Tempering Steel - explanation

Tempering Steel: Why Temper Through Hardened Steel?

<a href='https://www.imetllc.com/tempering-steel/'>Tempering Steel: Why Temper Through Hardened Steel?</a>

Tempering steel for battle

Tempering steel after quenching is part of the through hardening heat treatment. The purpose of tempering is to improve steel toughness.

Imagine you’re a warrior during the middle ages and it’s time to get a new sword. So, you go to a blacksmith and 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.

Attend the Steel Hydrogen Embrittlement webinar for FREE!
Use discount code STEELHE when registering

Steel Hydrogen Embrittlement

Webinar on July 11, 2024 1:00 - 1:45 pm CDT

Learn how hydrogen embrittlement occurs, steels affected, and component design and fabrication approaches to prevent hydrogen embrittlement.

Register Today!

Purpose of tempering steel

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, initially forming 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.

tempering steel
Tempered marteniste

Untempered martensite

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 strains within the martensite, 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.

What happens during tempering

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.

Amount of tempering required

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 tempering steel and 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.

Industrial Metallurgists, LLC

Providing metals engineering expertise for failure analysis and forensic investigations of metal components and products.
© 2024 imetllc.com — All rights reserved.