Hump on upper shelf of ductile-brittle transition temperature curve of a plain carbon steel

The ductile-brittle transition temperature (DBTT) in a plain carbon steel has been studied in a series of Charpy V notch samples. Both non-standard and standard samples were used in the as drawn, normalised and annealed conditions. In all cases, a hump was recorded on the upper shelf of the DBTT. Examination of fractures in the SEM in the vicinity of the hump showed typical microvoid coalescence. Low temperature fractures showed classical cleavage fractures with river markings. In the normalised and annealed conditions, both yield stress σ y and impact transition temperature T c varied inversely with square root of the ferrite grain size, in agreement with previous workers. However, the points for 'as drawn' samples lay off the linear plot. The reasons for this are discussed. The hump on the upper shelf of the DBTT is thought to be associated with dynamic strain aging.