Research On The High-temperature Steam Oxidation Behavior Of G115 Steel Used For Power Plant Boiler

G115 steel was a new type of martensitic heat-resistant steel developed independently in China.It was the only selectable material used for boiler header and main steam pipe in the new 630℃ ultra-supercritical power plant.The development of ultra-supercritical thermal power technology will strongly support China’s energy conservation and emission reduction,carbon peak,carbon neutralization strategy.Power plant boiler materials work in the high temperature and high pressure steam environment during the long-term service.Their oxidation problems were usually more serious than in normal enviroments.Accelerated oxidation will bring problems such as over-temperature,pipe blockage,and erosion of turbine blades that affect production safety.Study on high temperature steam oxidation behavior can provide data support for engineering application of G115 steel and safety operation of power plant.Therefore this research has important theoretical and engineering significance.The steam oxidation experiment was carried out in the high temperature steam oxidation test platform,in which G115 was exposed nearly 3 000 h.The experimental temperature was 650℃ the pressure was close to normal pressure,and the oxygen content was about 30 μg/L,with P92 steel as the reference material.According to the oxidation weight gain at different times,the steam oxidation kinetics results of G115 were obtained.The morphology and structural characteristics of the oxide scale were analyzed by SEM and TEM.The oxidation products were identified by EDS,XRD and SAED,and the high-temperature steam oxidation mechanism of G115 steel was explored.At the same time,the effects of steam temperature,grain size and alloy elements Co,Cu and Si on the high-temperature steam oxidation behavior were studied.Two pre-oxidation treatments were used to further improve the oxidation resistance of G115 steel at 650℃,and good results were achieved.Results showed that the high-temperature steam oxidation kinetics of G115 steel at 650℃ was close to or slightly lower than the cubic law,the oxidation rate constant was 1.8-2.6,and the time index was 0.235-0.337.The oxidation weight gain trend of G115 steel was very slow,and weight gain of G115 was much lower than that of P92 steel under the same experimental conditions.The analysis of oxide film structure showed that the oxide scale of G115 steel consisted of outer layer,inner layer and ’external oxidation’ layer.The ’external oxidation’ layer was a substantial Cr-rich layer,which was composed of FeCr2O4.The‘external oxidation’layer was continuous and dense,and acted as a barrier to the reactants,and therefore significantly improved the high temperature steam oxidation resistance of G115 steel.The existence of alloying elements Co,Cu and Si was beneficial to the formation of the ’external oxidation’ layer.Increasing steam temperature or decreasing grain size could improve the protection performance of the ’external oxidation’ layer and improved the long-term oxidation resistance of G115 steel.The results of this study confirmed for the first time that the external oxidation’layer in 9%Cr steel could be composed of FeCr2O4.According to the evolution characteristics of oxide film structure,the incubation period of the ’external oxidation’ layer was proposed for the first time,and it was pointed out that the incubation period was related to the critical Cr value of inner/outer oxidation transition.Combined with the results of oxidation kinetics and the evolution characteristics of oxide scale,it was pointed out that steam temperature,grain size and alloying elements could affect the protection performance of the ’external oxidation’ layer.