An Investigation On Microstructure And Properties Of Hot-Rolled Fe-36Ni Alloy

Fe-Ni based low expansion alloy has attracted much attention due to its low thermal expansion coefficient.But this kind of alloy has the problem of low strength.How to give a good balance between the low coefficient of thermal expansion and high strength and to apply the functional material as special structural materials have become a hot topic to researchers.In view of the above problem,the traditional method is to add alloying elements through the second phase to improve its strength,but it will often have a great impact on the low expansion properties of the alloy.In this paper,hot-rolled Fe-36Ni alloy as the research object.Improving the strength and plasticity of the alloy by asynchronous hot rolling(ASR)under the premise that the expansion coefficient of the alloy is not sacrificed.Furthermore,asymmetric rolling(ASR)is one of the methods of severe plastic deformation,which can be used to refine the grain.So this thesis has carried on conventional rolling(CR),asymmetric rolling(ASR)and solid solution heat treatment to comparative study the microstructure characteristics and mechanical properties of the alloy,such as average grain size,recrystallization volume fraction,annealing twin proportion and so on by optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),electron probe,phase change expansion instrument,electronic universal tensile testing machine,microhardness tester.The main search contents are shown as follows:(1)Using thermal expansion method,the thermal expansion coefficient of hot-rolled Fe-36Ni alloy was measured below the Curie temperature and analyzed the mechanism of abnormal thermal expansion behavior.(2)Using MMS-300 thermal simulation experiment machine for single pass compression test,the flow stress curves of the alloy were measured.The influence of the deformation temperature,strain rate and deformation degree on the flow stress and organization evolution of the alloy were researched.Recrystallization activation energy of the alloy was obtained by regression calculation and the constitutive equation of hot deformation was established.According to the deformation resistance of Jihua Zhou,the mathematical model of deformation resistance was established.(3)The effect of conventional hot rolling(CR)and different temperatures of solid solution heat treatment on the microstructure and properties of Fe-36Ni alloy was investigated.It was found that most of the grain rolled in the shape of the flattened elongated fibers along the rolling direction After solid solution heat treatment the grain was equiaxed coupled with the uniformity of microstructure was greatly improved,the proportion of recrystallization and the number of annealing twins increased gradually but the tensile strength decreased gradually with the increase of temperatur.(4)Different speed ratio(1.1,1.2)of asymmetric hot rolling(ASR)and different temperatures of solid solution heat treatment experiments were carried out.Firstly,the effect of conventional hot rolling and asymmetric hot rolling on microstructure and mechanical properties of Fe-36Ni alloy were studied.It was shown that ASR can not only effectively refine the grain size and make the grain size gradient distribution from the surface to the center layer,while grain size along the thickness direction of CR is almost no change.Also,the proportion of recrystallization and annealing twins were largely increased,and the strength combined with the plasticity of the alloy were greatly improved.In particularly,when the speed ratio is 1.2,tensile strength and elongation were increased by 22MPa,8.44%respectively.Moreover;the uniformity of microstructure of the asymmetric hot rolled plate was greatly improved by solution treatment,but the mechanical properties were affected to some extent.