Abstract
ASTM A193-12, grade B6 (Gr.B6), 3/4 in. (19.05 mm) in diameter by 4 in. (101.6 mm) in length, square-head bolts were breaking at the head to shank juncture during wedge tensile testing. Specimens from the same lot did meet the requirements of Gr.B6 for tensile strength, hardness, reduction of area, and elongation. Failure analysis was performed on the lot to indicate whether the discrepancy is with integrity of steel, forging or heading process, or the heat treatment. The investigation was carried out by dimensional inspection, visual inspection, chemical analysis, hardness and tensile test, scanning electron microscopy (SEM) analysis, and microstructure evaluation. SEM analysis indicated that the cause of fracture is because of embrittlement. The microstructure evaluation had showed the presence of tempered martensite in the core and needle martensite with retained austenite on the surface. Failure analysis concluded that cause of failure was because of the formation of unstable microstructure during heat treatment. The retained austenite with needle martensite structure had an average Vickers hardness (HV) of 301 at the surface and the tempered martensite structure had an average Vickers hardness of 262 HV at the core that led to a brittle failure on wedge tensile test. The lot was recommended to be re-tempered at 1180°F (637°C) with a holding time of 2.0 h to meet the requirements of wedge tensile testing.
