Prakt. Met. Sonderband 46 (2014) 309
hanism and austenite occurs above 500°C along with the precipitation of cementite and tempering
carbides as shown in figure 1.
The direct transformation of retained austenite will lead to formation of ferrite and
cementite or bainite [6]. Indirect decomposition involves precipitation of carbides from the
retained austenite and leading to formation of martensite upon cooling [7]. The purpose of
standard heat treatment (i.e tempering twice) in the steel industry is to avoid retained
austenite and to temper the fresh martensite. The fresh martensite is brittle and can
reduce the toughness while higher amount of retained austenite can produce dimensional
instability for the tools [8].
oduced with
tization was
wed by gas
Iffle furnace,
re produced.
the changes
was used to
t testing was
Figure 1: Microstructure of Caldie, austenitized at 1050°C tempered at 200°C, 2x2h
1050°C and showing undissolved primary carbides (left) and at 500°C, 2x2h (right) showing
| consists of precipitation of cementite. Etched with Pikral and 10% Nital for 3 s.
ery hard to
are fine and
orize the fine 3.2 HARDNESS:
3). They are The as-quenched steel has a high amount of carbon in the martensite thus producing a
distributed in high hardness. Atomic diffusion is very slow at room temperatures so tempering treatment
ich) and MeC is required to soften the steel. The tempering curve presented in figure 2 shows that for
are shown in low tempering temperatures, a higher austenitization temperature results in lower
hardness. The lower hardness is most likely due to the presence of a higher amount of
le tempering retained austenite.
d tool steels At 500-550°C, there is a peak in hardness known as ‘secondary hardening’. The extra
he tempering hardness is due to dispersion strengthening by tempering carbides such as MC, M,C [3,9].
molybdenum Above 525°C, the hardness drops dramatically as coarse cementite randomly distributed
in the SEM- in the microstructure forms. These coarse cementite will not contribute much to the
hardness [3].
ed austenite
dissolution of
the austenitic
er quenching
the retained
thin film type
ow tempering
n of retained