Investigation of reheat cracking susceptibility in CGHAZ of modified T23 steel

Abstract

T23 steel (2.25Cr–1Mo–1.6W–0.24V–0.05Nb, in wt.%) has been widely used as water walls in ultra-supercritical (USC) power plants. However, high reheat cracking susceptibility of T23 steel hazarded the safety of USC power plants. It is aimed to improve the reheat cracking susceptibility of T23 steel from the perspective of modifying chemical composition. Gleeble-3800 thermal simulator was used to simulate the coarse-grained heat-affected zone (CGHAZ) in T23 and modified T23 steels via thermal simulation of welding, and then evaluate the reheat cracking susceptibility in CGHAZ of T23 and modified T23 steels by the isothermal slow strain rate tensile test. The microstructure was systematically investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that the reheat cracking susceptibility of modified T23 steel is extremely improved. Compared with T23 steel, there are fewer M23C6 particles at the grain boundaries in modified T23 steel, and the degree of intergranular weakening is smaller. At the same time, not only MX in the grain interiors but also the solid-solutioned C and W in the matrix decrease in modified T23 steel, leading to the decline of intragranular strengthening. In addition, small grain size is beneficial to the improvement of the reheat cracking susceptibility of modified T23 steel. The decreased number of M23C6 at grain boundaries helps to retard the formation and propagation of reheat cracks because of decreased denuded zones.