AN INVESTIGATION OF METALLURGICAL FACTORS WHICH AFFECT THE FRACTURE TOUGHNESS OF ULTRA HIGH STRENGTH STEELS

Technical Report · Tue May 01 04:00:00 EDT 1973

DOI:https://doi.org/10.2172/950215· OSTI ID:950215

Wood, William E; Parker, Earl R; Zackay, Victor F

The relationship between microstructure, heat treatment and room temperature fracture toughness has been 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 l200 C, 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. The alloys 4130, 4140, 4340 all showed a severe intergranular embrittlement when austenitized at high temperatures and tempered above 200 C, while the alloys 4330 and 300-M exhibited no drop in toughness for the same heat treatment conditions. The as-quenched tensile specimens had a very low 'micro' yield strength which rapidly increased to the level of the 'macro' yield strength when tempered.

• Research Organization: Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA (US)
• Sponsoring Organization: Materials Sciences Division
• DOE Contract Number: AC02-05CH11231
• OSTI ID: 950215
• Report Number(s): LBL-1474
• Country of Publication: United States
• Language: English

Related Subjects

99 GENERAL AND MISCELLANEOUS
ALLOYS
AUSTENITE
BAINITE
EMBRITTLEMENT
FRACTURE PROPERTIES
GRAIN BOUNDARIES
HEAT TREATMENTS
IMPACT TESTS
METALLOGRAPHY
MICROSTRUCTURE
MORPHOLOGY
SCANNING ELECTRON MICROSCOPY
STEELS
STRAINS
YIELD STRENGTH