Microstructural Characteristics and m23c6 Precipitate Behavior of the Course-Grained Heat-Affected Zone of T23 Steel without Post-Weld Heat Treatment

Seong-Hyeong Lee,Youngson Choe,Kyong-Woon Lee,Chung Kang,Hye-Sung Na

doi:10.3390/met8030170

Abstract

The microstructural characteristics of a simulated heat-affected zone (HAZ) in SA213-T23 (2.25Cr-1.6W steel) used for boiler tubes employed in thermal power plants were investigated using nital, alkaline sodium picrate, and Murakami’s etchants. In order to investigate the microstructure formation process of the HAZ in the welding process, simulated HAZ specimens were fabricated at intervals of 100 °C for peak temperatures between 950 and 1350 °C, and the microstructural features and precipitate behavior at various peak temperatures were observed. The alkaline-sodium-picrate-etched microstructures exhibited a black dot or band, which was not observed in the natal-etched microstructure. As the temperature increased from 950 to 1350 °C, the black dot and band became wider and thicker. Experimental analyses using an electron probe micro-analyzer, electron backscatter diffraction, and transmission electron microscopy revealed the appearance of austenite in the black dot region at a peak temperature of 950 °C; its amount increased up to a peak temperature of 1050 °C and thereafter decreased as the peak temperature further increased. The amount of M23C6 decreased with an increase in peak temperature. Based on these results, we investigated the behaviors of austenite and M23C6 as functions of the peak temperature.

Highlights

Ultra-supercritical (USC) power plants are being operated under increasing pressure and high-temperature conditions to reduce fuel costs, environmental pollutants, and greenhouse gas emissions
transmission electron microscopy (TEM) micrograph and (b) schematic illustration of 1050 ◦ C. (c,d) Diffraction patterns at the regions denoted as X1 and X2. (e) EDX line-scan result of Y1 across particle
The findings are summarized as follows: (1) When the base metal of T23 was etched with alkaline sodium picrate, the ivory color obtained could be identified as tempered martensite and the brown color obtained was identified as bainite

Summary

Introduction

Ultra-supercritical (USC) power plants are being operated under increasing pressure and high-temperature conditions to reduce fuel costs, environmental pollutants, and greenhouse gas emissions. SA213-T22 (2.25Cr-1Mo steel), a conventional boiler tube material used in water-wall tubes in power plants, should be thickened for high-temperature and high-pressure applications [1,2]. If the tube becomes thicker than the ones commercially used, the construction costs will increase owing to the change in boiler design and the increase in the number of welding passes [3,4,5,6]. SA213-T23 (2.25Cr-1.6W steel), which has superior high-temperature creep properties compared to the existing material SA213-T22 (2.25Cr-1Mo steel), was developed. In order to enhance the high-temperature creep strength, T23 was developed by adding tungsten (1.6%) instead of reducing

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