Gas quenching method

Abstract

A gas quenching method of the present invention includes a first stage (t1 to t2) at which a workpiece is subjected to a rapid cooling by forcibly circulating a cooling gas, a second stage (t2 to t3) at which the circulation of the cooling gas is stopped and pressure is reduced inside the furnace to conduct heat insulation, and a third stage (as from t3) at which the workpiece is cooled again by the cooling gas. At the second stage, the workpiece is maintained at an intermediate temperature that is higher than martensite transformation start temperature, and, during this, temperature throughout the workpiece is made uniform. Therefore, it is possible to achieve a uniform quenching and suppress distortion caused by difference of the cooling speed.

1 . A gas quenching method in which a workpiece made of steel is heated and then cooled for quenching by allowing a cooling gas to flow around the workpiece in a furnace, the gas quenching method comprising:
 stopping supply of the cooling gas in the middle of the quenching before the workpiece reaches a martensite transformation start temperature;   reducing pressure inside the furnace and making temperature throughout the workpiece uniform by radiation cooling, while temperature of the workpiece is maintained at an intermediate temperature that is higher than the martensite transformation start temperature; and   resuming supply of the cooling gas after the temperature throughout the workpiece has been made uniform, thereby conducting the quenching to pass the martensite transformation start temperature.   
     
     
         2 . The gas quenching method as claimed in  claim 1 , wherein the temperature throughout the workpiece is made uniform, while the temperature of the workpiece is maintained at a temperature that is higher than the martensite transformation start temperature and lower than a bainite transformation curve. 
     
     
         3 . The gas quenching method as claimed in  claim 1 , wherein the workpiece has a surface that is previously subjected to a carburizing treatment. 
     
     
         4 . A gas quenching method, comprising:
 a first step of subjecting a workpiece made of steel to a rapid cooling in a furnace by a cooling gas from a heated condition;   a second step of stopping supply of the cooling gas to the workpiece and reducing pressure inside the furnace, such that, in the middle of a temperature lowering of the workpiece, the workpiece is maintained at an intermediate temperature that is higher than a martensite transformation start temperature; and   a third step of conducting a rapid cooling again by the cooling gas after temperature of the workpiece has been made uniform.