关键词: 模具钢, 加热温度, 保温时间, 奥氏体晶粒长大, 马氏体相变

Abstract: Austenite grain growth and martensitic transformation of P20 die steel under different heating temperatures (850-1200 ℃) and holding time were investigated via in situ observation using a high-temperature laser scanning confocal microscope. The results demonstrate that austenite grains of the P20 die steel exhibit extremely high sensitivity to heating temperature. As the heating temperature increases, the average austenite grain size significantly enlarges, rising from 18.82 μm at 850 ℃ to 126.89 μm at 1200 ℃. Notably, abrupt grain growth occurs at both 900 ℃ and 1200 ℃. This phenomenon is attributed to that the spheroidization of carbides is dominant at 900 ℃, while the dissolution of carbide particles is predominant at 1200 ℃, both of which reduce pinning effect on austenite grain boundaries. Additionally, the increase of austenite grain size promotes martensitic transformation, with the martensite start temperature (Ms) increasing from 263.2 ℃ (heated at 850 ℃) to 335.9 ℃ (heated at 1200 ℃). The martensitic transformation follows a selective initiation mode, rather than exhibiting fully explosive nucleation and growth. Furthermore, during cooling, under cooled austenite forms fresh martensite and secondary martensite. The secondary martensite nucleates and grows adjacent to the fresh martensite, typically forming approximately 60° angles with the fresh martensite.

Key words: die steel, heating temperature, holding time, austenite grain growth, martensitic transformation