Freshly created martensite is the steel’s hardest condition and the most brittle condition with low impact strength. In order to relieve the new stress created by fresh martensite, it is necessary that the steel be tempered to its appropriate operating-environment hardness.
The purpose of tempering is to create the optimum hardness value that will allow the heat-treated component to perform within its operating environment. The effects of tempering are numerous and are shown as follows:
- Reduction of hardness from the as-quenched value
- Improvement of impact strength
- Change in the tensile-strength properties
- Decomposition of retained austenite (if present)
- Precipitation of carbides, if the steel is a high-alloy steel
The more highly alloyed tool steels that contain carbide-forming elements within the matrix, such as chromium, molybdenum, vanadium, tungsten, titanium and silicon, will begin to initiate the precipitation of carbides at temperatures that are approximately at 700°F plus. These steels are known as secondary-hardening steels and are grouped as follows:
- Hot-work tool steels
- Dimensionally stable tool steels
- High-speed steels
- Some of the martensitic stainless steels such as 440C
The two types of temper embrittlement are known as tempered martensite embrittlement and temper embrittlement. These two conditions will occur when the steel is tempered at around 500°F followed by a slow cool at ambient temperature. If it is necessary to temper in this region for plain-carbon steel, it is recommended that one cools down to ambient temperature as quickly as possible.
If the freshly formed martensite is not addressed and dealt with by early tempering, there is a strong likelihood that the steel may crack. So, it is important to commence the tempering operation as soon as the steel is down to hand warmth (approximately 120°F). The golden rule is that one should not allow the steel to grow cold after successful austenitizing and rapid cooling.