Notch sensitivity of deformed thin sheets of an 18wt.% Ni-maraging steel

Studies were conducted to investigate the notch sensitivity of deformed thin sheets of an ultra high strength 18 wt. % maraging steel. Geometry and size of specimen and the corresponding holding fixtures were designed for heat treatment and subsequent mechanical testing. The geometry of the notch in the center notch specimens was modified to ensure the reproducibility of the results during its electro-discharge machining. The critical ratios of CC/L and CC/W were kept, as per ASTM standard as 0.7 and 2.6, respectively. CC is center to center distance of loading holes, L is the total length and W is the width of the specimen. Sets of specimens following different thermo-mechanical treatments were tested to establish the sensitivity of the technique. Here results on three series of experiments on specimens from thin sheets of an 18 wt. % Ni maraging steel are presented. Effects of (i) degree of deformation on the notch sensitivity (ii) annealing prior to aging and (iii) cyclic annealing and aging were determined. The technique has proved to be more sensitive than other mechanical testing techniques including hardness, tensile strength and phase analysis using x-ray diffraction. For example, change in the degree of deformation from 75 to 94% increases the hardness values by 12% while the notch strength decreases 2% but the notch strength increases by more than 35 %. Following an annealing aging cyclic, the change in hardness values is only 2-3 % while the value of notch sensitivity changes to more than 20%. (author)