Influence Of Solid-solution On Microstructure And Mechanical Properties Of Superalloy K325

At present,the issue of environment pollution and energy crisis become increasingly critical stems from the coal-fired power plants.It was imperative,accordingly,to develop a more efficient coal-fired plants to achieve the objective of energy-saving and emission reduction.The 700?advanced ultrasupercritical(A-USC)coal-fired power plants,with a greater energyefficiency and running parameters,was considered as the most advanced and economical power-generation technology currently.K325,a Ni-Cr based casting superalloy strengthened by solid-solution,has been designed as the material of cylinder and valve casing of boilers in 700? A-USC power plants resulting from itsunique mechanical properties,weldability and corrosion resistance under aggressive environments.Nevertheless,there were not mature research on heavy superalloy castings any more than the solid-solution process techonology in both domestic and aboard yet.Consequently,the effects of solid-solution treatment on microstructures and mechanical properties of alloy K325,combined with the solidification behavior,were investigated in this work to provide experimental evidence and theoretical basis for the formulation of heat treatment system.The conclusions were as follows:The dendritic structure was clearly exhibited in as-cast sample of both alloy 1 and 2.The stripy and blocky MC-type carbides highly enriching in Nb and Ti in alloy 1 while MC carbides along with ?/Laves eutectic in alloy 2,respectively,were observed in interdendritic volumes where Nb segregated severely.Meantime,the solidification sequence of alloy K325 could be written as L?L+??L+?+MC??+MC in alloy 1 and L?L+??L+?+MC?L+?+MC+?/Laves??+MC+?/Laves in alloy 2 respectively according the DTA results.The Laves phase,highly enriching in Nb and Mo,impaired the mechanical properties drastically.The microstructures and mechanical properties were influenced significantly by the temperature,holding time and cooling method of solution treatment.The microstructure of alloy 1,under various solution condition,were composed of ? phase and MC carbides.Initially,with solution temperature rising,the dendritic segregation turned alleviated and the proportionand size of carbides reduced,additionally,the ultimate tensile strength(UTS)at room temperature(RT)increased slightly whereas the UTS at 700?and plasticity showed no obvious changes.Then,with the extension of solution holding time,the proportionof carbides showed a slight decrease.Additionally,the UTS raised gradually while the plasticity had no change both at RT and 700?.The optimal solution holding time was roughly 2h.Finally,the amountand size of MC carbides,compared with water quenching method,increased slightlyafter air cooling.And MC carbides could be observed in grain interior mostly by above cooling method.The UTS and plasticity at RT showed little changes.Nevertheless,the percentageand size of carbides increased sharply under furnace cooling condition,meanwhile,the tensile properties declined obviously owing to the secondary carbides precipitated along grain boundaries continuously.It was worth noting that with cooling rate rising,the UTS increased significantly whereas plasticity remained constant at 700?.Additionally,the fracture mechanism of alloy 1 after water and air cooling displayed a ductile fracture in transgranular manner but mixedfracture mode after furnace cooling.In general,the water quenching was regard as the most favorable cooling method.Solution treatment was conducted on alloy 2 at different temperature.The microstructure showed that a bulk of mesh-like eutectic Laves phase transformed into block-like after solution treated at temperatures below 1225?.After solution treatment at 1225?and 1250?,MC carbides non-decomposed dissolved into the matrix mostly,furthermore,the incipient melting of Laves phase could be discernedobviously.The tensile experimental results indicated that alloy 2 solution treated below 1250?,with solution temperature elevating,plasticity at RT gradually improved remarkably but little change at 700?.In addition,the UTS at RT remained nearly constant except declined slightly under solution condition at 1200?and 1225?,while decreased evidently at 700?.The alloy heat treated below 1250?,characteristics of dendritic structure were displayed on the fracture surface of tensile samples,meanwhile,the samples presented ductile fracture in transgranular manner by microvoid coalescence.After solution at 1250?,excessive quantities of Laves phase melted were observed in grain interior and boundaries,a transition of the fracture mode from transgranular to intergranular,which give rise to the mechanical properties dropped drastically at RT and 700?.