Research On The Microstructure Evolution And Mechanical Properties Of Incoloy 800H During Hot Processing

Incoloy 800H is a typical nickel-base corrosion resistant alloy with solid-solution strengthening,which is characterized by good creep-rupture,good corrosion resistance and metallurgical stability at elevated temperatures.Therefore,Incoloy 800H is widely utilized in the field of chemical engineering,petroleum industry and nuclear power.The chemical composition of Incoloy 800H contains a large number of solution strengthening elements like Ni,Cr and Ti,which will contribute to the formation of different precipitates within a certain temperature range.It has a complicated effect on the hot deformation behavior.Furthermore,the main difficulties on hot processing of Incoloy 800H can be concluded as follows:a large deformation resistance,a narrow processing temperature range and a poor microstructure control.In recent years,the demand for high quality Incoloy 800H in China is rising continuously due to the rapid development of nuclear power industry and petroleum processing industry.It is of great importance to master the advanced manufacture and processing technology of Incoloy 800H.So there is theoretical significance and application value in the intensive research on the microstructure evolution and mechanical properties of Incoloy 800H during hot processing.In order to optimize the hot processing technology of Incoloy 800H,constant strain rate compression tests and strain rate jump compression tests were performed on thermal mechanical simulator and microstructure analyses were conducted by using optical microscopy(OM),scanning electron microscopy(SEM),electron backscatter diffraction(EBSD)technique,transmission electron microscopy(TEM)and electron probe micro-analyser(EPMA).The dynamic strain aging(DSA)behavior,which results in the plastic instability,was investigated to reveal the occurrence mechanism and determine the corresponding deformation conditions.The hot deformation behavior at different temperature ranges was studied to establish mathematical models which can be used to predict the evolution of microstructure and properties.Besides,the related deformation conditions for occurrence of adiabatic shear bands(ASBs)were concluded to avoid plastic instability during hot deformation.The hot deformation behavior with high strain rates was explored to reveal the nucleation mechanism of dynamic recrystallization(DRX).The isothermal aging treatments followed by hot deformation were performed to simulate the effect of applied loading stress on the alloy under working conditions.The morphology and distribution of precipitates were observed and discussed before and after deformation.Additionally,the microstructure and mechanical property were investigated at different temperature regimes of hot rolling and solution treatment by utilizing various microscopic analysis techniques.A great attention was paid to studying the evolution of micro-texture,E3n(1≤n≤3)boundaries and precipitation during hot rolling.The major innovative work can be concluded as follows:1.The DSA behavior during hot deformation was systematically investigated for the first time.It can be inferred that DSA takes effect at temperature range of 500～750℃ with a strain rate of 0.001s-1,at temperature range of 550～790℃ with a strain rate of 0.01s-1,and at temperature range of 600～840℃ with a strain rate of 0.1 s-1,.These deformation conditions should be avoided in order to optimize the hot processing technology.The strain rate jump tests were performed properly to calculate strain rate sensitivity values under different deformation conditions.The "Boundary slope method" was proposed to determine activation energy of DSA(194kJ/mol),indicating that DSA was caused by the pipe diffusion of substitutional solute atom(Ni)through the dislocation core.2.The stimulation temperature for DRX was selected as 1000℃ according to the dissolution temperature of carbides/carbonitrides(Cr23C6 and Ti(C,N))and variation of flow stress with deformation temperatures.Accordingly,the hot deformation behavior was studied at two different temperature ranges,namely low temperature regime(825～975 ℃)and high temperature regime(1020～1200℃).It is noted that DRX was accelerated with increasing strain rates larger than 1s-1 which could be attributed to the increased rate of dislocation accumulation and the adiabatic temperature rise.Based on the adiabatic correction for the true stress-true strain curves,an Arrhenius-type constitutive model coupled with the effect of true strain was established to predict flow stress at elevated temperatures.Based on the EBSD analysis of microstructure,it is proposed that the grains with a grain orientation spread(GOS)value less than 1° can be considered as DRX grains,which provides a reliable method to calculate volume fraction of DRX(Xdrx).It can be inferred that the average local misorientation(LM)is inversely proportional to Xdrx and the maximum LM values are located in the vicinity of grain boundary.3.The hot deformation behavior with strain rates of industrial application level(5-30s-1)was investigated systematically.The measured value of adiabatic heating increases with increasing strain rate or decreasing temperature.The deformation conditions related to the occurrence of ASBs,which contributes to the plastic instability during hot deformation,were determined to optimize the processing technology.At higher temperatures,repeated bulging of grain boundaries can be observed within original deformed austenite grains and DRX grains,which leads to the appearance of multiple DRX grains structure.A remarkable fraction of annealing twins can be found in the recrystallized regions,which can promote the expansion of the recrystallization front in the course of DRX and also help in separation of the bulged region from the parent grain.This would tend to suggest that a novel DRX nucleation mechanism-"multiple bulging and twinning" is the predominant nucleation process during the hot deformation at higher temperatures.Furthermore,it is clear that some newly formed DRX grains can be observed near the large TiN particles.4.In this work,the influence of hot rolling and solution treatment temperatures on the microstructure and property was studied.It is evident that some segregation bands of Ti element can be observed along the rolling direction.At the solution temperature of 1050℃,substantial fine Ti(C,N)particles form on the grain boundaries near the Ti segregation bands and impede the grain boundary migration,which can result in the alternative distribution of coarse grains layers and fine grains layers.Additionally,the evolution rule of E3" boundaries was proposed through investigation on the ratio of ∑3/∑(9+27)boundaries.It is suggested that the proliferation mechanism for ∑3n boundaries changes from ∑3 regeneration model to new twinning model with the increasing of solution temperature.5.The flow stress increases with increasing strain rate during hot deformation after isothermally aged at 750℃.Dynamic recovery plays a dominant role in the softening process.The peak stress decreases with the prolonged aging time due to the effect of precipitation on the grain boundary.After aging for 50h,an obvious segregation of Cr and C elements can be observed at the grain boundary,but the segregation level of Ti element is much weaker than the above two elements.Meanwhile,the depletion of Ni element can be found at the grain boundary.Most of the carbides are observed at incoherent ∑3 boundaries and general grain boundaries,some of which are needle-like carbides and grow parallelly with a certain angle at the both sides of grain boundary.The segregation of above-mentioned elements becomes heterogeneous and the depletion of Ni element becomes unconspicuous with sustained deformation.Meanwhile,the growth direction of needle-like carbides is not parallel any more.The distribution of carbides at the grain boundary can increase the LM and as well changes the misorientation profile along grain boundary.