The Study Of Fracture Mechanisms Of A Ferrite Heat Resistant Steel T91in Hot Rolling And Effects Of Heat Treatment On Its Microstructure And Properties

As a key material used in supercritical power plants, T91steel was firstly produced tenyears ago in China, but there are still many problems necessary to be solved in its fabricationprocess. Because of lacking an overall understanding of the formation of the microstructure inT91, it is difficult to make a reasonable processing procedure to ensure a high quality of theproducts.The basic problems, such as precipitated phases, Spinodal decomposition et al., arediscussed in this thesis, which may have important influence on mechanical properties of T91steel.The T91steel is a kind of alloy including many alloying elements which will affectproperties of T91steel by their interaction and their micro-distribution in T91.As is known thatM23C6, MX and other precipitated phases in T91,can strengthen the alloy. However, M23C6willbecome the resource of crack propagation while they aggregate. Lots of precipitates rich in Crhave been found in T91.In order to identify the precipitated particles, we have utilized all sorts ofexperiments as Optical Microscopic observation, SEM analysis, TEM analysis and X-raydiffraction analysis. It is found that these particles are spherical or sphere-like and theircrystalline-structure are proved non-cubic which is obviously different from that of M23C6. Theidentification of these precipitated phases in T91may need for further study. But here, it can beconcluded that the aggregation of these kind of unknown particles is the mainly reason for thecracks in the factory.After tempering the steel at480℃, many precipitated spherical particles rich in chrome canbe observed within grains, but this phenomenon is not found after tempering the steel at450℃,500℃and550℃.The alloy tempered at480℃has the highest micro-hardness and itsmicro-hardness is also much higher than that in as-received steel. This means that thetemperature,480℃,is a favorable temperature to for the precipitation and aggregation of thespherical phases and, so it is critical to keep T91steel away from staying long duration at480℃.