The quenching temperature is mainly determined according to the critical point of the wear resistant lining of high chromium cast iron.Subeutectoid steel is usually heated to 30~50℃ above Ac3, while eutectoid steel and hypereutectoid steel are heated to 30~50℃ above Ac1, and in some cases to 50 ~ 70℃ above ^, as shown in the figure.If the quenching heating temperature of subeutectoid steel is lower than that of Ac3, then the steel has not been fully austenitized at this time, and some untransformed ferrite exists, and the ferrite remains in the quenching structure after quenching.The hardness of ferrite is low, so that the hardness after quenching can not meet the requirements, but also affect other mechanical properties.If the subeutectoid steel is heated to the Ac3 temperature for quenching, the austenite grain will be significantly larger, thus damaging the properties after quenching.The eutectoid steel was heated above Ac1 at 30 ~ 50℃.After quenching, only a few residual austenite particles were found in vitro, and the structure was small carbide particles evenly distributed on the lamellar martensite matrix.Such tissues have high hardness, good wear resistance and relatively low brittleness.The quenching temperature of hypereutectoid steel cannot be lower than that of Ac1, because the steel has not been austenitized at this time.When heated to a temperature slightly higher than the temperature, the pearlite completely transforms into austenite and a small amount of cementite dissolves into austenite.The austenite grains were fine and the carbon mass fraction was higher than that of eutectoid.If the temperature continues to rise, the secondary cementite will continue to dissolve into austenite, resulting in the continuous growth of austenite grains and the increasing carbon concentration, which will lead to the increase of quenching deformation tendency, quenching microstructure microcracks and brittleness.At the same time, due to the high austenite carbon content, the amount of residual austenite increased after quenching, reducing the hardness and wear resistance of the wear-resistant liner.
Therefore, the quenching heating temperature of hypereutectoid steel is not suitable to be too much higher than Ac1, and heating to the temperature of fully austenitized Acm or above is even more inappropriate
