Detection of embrittlement in low alloy steels due to thermal aging by small punch test

The applicability of miniaturised small punch tests (SPT) to capture thermal aging degradation of two commonly used low alloy steels (LAS) viz. Mn-Ni-Mo and Cr-Mo-V steels, was investigated. This was done by comparing the SPT results to the results of the conventional Charpy V-notch (CVN) tests, fractographic analysis and results of chemical etching test. Both the steels were subjected to accelerated thermal aging at 450 °C (8400 h for Mn-Ni-Mo steel and 5600 h for Cr-Mo-V steel). For both LAS, conventional CVN tests showed reduction in the impact energy and fractographic analysis indicated increase in the percentage brittle fracture with thermal aging. Fractographs indicated intergranular (IG) brittle fracture for the Mn-Ni-Mo steel and transgrannular (TG) brittle fracture for the Cr-Mo-V steel. This was corroborated by the chemical etching tests that showed preferential attack at prior austenitic grain boundaries (PAGBs) for Mn-Ni-Mo steel and conspicuous absence of such IG attack for Cr-Mo-V steel. SPT could clearly capture the embrittlement due to thermal aging for both the LAS as investigated by the conventional CVN testing. However, SPT could not establish the ductile to brittle transition temperature (TSP) as the lower shelf energy was not reached even at the lowest possible test temperature (-196 °C). SPT has been successful in capturing the indications of embrittlement due to thermal aging for both the LAS i.e. it is able to show the loss of ductility and increase in the fraction of brittle fracture. However, the use of SPT, at least in the present form, as an alternative to the most commonly used CVN test, for the materials studied is not recommended.