Effect of Aging Temperature and Time on the Structure and Precipitated Phase of TP304H Stainless Steel
TP304H stainless steel has high thermal strength and good oxidation resistance, widely used in the high temperature section of boiler superheaters and reheaters over 600℃, and the maximum operating temperature can reach to 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 structure 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 online, and improving the material itself. For this reason, through the high temperature aging simulation test, the influence of aging temperature and time on the structure and precipitation phase of TP304H stainless steel is studied, which provides a reference for the safe service of TP304H stainless steel.
The delivery condition of the test material is solution annealing, that is, air-cooled or air-cooled 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 with an optical microscope, and the structure is analyzed by 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 is corroded by alkaline potassium permanganate solution, and the presence of σ phase after aging of TP304H stainless steel is determined by observing whether there are orange-red spots on the surface of the sample under the metallographic microscope. Research indicates:
- 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.
- 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, and the area fraction of precipitated phases, namely the total amount of precipitated phases, respectively conforms to the functions S650=0.084t0.454, S700= 0.281t0.327, S750=0.313t0.338.
- After aging of TP304H stainless steel at 650℃ and 700℃ for 30 days, the precipitated phases are mainly carbides. After 60 days of aging, there are very few σ phases 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.
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