| 摘要 |
PURPOSE: A method of heat treatment is provided to prolong the life time of heat treated steel in use for gauge, medical instrument, bearing and tool up to 2 to 10 times by including very low temperature sub-zero treatment. CONSTITUTION: A method of heat treatment includes pre-heating, quenching and tempering steps. A short quenching step is carried out at the end of quenching at a temperature of -150 to -270deg.C. Very low temperature sub-zero treatment is carried out for a certain time period at the temperature of short quenching. The treatment time of very low temperature sub-zero treatment is different according to the thickness of steel. The temperature ranges from -150 to -270deg.C since a complete martensite structure does not appear over -150deg.C and heat treated steel is broken under -270deg.C. Examples of coolant for very low temperature sub-zero treatment include liquid methane, liquid nitrogen, liquid oxygen, liquid hydrogen, liquid helium and the like.
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