SULFUR-INDUCED DYNAMIC EMBRITTLEMENT IN A LOW-ALLOY STEEL

The rate of crack growth by apparent intergranular decohesion of a sta tically loaded MnMoNiCr steel at 535-600 degrees C in vacuum was measu red using the d.c. potential-drop method. Slow-displacement-rate testi ng of notched tensile bars at the same temperatures in an inert atmosp here was also carried out. The steel had been air-cooled from 1300 deg rees C to simulate a weld heat-affected zone. Interrupted tests follow ed by low-temperature brittle fracture were used to clarify the detail s of the cracking process, which occurred more rapidly in some grain b oundaries than in others, giving rise to oscillations in the measured crack velocity. Striations on the intergranular facets were consistent with crack growth in steps of about 0.1 mu m. SEM observations indica ted that the tip of the main crack had a radius on the sub-micron scal e. The cracking behavior of the tensile bars was qualitatively similar to that of the CT specimens and was found to have a temperature depen dence of about 55 kcal/mol. The similarity of this phenomenon to crack ing in other alloy systems is pointed out. The three contending models for the brittle mode of stress-relief cracking are compared in the li ght of the present results; a detailed treatment of a proposed theory is given in a separate paper.