The heat treatment of trapezoidal thread measuring tool steel is introduced. The main feature of heat treatment of measuring tool steel is to take various measures to keep the measuring tool size stable in long-term use while maintaining high hardness and high wear resistance. The phenomenon that the size of the measuring tool changes with time in use is called the aging effect of the measuring tool. This is because the hypereutectoid steel used for measuring tools contains a large amount of retained austenite after quenching, and the volume expansion is caused by the transformation of retained austenite into martensite. Martensite continues to decompose in use, cubic reduction leads to volume shrinkage. 3. The existence and redistribution of residual internal stress make elastic deformation partly transformed into plastic deformation and size change. Therefore, in the heat treatment of measuring tools, the following heat treatment measures should be adopted for the above reasons:
(1) conditioning treatment. The aim is to obtain tempered sorbite structure to reduce quenching deformation and improve the cleanliness of mechanical processing.
(2) Quenching and low temperature tempering. Measuring tool steel is hypereutectoid steel. Normally, the quenching temperature is lowered as far as possible and preheated to reduce the temperature difference and quenching stress in the process of heating and cooling, on the premise that the hardness is guaranteed now without quenching and tempering at low temperature. The quenching mode of measuring tool is oil-cooled (20-30 C). It is not suitable to use step quenching and isothermal quenching. Only in special cases can it be considered. Generally, low temperature tempering is used, tempering temperature is 150-160 C, tempering time should not be less than 4-5 H.
(3) Cold treatment. High precision gauges are cooled after quenching to reduce the amount of retained austenite and increase dimensional stability. The temperature of cold treatment is generally 170-80 (?) C and it is carried out immediately after quenching and cooling to room temperature in order to avoid the ageing and stabilization of retained austenite.
(4) Aging treatment. In order to further improve dimensional stability, after quenching and tempering, the residual internal stress can be eliminated by aging treatment at 120-150 C for 24-36 h, which greatly increases dimensional stability without reducing its hardness. In a word, besides the normal heat treatment of hypereutectoid steel (no quenching and 10 low temperature tempering), there are three additional heat treatment processes, namely, quenching before quenching, cooling between normal quenching and aging after normal heat treatment.