An Investigation of Metallurgical Factors Which Affect Fracture Toughness of Ultra-High Strength Steels
Final rept. 26 Jun-26 Dec 1972
CALIFORNIA UNIV BERKELEY LAWRENCE BERKELEY LAB
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DETERMINED FOR THE LOW ALLOY ULTRA-HIGH STRENGTH STEELS 4130, 4330, 4340, 4140 AND 300-M. Optical metallography, microprobe analysis, and scanning electron microscopy were used to characterize the structure and morphology, while both Charpy V-notch impact tests and plane strain fracture toughness tests were used to determine the fracture properties. The normal commercial heat treatment resulted in the formation of some bainite in all the alloys. MnS inclusions on prior austenite grain boundaries were found to initiate cracks during loading. By increasing the austenitizing temperature to 1200C, the fracture toughness could be increased by at least 60. For some alloys increasing the severity of the quench in conjunction with the higher austenitizing temperatures resulted in further increases in the fracture toughness, and the elimination of any observable upper bainite. There was no correlation between the Charpy impact test results and the fracture toughness results.
- Properties of Metals and Alloys