Imagine you’re a warrior during the middle ages and it’s time to get a new sword. So, you go to a blacksmith to buy a sharp, shiny long sword. A few weeks later you’re in a battle, fighting at the front of the shield wall. You take a huge swing at the enemy, who meets your blow with his sword, and your sword shatters into several pieces. Unfortunately for you, your blacksmith outsourced a batch of swords to a blacksmith on the other side of town who didn’t have time to temper the swords. As a result, the swords were strong, but brittle. Their lack of toughness meant that they could not absorb much of an impact before fracturing.
Tempering is used to improve toughness in steel that has been through hardened by heating it to form austenite and then quenching it to form martensite. During the tempering process the steel is heated to a temperature between 125 °C (255°F) and 700 °C (1,292 °F). At these temperatures the martensite decomposes to form iron carbide particles. The higher the temperature, the faster the decomposition for any given period of time. The micrograph shows a steel after substantial tempering. The black particles are iron carbide.
Untempered martensite is a strong, hard, brittle material. The stronger and harder it is, the more brittle it is. The strength and hardness is a due to elastic strain within the martensite, which is a result of too many carbon atoms being in the spaces between the iron atoms in the martensite. As the amount of carbon in a steel increases (up to about 0.8 weight percent carbon) the martensite strength and hardness increases.
During the tempering process, the carbon atoms move out of the spaces between the iron atoms in the martensite to form the iron carbide particles. The strain within the martensite is relieved as the carbon atoms move out from between the iron atoms in the martensite. This results in an improvement in the steel toughness, at the expense of reduced strength.
The amount of tempering required depends on the particular application in which the steel will be used. In some cases, toughness is not important, so tempering at a low temperature for a short period of time is acceptable. In cases where very strong and tough steel is required a high carbon steel tempered at a high temperature might be used.
Learn more about about steel heat treating is in our Steel Metallurgy and Steel Through Hardening courses. The book Steels: Processing, Structure, and Performance by George Krauss provides a comprehensive discussion of steel heat treating.