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

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.

Analysis of Mechanical Properties of Welded Joint Metal from TPP Steam Piping after Its Operational Degradation and Hydrogenation

“…A more intense degradation of WM relative to BM was associated with the well-known inhomogeneity of the chemical composition of WM and its microstructure, the higher hydrogen content in WM, and the effect of residual stresses inside the WJ [47]. It was also noted that the redistribution of thermal stresses in the pipe section (with active maneuvering of the operating mode of TPP units with a significant number of their shutdowns) could also contribute to the degradation of WM [14]. An important factor intensifying the WM degradation of WM is also its hydrogenation from a high-temperature process environment [33].…”

“…The main factors of the degradation of heat-resistant steels operated at TPPs include high temperature and stress, supplemented by the influence of a hydrogen-inducing technological environment [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. During the long-term operation of high-and low-alloy heat-resistant steels, the carbides form and coagulate along the grain boundaries.…”

“…This promotes the formation of intergranular pores and weakens the cohesion of neighboring grains. As a result, the fracture of the structural elements is facilitated due to creep (especially in a hydrogenating medium [30][31][32]) and thermal cycling (in particular, due to frequent shutdowns of the technological process) [14,33,34].…”

“…In general, hydrogenation of heat-resistant steels (like most structural materials) contributes to their embrittlement [38][39][40]. This is especially important during frequent shutdowns of technological processes [14] when the solubility of hydrogen in the metal decreases and its forced desorption occurs. Based on the relationship between the hydrogen content in 2.25Cr1Mo0.25V steel and its characteristics during tensile tests, the loss of its ductility under the influence of absorbed hydrogen was used to assess its susceptibility to hydrogen embrittlement [38].…”

Service Life of Steam Turbine Rotor Material Made of Low-Alloy Cr-Mo-V Steels after Long-Term Use outside the Design Operating Time of Power Unit

“…in conditions where creep is the main or significant destruction process that determines changes in the material during operation, include components of the pressure part of the boiler and turbine, which include rotor shafts. It is also important to determine the type of carbides and their impact on the performance properties of rotor steels [11][12][13]. The paper covers the issues of assessing the suitability of the rotor material for operation after exceeding the design operating time, assessing their microstructural state and the set of functional properties corresponding to this structure.…”