Tempering

Once the tool steel has been quenched and has cooled down to a temperature of (approximately) 150°F, it is critical to temper the tool steel immediately. The next thing heard is usually “We don’t have a tempering furnace available.” If that is the case, then it is mandatory to at least keep the tool warm and not allow it to cool down to room temperature. Remember that the purpose of quenching from the austenitizing temperature is to transform the austenite into fresh martensite. Once martensite has been formed, the steel is in its most unstable condition because the fresh martensite can, and most probably will, crack. Then the tool is completely scrapped.

If one cannot temper the austenitized and quenched tool, then it could be "flash tempered." This means a tempering procedure at any temperature that is BELOW the final required tempering temperature. This will remove the risk of the potential for cracking resulting from the formation of fresh martensite.

There is also a procedure known as double tempering, which simply means tempering a minimum of two times. The purpose of this procedure is to help decompose any retained austenite that might be present with each subsequent tempering procedure. On average, each tempering procedure will decompose any retained austenite by approximately 50%. So, the more tempers the austenitized and quenched tool steel can be given, the more the retained austenite would be reduced.

Double tempering is sometimes referred to as secondary hardening. This will be discussed in Part 4.