Research On Intergranular Corrosion Resistance Of A Novel Supcr304H Heat-resistant Stainless Steel For Boiler

The pressure and temperature of steam in boilers has been increasing constantly with thedevelopment of thermal power plants, and thus higher requirements for the properties ofheat-resistant steels used for boiler tubes are brought forward. Therefore, a novelheat-resistant stainless steel, the Super304H steel, was firstly developed by Japan in recentyears. Nowadays it has been widely applied in ultra supercritical fossil fired boilers tofabricate superheater and reheater tubes because of its more superior high temperatureperformance and welding performance compared with traditional TP304H steel. But the highcarbon content and relatively complicated heat treatment processes have made Super304Hmore susceptible to intergranular corrosion （IGC）and up to now little systematic researchconcern has been focused on this issue.In this paper, the as-received17Super304H tubes from different batches supplied bySumitomo Metal Industries of Japan were concentrated as the research target. Firstly, thechemical compositions, microstructures, IGC states and hardness of Super304H tubes wereinvestigated and discussed by spectrometer, immersion test in H₂SO₄-CuSO₄-Cu, OpticalMicroscope （OM）, Scanning Electron Microscope （SEM）, Energy Dispersive Spectrometer（EDS）, X-ray Diffraction （XRD） and durometer. Then the influence of chemical compositionincluding carbon content, niobium content and Nb/C mass ratio on IGC susceptibility ofSuper304H was systematically analyzed with statistical method. Finally, the effect of heattreatments including solution, stabilization and softening processes on microstructures andIGC resistance of Super304H were deeply investigated through microstructure analysis,carbides extraction and electrochemical experiments.Research results in this paper suggest that there exist four IGC levels in the as-receivedSuper304H tubes. In details, seven of the as-received tubes, whose IGC status is level Ⅱ,have a relatively high IGC susceptibility. And two tubes have IGC levels of Ⅲ and Ⅳrespectively, while the rest eight tubes have no IGC sensibility, with IGC state to be levelⅠ.The microstructures of Super304H tubes with different IGC states are all made up ofmatrix γ-phase and a lot of precipitates. The grain sizes of matrix γ-phase are maintainedbetween grade8and10and the precipitates include amounts of dispersed carbides andlath-shaped Nb-rich phase with superior high-temperature stability. Besides, there are a lot ofglobular M₂₃C₆（M=Fe, Cr） carbides forming on grain boundaries in the tubes with the Ⅲ andⅣ IGC levels. The hardness of as-received Super304H tubes vary from HV227to293. The change of IGC susceptibility is revealed to be irregular with the increase of Nbcontent under the same level of C content. In addtion, the IGC resistance also tends to beirregular with the increase of C content under the same level of Nb content. Judging from theinfluence of Nb/C ratio on IGC susceptibility of Super304H, it can be concluded that thesample would have no IGC susceptibility when its Nb/C ratio reaches8.15and above,indicating that the minimum Nb/C mass ratio needed to entirely suppress IGC susceptibility is8.15while the sample with Nb/C less than5.00would be easily sensitized. Meanwhile, theIGC states of as-received tubes tend to be diverse when Nb/C ratio vary between5.00and8.15and the IGC states of Super304H in this Nb/C region are determined by the extent ofNb（C,N） precipitation during the process of as-received heat treatment.The IGC susceptibility of4#tube, whose Nb/C mass ratio is7.22, can be eliminatedthoroughly after solution treatment at1150℃for15minutes, while6#tube with Nb/C of5.68is still subjected to high IGC susceptibility after solution treatment. After being stabilizedfrom950℃to1100℃on the basis of solution treatment, the grain level of6#tube can beretained at the range of grade7to9. There are many globular M₂₃C₆（M=Fe, Cr） precipitatesdistributing along grain boundaries when stabilized at950℃, while the precipitates are foundto dissolve into the matrix at1000℃and higher temperature. What is more, compared withthe solution state, the amount of dispersed Nb（C,N） precipitates increases significantly afterstabilization treatment and its precipitation speed reaches the maximum at1100℃. In theaspect of IGC susceptibility, the IGC resistance has been improving with the increase ofstabilization temperature. While950℃is still a sensitizing temperature for M₂₃C₆（M=Fe, Cr）precipitation for Super304H steel, the IGC resistance is obviously superior to that of thesolution state when stabilizing temperature reaches1100℃, which is in the range of solutiontemperature （1050℃to1150℃） for Super304H. Therefore, it can be drawn that the deliverystate solution treatment of Super304H plays a dual function of solution and stabilization.In addition, the lath-shaped Nb-rich phase in Super304H samples inclines to graduallydissolve into the matrix when softened at1300℃. But it should be paid attention that theNb-rich phase cannot be dissolved absolutely unless the soaking time of softening treatmentmaintains beyond60minutes.