Effects of Long-Term Service on Microstructure and Impact Toughness of the Weld Metal and Heat-Affected Zone in CrMoV Steel Joints

Qixing Sun,Kejian Li,Shanlin Li,Jiluan Pan,Chaoyu Han,Dangxun Gao,Xiaogang Li,Zhipeng Cai

doi:10.3390/met12020278

Abstract

The microstructure and impact toughness of weld metals (WMs) and heat-affected zones (HAZs) of a low-alloy CrMoV steel gas turbine rotor which had served for 14 years were investigated. The ex-service joints in the turbine part (serving at 500–540 °C) and the compressor part (serving below 300 °C) of the rotor were selected for comparative research. The microstructure of the WMs and HAZs between the turbine part and the compressor part was similar, indicating that there was no significant deterioration in microstructure of the turbine part during service. However, compared with the compressor part WM, the impact energy of the turbine part WM decreased significantly, and FATT50 increased greatly. The degraded toughness of turbine part WM was related to more serious intergranular cracking caused by higher segregation level of phosphorus (P) at prior austenite grain (PAG) boundaries. Welding and post-weld heat treatment led to obvious segregation of P at PAG boundaries in WMs, and the segregation of P in turbine part WM was further aggravated during serving at 500–540 °C. Additionally, the inhomogeneous microstructure of the WMs also aggravated the segregation of P. The toughness of the HAZs in both turbine part and compressor part was high, which was because of fine grains. Furthermore, due to there being more grain boundaries and low P content, the segregation of P in HAZs was slight and its adverse effect on the toughness could be ignored.

Highlights

Introduction iationsDue to the excellent elevated temperature strength, the good oxidation resistance, and the high hydrogen embrittlement resistance, low-alloy CrMoV steels are widely used for high-temperature applications in the power, chemical, and oil industries [1,2,3]
We investigate the impact toughness and FATT50 of the weld metals (WMs) and heat-affected zones (HAZs) in the turbine part and the compressor part of an ex-service gas turbine welded rotor
Compared with the compressor part WM and HAZ, there was no obvious difference in microstructure of the turbine part WM and HAZ, indicating that the microstructure in the turbine part WM and HAZ did not significantly degrade during service at

Summary

Introduction

Due to the excellent elevated temperature strength, the good oxidation resistance, and the high hydrogen embrittlement resistance, low-alloy CrMoV steels are widely used for high-temperature applications in the power, chemical, and oil industries [1,2,3]. These steels are extensively used to manufacture critical components operated under the circumstances of high temperature, high pressure, and dynamic loads [1,2,3]. The microstructure and mechanical properties of these steels might deteriorate during service, leading to threats to the service safety of critical components [1,2,3,4,5]. Investigating the effects of service on the microstructure and the mechanical properties of low-alloy CrMoV steels is meaningful for ensuring service safety and assessing the remaining service life of critical components in generating units

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Results

Conclusion