Effect of Postweld Heat Treatments on Type IV Creep Failure in the Intercritical Heat-Affected Zone of 10% Cr Martensitic Steel Welded with Haynes 282 Filler

Namkyu Kim,Jinhyeok Bang,Yongjoon Kang,Namhyun Kang,Chung-Yun Kang,Seong-Moon Seo,Sangwoo Song

doi:10.3390/met11050726

Namkyu Kim, Jinhyeok Bang + Show 5 more

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https://doi.org/10.3390/met11050726

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Abstract

This study investigated the effect of postweld heat treatment (PWHT) conditions on Type IV failure behavior of 10% Cr martensitic steel welds using Haynes 282 filler. The welded joints were subjected to PWHT at temperatures of 688, 738, and 788 °C for 4 and 8 h. Creep tests were carried out at 600 °C under a stress of 200 MPa. The as-welded joint without PWHT showed Type IV cracking due to growth of voids around Laves phase by localized creep deformation in the intercritical heat-affected zone (ICHAZ). The creep properties of the PWHTed joints at 688 °C were similar to those of the as-welded joints without PWHT. On the other hand, the PWHTed joints at 738 °C exhibited a significantly longer creep life by a lower amount of Laves phase in the ICHAZ than those at 688 °C; this could be a result of the homogenization of ICHAZ microstructure during PWHT at 738 °C. However, the PWHT at 688 and 738 °C showed the same Type IV creep failure mode. Meanwhile, the PWHTed joints at 788 °C exhibited the shortest creep life in this study. The failure location was shifted to the base metal away from the HAZ, and severe plastic deformation occurred due to the softened matrix by excessive tempering.

Highlights

Dissimilar metal welds (DMWs) between Ni-based superalloys and 9–12% Cr martensitic steels are inevitable in USC and advanced USC (A-USC) power plants
Several studies have reported that postweld heat treatment (PWHT) is effective in retarding Type IV cracking and increasing creep life of the HAZ of 9–12% Cr martensitic steels [20,21,22,23]
200 MPa, and it was found that the proper selection of PWHT temperature and holding time can suppress formation of Laves phase during creep test and results in retardation of

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. For successful long-term operation under a high temperature and pressure environment, it is necessary to investigate the creep behavior of the DMWs. The creep rupture of DMWs between Ni-based superalloys and ferritic/martensitic steels occurred in the steel side (i.e., base metal (BM), heat-affected zone (HAZ), and fusion line) because of its relatively lower creep strength [13,14,15]. For 9–12% Cr martensitic steel welded joints, it is reported that creep rupture normally occurs in the fine-grained HAZ (FGHAZ) or intercritical HAZ (ICHAZ) with relatively little total crossweld strain under long-term creep conditions, commonly known as Type IV failure. Several studies have reported that PWHT is effective in retarding Type IV cracking and increasing creep life of the HAZ of 9–12% Cr martensitic steels [20,21,22,23]. The mechanism to increase the creep properties is discussed based on the microstructural observation occurring in the ICHAZ during the creep testing

Experimental Procedures

Creep and Mechanical Testing

Microstructural Characterization

Creep and Rupture Behavior for Various PWHT Conditions

Type IV Failure Mechanism in the ICHAZ

Conclusions