Limitation of TP304H steel group and the influence of precipitated phase
TP304H austenitic stainless steel with high temperature and oxidation resistance, widely used in more than 600℃ high temperature of boiler superheater and reheater section, the highest temperature can reach 760℃. TP304H steel use, to a certain extent solved due to the chamber of a stove or furnace flue gas temperature difference and cause of overtemperature tube, improves the safety of boiler operation.
But TP304H steel at high temperature for a long time running process organization change, lead to material aging. So the TP304H austenitic stainless steel under the condition of high-temperature operation of the organizational transformation and its influencing factors, for the running time of reasonable arrangement of materials, online monitoring of pipeline damage degree and to improve the material itself is of great significance in such aspects.
Therefore, through the high-temperature aging simulation test, the aging temperature and time on microstructure and the influence of precipitated phase TP304H steel, provide a reference for TP304H steel security service.
High-temperature aging simulation test in power plant with TP403H steel tube, its main chemical components such as table 1.
Table 1 test with TP304H steel chemical composition (mass fraction, %)
| C | Mn | Si | Ti | S | Mo | Cr | Ni | Fe |
|---|---|---|---|---|---|---|---|---|
| 0.08 | 1.53 | 0.44 | 0.15 | 0.03 | 0.31 | 18.05 | 7.95 | Balance |
Material delivery status after solid solution treatment, namely 1060 ~ 1070℃ heat preservation for 15 ~ 30 min after air cooling or air cooling, for single-phase austenitic organization. This test by raising temperature accelerates TP304H steel aging and aging temperature for 650, 700, and 750℃, the aging time is 30, 60, and 150 d, respectively by aging simulation studies in the organizational change features of the TP304H steel in the long run.
High-temperature aging simulation sample and original sample after grinding, polishing, aqua regia corrosion, using the optical microscope observation of grain size, two 400 scanning electron microscopy (SEM) is adopted to improve the organization analysis, so as to observe the sample organization form, and the Image-Pro Plus software for quantitative analysis of microstructure, distribution and characteristics of precipitated phase contrast, were analyzed by using SEM with eds.
Using alkaline potassium permanganate solution corrosion specimens, metallographic microscope specimens were observed by surface whether their orange spots, to determine the existence of the σ phase after TP304H steel aging. The results show that:
(1) original TP304H steel group for austenitic, twin boundary is clearly visible; After high-temperature aging is gradually increasing grain size, grain boundary coarsening, twin, abnormal grew up grain increase.
(2) TP304H steel in 650, 700, and 750℃ aging process, the amount of precipitated phase extension of the power function increased with time, the total score that precipitate phase precipitated phase area respectively in accordance with function S650 = 0.084 t0.454, S700 = 0.281 t0.327, S750 t0.338 = 0.313.
(3) TP304H steel aging after 30 d, respectively, in 650, 700℃, mainly for carbide precipitation, precipitation after aging 60 d division, still have very little sigma phase carbide, main ingredients for Fe, Cr; After 750℃ aging 30 d, precipitated phase number increased significantly, mainly for carbide, with a small amount of σ phase.