Chemical Composition Optimization, Microstructure And High Temperature Strengthening Mechanisms Of Nimonic80A For Ultra-supercritical Steam Turbine Blade

Due to the high efficiency, reduced environmental pollution and high reliability,ultra-supercritical coal-fired technology becomes the world leading andcommercialized power generation technology. The ultra-supercritical technology withhigh capacity, parameters and efficiency is the development trend of coal-fired powerplants. With the development of ultra-supercritical plants, it requires advanced hightemperature materials which can be used at higer temparture and stress. Nickel-basedsuperalloys show high strength and have wide applications at high temperature due tothe stable microstructure. Nimonic80A has been developed for more than half acentury and is a kind of precipitate strengthening alloy with excellent creep andoxidation resistance properties. Nimonic80A was mainly used as blade material inaero-engine which required shorter service time than that in steam turbine in the past.The technical requirements for ultra-supercritical steam turbine blade are very strictand the stress-rupture life of Nimonic80A at750°C/310MPa must be higher than100h in factory. So the chemical composition and heat treatment have been optimized inorder to form a stable manufacturing process for mass production in ultra-supercriticalsteam turbine.Atom probe tomography （APT）, high resolution transmission electronmicroscopy （HRTEM）, scanning electron microscopy （SEM）, X-ray diffraction （XRD）and thermal-calculation were employed to study the effects of alloying elements onroom temperature mechanical properties, stress-rupture properties at750oC/310MPa,long-term stress-rupture properties at600°C/450MPa and creep properties at750oC/240MPa of Nimonic80A. The chemical compositions have been optimizedand the effects of heat treatments on mechanical properties have also beeninvestigated. The conclusions are shown as follows:1) The volume fraction of γ′phase increases with the increase of Al+Ti contentfrom2.8to4.5wt.%, which improves room temperature tensile strength. Thestress-rupture life is the longest when Al+Ti content is about4.05wt.%. Room temperature tensile strength increases significantly with the increase of Ti/Al ratiofrom0.14to4.0, but stress-rupture life first increases then decreases and the longeststress-rupture life appears when Ti/Al ratio is about1.22. The β-NiAl phase canprecipitate in the grain interior when Ti/Al ratio is low, which can easily lead to thecracking in the grain interior and finally the fracture of alloys at high temperature. Theη-Ni₃Ti phase can precipitate at grain boundary when Ti/Al ratio is high, which canlead to the grain boundary cracking. Hence, low or high Ti/Al ratio is harmful tostress-rupture life and the ductility at high temperature is also bad for the alloy withhigh Ti/Al ratio. Room temperature tensile strength increases slightly with theincrease of Al contents from1.4to1.8wt.%, while the corresponding elongation andreduction in area decrease slightly. Stress-rupture life increases significantly with theincrease of Al content, and the high temperature ductility is also improved. Theincrease of Ti content from1.8to2.7wt.%can increase the volume fraction of γ′phase, which improves room temperature tensile strength, however, stress-rupture lifefirst increases then decreases. The longest stress-rupture life appears with good hightemperature ductility when Ti content is about2.25wt.%. But the η-Ni₃Ti phase canprecipitate when Ti content is too high. The η-Ni₃Ti phase can precipitate at grainboundary when Al+Ti, Ti/Al ratio or Ti contents are too high, the brittle η-Ni₃Ti phasecan easily lead to grain boundary cracking, which reduces stress-rupture life but doesno harm to room temperature tensile properties.2) The content of Cr₂₃C₆carbide at grain boundary and in the grain interiorincreases with the increase of C content from0.01to0.10wt.%, which suppressesgrain boundary sliding and growth of grains, so room temperature tensile strength andstress-rupture life can be improved. The grain boundary Cr₂₃C₆carbide exhibits anorientation relationship with γ matrix when C content is about0.10wt.