Grain and phase stress criteria for behaviour and cleavage in duplex and bainitic steels

Stress analyses by X-ray diffraction are performed on a cast duplex (32% ferrite) stainless steel elbow and a bainitic (95% ferrite) pressure vessel steel. During an in situ tensile test, micrographic observations are made (visible glides and microcracks) and related to the stress state determined in the individual ferritic grains (aged duplex) and the ferritic phase (bainite loaded at low temperatures). Several material parameters have been identified at different scales, as for example, the critical resolved shear stress of 245 MPa for the aged ferritic grain (duplex) or 275 MPa for bainite (–60 ◦C), a crystallographic cleavage propagation criterion of 465 MPa (stress normal to {100} planes), and a fracture stress of approximately 700 MPa in the ferritic phase. Even though the two steels are different in many respects, the macroscopic fracture strains and stresses are well predicted by the polycrystalline model developed for bainite, whatever the temperatures tested (considering 7% of the grains reaching the local criterion)

Determination of inhomogeneous residual stress states in surface layers of machined engineering ceramics by synchrotron X-rays

Ceramic components for engineering applications must in most cases be ground after sintering in order to achieve a sufficient precision of the final dimensions as well as a sufficient surface quality. Since the surface layers influenced by grinding are very shallow, the determination of near-surface grinding residual stresses and their gradients within the penetration depth of the X-rays is difficult. In this paper, a procedure is applied which allows the investigation of residual stress states in ground ceramic surfaces by X-ray diffraction using synchrotron radiation and parallel beam optics. Three different ceramics were examined. In all cases, compressive residual stresses were found in the surface layers of the materials investigated. The magnitudes and gradients of the grinding residual stresses markedly depend on the grinding parameters and on the resulting mechanism of materials removal