308 Prakt. Met. Sonderband 46 (2014) 
studied. This knowledge will be useful to further understand the failure mechanism and austeni 
therefore facilitate future development of improved tool steels. carbide 
The di 
cement 
2. EXPERIMENTAL retainet 
standat 
The chemical composition of the steel is given in table.1 austeni 
Table 1: Chemical composition of Caldie in wt.% reduce 
Lo I instabili 
C Dr Mo V Si "Mn 
0.7 5.0 2.3 0.5 0.2 vu 
Test specimens had a dimension of 177mm x 17mm x 35mm and were produced with 
austenitization temperatures of 1020°C, 1050°C and 1075°C. The austenitization was 
carried out in a vacuum furnace with a holding time of 30 minutes followed by gas 
quenching with tgs = 300 s. The tempering treatment was carried out in a muffle furnace. 
two times, each for 2 hours (2x2h). 
Specimens with a range of tempering temperatures (from 200°C to 600°C) were produced. 
The samples were then investigated with SEM (Joel, JSM 6010 LA) to study the changes 
in the distribution of precipitates. X-Ray Diffraction (GE, SEIFERT XRD-3003) was used to 
quantify the volume fraction of retained austenite. Un-notched Charpy impact testing was 
conducted for low and high tempering temperatures (200°C and 525°C). 
3. RESULTS AND DISCUSSION 
3.1 MICROSTRUCTURE AND RETAINED AUSTENITE: 
Fi 
SEM micrographs showing the microstructure of Caldie austenitized at 1050°C and chow 
tempered at 200°C and 500°C are shown in figure 1. The matrix of the steel consists of precir 
tempered martensite with some retained austenite and bainite. It is very hard to 
differentiate between retained austenite and lower bainite with SEM, as they are fine and 
mixed. More detailed investigation would further be required to fully characterize the fine 3.2 HAI 
details of the microstructure. 
The microstructure also contains undissolved primary carbides (i.e M;Cj). They are The as 
present along grain boundaries, triple point grain boundaries and unevenly distributed in high ha 
the matrix. Thes carbides ranges in size from 1-8 ym. Some MC (vanadium rich) and MeC is requi 
(molybdenum rich) carbides were also found. Examples of primary carbides are shown in low ter 
figure 1. hardne: 
When tempering above 500°C along with the formation of cementite, nanoscale tempering retainec 
carbides are also precipitated. Previously published work on similar alloyed tool steels At 500- 
proposed that the nano precipitates are finely distributed in the matrix [3]. The tempering hardne: 
carbides could be MC and MC type carbides with solubility for chromium, molybdenum Above 
and vanadium [3]. The tempering carbides are too small to be visible in the SEM- in the 
micrographs. hardne 
The retained austenite content is presented in figure 2. The amount of retained austenite 
increases with higher austenitization temperature. At this temperature, the dissolution of 
carbides takes place at a higher rate, dissolving more alloying elements in the austenitic 
matrix, lowering the Ms-temperature resulting in more retained austenite after quenching 
[4]. Studies done on retained austenite by Yaso et.al [5] has characterized the retained 
austenite to be of the block type present within the packets of martensite and thin film type 
present between the laths of martensite. The retained austenite is stable at low tempering 
temperature and starts to transform around 475°C. Complete transformation of retained