Effects of Carbon Variation on Microstructure Evolution in Weld Heat-Affected Zone of Nb-Ti Microalloyed Steels
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EXCELLENT combinations of strength and toughness are obtained in modern pipe line steels by thermo-mechanically controlled processing, in conjunction with Ti and Nb microalloying, to produce a fine, uniform microstructure.[1,2] However, the microstructure obtained in the base metal is upset by the thermal cycles brought about by the welding during pipe making and joining, resulting in areas of poor toughness in the heat-affected zone (HAZ).[3] One of the causes of embrittlement is the formation of undesirable XIAOPING MA is with the Department of Materials Science and Engineering, McMaster University, Hamilton, Canada, L8S 4L7, Canada and also with Algoma Steel Inc., Sault Ste. Marie, P6A 7B4, Canada. Contact e-mail: [email protected] XUEDA LI is with the College of Mechanical and Electronic Engineering, China University of Petroleum (East China), Qingdao, 266580, China. BRIAN LANGELIER and SUNDARESA SUBRAMANIAN are with the Department of Materials Science and Engineering, McMaster University. BAPTISTE GAULT is with the Department of Microstructure Physics and Alloy Design, Max-Planck-Institut fu¨r Eisenforschung GmbH, Max-Planck-Straße 1, Du¨sseldorf, 40237, Germany. LAURIE COLLINS is with EVRAZ North America, Regina, S4P 3C7, Canada. Manuscript submitted December 17, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
microstructures resulting from weld thermal cycles and the base chemical composition of the steel.[4] Since the HAZ microstructure is heterogeneous and has many sub-zones, it is very difficult to establish the relative contribution of each sub zone to the properties, but the area most frequently highlighted in literature for impairing toughness is the coarse grained HAZ (CGHAZ). CGHAZ refers to the region immediately adjacent to the weld fusion line, where the base metal was subjected to a high thermal cycle peak temperature above the solvus for microalloying carbo-nitrides and significant austenite grain coarsening occurs. The extent of austenite grain coarsening depends on the thermal cycle experienced, and the chemical composition of the steel, as well as the size and dispersion of second phase particles and their potential for dissolution.[5-7] In the absence of thermo-mechanical deformation, the resulting microstructure morphology and crystallography in CGHAZ upon subsequent cooling depend on the temperature at which phase transformation occurs, which, in turn is determined by the hardenability and the cooling rate.[8] Additionally, martensite-austenite (MA) constituent has been identified as a distinct microstructural feature influencing HAZ toughness.[4,9,10] It is widely accepted that coarse MA
deteriorates toughness by initiating brittle micro-cracks, either by the cracking of MA particles or their decohesion with the matrix, in accordance with the Cottrell–Petch model for brittle fracture.[11] Refinement of MA is thus important to improve HAZ toughness, by preclusion of brittle crack initiation. Although each of the abovementioned morphological and crystallographic parameter has been ex
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