The first heat
treatment is heating the high chromium cast iron to a temperature between 300
and 400°C. This temperature range produces no change in the microstructure of
the material. There is no precipitation of secondary carbides and no
transformation of the matrix. However, hardness of material can be slightly affected
and reduced. This caused by relieving of internal residual stresses partial
tempering of martensite. This can lead to have different microstructures, such
as epsilon-carbides formation and presence of cementite. The change in phase is
a function of many parameters such as time of heating, composition of material
and temperature range 11.
3.7.2 Heat treatment at 500-600°C:
as-cast irons increased to withstand aggressive environments in slurry pumps.
Heating the high chromium white iron to 550°C for 2-6 hours will relieve the
stresses in the iron. Literature results show that heating in a range of 2-6
hours will slightly increase hardness. Heating the material for more than 6
hours can cause a significant change where the microstructure change to a
mixture of ferrite and cementite, causing the drop of hardness. At this
temperature range, there is no possibility for any secondary carbides to form,
enhancing significantly the hardness 11.
3.7.3 Heat treatment at 600-850°C:
that heat treatment above 600 °C results in a decrease of hardness. This is
because of the transformation of the matrix (austenite and/or martensite) to
different microstructures with lower hardness, such as ferrite and pearlite
11. Figure 10 illustrates that quenching the material at 850oC
will increase its hardness; hence, its wear resistance will increase. In
contrast quenching the media at 750oC before its fully forming
austenite will cause the material to have low hardness. Literature was
conducted to improve the abrasive wear resistance of the high chromium cast
iron ASTM A-532 through thermal treatment cycles.
Figure 10 The change of temperature with hardness
3.7.4 Heat treatment at 900-1000°C (Destabilizing):
In order to obtain
maximum hardness of high chromium cast iron and highest wear performance,
as-cast cast irons are subjected to destabilizing heat treatment. The idea
behind it is to heat the cast iron in the range of 950-1065°C for 2-4 hours in order
to get secondary chromium carbides. This will lead the austenitic matrix to
lose carbon becoming less stable at room temperature, while martensitic
starting temperature (Ms) is been raised. Upon cooling the unstable austenite
matrix will transform into martensite, which is harder and have a better
performance in abrasive applications such as slurry pumps.
been known that high chromium cast iron in as-cast condition can have a matrix
of either austenite or martensite. Destabilization in cast iron with austenitic
microstructure has been explained. In as-cast martensitic microstructure there
are retained austenite and they have a negative effect on hardness levels. To
quench the material after destabilization procedure. Different quenching media
could be applied to show the change of the mechanical properties and
determining the increase of hardness. The quenching media are water, oil, air
and air & dry air as shown in the figure 8.