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High stresses experienced in rolling cutter rock bits during the drilling of large-diameter, deep oil wells require steels having extra high levels of hardenability. Nickel-molybdenum steels provide the needed hardenability and are being used successfully in the drilling industry. New EX-grades of steel were evaluated in the carburized condition by means of several laboratory tests and compared to the high-nickel SAE 4800 or SAE 9300 grades. The tests included impact fatigue, high-cycle fatigue, impact fracture stress, and plane-strain fracture toughness. In each test, the excellent performance of EX55 (0.87Mn-0.58Cr-1.85Ni-0.75Mo) was demonstrated.
Several compositional variables of EX55 were evaluated to determine their importance on mechanical properties. The variables included core carbon content as well as alloy modifications of chromium, nickel, and molybdenum. Steels having a core carbon content of 0.14 to 0.17 percent were observed to have more potential for exhibiting high impact fatigue resistance than 0.20 percent carbon steels.
steels, carburized steels, high hardenability, mechanical tests, high-cycle fatigue, impact fracture stress, impact fatigue, fracture toughness, microstructure
Staff metallurgist, Ann Arbor Research Laboratory of Climax Molybdenum Company of Michigan, a subsidiary of American Metal Climax, Inc., Ann Arbor, Mich.