Abstract
Appropriate post weld heat treatment (PWHT) is usually obligatory when creep resistant steels are welded for thermal power plant components that operate at elevated temperatures for 30-40 years. The influence of different PWHTs on the microstructure, hardness, and impact toughness of simulated heat affected zone (HAZ) subzones was studied. Thereby, coarse grained HAZ, two different fine grained HAZ areas, and intercritical HAZ were subjected to 20 different PWHTs at temperatures 740–800 °C and durations 0.5–8 h. It was found that the most commonly recommended PWHT, of 3 h or less at 760 °C, is insufficient with respect to the hardness and impact toughness of coarse grained HAZ. To obtain a Vickers hardness ≤ 265 HV and impact toughness at least equal to the impact toughness of the base metal (192 J) in the coarse grained HAZ, it took 8 h at 740 °C, 4 h at 760 °C, more than 1 h at 780 °C, and 0.5 h and 800 °C. Even after 8 h at 800 °C, mechanical properties were still within the target range. The most recommendable post weld heat treatments at 780 °C for 1.2–2 h or at 760 °C for 3–4 h were identified. All specimens subjected to these treatments exhibited appropriate hardness, impact toughness, and microstructure.
Highlights
Numerous important thermal power plant components, e.g., steam pipelines, boilers, and heat exchangers etc., operate at elevated temperatures and high pressures
The hardness and impact energies of simulated heat affected zone (HAZ) zones before post weld heat treatment (PWHT) are summarized in According to EN ISO 15614-1 Specification and qualification of welding procedures for metallic materials—Welding procedure test, Part 1, hardness up to 350 HV is acceptable for heat treated weld joints of materials belonging to group 6
At temperatures of 760 ◦ C and 740 ◦ C, 4 h was not enough. These results show that, in contrast to the reports found in the literature, at temperatures below 780 ◦ C, adequate hardness cannot be obtained in all HAZ subzones in 3 h
Summary
Numerous important thermal power plant components, e.g., steam pipelines, boilers, and heat exchangers etc., operate at elevated temperatures and high pressures. Considering all this, simulated microstructures are more suitable for studying the influences of different PWHTs on the mechanical properties of individual HAZ subzones, and the results of Charpy impact tests on simulated test pieces can be regarded as more reliable than those obtained with test pieces cut from real weld HAZs. simulated material for the study of HAZ subzones was used less frequently than expected. As the HAZ in real welds is a narrow and inhomogeneous transition zone, the results of Charpy impact tests must be regarded as an average of all the influences exhibited by different microstructures through which a crack propagated. The presence of such small areas cannot be reliably detected by the tests of real weld specimens To confirm their presence after PWHT, mechanical tests of individual HAZ subzones are necessary. The results of PWHTs comparable to the one certificated for the defective welds were compared with the results of mechanical tests of real weld HAZs obtained in the process of the welding procedure certification
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