Comparison of the fatigue limit of heat-resistant nickel alloys in bending and tension-compression

Abstract

Specimens of heat-resistant nickel alloys were fatigue .tested in pure bending with rotation on machine type NUM. The frequency of the variable load was 50 Hz; the test base was 20-200 million cycles ( ~1001000 19. The tension-compress ion tests were carried out on MIR-ST machines with a frequency of the variable load of 35 Hz [6, 7]. The test base was taken equal to 10-50 million cycles (80-400 h). The specimens were heated to the test temperature in electric furnaces with omega-shaped heating elements. The fatigue tests of the heat-resistant nickel alloys at high temperatures were carried out by standard methods. The blanks for the fatigue-test specimens of formed alloy EI867 were cut out of rods; the specimens of cast alloys VZhL12U and ZhS6K were made by precision casting in vacuum. The blanks thus obtained were heat treated under standard conditions. The specimens were made by turning with subsequent grinding and polishing of the working part to V 9-10. The chemical composition and mechanical properties of the material of the specimens conformed to the standard. The fatigue tests in bending with rotation of the alloys EI867 and ZhS6K were made with specimens with a toroidal working part of 7.0 mm diameter and ofthe alloy VZhL12U, withcylindrical working part of 7.5 mm diameter and length [p = 5d. Fatigue tests with tension-compress ion were carried out with specimens with a cylindrical working part of 7.5 mm diameter and length lp = 5d. Before the tests, the specimens were subjected to visual and instrumental inspection, and the diameters of their working part and heads were measured. For the purpose of verifying the homogeneity of the material, selective hardness measurements of the specimens were carried out. The results of the fatigue tests were processed by linear regression analysis on a BF.SM-4 computer. The obtained data were approximated in coordinates log ~a log Np by straight lines (in case of a break by two straight lines).