| Incoloy series nickel base corrosion resistant alloys have good acid and alkali corrosion resistance,stress corrosion cracking resistance and oxidation resistance,and they are widely used in nuclear industry,chemical industry,boiler pipes,medium heat exchangers and condensers.Nickel based corrosion resistant alloy has high element content,large deformation resistance,easy to produce cracks and inhomogeneous structure in hot piercing,which seriously restricts the subsequent processing.There are a lot of twins in the structure of nickel base corrosion resistant alloy,and the characteristic distribution of grain boundary has a significant effect on its mechanical properties and corrosion resistance.In this paper,the microstructure evolution of nickel based corrosion resistant Incoloy825 alloy during hot deformation was studied,the recrystallization mechanism of its mixed crystal structure region was revealed,and the optimal process parameters for obtaining uniform and fine hot deformation structure were established;By means of deformation and heat treatment,the characteristic distribution of the grain boundary of Incoloy825 alloy was adjusted and the layered heterogeneous structure was prepared.In-situ EBSD was used to observe the interaction between twins and dislocations during deformation,as well as the evolution of the heterogeneous interface,and its influence on the mechanical properties of the alloy was analyzed;The mechanism of isothermal oxidation and molten salt hot corrosion of Incoloy 825 alloy was studied,and the influence of special grain boundaries on the propagation path of molten salt hot corrosion was discussed.The main conclusions of the paper are as follows:The strain compensated Arrhenius equation can accurately describe the stress-strain curve of Incoloy825 alloy during hot deformation,and the correlation coefficient is 0.98861.The instability zone of Incoloy825 alloy is mainly located in the low temperature and high strain rate zone(950-1050℃,0.1-10 s-1),which is easy to form mixed crystal structure.The optimal hot working process parameters are:deformation temperature 1150℃+strain rate 1s-1.The discontinuous dynamic recrystallization mechanism is dominant during the hot deformation process of the alloy,and fine and uniform equiaxed recrystallization structure can be obtained.There are three dynamic recrystallization mechanisms in the mixed crystal structure:discontinuous dynamic recrystallization mechanism,continuous dynamic recrystallization mechanism and twin dynamic recrystallization mechanism.By cold rolling 5%+1000℃/15 min（GBE）treatment,the special grain boundary fraction in Incoloy825 alloy can be increased from 48%to 72.8%,and the grain size can be keep basically the same（～200μm）.At low strain rate(1.25×10-4 s-1),the special grain boundary has little effect on the strength of the alloy,the slip band can pass through the twin boundary,and the yield and tensile strength of GBE and solid solution sample are basically the same.At high strain rate(4.16×10-3s-1),the twin boundary obviously hinders the movement of dislocations,leading to the stacking and entanglement of dislocations near the twin boundary,forming a high stress concentration zone.The yield strength of GBE sample reaches 285 MPa,which is 16%higher than that of the solid solution state.With the increase of strain accumulation,the twin boundary rapidly transforms into a random grain boundary,and dislocations pass through the grain boundary to complete deformation.Incoloy825 alloy layered isomeric material can be prepared by large strain cold rolling+incomplete recrystallization process.Its yield strength is up to 550 MPa,and uniform elongation is up to 25%,with good strength plasticity matching.The heterogeneous structure is composed of soft and hard phase.The soft phase is recrystallized grain,and the hard phase is strip shaped deformation recovery structure.In the process of plastic deformation,the soft/hard interface is prone to cause stress concentration,forming the accumulation of a large number of geometrically necessary dislocations.Significant back stress strengthening occurs in the soft phase,while the hard phase produces a pre-stress that is balanced with the back stress.The combined effect of back stress and pre-stress makes heterogeneous materials have better strength plasticity matching than homogeneous materials.In the process of tensile deformation,the soft/hard interface is the origin of cracks in layered heterogeneous structure,and cracks propagate along the interface,leading to material fracture.At 950℃,the oxidation weight gain and oxidation time of Incoloy825 alloy show a parabola law,while the hot corrosion of 75%Na2SO4+25%Na Cl wt%mixed molten salt can be divided into three stages:the mixed salt film on the alloy surface begins to melt and permeate through the randomly large angle grain boundary;The molten salt starts to decompose,producing sulfur dioxide and sulfur trioxide.Chromium and titanium combine with oxygen to form a protective oxide film under the salt film;The oxidation sulfurization and oxidation chlorination reactions are carried out cyclically.Sulfur and chlorine elements constantly penetrate inwards along the random grain boundary,generating sulfide and chloride,forming holes and cracks in the oxide film and metal matrix,causing dissolution and fracture of the oxide film,and serious hot corrosion.The large angle grain boundary in the microstructure of Incoloy825 alloy is the fast channel of molten salt hot corrosion.During the corrosion process,obvious Cr depletion occurs at the large angle grain boundary.A high proportion of special grain boundaries can block the penetration path of molten salt hot corrosion.The existence ofΣ3 boundary is the key to block the corrosion path. |
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