Crack mechanism understanding and failure avoiding treatment of T24 tube material in advanced super critical coal fired steam generators (CRAMUFAT24) : final report

In order to determine the susceptibility of T23 and T24 against stress relief cracking (SRC) and stress corrosion cracking (SCC), tube butt welds and simulated heat-treated base metal against were investigated. Experiments on T23 and T24 steel grades with standard carbon contents and T24 with reduced carbon comprised SRC-tests in hot air at 550 C. SCC-tests in conditioned and pressurized feed water at 180 °C and 300 °C and tests on tube butt welds in model panels and in special C-ring-specimens. The heat input during welding can decisively influence the susceptibility to SRC and SCC, i.e. an optimised multi-layer welding technique can reduce the susceptibility to both failure mechanisms. The metallurgical influence of a simulated boiler heating at 470 °C for 48 h on both failure mechanisms was low, thus the improvement obtained by boiler heating procedures seems to be restricted to the reduction of residual stresses. The SCC-susceptibility also depends on the oxygen content and the temperature of the feed water. Crack faces showed a predominantly intergranular topography. TEM-investigations revealed that untampered martensite in tube butt welds and in simulated heat-affected zones is the reason for a relatively high susceptibility of T23 and T24 to SRC and to SCC. Numerical calculations for temperature distributions from the coal burning process and the water-steam-circuit and subsequent structural calculations yielded integral and local stresses and strains in the membrane walls and in the tube butt welds. Real strain figures were used for validation. It could be concluded that the stresses imposed exclusively by heat flux cannot be considered as the main driving mechanism for any possible T23/T24 cracking.