Patent ID: 8152935
Filing Date: 2012-04-10
Classification: C23C

Abstract:
1. A vacuum carburization method comprising: a preheating process of heating a workpiece in a heating chamber to a first temperature; a carburizing process of supplying carburizing gas into the heating chamber to carburize the workpiece in a state where the inside of the heating chamber is depressurized into an extremely low pressure; a diffusion process of stopping the supply of the carburizing gas so as to diffuse carbon from the surface of the workpiece to the inside thereof; and a quenching process of quenching the workpiece so as to cool the workpiece from a second temperature, the method further comprising performing, between the diffusion process and the quenching process, the following processes: a normalizing process comprising a continuous step cooling sequence comprising: a first temperature lowering treatment followed without any intervening step by a first temperature maintaining treatment, and wherein the first temperature maintaining treatment is followed without any intervening step by a second temperature lowering treatment that is followed without any intervening step by a second temperature maintaining treatment so that a temperature history from the first temperature to a predetermined temperature satisfies a predetermined condition an after-normalizing maintaining process of maintaining the temperature of the whole workpiece for a predetermined time after the normalizing process so that the whole workpiece reaches the predetermined temperature, thereby producing fine crystal grains of the workpiece; and a reheating process, performed after the after-normalizing maintaining process, of raising the temperature of the workpiece to the second temperature.