Torsion Testing in an Inert Atmosphere

Abstract

As part of a research project investigating metallographic methods of life prediction in low alloy ferritic steels, a creep testing programme is being carried out on 1%Cr-0.50025Mo steel in a simulated weld heat affected zone condition. The major creep damage mechanism in this fully bainitic material is the nucleation and growth of cavities at prior austenite grain boundaries, resulting in low ductility intergranular fracture (uniaxial failure strains are typically less than 2%). Since cavity growth is considered to be driven by the maacimum principal stress σ _l, and creep deformation to be controlled by equivalent stress $\bar \sigma$, a torsion creep test programme was initiated to quantify the effects of stress state on damage accumulation and failure. Choice of a “thin-wall” specimen to simplify comparisons between uniaxial and biaacial tests dictated the use of an inert atmosphere to combat oxidation effects. This requirement was reinforced by the use of test temperatures up to 675°C for test acceleration in order to maintain the operational creep mechanism (Cane and Townsend, 1984). Existing torsion-tension creep machines were modified to accept gas boxes and torsion testpieces with threaded ends. Methods of strain monitoring and gas sealing were adapted for torsion testing.