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Title: Type IV cracking in 1¼Cr - ½Mo low alloy steel welds
Author: Stratford, G. C.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1995
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Service experience has shown the main form of medium to long term damage and failure in low alloy steel weldments to be "TYPE IV" creep cracking in the intercritically transformed region of the heat affected zone (HAZ). This research programme aimed to define the fabricational, loading and microstructural factors which lead to this form of damage. Research welds, which were an accurate model of the geometry and microstructure of steam pipe weldments, were fabricated in tubular testpieces using standard welding techniques. The welds were subjected to a post-weld heat treatment (PWHT) of 700oC or 750oC for two hours. Whilst the heat treatment reduced the hardness of the welds, significant changes in microstructure were only seen in the 750oC PWHT weld. Uniaxial creep tests were performed on base metal and cross-weld specimens. Post-weld heat treatment increased the creep deformation and reduced the failure life of base metal specimens. For cross-weld specimens in the as-welded condition, the susceptibility for low ductility TYPE IV failure in the HAZ was invariably found to be linked to the sub-surface development of creep cavities and cracks. All specimens tested in the as-welded condition failed in a low ductility TYPE IV mode. For all cross-weld specimens, the susceptibility to low ductility failure was linked to factors which affect the base metal ductility, such as PWHT and test temperature. Thus, specimens with a PWHT of 750oC or those with a PWHT of 700oC tested at a temperature greater than 580oC were found to fail in a high ductility manner in the base metal region. Tubular specimens were tested at elevated temperature under internal pressure and applied end-load conditions. Results showed that deformation and fracture were dependant on temperature and axial stress and that sub-surface TYPE IV damage had developed in the HAZ of 700oC PWHT welds.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available