Study On Subcritical Normalizing On T24 Steel Rcheat Cracking Susceptibility

Bainitic T24 steel is widely used in ultra supercritical boiler water wall with excellent high temperature oxidation resistance. corrosion resistance. low cost and easy processing. Whereas, it often prodeces reheat cracking in CGHAZ (coarse grain heat affected zone) during operation. Reheat cracking initiate and expand under the welding residual stress. That is because of strain can not be released within crystal due to intragranular strengthening, M/A islands at grain boundaries break down and prodece ferrite and carbides during operation. which cause the grain boundary weakening and stress concentrate at the softening phase of ferrite. Therefore. it is particularly important for T24 steel reheat cracking susceptibility to control the content. size and distribution of M/A islands.T24 steel of the delivery state (normalizing I tempering) is treated by 850 ℃,900 ℃ subcritical normalizing and 950 ℃ normalizing. The microstructure and properties are studied by OM、SEM、XRD、TEM、EDS、hardness、impact、tensile and so on, and the role of M/A islands in T24 steel is analyzed. Then, in order to get the tissue of CGHAZ, T24 steel of normalized, delivery state and subcritical normalized are treated by welding thermal simulation. Finally. the microstructure and properties of tissue in CGHAZ are analyzed. At the same time the reheat cracking susceptibility is studied. Research indicates that (1) M/A islands in CGHAZ is the most important effected factor for T24 steel reheat cracking susceptibility, that is because of M/A islands decomposed to generate M3C and the softening zone of ferritic. (2) The reheat cracking susceptibility reduced after T24 steel (supply state) was treated by subcritical normalizing or normalizing. (3) The decrease of content and size of M/A islands in CGHAZ is the most important reason, after delivery state T24 suffered subcritical normalizing or normalizing was treated by welding thermal simulation. (4) T24 (supply state) suffered subcritical normalizing or normalizing was dealt with welding thermal simulation, and the content of alloying elements in austenite in CGHAZ is increasing than T24 (supply state) under the effect of thermal cycles, and it will makes the hardenability higher, which results in the decrease of content and size of M/A islands.Therefore the reheat cracking susceptibility reduced after T24 (supply state) suffered subcritical normalizing or normalizing was treated by welding thermal simulation, that is because of the decrease of content and size of M/A islands in CGHAZ lead to the reduction of softening zone of ferritic.