Engineering steels 257of cooling can then be increased. This treatment results in the formation of a
ferrite matrix with finely dispersed carbide particles. However, the large primary
carbides remain virtually unaffected by this treatment. After annealing, the hard-
ness of the standard grades of high-speed steel is less than 300 HB.
As indicated earlier, elements such as molybdenum, tungsten and vanadium
form stable carbides which have only limited solubilities in steel. However, in
order to produce a high-carbon martensite, with good tempering resistance and
the facility for secondary hardening, it is essential that a proportion of these
carbides is taken into solution. Solution treatment temperatures very close to
the solidus temperatures are therefore employed, i.e. temperatures in the range
1200-1300~ depending upon the grade. According to Hoyle, 2s the solution
of M23C6 (Cr-based) carbide begins at temperatures just above 900~ and is
complete at about 1100"C. The solution of M6C (Mo- and W-based) carbides
begins at about 1150~ and continues until the solidus is reached. On the other
hand, MC (V-based) carbide is extremely stable and little solution is achieved,
even at temperatures close to the solidus.
In order to minimize thermal shock, the steel is preheated slowly to a temper-
ature of about 850~ This is generally carried out in one furnace and the steel
is then transferred to a high-temperature furnace with a neutral atmosphere.
However, the soaking time at the hardening temperature must be short, e.g. two
to five minutes, in order to minimize decarburization and grain growth.
Following solution treatment, the cooling operation can be carried out in air
or by quenching into oil or a salt bath. Salt bath temperatures of 500-600~ are
employed and the treatment is carded out in order to reduce temperature gradients
and thereby reduce distortion and the risk of cracking. However, the salt bath
treatment should only be long enough to allow the material to attain a uniform
temperature and the component should then be air cooled. Transformation to
martensite will begin at a temperature below 200~ and will be about 80%
complete at room temperature. Whereas it is important to ensure that the steel
reaches ambient temperature in order to achieve a high degree of transformation,
tempering treatments must take place immediately after the hardening cycle in
order to prevent stabilizing effects in the residual retained austenite.
Tempering treatments are carded out at 530-570~ and serve two purposes:
- To produce a tempered martensitic structure which will be hard but stable at
elevated temperatures. - To destabilize the retained austenite such that martensite will form on cooling
to room temperature.
As indicated in Figure 3.36, a secondary hardening reaction takes place with
a peak at a temperature of about 550~ i.e. the typical tempering temperature
for high-speed steel. Therefore, as well as providing a structure which will be
stable in the short term up to this temperature, the tempering treatment also
produces the maximum level of hardness. During the first tempering treatment,
carbide precipitation takes place in both the martensite and retained austenite
that were present in the structure after the hardening treatment. In the latter case,
carbide precipitation depletes the phase in carbon and alloying elements, such