%, which isbeneficial to grain boundary strength at high temperature. What is more, the increaseof C content can also increase the lattice misfit of γ′/γ phases. The longeststress-rupture life appears with high ductility when C content is about0.10wt.%.3) Nb can increase the precipitate temperature and volume fraction of γ′phase inNimonic80A, which improves room temperature mechanical properties, stress-rupture properties at750°C/310MPa and creep properties at750°C/240MPa.The replacement of Ti by Nb and the precipitate of （Nb, Ti） C reduce Ti content in γ′phase. The diffusion of Nb element from γ to γ′phase makes the co-existance of fineγ′2phase with coarse γ′1phase at high temperature. The δ-Ni₃Nb phase precipitates inγ matrix with an orientation relationship with （Nb, Ti） C carbide on {100} atomicplanes at845°C:（100）δ||（100）MC&[010]δ||[010]MC. The precipitate of （Nb, Ti） Ccarbide at grain boundary or in the grain interior can respectively suppress grainboundary sliding and growth of γ′phase. At the same time, precipitate of （Nb, Ti） Ccarbide can also suppress the connection of grain boundary cavitation.4) Four heat treatments T1（1070oC×8h, AC+700oC×16h, AC）, T₂（1070oC×8h, AC+980oC×4h, AC+700oC×16h, AC）, T3（1070oC×8h, AC+845oC×24h, AC+700oC×16h, AC） and T4（1070oC×8h, AC+980oC×4h, AC+845oC×24h, AC+700oC×16h, AC） have been designed to study the mechanicalproperties and microstructures of Nimonic80A. The results show that heat treatmentT₂or T3can increase room temperature tensile strength and that room temperaturetensile strength is highest after heat treatment T3compared with that after heattreatment T1. However, room temperature tensile strength decreases after heattreatment T4. Stress-rupture life after heat treatment T₂has slightly decreased, andstress-rupture life decreases significantly after heat treatments T3and T4. Althoughstress-rupture life decreases after heat treatments T₂and T4compared with that afterheat treatment T1, heat treatments T₂and T4can significantly improve hightemperature ductility. The γ′phase exhibits a coherent orientation relationship with γphase after heat treatments and stress-rupture test. The fine γ′phase shows anapproximately spherical shape and the coarse γ′phase （>75nm） shows anapproximately cubic shape. The increased volume fraction of γ′phase with averagesize about16²0nm can increase stress-rupture life. The precipitate of blocky carbideat grain boundary can suppress grain boundary sliding and hence increasestress-rupture life. The coarse γ′phase facilites the movement of dislocations andimproves high temperature ductility. The precipitate of multi-modal size distributionsof γ′phases benefits stress-rupture life and ductility at high temperature. 5) The long-term stress-rupture life at600°C/450MPa increases from4512h ofsample with non-homogeneous grain structure to6863h of sample with abnormalgrain structure and10765h of sample with homogeneous grain structure. The stable γ′phase exhibits a coherent orientation relationship with γ phase during long-termstress-rupture test. The Cr₂₃C₆carbide firstly precipitates at grain boundary thenprecipitates in the grain interior. With the prolonged stress-rupture life, plate likeCr₂₃C₆carbide transfers into strip like and approximately spherical ones, and Cr₂₃C₆carbide exhibits an orientation relationship with γ matrix. The plate Cr₂₃C₆carbidecan easily induce grain boundary cracking at the transitional area of large and smallgrain regions. The average size of TiC carbide at grain boundary grows up slightlywith the increase of long-term stress-rupture life. However, the TiC carbide in thegrain interior is very stable. The microstructure of homogeneous grain structure isbeneficial to long-term stress-rupture life of Nimonic80A.The above results indicate that the suggested optimum chemical compositions ofNimonic80A are: C0.06⁰.08wt.%, Al1.7¹.8wt.%, Ti2.0².5wt.%, Ti/Al ratio1.2¹.4, Cr19²0wt.%, B80ppm, Mg50ppm. And the further addition of1.5wt.%Nb can also improve the high temperature mechanical properties.