PSI - Issue 14

Afroz Shaikh et al. / Procedia Structural Integrity 14 (2019) 782–789 Afroz Shaikh / Structural Integrity Procedia 00 (2018) 000–000

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The microstructure obtained after HT 2 is shown in Fig. 4. Water cooled specimen shows microstructure consisting of acicular ά martensite and primary α (Fig. 4a-b) whereas a duplex type of microstructure consisting of primary α and lamellar mixture of α and β is obtained after air cooling (Fig. 4c-d). Upon furnace cooling a coarse duplex type of microstructure is obtained (Fig. 4e-f).

a

b

Primary α

c

d

Acicular α

f

e

Fig. 4. Microstructure of heat treated samples (a-b) 950 °C & WQ; (c-d) 950 °C & AC; (e-f) 950 °C & FC.

The microstructure obtained after HT 3 is shown in Fig. 5. Water quenched specimen shows a microstructure which consists of acicular ά martensite and broken lamella of α (Fig. 5a-b) whereas microstructure consisting of broken blocky lamella of α and plate-like acicular α and β is obtained after air cooling (Fig. 5c-d). Upon furnace cooling, coarse broken lamellar mixture of α and β with grain boundary α on the prior β grains is obtained (Fig. 5e f).

3.2. Hardness results

The result of hardness test is shown in Fig. 6. In all heat treatment cycles, faster cooling (WQ) results in higher hardness and least hardness is observed in case of furnace cooled specimen. The hardness values of WQ specimen is approximately 19% - 26% higher as compared to the AC specimen. Hardness of AC specimen is approximately 3%