The causes of insufficient hardness of the quenched workpiece are as follows.
(1) The heating temperature is too low and the holding time is insufficient. When examining the metallographic structure, undissolved ferrite can be seen in the hypoeutectoid steel, and more undissolved carbides can be seen in the tool steel.
(2) "Surface decarburization causes insufficient surface hardness. The hardness measured after the surface layer is removed is higher than that of the surface.
(3) The cooling rate is not sufficient. Black bainite can be seen along the grain boundaries on the metallographic structure.
(4) The hardenability of the steel is insufficient, and the large section is not hardened.
(5) In the case of interrupted quenching, the residence time in water is too short, or after being taken out of water, the residence time in the air is too long and then transferred to human oil. The hardness is reduced due to insufficient cooling or self-tempering.
(6) The quenching temperature of tool steel is too high, and the amount of retained austenite is too much, which affects the hardness.