Investiagtion Of Creep Properties And Fracture Mechanism Of Sanicro25 Steel Using Small Punch Test

The small punch test is a miniaturized testing method,which can evaluate material properties.It is suitable for evaluating the mechanical properties of complex components and limited materials.In this paper,Sanicro25 steel is taken,and the creep properties,microstructure evolution and fracture mechanism during small punch creep tests are studied by the means of experiment and finite element methods.The main contents of this paper are as follows:(1)Small punch creep(SPC)test and uniaxial creep(UC)test were conducted,and the load-stress transformation relationship between SPC test and UC test was established based on experimental data.It is found that there is a similarly trend between SPC curves and UC curves.The load-stress correlation was established according to CWA and EFS standards,and there is still some error compared with the expermental results.In addition,a modified sphere-cone model was proposed after analyzing the variation of the thickness of Sanicro25 specimens during SPC deformation.The calculated results were in good agreement with the calculated results of FE method.It was reasonable and feasible,and can simplify the calculated process.(2)Scanning electron microscopy technology,transmission electron microscopy technology and energy spectrum technology were carried out to determine the type of precipitates of Sanicro25 steel during creep.It is found that after the creep process,Z phase,Cr₂₃C₆ phase and Laves phase are precipitated at the grain boundary,and Laves phase and Sigma phase are easily precipitated in the grain.The Z phase and Cr₂₃C₆phase at the grain boundary are easy to form creep voids;the Laves phase precipitated and the Sigma phase in the grain can reduce the toughness of the material significantly.(3)Modified Kachanov-Rabotnov creep damage(K-R)model,Liu-Murakami creep damage(L-M)model and creep ductility exhausted(M-C-D)model based on microporous action,multiaxial creep ductility and creep process are used in finite element method.The creep life of SPC tests under different loads is predicted,and it is found that the results in M-C-D model are more accuracy,and fewer parameters are required in finite element analysis.It can be adpoted in engineering application in the future.(4)Based on M-C-D model and crystal plasticity finite element method,the fracture mechanism of SPC specimens with different thicknesses was studied based on the fracture morphology of Sanicro25 steel.It is found that in SPC test,cracks are initiated on the outer surface of the specimen and are controlled by the stress triaxiality.Crack growth fracture behaviors were controlled by the stress and stress triaxiality distributions.Under low stress,cracks extended via an intergranular pattern.In contrast,transgranular cracks were formed under a combination of high stress and stress triaxiality.