Tempered steels are unalloyed and alloyed machine manufacturing steels whose chemical compounds are suitable for hardening especially due to carbon content and display high firmness against a certain tensile strength after the tempering process.
In tempered steels, high strength and ductility is desired as well as hardenability properties. To be able to obtain adequate level of hardness, tempered steels contain relatively more carbon. Since hardness depth is the most important criterion for thick cross-section parts, these parts are manufactured from alloyed tempered steels.
Part dimensions and strength values are important in selecting tempered steels. Unalloyed tempered steels can only be efficient in small cross section parts. In the case of thick cross section parts, the homogeneity of hardness distribution depends on whether or not they are alloyed. The changes in hardness distribution according to material alloys can be observed by Jominy testing results. Jominy testing simply refers to the hardening values of a material shaped into a rod heated to hardening temperature and cooled from a single end in terms of distance to the cooled end.
Tempered steels can be hardened by flame or induction, even after the tempering process. In selecting the material that will be subjected to such a heat treatment, chemical composition as well as the hardness value to be obtained on the surface and hardening depth are considered. In the case of unalloyed steels, hardness depth can be 3-4 mm. This depth reaches 10-12 mm for alloyed steels. Also, since high manganese would create a cracking risk during hardening through induction, it would be more suitable to use Cf grade steels with high carbon and low manganese content. What’s more, decreasing the risk of cracking is closely related with the material grain structure being small.
Tempering process is described as the combination of hardening and then damascening processes in which the steel part will gain high firmness properties. Tempered steels are widely used for manufacturing of various machinery and engine parts, forged parts, including a great variety of bolts, nuts and studs, crankshafts, axles, controller and drive parts, piston arms, several types of axles, teeth, etc. due to their superior mechanical properties at the end of the tempering process. Thus, tempered steels are the most manufactured and used type of steel after construction and unalloyed steels.
Tempered steels are gathered into 4 main groups by chemical composition.
- Unalloyed tempered steels
- Manganese-alloyed tempered steels
- Chromium-alloyed tempered steels
- Chromium-molybdenum alloyed tempered steels
In the case of unalloyed steels, the tempering strength increases in proportion to carbon content. For a diameter of up to 16 mm, the minimum yield strength ranges from 370 N/mm^2 (%C:0,25) to 570 N/mm^2 (%C:0,50). For a diameter of 16-40 mm, this value decreases by 50-80 N/mm^2.
Since manganese increases the hardenability of manganese-alloyed tempered steels, the minimum yield strength in 30Mn4 and 40Mn4 steels displays the characteristics in C60 steel.
In the case of chromium-alloyed tempered steels, chromium element significantly increases hardenability and also positively affects plasticity as well. For instance, in the case of 40Cr4 steels with a diameter of 16-400 mm, the minimum yield strength is 700 N/mm^2.
Molybdenum increases the hardenability capability more compared to chromium. It also increases the tempering strength and decreases the possibility of tempering fragility.
The tempering process is described as a hardening process followed by a temper process. The hardening and temper processes are explained separately for steels as follows.