Alloy Steel Dynamic Strain-Aging and Notch Brittle Fracture

Transverse V-notch Charpy specimens prepared from sheet bar of 1045, 4140, and 4340, each in four different metallurgical conditions, were deformed to fracture in slow bending at four diverse combinations of deflection rate and temperature within the blue-brittle range. In more than half of the instances, after reaching the maximum load condition, load-deflection curves showed instantaneous load drops due to the occurrence of rapidly propagating fractures. Fracture surfaces were replicated and examined by electron fractographic techniques. The rapidly propagating cracks involved cleavage or quasi-cleavage, conclusively demonstrating that notch brittle fracture can occur with alloy steels undergoing dynamic strain-aging at considerably elevated temperatures.