Effects of Thermal History and Microstructure on Segregation of Phosphorus and Alloying Elements in the Heat-Affected Zo
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INTRODUCTION
THE toughness of low alloy steels (LAS) can be degraded by aging, which increases the ductile–brittle transition temperature by an amount dependent on material, prior heat treatment, aging temperature, and time. This embrittlement is associated with segregated elements at grain boundaries (GB), essentially phosphorus (P).[1,2] Due to the specific thermal cycles induced by multi-pass welding, complex phase transformations take place in the heat-affected zone (HAZ) of LAS, especially in the close vicinity of the fusion boundary, resulting in coarse-grained (CG), fine-grained (FG), intecritically reheated coarse-grained (ICCG), and subcritically reheated coarse-grained (SCCG) HAZ.[3,4] Variations of solute segregation at GBs are also expected in these microstructures. Although the fundamentals of the interactive co-segregation of phosphorus and alloying elements in thermally aged ferritic steels have been intensively studied,[5–10] little is known about the actual segregation behaviors in real weldments with complex thermal histories. In order to evaluate and mitigate the risks of temper embrittlement, a better understanding on the segregation in real weldments is necessary. In this study, the GB segregation of phosphorus and alloying elements in the HAZ of a LAS was investigated ZIQING ZHAI, Ph.D. Candidate, HIROSHI ABE, Assistant Professor, and YUTAKA WATANABE, Professor, are with the Graduate School of Engineering, Tohoku University, 6-6-01-2 Aoba, Aramaki, Aoba-Ku, Sendai 980-8579, Japan. Contact e-mail: ziqing. [email protected] YUICHI MIYAHARA, Staff Member, formerly with the Material Science Research Laboratory, Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa 2400196, Japan, is now with the Planning Group, Central Research Institute of Electric Power Industry, 1-6-1 Otemachi, Chiyoda-ku, Tokyo 100-8126, Japan. Manuscript submitted May 30, 2014. Article published online September 30, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
quantitatively using atom probe tomography (APT), with focuses on the effects of thermal history and HAZ microstructure. Characteristics of interactive co-segregation between phosphorus and some alloying elements were also examined.
II.
EXPERIMENTAL
A. Material and Specimen Preparation The HAZ investigated in this study was fabricated by filling Ni-base alloy 182 into a machined groove in an A533B plate using shielded metal arc welding. Two types of Alloy 182 filler metals were used: Inconel 182 for the first 4 layers from bottom of the groove up to a height of 7.2 mm, and Yawata Weld 182 for the subsequent 11 layers. The chemical composition of the Ni-base alloys and LAS are given in Tables I and II, respectively. A typical post-weld heat treatment (PWHT) was applied to reduce residual stresses present after the welding and fast cooling process, of which the procedure is shown in Figure 1. An additional step cooling treatment,[11–13] was conducted on part of the weld joint to produce enhanced P segregation at GBs that would be attained
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