310 Prakt. Met. Sonderband 46 (2014) 
poe 200 
[AS quenched T0728 % x 
160 
140 
- 120 
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i 80 
£ 60 
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Tusmparing Tomparatur C. > Tempering nA C. m2 ao 
Figure 2: The effect of austenitization and tempering temperatures on retained Fi 
; gure 
austenite and hardness at room temperature. Microgr 
directio 
temper 
3.3 IMPACT TOUGHNESS: 
Figure 3 shows the results from the impact toughness testing. Austenitization at a lower 
temperature resulted in higher toughness for all tempering temperatures. The fracture 
surfaces for low austenitization temperature show a ductile fracture with fibrous and dimple 
type fracture. On the other hand, the specimen austenitized at higher temperature loses 
toughness and becomes brittle with a quasi-cleavage type fracture. Figure 4 shows the 
coarsening of the austenite grains with increasing austenitization temperature. It is evident 
that lower values of toughness were found for coarser prior austenite grains. 
It is well known that a fine grained material gains significantly higher resistance to plastic 
deformation than coarse-grained [10]. Retained austenite is also known to contribute to 
better impact toughness. The retained austenite is quite stable at low tempering Figure + 
temperature and can hinder crack growth and improve toughness [11]. In this study it 1020°C 
appeared that the effect of coarser prior austenite grains was larger than the effect of for 3s. 
increasing amount of retained austenite for high austenitization temperatures. 
Other factors that could affects the toughness is ultra-thin carbon enriched retained 
austenite present between the laths of martensite. This can lead to carbide precipitation at 4. CO! 
lath boundaries during tempering [12] which can reduces toughness. Also, the presence of 
large amount of irregular shaped primary M;C3 carbides will make the material less tough. The eff 
These carbides have high hardness (about 1600 HV) [1,13] and provides good wear and im; 
properties [1]. However, they have a misfit with the softer matrix and can favor the crack conclus 
propagation in the interface between the carbide and the matrix. Further studies are 
needed to verify if any of these possible mechanisms are important for the resulting 
toughness.