Unravelling Microstructure Evolution and Grain Boundary Misorientation in Coarse-Grained Heat-Affected Zone of EH420 Shi
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ng to an array of properties, including excellent weldability, adequate tensile strength, and satisfactory low temperature impact toughness, EH420 steel grade has been widely applied in sectors such as ship construction and offshore engineering.[1] However, such appealing performance is generally upset when high heat input welding is applied.[2–4] Under most of the circumstances, the resultant heat-affected zone (HAZ) is often the most critical region susceptible to failure. In particular, coarse-grained HAZ (CGHAZ) adjacent to the welding fusion line, incurred by rapid heating and varied cooling rate with higher peak temperature, always shows the worst impact toughness.[5–8]
XIAODONG ZOU is with the School of Metallurgy, Northeastern University, Shenyang 110819, China and also with the Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan. JINCHENG SUN and CONG WANG are with the School of Metallurgy, Northeastern University. Contact e-mail: [email protected] HIROYUKI MATSUURA is with the Department of Materials Engineering, The University of Tokyo. Manuscript submitted August 25, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
It is well-known that welding heat input would greatly affect the transformation of austenite into different microstructures in steels, which ultimately determine the final toughness.[9,10] Lower heat input condition could lead to higher cooling rate, often specified in terms of the time for cooling through the transformation range from 1073 K to 773 K (t8/5).[11,12] In general, with the increase in cooling rate, the nature and morphology of ferrites would change from polygonal to plate type, and to lath and acicular type.[13] In addition, as heat input decreases, shorter cooling time may likely enhance the nucleation rate and restrict the growth of grains due to the impingement of intergranular and intragranular ferrites.[14] As a result, ferrite grain refinement and low temperature transformation products, notably bainites and acicular ferrites (AFs), can be achieved, which are of practical importance tuning toughness.[15–18] On the other hand, the change in the transformation products controlled by cooling rate has a direct influence on the fraction of high angle grain boundary (HAGB),[19] which is beneficial to deflect or even arrest the propagation of cleavage cracks, thus enhancing the impact toughness.[20] An appropriate heat input could facilitate fine lath bainite and AF transformation to obtain desired microstructures with numerous HAGBs. Instead, an inappropriate cooling rate generally has a deleterious effect on the toughness of CGHAZ due to the formation of coarse microstructure with quite few HAGBs, such as granular bainite.[21] Therefore, it is expected to achieve target transformation products with high fraction of HAGB by varying heat input. In a separate study, the effect of heat input on the nucleation and growth of ferrites in HAZ of EH36 shipbuilding steel has been well documented and it was found at 120 kJ/cm heat input AFs with large amount of HAG
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