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