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.

The high-alloy tool steels such as A series, D series, H series and HSS series will begin to show a reduction in the hardness value. Then (depending on the tool-steel analysis) hardness will increase at tempering temperatures from approximately 700˚F up to approximately 1100˚F.

The temperature selection will be dependent on the hardness required in relation to the material chemistry. The tempering-data graphs that are published are based on a 1-inch maximum cross section. 

The Jominy end quench test of hardenability will give a good indication to the as-quenched hardness value for cross sections that are greater than 1 inch. However, one can make up their own tempering diagrams simply by keeping an as-quenched/as-tempered log to keep a record of:

• The material being tempered
• The maximum cross section
• The temperature selected for the tempering procedure
• The material’s as-quenched hardness
• The time in the furnace
• The time up to the tempering temperature
• The time out of the furnace
• The tempered hardness results

From the acquired and collected data, you will be able to make up very accurate tempered hardness details for any of the steels that you deal with in your facility. The next article in the tempering series will be the concluding presentation dealing with the influence of the tempering procedure.