to Heat treatment technologies
Dr. Dmitri Kopeliovich
Tempering is a heat treatment operation involving reheating hardened steel to a certain temperature below the lower critical point (A1) followed by soaking and then cooling.
The steel structure after hardening consists mainly of martensite which is hard and brittle. Tempering is carried out in order to change the martensite structure and obtain a desired combination of strength and ductility.
The object of tempering is also to reduce the internal stresses caused by quenching.
Depending on the tempering temperature, the following stages of tempering take place:
Tempering at temperatures 300°F - 480°F (150°C - 250°C)
. The soaking time is commonly about 1-3 hrs. At these temperatures low carbon
(0.25%) tempered martensite and fine dispersed carbides
form. The internal stresses are partially reduced and some softening (by 2-3 HB) occurs at this stage.
Tempering at temperatures 570°F - 750°F (300°C - 400°C)
. Soaking time varies from 2 to 8 hours depending on the parts size. At these temperatures martensite transforms to trostite
(very fine mixture of ferrite
). Trostite is softer than martensite and more ductile
Tempering at temperatures higher than 750°F (400°C) but lower than lower critical point(A1)
. Soaking time varies from 2 to 8 hours depending on the parts size. At these temperatures martensite transforms to sorbite
(fine mixture of ferrite and cementite). Sorbite and trostite are principally similar structures differing only in the particles size. Sorbite is more ductile and less strong than trostite. This kind of tempering is used for the parts exposed to impacts.
If the tempering temperature is above 1020°F (550°C) strength decreases sharply without any notable increase of ductility.
Batch type furnaces ( either air atmosphere or liquid bath) are used for the tempering heat treatment.
Oil baths are widely used for tools tempering at relatively low tempering temperatures 300°F - 600°F (150°C - 315°C).
In order to prevent cracking the steel part should be preheated before immersing to hot oil.
Molten salt baths are used for tempering at temperatures 400°F - 1020°F (200°C - 550°C). Mixtures of sodium nitrate and potassium nitrate are suitable as the bath medium.