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Zhang, Y.*; 丸澤 賢人*; 工藤 航平*; 諸岡 聡; Harjo, S.; 宮本 吾郎*; 古原 忠*
ISIJ International, 64(2), p.245 - 256, 2024/01
被引用回数：0As-quenched martensite in carbon steels needs to be tempered to restore its ductility and toughness for practical applications. During tempering of martensite, microstructural evolutions induced by a series of reactions relevant to carbon diffusion is known to occur. In this study, multi-aspect characterization using advanced techniques such as in-situ neutron diffraction, transmission electron microscopy and three-dimensional atom probe tomography, was performed to investigate the changes in tetragonality, physical properties, microstructure and solute carbon content in high-carbon martensite, with an aim to clarify its low-temperature tempering behaviors. A binary alloy with a chemical composition of Fe-0.78 mass%C was austenitized and quenched to prepare the as-quenched martensite, followed by tempering in continuous heating at different heating rates. It was found that various reactions occurred sequentially during tempering, starting from the structure modulation generated by carbon clustering in the 0th stage, then followed by the precipitation of metastable 
-carbide particles on linear features in the 1st stage, towards the later decomposition of retained austenite and precipitation of cementite in the 2nd and 3rd stages, respectively. After analyzing the experimental results, the solute carbon content in martensite tempered under various conditions was found to be in good agreement with that estimated from the lattice volume expansion, whereas the evaluation based on the tetragonality might lead to some underestimation of the solute carbon content in martensite tempered at high temperatures.
Zhang, Y.*; 丸澤 賢人*; 工藤 航平*; 諸岡 聡; 宮本 吾郎*; 古原 忠*
no journal, ,
As-quenched martensite in carbon steels needs to be tempered to restore its ductility and toughness for practical applications. During tempering, a series of reactions relevant to carbon diffusion are known to occur sequentially, causing changes in microstructure in tempered martensite. In this study, multi-aspect characterization using various advanced characterization techniques were performed, with an aim to clarify the low-temperature tempering behaviors of high-carbon martensitic steels. An Fe-0.8 mass% binary alloy was mainly used in this study, and 4 ternary alloys with further 2 at% addition of Mn, Si, Cr or Al, were also investigated for comparison. All the alloys were water quenched after austenitization to obtain the as-quenched martensite as the starting microstructure. Tempering processes were performed either by continuous heating or isothermal holding under various conditions. Afterwards, the changes in physical properties of tempered martensite were analyzed via calorimetry, dilatometry, and resistometry, whereas the microstructural evolutions were characterized via transmission electron microscopy, in-situ neutron diffraction, and three-dimensional atom probe tomography. The experimental results revealed the continuous occurrence of different tempering stages, including carbon clustering, precipitation of metastable iron carbide, decomposition of retained austenite, and precipitation of cementite. In addition, the tetragonality of martensite became continuously lowered due to the reduction in solute carbon content by tempering. Among all the investigated elements, the addition of Al was found to have the largest retardation effects on the tempering kinetics, which was caused by its suppression effect on carbon diffusivity.
Zhang, Y.*; 宮本 吾郎*; 古原 忠*; 丸澤 賢人*; 工藤 航平*; 諸岡 聡
no journal, ,
In this study, following our previous work on low-temperature tempering kinetics, multi-aspect characterization (in-situ neutron diffraction, TEM and 3DAP) via combining various traditional and advanced experimental approaches was performed, with an aim to thoroughly elucidate the low-temperature tempering behaviors and microstructural evolutions in martensite. As the result, the lattice parameters of a- and c-axes of martensite became increased and decreased, respectively, during continuous heating, resulting in continuous weakening of its tetragonality especially in the temperature range of the 1st stage of 
/-carbide precipitation (340 K 
500 K). On the other hand, the lattice parameter of austenite stopped increasing at 450 K much earlier before the onset of the 2nd stage, indicated the occurrence of carbon depletion.
丸澤 賢人*; 工藤 航平*; Zhang, Y.*; 宮本 吾郎*; 古原 忠*; 諸岡 聡
no journal, ,
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.
