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Title: The fracture toughness of some heat-treated alloy steels
Author: Johnson, R.
ISNI:       0000 0001 2448 1572
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 1974
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A study has been carried out on the mechanical and microstructural properties of seven alloy steels. These were heat-treated to produce a wide range of strength levels, and fracture toughness and tensile tests were performed. The fracture surfaces and microstructures were examined by scanning and transmission electron microscopy. The particles that affected the fracture toughness to the greatest extent were the large type II and type III sulphide inclusions, but their volume fractions were too low to allow the fracture to proceed entirely by void coalescence around these inclusions. The overheating sulphides could nucleate complete void coalescence but did not affect the fracture toughness to the same degree. Apart from the very low strength specimens, where overheating facets dominated the fracture surface of some steels, the fracture process was controlled by the matrix failure around precipitate particles, although only the voids around these particles were visible on the fracture surfaces. For correlating fracture toughness and tensile properties in the quenched and the quenched and tempered specimens it was found that the u. t. s. was the most consistent strength parameter, the proof stresses being affected by the internal quench stresses of the martensitic transformation. Using a corrected reduction of area term, that assessed the effect of second phase particles upon the ductility of the steel, and the u. t. s. an empirical equation relating them to the fracture toughness was derived. Of the existing theoretical equations it was found that the model of Krafft was too simple to cope with these complex steels, but that a modified Hahn and Rosenfield equation was in good agreement with the experimental results.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available