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Title: Low temperature fracture mechanisms in spheroidised ferritic steels
Author: Kumar, Akshay
ISNI:       0000 0001 3602 8732
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2007
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Low temperature fracture mechanisms of annealed (40h at 650°C) A533B (spheroidised ferritic) steel and heat-treated (lh at 316°C) 4340 (spheroidised ferritic) steel were investigated. Uniaxial tensile tests over the temperature range of -190°C to 60°C were used to identify crack nucleation sites, whether these cracks nucleate at early stages of loading or just prior to fracture, and whether crack growth is continuous. Additionally, electron probe microanalysis and electron back scattered diffraction were used to characterise fracture nucleation sites and microcracks in annealed A533B steel; and Charpy impact tests and electron probe microanalysis were used to investigate fracture nucleation site properties in heat-treated 4340 steel. These results were then compared with the low temperature fracture mechanisms observed in as-received bainitic A533B steel. This study found that at the lowest test temperatures both spheroidised A533B steel and heat-treated 4340 steel fail by 'quasi-cleavage' while as-received A533B steel fails by 'true transgranular cleavage'. Quasi-cleavage fracture surfaces are characterised by voids separating planar facets. Unlike in true transgranular cleavage, quasi-cleavage fracture is preceded by significant plastic deformation even at the lowest test temperatures. This study also found that fractured brittle second phase particles played no role in fracture nucleation in both the heat-treated steels. In heat-treated 4340 steel, fracture nucleated at a localised region of microvoids. In spheroidised A533B steel, 3 types of nucleation sites were seen: (a) a single void smaller than 2.5 J.lm in diameter, (b) single voids larger than 2.5J.lm in diameter and (c) a region of microvoids in a sulphur-rich area.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available