Influence of denuded zones on the creep strength and ductility of 20Cr-25Ni/Nb stabilized steel

The uniaxial creep properties of 7% cold worked and aged, 20/25/Nb stabilized stainless steel have been investigated at 800 deg C employing samples with grain sizes of 45, 100 and 260 μm. The heat treatment, which precipitates Nb(CN) on the cold worked dislocation structure, leads to a high creep strength within grain interiors but also generates narrow precipitate-free zones at grain boundaries which are of low creep strength. At stresses exceeding 80 MPa, the stress exponent of the creep rate was large (approx. 10), but below 80 MPa the exponent fell to a low value (approx. 2). Creep rates and rupture ductility were similar for all grain sizes at high stresses but decreased with increasing grain size at low stresses. Very low failure strains, typically less than 2%, were observed. Measurement of the activation energy and the grain size dependence of creep rates suggests that recovery creep is controlling the deformation process at low stresses. Electron metallography of creep samples and scanning microscopy of fracture surfaces confirm that much of the creep deformation, particularly at stresses < 80 MPa, occurs in zones near grain boundaries. The influence of denuded zones upon creep strength and ductility is discussed. (author)