Investigation of the Hydrogen Embrittlement Susceptibility of T24 Boiler Tubing in the Context of Stress Corrosion Cracking of its Welds

For the membrane and spiral walls of the new USC boilers, the advanced T24 material was developed. In 2010, however, extensive T24 tube weld cracking during the commissioning phase of several newly built boilers was observed. As the dominant root cause, Hydrogen Induced - Stress Corrosion Cracking was reported. An investigation into the interaction of the T24 material with hydrogen was launched, in order to compare its hydrogen embrittlement susceptibility with that of the T12 steel commonly used for older boiler evaporators. Both base materials and simulated Heat Affected Zone (HAZ) microstructures were tested. Total and diffusible hydrogen in the materials after electrochemical charging were measured. Thermo Desorption Spectrometry was used to gain insights into the trapping behaviour and the apparent diffusion coefficient at room temperature was determined. Based on the hardness and the diffusible hydrogen pick-up capacity of the materials, it was concluded that T12 is less susceptible to hydrogen embrittlement than T24, as base material as well as in the HAZ condition and that the HAZ of T24 is more susceptible to hydrogen embrittlement than the base material, both in the as welded and in the Post Weld Heat Treated (PWHT) condition. However, based on the results of this investigation it could not be determined if the T24 HAZ is less susceptible to hydrogen embrittlement after PWHT.