Quenched and tempered steels are mainly employed for components that require specific characteristics in order to handle not only high mechanical stresses, but also deflection, strain, compression-shear and twist.
The ability to resist to mechanical stresses must be combined with high toughness that allows to avoid fractures for brittleness.
For those parts that are destined to power transmission, it is also required a good wear resistance and a good resistance to dynamic stresses.
Chemical composition and heat treatment are strictly connected with steel's micro-structure and its employment.
The yield strength and the elastic limit depend on the material's structure after hardening and on the tempering temperature; wear resistance is obtained by carrying out a superficial hardening or nitriding treatment.
ELIGIBILITY FOR HEAT TREATMENT
Quenched and tempered steels must have an adequate hardenability as a function of the mechanical characteristics that are to be obtained after a heat treatment. It is important to know the Jominy curve of the steel that you would like to employ.
All out steels have a controlled and certified hardenability.
They are steels destined to the production of parts that require demanding processes. machinability becomes essential for what concerns the economic aspect.
Machinability depends on hardness, inclusion, chemical composition, micro-structure and toughness. It is improved by the presence of sulphur, lead, selenium and sometimes they are treated with calcium.
CHOICE CRITERIA FOR QUENCHED AND TEMPERED STEELS
Turnings removal is generally the most expensive operation, for this reason it is necessary to take into account steels with an improved machinability, in order to save time during processing.
The mechanical characteristics, which will have to be obtained, will be evaluated in terms of steel hardenability and in this way hardness and tensile strength of finished parts will be evaluated.
It is fundamental that heat treatment is carried out in the best way, so that a correct micro-structure and relative mechanical characteristics can be obtained.
QUENCHED AND TEMPERED STEELS (EN 10083-3)
|25CrMo4 hardened and tempered
|30CrNiMo8 euenched and tempered stress relieved
|42CrMo4 hardened and tempered
drawn / peeled h11
|42CrMo4 hardened and tempered high machinability
|39NiCrMo3 hardened and tempered
drawn / ground
|39NiCrMo3 hardened and tempered high machinability
|39NiCrMo3+Pb hardened and tempered
drawn / peeled h11
draw / ground
|C45 hardened and tempered
|hardened and tempered