Stress Relief Cracking on the Weld of T/P 23 Steel

Abstract

T/P 23 weld failures occurred in coal fired supercritical power plants were investigated and possible causes were considered. Susceptibility of stress relief cracking was tested through mechanical properties varying heat treatment conditions in weld and intrinsic stress level in the pipe system was calculated by FEM analysis using displacement data. We also investigated what elements in the filler composition affected stress relief cracking examined. The results showed any serious susceptibility of stress relief cracking in T/P 23 steel. Post weld heat treatment (PWHT) about 740°C, 1hr specified in welding procedure produced good tensile and ductility, and cause reasonable reduction of hardness. Finite element analysis for the system has shown that the stress ranges of inlet header, link pipe and outlet header were 83∼96Mpa, 95∼104Mpa, 40∼41Mpa, respectively. The ratio of maximum stress for the design allowable stress was only 0.565 in link pipe. In the microstructural examination of T/P 23 weld, an interesting result was found that Mo content in the filler composition was significantly involved in the boundary behavior which affected stress relief cracking. Mo content close to the high level of ASME range has shown good microstructural stability and low susceptibility tostress relief cracking.