453 K, whose kinetics were evaluated by calorimetric, dilatometric and resistometric analyses, respectively. Furthermore, in-situ neutron diffraction experiment and microstructure observation using TEM and 3DAP were conducted. As the result, the tetragonality is found to be gradually decreased during heating, especially in the temperature range of the 1st stage of tempering. Its decreasing rate is strongly retarded by Al addition, whereas the effects of other alloying elements are relatively small. Such effects were also recognized by the calorimetric and dilatometric analyses. In these specimens, heterogeneity in carbon distribution is induced by precipitation of metastable iron carbide. When the parameter (deviation from the binomial distribution) is used to evaluate the degree of heterogeneity, a much lower value in the Al alloy than the other alloys indicates retarded carbide precipitation by Al addition. This is also consistent with the smaller reduction in solute carbon content in the Al alloy. By assuming the linear relationship between tetragonality and solute carbon content, this observation corresponds to the retardation effect on decreasing rate of tetragonality.