Correlation Between Microstructure and Low-Temperature Impact Toughness of Simulated Reheated Zones in the Multi-pass We
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RODUCTION
RECENTLY, as various structures (e.g., pipelines and offshore construction) have become larger and their environments have become harsher, the development of steel plates with a combination of excellent strength and low-temperature toughness has accelerated. However, compared to base metals with superior mechanical properties, weld metals and the heat-affected zone (HAZ) generally exhibit poor mechanical properties, especially in terms of toughness, which decreases the reliability and stability of the structures. Hence, many studies have focused on improving the toughness of weld metals and HAZs of steels. Among the various regions in the HAZ near the single-pass weld bead, the coarse-grained HAZ (CGHAZ) adjacent to the fusion line has the lowest toughness due to the considerable growth of prior austenite grains. Consequently, there have been many
YONGJOON KANG is with the Joining Technology Department, Korea Institute of Materials Science, Changwon 51508, Korea. GITAE PARK, SEONGHOON JEONG, and CHANGHEE LEE are with the Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Korea. Contact e-mail: [email protected] Manuscript submitted December 20, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
efforts to suppress austenite grain growth via the pinning effect, which is caused by microalloying precipitates that are stable at high temperatures.[1,2] In addition, ferrite grain refinement has been attempted by promoting intragranular ferrite nucleation on the finely dispersed particles that are formed during the steelmaking process.[3,4] During multi-pass welding, the HAZ experiences a more complicated thermal history compared with single-pass welding. In this situation, the intercritically reheated CGHAZ (ICCGHAZ), where CGHAZ is reheated to the intercritical temperature range (between Ac1 and Ac3) by subsequent welding passes, has the poorest toughness due to the formation of martensite–austenite (M–A) constituents along the prior austenite grain boundaries of the CGHAZ.[5,6] Also, it has been demonstrated that the toughness of ICCGHAZ is mainly affected by the distribution and morphology of the M–A constituents. The toughness of weld metals can be improved by the formation of the interlocking structure of acicular ferrite. Thus, many researchers[3,7-10] have focused on determining the optimum welding process parameters and weld metal compositions that encourage the development of acicular ferrite. When multi-pass welding is applied, large thermally affected regions are formed in both the base metal and the weld metal. The thermally affected region in the weld metal is usually referred to as the ‘‘reheated’’ region. However, only a few studies have
investigated the mechanical properties of reheated weld metals, and their results have been inconsistent. Some studies[7,9,10] have reported that a higher fraction of the reheated zone (RZ) resulted in higher toughness of the weld metal due to the refined microstructure. Other studies[11,12] have indicated that coarse-grained RZ (CGRZ) subj
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