Fatigue, monotonic and fracture toughness properties of a Cr–Mn–N steel

The low-cycle fatigue, monotonic and fracture toughness behaviour of E3949, a Cr–Mn–N austenitic stainless steel, used for drillcollar connections was studied. Low-cycle fatigue tests were carried out at room temperature under total strain control in the range of 0.40 to 1.50% using Companion Specimens Test (CST) and Incremental Step Test (IST) methods. Cyclic softening without saturation was observed in all tests. Massing cyclic stress–strain behaviour was observed only with the IST method. The fatigue life behaviour obeyed Basquin and Coffin–Manson relationships and the high value obtained for ε′f imparts a significant improvement in fatigue resistance of this alloy compared to AISI 304LN. The J–R curves and JIC values were obtained at room temperature and at 150°C by using single specimens and the elastic compliance technique for crack length measurement. The observed decrease in crack initiation fracture toughness at 150°C is proposed to be due to a dynamic strain ageing effect, which impairs ductility.