The high carbon content of the steel gives it a strong tendency for decarburization at elevated temperatures unless treatment is done under vacuum or alternative surface protection. Decarburization can be reduced by using any of these methods.
- Wrap in stainless steel foil with a smear of oil on the inside face of the foil so as to burn up oxygen that is entrapped inside of the foil. This will reduce the risk of surface oxidation and decarburization.
- Atmosphere heat treatment. Care must be given to create and maintain equilibrium conditions with the carbon potential of the furnace atmosphere and the carbon content of the steel.
- Salt-bath heat treatment, which (if neutral) will protect the work surfacefrom both oxidation and decarburization.
In general, D2 steel does have good oxidation resistance due to the high chromium content. The steel will polish very well after austenitizing, which is often required in die-casting manufacturing.
It is mandatory that the steel be very carefully preheated. This steel has a low heat conductivity and a poor ability to absorb heat. If it is heated too fast, then there is a very strong likelihood that the steel will possibly crack during the heat-up phase. Therefore, ramp and soak is very necessary for preheating. (This also applies to heat-up for forging.) The ramp-up time could be adjusted as required. More complex D2 die forms would be preheated very slowly.
A suggested procedure is:
- Ramp up to say 500°F (260°C), and hold for equalization.
- Ramp to 1200°F (650°C) and equalize throughout the cross section.
- Ramp to the austenitize temperature at 1790-1850°F (975-1010°C). Hold at the temperature and soak for one minute per 1 mm of maximum cross-sectional area, and follow with the quench.
- Do not over-soak at the austenitize temperature because grain growth will occur.
- Do not go to too low an austenitizing temperature to avoid size growth. Additionally, insufficient carbides will dissolve, resulting in (perhaps) a lower hardness value.
- Conversely, do not select too high an austenitizing temperature. Otherwise there is a risk of too many carbides dissolving, which can lead to retained austenite.
The above procedure is the most common practice of austenitizing D2 steel.
These steels (D series) are very susceptible to retained-austenite conditions. This is due to the high-carbon and high-chromium chemistry of the steel.