Nonisothermal Austenite Grain Growth Kinetics in a Microalloyed X80 Linepipe Steel
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TO transport natural gas from the gas fields to the consumers, long distance pipelines are used. For these pipelines, increasingly higher strength linepipe grades, i.e., X80 and X100, are required to reduce the cost by thinner wall thickness and enhance the gas transportation efficiency by increased operating pressure.[1] In addition, novel welding procedures are crucial to produce high-quality welds that allow safe operation of the pipeline but that also enable cost efficient welding in the field. Controlled hot rolling and accelerated cooling are usually employed for producing linepipe steels. Weld strength is a dominating factor in assessing the overall performance of these materials for the manufacturing of pipelines, which, to a large degree, is controlled by the austenite grain size in the heat affected zone (HAZ). Large austenite grains near the fusion line may promote the formation of martensitic or bainitic transformation products with adverse effects on weld properties, e.g., fracture toughness.[2–4] For example, Hamad et al.[4] found that maintaining a relatively fine austenite grain size in the HAZ is critical to enhance the crack tip opening displacement (CTOD) values such that a minimum target CTOD value of 0.15 mm at 253 K (20 °C) can be realized in X80 linepipe steels. Thus, KUMKUM BANERJEE, Researcher, is with the Research & Development Department, Tata Steel Ltd., Jamshedpur 831007, India. MATTHIAS MILITZER, Professor, is with The Centre for Metallurgical Process Engineering, The University of British Colombia, Vancouver, BC, Canada V6T 1Z4. Contact e-mail: [email protected] MICHEL PEREZ, Professor, is with the Universite´ de Lyon, INSA Lyon, MATEIS, UMR CNRS 5510, Lyon, France. XIANG WANG, Research Associate, is with the Materials Science and Engineering Department, McMaster University, Hamilton, ON, Canada L8S 4L7. Manuscript submitted March 29, 2010. Article published online August 10, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
reducing the migration rates of austenite grain boundaries during welding, with an aim of achieving fine austenite grains, is an important metallurgical phenomenon to control the resultant properties favorably in high strength microalloyed steels. Finely dispersed particles impose pinning forces that restrict the boundary movement and grain growth during heat treatment or welding.[5–9] Sufficiently finely dispersed particles containing the microalloying elements, niobium, titanium, or vanadium, have been found to inhibit austenite grain growth effectively.[6,10–13] During welding, particle dissolution or coarsening may occur in the HAZ depending upon the thermal cycle. The associated change in average particle size and volume fraction reduces the pinning force and may, thus, play a crucial role in controlling the austenite grain size in the HAZ.[13,14] A plethora of research work has been carried out on austenitization and the control of the grain size for more than 50 years.[6,11,12,15–20] It is well known that higher temperatures favor grain growth.[19] The role of
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