Abstract
In Europe between 2006 and 2012 several ultra-super-critical (USC) coal-fired power plants were built employing T24 (7CrMoVTiB10-10 / DIN EN 10216-2:2014-03 / VdTÜV sheet 533/2) in membrane walls. During commissioning stress corrosion cracking (SCC) on the tube-to-tube butt welds appeared. The widespread damages required the development of a new patented commissioning procedure to avoid recurring damages. Although this commissioning procedure was employed successfully and the power plants are in operation since then, a debate about the implementation of a hardness limit for such butt welds was initiated. According to the European standards butt welds of T24 boiler tubes with wall thickness < 10 mm (0.3937 in) do not require any post-weld heat treatment (PWHT) and no hardness limits are given. When looking at manufacturing related issues such as an imminent risk of cold cracking after welding of micro-alloyed steels a widely applied but coarse hardness limit is 350 HV. Based on laboratory tests, some authors reallocated this 350 HV hardness limit for addressing SCC susceptibility of low-alloyed steels. This article describes typical hardness levels of T24 boiler tube TIG butt welds and the SCC behavior in high temperature water. Further the effect of the stress relief heat treatment (SRHT) of the boiler membrane walls between 450 °C and 550 °C (842 °F and 1022 °F) on its hardness values and on the SCC behavior is discussed, showing that the hardness values should not be used as an indicator for SCC susceptibility of T24 boiler tube butt welds.
