Microsoft Word 2012, Källén, M.,-Energy Efficiency Opportunities within the Heat Treatment Industry

2.2.1  Hardening and Tempering 

Hardening is a generic name for all processes that make the steel harder and/or increase the 

durability of the steel by creating a martensite or bainite structure. The steel is first heated to 

its hardening temperature and austenite is formed. The hardening temperature and the holding 

time are determined by the hardenability of the steel and the dimensions of the steel object. 

The steel is then cooled down, either directly or in stages. It is the cooling course that defines 

the hardening process. [1] 

The direct cooling will cause formation of martensite and the process is called direct 

hardening. When the cooling is performed in stages, so called martempering, the steel is first 

cooled to a temperature slightly above the temperature at which martensite will be formed. 

The steel is kept at this temperature until the temperature is constant throughout the steel 

object. The steel is then cooled to room temperature and martensite is formed.  The steel will 

get almost the same structure from direct hardening and martempering, but martempering will 

give less residual stress. If the steel needs to become more ductile, tempering is performed. 

[1] 

The fast cooling in hardening processes causes martensite to form instead of ferrite because 

the cooling is faster than the diffusion of carbon. The carbon atoms will then slowly diffuse 

from the martensite at room temperature after the cooling is completed. This will cause a 

variation in time of the properties of the steel. To avoid this, a process called tempering is 

performed. The steel is heated to 160-650°C to cause a faster precipitation of the carbon 

atoms. It should be carried out shortly after the steel has cooled down after the hardening. 

Hardening followed by tempering will give steel with a desired hardness and toughness. [1]