What is meant by the terminology of secondary hardening?
Secondary hardening is a term that is given to the phenomena that occurs in the tempering of high-alloy tool steels, and it is indicated by an increase in the as-quenched hardness condition. It is generally associated with steels that contain one or more principle carbide forming elements (there are others, but of a lesser concern) such as:
These elements interact with carbon at the appropriate austenitizing temperature to form metallic carbides. After soaking at the austenitizing temperature followed by rapid cooling, the resulting hardness is generally lower than what could be theoretically expected. One reason for this is that there is usually a strong potential for retained austenite formation, which will exhibit a low as-quenched hardness.
While that may be true, and during the tempering procedure the austenite decomposes to fresh martensite, the real reason is that the carbon content of the steel has interacted with the particular carbide-forming element/elements to form very fine alloy carbides that will increase in hardness on tempering. This is particularly noticeable in tool steels such as:
- A series (generally considered to be air-hardening tool steels)
- H series (generally considered to be 0.4% carbon and 5% chromium)
- D series (generally considered to be 2.0% carbon and 12% chromium)
- M series (generally considered to be 1.0% carbon with chromium, vanadium, molybdenum and cobalt in varying percentages)
- T series (generally considered to be 1.0% carbon with chromium, vanadium and cobalt in varying percentages)