The high chromium content of both of these tool-steel groups is to promote deep hardening, particularly on large cross-section tool-steel dies. The high carbon is there to work with the chromium to form hard-wearing chromium carbides.
It is interesting to note that some of the A series (A4) steels with a high manganese content can be successfully hardened at similar austenitizing temperatures in the same temperature range as some of the plain-carbon steels, thus reducing the risk of distortion.
There is a direct similarity of the two groups (A and D series) in terms of dimensional stability during heat treatment. Both groups will show a slight resistance to heat softening and will be limited to application with low operating temperatures. However, they will run the risk of edge chipping at high hardness values.
It is strongly recommended that careful preheating is performed to reduce both thermal shock and distortion when ramping to austenitizing temperature. This allows the phase transformation from ferrite to austenite to occur in a more uniform manner across the tool cross section.
The soaking time at the selected austenitizing temperature is also a critical phase of the heat treatment. This is to allow the carbide dissolution to occur for the maximum as-quenched hardness value. The selection of the austenitizing temperature is critical. If the austenitizing temperature is too high, excessive retained austenite can occur at the quenching procedure. Retained austenite is an unstable condition for the steel to be in. The reason is that the retained austenite will progressively transform into untempered martensite. This will result in both an increase in the hardness as well as progressive dimensional changes.
Tempering should be accomplished immediately after the tool is at a hand-warmth temperature of around 150°F and not allowed to grow cold. This can result in catastrophic cracking of the tool. If retained austenite is present (indicated by a low as-quenched hardness value), then cryogenic treatment should be conducted followed by tempering as soon as the steel has warmed up to room temperature from the cryogenic treatment.
The cryogenic treatment will give a reasonable assurance of dimensional stability after tempering has been conducted.
The decomposition of the retained austenite can be accomplished by multiple tempers. It should be noted that with each subsequent temper, the retained austenite will decompose by approximately 50%. Therefore, multiple tempers are necessary (at least two, but preferably three tempers).