Microstructure And Property Evolution Of Super304H Steel For Ultra Supercritical Boilers During High Temperature Creep

Super304H austenitic heat-resistant stainless steel has been widely used for superheater and reheater tubes in ultra supercritical(USC) unit since its developedment in nineteen eighties and put into use in the nineties of the 20 th century. It is well known that the superheater & reheater tubes in USC boilers are prone to microstructure degradation and oxidation, which will lead to mechanical property decrease and may cause tube failure. Because of the relatively short period of practical application of Super304 H steel, it is difficult to obtain its specimens of long time service for research. Therefore, no systematic microstructure observation and property evolution at different service time of Super304 H steel were reported, especially the evolutions of carbides and creep holes or cracks, which are very essential for judging the degradation level of the in-service Super304 H tubes in USC boilers.In this thesis, microstructure and property evolutions of Super304 H steel were investigated from the following two aspects:on one hand,two serviced specimens were collected from power plants and were studied in details. On the other hand, accelerated creep tests were carried out to study the evolutions of microstructure and properties of Super304 H heat resistant steel during high temperature creeping. The microstructural changes of Super304 H steel at different creeping time were investigated to find out the evolution rules of microstructure and creep damage during creeping, which aims to establish a reference metallographic atlas for evaluation of degradation and creep damage levels of Super304 H steel.Research on serviced Super304 H steel tubes shows that grain size of the tube after 36891 h service is extremely large with grade 2, while the tube after 41000 h service and the as-received tube both have a small grain size of grade 7-8. M23C6 carbides precipitate in a chain distribution along grain boundaries of serviced tubes. A small amount of ferromagnetic phase distribute along part of grain boundaries and magnetization has been observed in the tube after 36891 h service whose magnetization intensity is higher than the tube serviced for 41000 h and as-received tubes. The morphologies after DL-EPR test show that the intergranular corrosion susceptibility of the 41000 h serviced tube was higher, but the depth and width of the Cr-depleted zone at grain boundaries were significantly smaller than the 36981 h serviced tube. According to the analysis, we believe that the precipitates will not cause significant changes in the magnetic properties of Super304 H after service. Grain coarsening, which leads to reduction of the diffusion channels, makes it difficult for Cr to diffuse from the matrix to Cr-depleted zones. As a result, the Cr content in the Cr-depleted zones maintains a lower level, which will lead to the increase of Ms of the Cr-depleted zone. Thus it finally forms ferromagnetic martensite during cooling.Accelerated creep experiments of Super304 H steel crept at 650 ℃ /195 MPa of Super304 H steel were carried out and the results show that the microstructure of both as-received and crept specimens is composed of γ phase with a similar grain size of grade 7-8 and a small amount of precipitates, and no obvious grain coarsening was observed. M23C6 carbides precipitated along grain boundaries in a chain distribution and got coarsened with creeping time increase. Creep holes formed near the surface when the specimen crept for 2500 h. Afterward creep holes increased, developed, interconnected and finally formed micro cracks over the whole grain boundaries in Super304 H steel when it crept over 3500 h. Intergranular corrosion susceptibility of Super304 H steel increased first and then gradually declined, indicating that the steel had experienced a sensitization- desensitization process during the creeping process. The hardness of Super304 H steel increased dramatically first and then tends to reached a stable state during the creeping process. Hardness of the loadingsection specimen were higher than the clamping-end of creep specimens because of work hardening in the loading section during creeping. Reference metallographic atlas for evaluation of microstructure aging degradation and creep damage levels were preliminary established according to the microstructure and property evolutions of Super304 H steel during accelerated creeping.