Study On Precipitation Kinetics Of ? Phase In HR3C Austenitic Stainless Steel

HR3C steel, as a new type of austenitic heat resistant steel, is widely used in supercritical unit as superheater and reheater tube for its high creep strength, especially excellemt high temperature oxidation resistance. Since the life of ultra-supercritical power plant equipment is 30 to 40 years, the component steels should have good stability. But when aging for long time under high temperature and high pressure service condition, microstructure and phase structure of component steel would degraded, resulting in the loss of operation safety and reliability, even early failure. For this reason, it is of great importance to study microstructure stability, mechanical properties changes and reliability during the process of serving of HR3C steel.At present, there have been a lot of researchs on precipitation of NbCrN, M23C6 and Nb(C, N) phases during serving and their influences on the properties of HR3C steel, only a few reseachers have reported a phase precipitation issue. The precipitation of a phase in HR3C steel was regarded to be restrained due to the composition design, but recent studies have revealed its problem of turning brittle after service. Our previous work focused on Sumitomo HR3C steel aged at 700?-800? and found the precipitation of a phase. It is generally believed that precipitation of a phase would significantly reduce mechanical properties and creep performace of materials, so more investigations needed to be conducted on a phase precipitation behavior and its effect on properties in HR3C steel.This work confirmed the precipitation of a phase in HR3C steel aged at high temperature by metallographic microscope, X-ray diffraction analyzer, scanning electron microscope and so on, and analyzed the kenitics behavior of ?-phase precipitation by chromogenic corrosion semi-quantitative method combined with Avrami equation. Besides, properties changes of HR3C during aging have been studied, and heat treatment process which could retard ? phase formation has also been preliminarily explored.The results showed that brittle ? phase would precipitate along grain boundaries after aging for enough hours at temperature range from 650? to 800?; The precipitation kenitic curves of a phase were obtained by chromogenic corrosion semi-quantitative method combined with Avrami equation. It indicated that the tip temperature of a phase formation "C" curves was around 750?, and when aged at 650? closed to actual service temperature, the transformation volume of a phase can reach 5% within 10500 hours; It was also found that ? phase precipitated more easily in Domestic HR3C samples than Sumitomo HR3C samples under same aging conditions; And when Domestic HR3C samples were pre-solution treated at 1200"C for 1 hour, precipitation of ? phase could be delayed in subsequent aging process; Through hardness test, impact toughness test and creep durable test on different states HR3C samples aged at 700?, it was found that when initial HR3C sample was directly aged up to 2000 hours, its hardness increased, impact toughness sharply decreased and creep rupture life considerably reduced; And when initial sample was aged after solution treated at 1200?, its impact toughness didn't become better, but creep rupture life was significantly improved.