Effect of Aging Temperature and Time on Structure and Precipitated Phase of TP304H Stainless Steel
304H stainless steel has high thermal strength and oxidation resistance. It is widely used in the high temperature section of boiler superheaters and reheaters over 600℃, and the maximum service temperature can reach 760℃. The use of TP304H stainless steel, to a certain extent, solves the over-temperature tube burst caused by the large temperature difference of the furnace smoke and significantly improves the safety of the boiler operation. However, TP304H stainless steel is prone to structural transformation during long-term high temperature operation, resulting in material aging. Therefore, studying the microstructure transformation of TP304H austenitic stainless steel and its influencing factors when operating under high temperature conditions is of great significance for rationally arranging the running time of the material, monitoring the damage degree of the pipeline on-line, and improving the material itself. For this reason, through high-temperature aging simulation tests, the effects of aging temperature and time on the structure and precipitates of TP304H stainless steel are studied, which provides a reference for the safe service of TP304H stainless steel.
The supply state of the material is Solution Annealing , that is, air cooling or air cooling after holding at 1060～1070℃ for 15～30min, and the structure is single-phase austenite. This experiment accelerates the aging of TP304H stainless steel by increasing the temperature. The aging temperature is 650℃, 700℃ and 750℃, and the aging time is 30d, 60d and 150d, respectively. The structure change characteristics of TP304H stainless steel pipe in long-term operation are studied by aging simulation.
After the high-temperature aging simulation sample and the original sample are ground, polished, and corroded by aqua regia, the crystal grain size is observed by an optical microscope, and the structure is analyzed by the QUANTA 400 scanning electron microscope to observe the structure of the sample, and the Image-Pro Plus software is used Quantitatively analyze the microstructure, compare the distribution and characteristics of the precipitated phases, and use the energy spectrometer attached to the SEM for component analysis. The sample was corroded by alkaline potassium permanganate solution, and the presence of σ phase after aging of TP304H stainless steel was determined by observing whether there were orange-red spots on the surface of the sample under the metallographic microscope. Research indicates:
(1) The original structure of TP304H stainless steel is austenite, and the twin grain boundaries are clearly visible; after high temperature aging, the grain size gradually increases, the grain boundaries become coarser, the twins decrease, and the abnormally grown grains increase.
(2) During the aging process of TP304H stainless steel at 650, 700 and 750℃, the total amount of precipitated phases increases with the extension of time. The area fraction of precipitated phases, that is, the total amount of precipitated phases, respectively conform to the functions S650=0.084t0.454, S700= 0.281t0.327, S750=0.313t0.338.
(3) After aging of TP304H stainless steel at 650 and 700℃ for 30 days, the precipitated phase is mainly carbides. After 60 days of aging, there is a very small amount of σ phase in addition to carbides. The main components are Fe and Cr; aged at 750°C for 30 days Later, the number of precipitated phases increased significantly, mainly carbides with a small amount of σ phases.
General Properties
Chemical Composition
Resistance to Corrosion
Heat Resistance
Physical Properties
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