Study On Mechanical Properties Of Two Typical High-temperature Structural Materials Using Miniature Specimens

In recent years,the safety and reliability of key components such as nuclear power equipment and turbine blades have attracted great attention from scholars.Compared with the standard mechanical properties test,the technique of test by using miniature specimens can effectively avoid serious damage to the in-service components,and it has received more and more attention in the study of material properties.The cantilever beam bending fatigue test and the small punch creep test are suitable for predicting the fatigue performance and high temperature creep performance of in-service equipment and key structural components because of using miniature specimens.The results of the test by using miniature specimens are similar to the conventional fatigue test and creep test,and this test has been gradually applied to nuclear power,aerospace and other fields.In this thesis,two typical high-temperature structural materials,316 stainless steel and Inconel 718 alloy,were selected as the research objects.The cantilever beam bending fatigue test and the small punch creep test using miniature specimens were used to study the fatigue properties and creep performance of materials.The main results are as follows:The tensile test and cantilever beam bending fatigue test using miniature specimens of annealed,pre-strained and cold-rolled 316 stainless steel indicate that the cold-rolled 316 stainless steel has the highest strength and fatigue limit,followed by the pre-strained 316 stainless steel,and the lowest in the annealed 316 stainless steel.However,under the low-cycle fatigue test conditions,the fatigue damage in the cold-rolled 316 stainless steel is the lowest,followed by the pre-strained 316 stainless steel,and the highest in the annealed 316 stainless steel.With the strain amplitude decreases,the fatigue damage of the three specimens decreases gradually,which is consistent with the results of fatigue test using standard specimens.The tensile test and cantilever beam bending fatigue test by using miniature specimens can quickly evaluate the tensile properties and fatigue properties of 316 stainless steel.The small punch creep tests using miniature specimens of annealed,pre-strained and coldrolled 316 stainless steel reveal that the steady state creep rate increases and creep life decreases with the increase of pre-deformation degree.The creep characteristics of the annealed and prestrained specimens are wedge crack,micropore aggregation,intergranular fracture,transgranular dimple and uniform cracking in all directions.However,the creep fracture of the cold-rolled specimen is smooth,and the cracking direction is parallel to the rolling direction.The small punch creep test by using miniature specimens can quickly evaluate the creep properties of 3 16 stainless steel.Experimental results of the cantilever beam bending fatigue test of Inconel 718 parts by using miniature specimens show that the scattering of fatigue life under the same strain amplitude is related to the position,size and number of pores in materials.The decrease in the size/the number of pores and/or the increase in the distance from the pore center to the specimen surface can increase the fatigue life.The fatigue life was most closely related to the location of pores,and the correlation coefficient is as high as 0.72.The influence of the number of pores on fatigue life is second,and the correlation coefficient is-0.67.The pore size has the weakest influence on fatigue life,and the correlation coefficient is only-0.11.It is feasible to use miniature specimens experimental technique to explore the quantitative relationship between the fatigue life and pore characteristics of the selective laser melting(SLM)-fabricated Inconel 718,which has been widely studied.