Gas quenching tool steels

Alloy steel parts are increasingly being heat treated in vacuum furnaces using gas overpressure quenching. The maximum gas pressure typically is 6 bar absolute, and nitrogen usually is the quenching gas. This is adequate for through-hardening tools and other parts made of high-speed, hot-work, and, to a limited extent, cold-work steels. However, the quenching speed (cooling rate) is often too low to permit the successful heat treating of: (1) tools and other parts made of medium- and low-alloy steels; (2) large forging and plastics molding dies made of medium-alloy steels; (3) large, densely loaded batches of identical parts; (4) parts made of austenitic steels and alloys that require a rapid quench after they are solution annealed; and (5) cold-work steel tools and other parts having diameters larger than 100mm (4 in.). Oil quenching is required for these applications. However, its use increases the potential for cracking and can cause distortion. To extend gas quenching to these materials, Leybold Durferrit GmbH, Hanau, Germany, has developed a new generation of vacuum furnaces that combines convective heating and high-pressure gas quenching at pressures up to 20 bar (2 MPa, 290 psi).