The 440C martensitic stainless steel is considered to be among the hardest steels, owing to its high carbon content. Careful heat treatment of this material introduces multiple carbide particles, which can alter microstructure and mechanical properties. This study focused on the effect of austenitisation temperature on the microstructure and tensile properties of 440C steel. Austenitisation was performed on the austenite + carbide region, because 440C steel lacks a single-phase region. The steel was austenitised at two different temperatures; namely, 1160 °C and 950 °C, and subjected to oil quenching. The as-quenched samples showed a typical lath martensite structure with retained austenite phase. The treatments at 1160 °C and 950 °C promoted the formation of M₇C₃ and M₂₃C₆ carbides, respectively. The austenite grains in the sample treated at 1160 °C showed a higher growth rate than those in the sample treated at 950 °C. The sample treated at 1160 °C showed low-fraction and a large-size carbide phase. The Zener pinning force decreased, thereby increasing the austenite grain growth in the sample treated at 1160 °C. The hardness and 0.2% proof stress of the sample treated at 950 °C were higher than those of the sample treated at 1160 °C, owing to the higher martensite content in the former. The strength–ductility balance of the sample treated at 950 °C was higher than that of the sample treated at 1160 °C. The decreased austenitisation temperature resulted in improved mechanical properties of the steel. Therefore, the austenitisation temperature alters the microstructure and mechanical properties of 440C steel.

中文翻译：

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部分固溶处理温度对440C马氏体不锈钢组织和拉伸性能的影响

440C马氏体不锈钢由于碳含量高而被认为是最坚硬的钢之一。对该材料进行仔细的热处理会引入多个碳化物颗粒，这会改变微观结构和机械性能。这项研究的重点是奥氏体化温度对440C钢的组织和拉伸性能的影响。因为440C钢缺少单相区域，所以在奥氏体+碳化物区域进行了奥氏体化。钢在两个不同的温度下被奥氏体化。即在1160℃和950℃下进行油淬。淬火后的样品显示出典型的板条马氏体结构，并保留了奥氏体相。1160°C和950°C的处理促进了M ₇ C ₃和M _23的形成C ₆碳化物。在1160°C处理的样品中，奥氏体晶粒的生长速率比在950°C处理的样品中的奥氏体晶粒高。在1160°C下处理的样品显示出低分数和大尺寸碳化物相。齐纳钉扎力降低，从而增加了在1160°C下处理的样品中的奥氏体晶粒长大。由于前者中马氏体含量较高，因此在950°C下处理的样品的硬度和0.2％屈服强度高于在1160°C下处理的样品。在950°C下处理的样品的强度-延性平衡高于在1160°C下处理的样品。降低的奥氏体化温度导致钢的机械性能提高。因此，奥氏体化温度改变了440C钢的组织和力学性能。