190 CHAPTER 9 THE TEMPERING OF MARTENSITE
9.2.5 Role of carbon content
Carbon has a profound effect on the behaviour of steels during tempering.
Firstly, the hardness of the as-quenched martensite is largely influenced by the
carbon content (Fig. 9.6), as is the morphology of the martensite laths which
have a {111}
γ
habit plane up to 0.3 wt% C, changing to {225}
γ
at higher car-
bon contents. The M
s
temperature is reduced as the carbon content increases,
and thus the probability of the occurrence of auto-tempering is less. During
fast quenching in alloys with less than 0.2 wt% C, the majority (up to 90%)
of the carbon segregates to dislocations and lath boundaries, but with slower
quenching some precipitation of cementite occurs. On subsequent tempering
of low-carbon steels up to 200
◦
C further segregation of carbon takes place, but
no precipitation has been observed. Under normal circumstances it is difficult
to detect any tetragonality in the martensite in steels with less than 0.2 wt% C,
a fact which can also be explained by the rapid segregation of carbon during
quenching or because M
s
exceeds the Zener ordering temperature.
The hardness changes during tempering are also very dependent on carbon
content,as shown in Fig. 9.6 for steels up to 0.4 wt% C.Above this concentration,
an increase in hardness hasbeen observedin thetemperature range 50–150
◦
C,as
ε-carbide precipitation strengthens the martensite. However, the general trend
is an overall softening, as the tempering temperature is raised. The diagram
indicates the main physical processes contributing to the change in mechanical
properties.
9.3 MECHANICAL PROPERTIES OF TEMPERED PLAIN
CARBON STEELS
The intrinsic mechanical properties of tempered plain carbon martensitic steels
are difficult to measure for several reasons. Firstly, the absence of other alloying
elements means that the hardenability of the steels is low, so a fully martensitic
structure is only possible in thin sections. However, this may not be a disadvan-
tage where shallow hardened surface layers are all that is required. Secondly,
at lower carbon levels, the M
s
temperature is rather high, so auto-tempering is
likely to take place. Thirdly, at the higher carbon levels the presence of retained
austenite will influence the results. Added to these factors, plain carbon steels
canexhibit quench crackingwhichmakes it difficulttoobtain reliable testresults.
This is particularly the case at higher carbon levels, i.e. above 0.5 wt% carbon.
Provided care is taken, very good mechanical properties, in particular proof
and tensile stresses, can be obtained on tempering in the range 100–300
◦
C.
However,the elongation is frequently low and the impact values poor.Table 9.2
shows some typical results for plain carbon steels with between 0.2 and 0.5 wt%
C, tempered at low temperatures.
Plain carbon steels with less than 0.25 wt% are not normally quenched and
tempered, but in the range 0.25–0.55 wt% C heat treatment is often used to