Uniaxial step loading test setup for determination of creep curves of oxidation-sensitive high strength materials in vacuum under tensile and compressive load

Abstract

For long-term applications of components, such as in turbomachinery or automotive engineering, knowledge of creep behavior under increased load and temperature is of interest. Creep tests are commonly used to investigate the creep behavior of materials at a constant test temperature above room temperature under a constant force. The present work describes a so-called uniaxial step loading creep test setup and first results for a WC-Co hard metal under isothermal conditions at 700 °C in vacuum. Heating and temperature control within the tested specimen's gauge length were performed by induced eddy currents and a thermocouple, respectively. In contrast to conventional creep tests, the mechanical load is increased stepwise and the stress at each level is kept constant for 500 s. Displacement of the strain gauge markings was measured contactless with a laser extensometer. First tests were carried out for a WC-Co hard metal under compression and tensile loading. In order to avoid buckling of the high-strength material under compression, a special specimen geometry with non-constant specimen diameter was used. The minimum creep rate was determined for each applied tensile and compressive stress level. Under tensile load, minima of the creep rate were observed above a stress of 500 MPa that are interpreted as the secondary creep rates. Under compressive load, the respective creep rate minima were observed above a stress of −700 MPa.