PSI - Issue 71

Ganesh Nigudage et al. / Procedia Structural Integrity 71 (2025) 218–225

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1800h aged was 208VHN. Variation of Vickers hardness with thermal ageing is indicated in Fig.4.

Fig.4. Vickers hardness in as-fabricated and thermal aged specimens.

3.4. Microstructural examination The optical micrographs of as-fabricated and thermal aged specimens after chemical etching using Nital are shown in Fig.5. The typical microstructure in as-fabricated condition is tempered bainite. The lath features of bainite are arranged in packets inside prior austenite grains. The average prior austenite grain size was measured using Image J software and variation of prior austenite grain size of as-fabricated and thermal aged specimens is shown in Fig.6. No change in grain size is observed for the thermal ageing time studied.

c

a

b

Fig.5. Optical micrographs of a) As fabricated base, b) 1200h aged, c) 1800h aged.

Fig.6. Prior austenite grain size in as-fabricated and thermal aged specimens.

3.5. Fractography of the SPT samples The fracture surface of as-fabricated and thermal aged specimens was observed in SEM after SPT and are shown in Fig. 7. The fracture surface of as-fabricated and 1200h thermal aged specimen was dominated by ductile fracture with no intergranular fracture. Mixed mode fracture mechanism was observed for 1800h thermal aged specimen. For the 1800h thermal aged specimen, the size of intergranular fracture feature observed on fracture surface is similar to size of prior austenite grain observed under optical microscope. The presence of this intergranular fracture feature indicates the reduction of grain boundary cohesive strength which occurs primarily due to presence of phosphorus at grain boundary. Also, ductile fracture consisting of dimples was observed on fracture surface surrounding the intergranular fracture. These results are in agreement with (B. Gurovich, 2013) which observed brittle intergranular fracture on fracture surface of Charpy specimens due to segregation of phosphorus at grain boundaries.