Abstract
The failure by cracking of two ultra-high strength steels, Rex. 539 and En. 40C, under conditions of stress-corrosion has been studied. The steels were heat-treated to strength levels near 120 t. s. i. . Application of sodium chloride and other corrodents to notched tensile test-pieces loaded at 33, 50, 70 and 80% of their breaking load resulted in fracture within a few weeks, or even within an hour, except at the lowest stress when life exceeded 8 weeks. The higher silicon steel, Rex. 539, proved the less susceptible (i. e. had longer life). For both steels a wide scatter in stress-corrosion lives seemed to be a feature of the phenomenon studied. The life-range/stress relationship and other characteristics could best be explained by a mechanism based on embrittlement by hydrogen resulting from corrosion, the stress-concentration at the notch promoting its absorption. The time-to-failure by this mechanism is at least partly controlled by the rate at which hydrogen accumulates by diffusion at the zone of maximum triaxial stress just beneath the notch-root. Thus the mechanism applies strictly to notched test-pieces only. Similar failures were observed with cadmium---plated specimens which had been de-embrittled by baking. Very thin coatings accelerated cracking by galvanic action, but thicker coatings delayed cracking by protecting the steel from corrosion.