The heat treatment of tool steels (and any other steel heat-treatment procedure) relies on a given set of metallurgical principles. For example, the formation of martensite relies on:
  • The carbon content of the steel
  • The alloying elements contained in the steel
  • The appropriate austenitizing temperature
  • The appropriate rate of cooling
It does not matter if one heats the tool or steel by an oxy-acetylene gas torch or a vacuum furnace or a fluid-bed furnace, the steel will respond if all of the above parameters are met. The principles of metallurgy suggest that steel does not care from where it receives the temperature, it will respond to the temperature and cooling rate if there is sufficient carbon present to form martensite.

The method of vacuum heat treatment has been with us for many years, but only in the past 25 years or so has the heat-treatment industry (captive or commercial) seen a significant growth in the use of vacuum-processing technology, particularly with the ability to capture and store data as a result of the growth of PC/PLC technology.

When one considers process technology and its growth as a result of vacuum technology, it supports a paper that was presented (by the writer of this article) at the first ASM European conference, held in The Netherlands in 1991. The writer forecast that the future of the heat-treatment industry was to “make a silk purse from a sow’s ear” (using low-alloy materials to enhance the surface characteristics as well as the core properties). Consider the following processes that are now conducted in low-pressure (vacuum) conditions:
  • Low-pressure nitriding
  • Plasma nitriding
  • Ferritic nitrocarburizing
  • Austenitic nitrocarburizing
  • Low-pressure carbonitriding
  • Low-pressure carburizing
  • High-temperature/low-pressure carburizing
  • Tool-steel heat treatment with high-pressure gas quenching
  • Quench technology utilizing blended gases of nitrogen/hydrogen or nitrogen/helium
  • Thin-film hard coating
  • Low-pressure brazing of steels and exotic materials
  • Low-pressure brazing of aluminum alloys
  • Heat treatment of stainless steels (martensitic)
  • Heat treatment of precipitation-hardening stainless steels
  • Heat treatment of titanium, cobalt and other alloys
  • And many others not yet even considered
For tool-steel heat treatment, however, vacuum offers a distinct advantage (provided that the vacuum furnace has a very low leak-up rate), which is the absence of oxygen.

There can be no surface oxide attack at the surface of the steel (provided that the oxygen source is not already present or has been carried into the process chamber). This means that there can be no intergranular surface oxidation (IGO). It also means that, provided the appropriate stress relieving steps have been considered and there has not been any abusive machining practice, the distortion will be kept to a minimum with only a minimum grind-stock allowance being made.

The transformation from austenite to martensite will now be dependent on selecting the appropriate rate of cooling. This will be the subject of part 2 of this series.