The rate of crack growth by apparent intergranular decohesion of a statically loaded MnMoNiCr steel at 535-600°C in vacuum was measured using the d.c. potential-drop method. Slow-displacement-rate testing of notched tensile bars at the same temperatures in an inert atmosphere was also carried out. The steel had been air-cooled from 1300°C to simulate a weld heat-affected zone. Interrupted tests followed by low-temperature brittle fracture were used to clarify the details of the cracking process, which occurred more rapidly in some grain boundaries 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 μm. SEM observations indicated that the tip of the main crack had a radius on the sub-micron scale. The cracking behavior of the tensile bars was qualitatively similar to that of the CT specimens and was found to have a temperature dependence of about 55 kcal/mol. The similarity of this phenomenon to cracking in other alloy systems is pointed out. The three contending models for the brittle mode of stress-relief cracking are compared in the light of the present results; a detailed treatment of a proposed theory is given in a separate paper.
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