A tempering furnace is an industrial oven designed to heat treat ferrous metal products and increase their toughness. In metallurgical terms, the toughness of an alloy describes its ability to elastically deform and absorb energy before the material fractures.
It is part of the complex relationship between ductility and strength in metals, and high toughness values require a subtle intersection of the two. Typically, high-strength materials are not ductile and vice versa. But heat-treating certain materials in a tempering furnace can unlock beneficial value on both axes of this mechanodynamics.
When we temper parts, we tend to focus on one parameter, hardness. As a determinant of the success or failure of a tempering operation. We must broaden our horizons and understand that tempering is to "toughen" previously hardened or normalized steel.
The post-quench microstructure of predominantly martensitic steels is very unstable and in a strain-induced state. The transformation of martensite to a mixture of cementite and ferrite during tempering usually results in an increase in grain size, while volume decreases with increasing tempering temperature.