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Title: Effects of microstructure and prestraining on ductile to brittle transition in carbon-manganese weld metals
Author: Novovic, Milorad.
ISNI:       0000 0001 3450 1477
Awarding Body: University of Birmingham
Current Institution: University of Birmingham
Date of Award: 2001
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The effects of microstructure and prestraining on toughness of a multipass carbon-manganese (C-Mn) weld metal have been investigated. The toughness has been assessed in terms of both Charpy impact energy and Crack Tip Opening Displacement values. An extensive Charpy impact test experimental programme has been carried out on the C-Mn pressure vessel steel weld metal resulting in comprehensive databases for the weld metal in as-received and prestrained conditions. Charpy specimens were machined from multipass submerged arc weld metals with various notch tip locations and orientations relative to the weld metal. The impact energy was evaluated over a range of temperatures from -196 to 150°C. Mechanical prestraining levels of 2,9, and 12% have been applied to impact specimens to provide the trends in impact toughness that approximate some effects of irradiation embrittlement in service. Statistical interpretation of the results, in conjunction with the measurements from the fracture surfaces of Charpy specimens and fractography, has been carried out to provide a quantitative assessment of transition temperature shifts from the full and sub-populations of impact energy data pertaining to a particular notch root location relative to the weld microstructure. Impact energy values of a multipass submerged-arc C-Mn weld metal in both as-received and prestrained conditions are found to be subject to substantial scatter in the transition and upper shelf regions. Lower shelf impact energy values are found to be insensitive to microstructure and prestraining. The microstructure is found to have a significant influence on the ductile to brittle transition curve. Impact toughness for specimens with notch root locations in the reheated microstructure is greater than the impact toughness of specimens with notches located in the as-deposited microstructure. This trend is observed in both transition and upper shelf regions and is valid for both as-received and prestrained material conditions. The OT40Js hift between the transition curves corresponding to the as-received specimens with notch root locations on the centreline of the last deposited bead and in reheated microstructure has been estimated to be approximately 50°C. Prestraining (cold deformation) is found to provide the same trends in Charpy impact energy transition curve as irradiation embrittlement. Prestraining thus reduces the as-received upper shelf energy level and shifts the as-received transition curve towards higher temperatures. The magnitude of these effects depends on the amount of prestraining. The as-received and 2% prestrained datasets are found to be very similar, as are the datasets obtained after 9 and 12% prestraining.The transition curve for specimens with the notch root located in the reheated microstructure in the as-received material condition and transition curve for the specimens with notch root located on the centreline of the last deposited bead in the 9% prestrained material condition form the boundaries of the region of all possible locations for the transition curves along the temperature axis. The width of this "window" at the 40 J energy level, i. e. the OT4o4 temperature shift between the "best" (upper bound) dataset in as-received condition and the "worst" (lower bound) impact energy dataset in 9% prestrained material is estimated to be 102°C. The microstructure is also found to have a marked influence on the CTOD values of the as-received material with reheated microstructure showing better CTOD toughness across the whole temperature range examined. Prestraining at the 9% level shifts the onset of plastic collapse to lower values of the opening displacement resulting in the reduction in upper shelf CTOD values of more than 50% relative to the as-received upper shelf CTOD values. The transition region is also shifted along the temperature axis.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Charpy