The purpose of tempering is to develop both the toughness and ductility of the austenitized and quenched steel implement.
Tempering can also be considered a stress-relieving procedure to relieve the brittle nature resulting from the austenitizing and quenching procedure. Depending on response of the steel to the austenitizing temperature, we must now select the appropriate tempering temperature to ensure functionality of the as-quenched implement. It is that tempering temperature and the time at that particular temperature that will determine the final hardness.
Generally, there is usually no noticeable change in hardness to the steel (through-hardened or surface treated by carburizing) at a temperature of say 300°F. The final and functional tempered hardness will depend on:
- Selection of the tempering temperature
- Residence time of the steel at the tempering temperature
- Material chemistry (carbon and alloy content present in the treated steel implement)
- Type of martensite present in the microstructure
- Retained austenite (if present after quenching)
The type of martensite that is present will be determined by the carbon content of the steel. The two types of martensite are:
- Lath martensite (steels with low carbon content present)
- Plate martensite (steels with carbon content above 0.60%)
You will find that most tempering data related to hardness will display a decrease in hardness and an increase in toughness. This will apply to the plain-carbon steels as well as the alloy steels and some of the tool steels.
Steel Response during Tempering
Some high-alloy tool steels (such as A series, D series, H series and HSS series) will require a double temper, whereas the AISI alloy steels will only require a single tempering treatment. These steels will begin to exhibit an increase in hardness at tempering temperatures from approximately 800°F up to approximately 1100°F. The temperature will depend, of course, on the hardness required in relation to the material chemistry and mechanical and metallurgical properties required.
The tempering data graphs that are published by steelmaking companies are based on a 1-inch maximum cross section. The study of the Jominy End Quench hardenability tests for that particular steel analysis will give a good indication as to the as-quenched hardness value for cross sections that are greater than 1 inch.
However, you can make up your own tempering diagrams simply by keeping an as-quenched/as-tempered log to keep a record of:
- The material being tempered
- The material maximum cross section
- The temperature selected for the tempering procedure
- The material as-quenched hardness
- The time into the furnace
- The time up to the tempering temperature
- The time out of the furnace
- The tempered hardness results
From this data, you will be able to make up reasonably accurate tempered hardness data for any of the steels that you deal with in your facility.