the size of grains growbigger, and more austenite occur, it causes lower hardness. When the holding time is60min, the microstructure is uniform, size of grain is small, saturation of martensite is higher, and has the maximum hardness of 39.1 HRC. The results of different aging temperatureexperiment show that:the microstructure of 15-5PH not only containsmartensite, but also austenite, ferrite and precipitated phases. When the aging temperature is450 ℃, there are many quenched martensite in the microstructure and little carbide and intermetallic compound (IMC) has been occurring from martensitic matrix. It causes higher strength and lower plasticity.With the increase of aging temperature, quenched martensite has been posed gradually, the tempered
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