석탄화력 발전소용 초임계변압(SOVR)보일러 수냉벽 튜브 용접부의 열처리에 따른 신뢰성 향상

Abstract

ABSTRACT Investigate the Influence of Post Weld Heat Treatment on Weld Characteristics of Water-Cooled Wall Tube Used in Stationary Once-Through Variable Pressure Reheat (SOVR) Boiler

Kim Sun-In Advisor : Prof. Bang, Hee-Seon, Ph.D. CO-Advisor : Prof. Bang, Han-Sur, Ph.D. Department of Welding and Joining Science Engineering, Graduate School of Chosun University

In Central and South America, the cheapest coal in Stationary Once-Through Variable Pressure Reheat (SOVR) boiler can generate power around 350 MW, steam of 5000 T/H and pressure of 250 MPa. SOVR boilers are being built and operated, and large-scale SOVR boilers will continue to be built in Central and South America. In particular, Cr-Mo steels are widely used to improve the heat resistance, wear resistance and corrosion resistance of the power plant internal structures such as the re-heater tube which works at high temperature and high-pressure conditions. However, joining of re-Heater tubes with retaining high tensile strength is a crucial issue.

The purpose of this study is to investigate the effect of heat input in gas tungsten arc welding (GTAW) on the appearance of weld cracks due to improper bead shape geometry and post-weld heat treatment. Hardness test, chemical analysis, non-destructive test and post-heat treatment are performed for in detail analysis.

In the present work, Martensitic steel SA213-T91 was welded by GTAW process. No defects were observed in the welded parts, and the condition of back bead weld was also excellent. The heat treatment was performed for both specimens. No external defects or internal defects were found with no evidence of localised high-temperature exposure. Also, inside and outside of the tube confirmed that there was no deformation due to stress relaxation or exposed to air during long-time heat treatment and that no metal surface was oxidised. The metal biopsy test of both specimens did not show any microscopic cracks in the heat affected zone and the base metal, and both samples were free from hydrogen embrittlement cracks after post weld heat treatment (PWHT). The specimens which passed the visual inspection after the heat treatment was subjected to RT Test based on ASME Sec V.

Pendant Super-Heater for the water-cooled wall of SOVR Boiler was manufactured by 9% Cr-1Mo SA213-T91 Steel by heat treating specimens at 720℃~750℃. The PWHT for an additional 120 minutes was considered to be safe as it eliminated the lamination defect formed due to work hardening in the structure. The PWHT showed the reduction of the hardness of the welds, HAZ, and base metal of SA213-T91 steel. It is therefore considered that the post-weld heat treatment temperature is required to be 730°C ~ 750°C or more to lower the hardness of the weld metal and HAZ.