EFFECT OF AGING ON MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF COBALTFREE 18-PERCENT-NI (250-GRADE) MARAGING-STEEL

The influence of aging on the microstructure, tensile properties, frac ture toughness, and fractographic features of a cobalt free 18%Ni 250 grade maraging steel has been investigated. This steel, designated as T250 steel, with 17.1%Ni, 2.25%Mo, and 1.39%Ti as the main alloying el ements, contains no cobalt and less molybdenum than the widely used 18 Ni-8Co-5Mo type 250 maraging steel (M250). The present investigation s hows that tensile properties as well as plane strain fracture toughnes s values comparable with those of M250 can be obtained in the T250 ste el with marginal change in aging cycle. The Ni3Ti phase precipitating in the present steel has been found to have excellent resistance to co arsening at the normal aging temperatures. The condition for maximum y ield and tensile strengths is also the one that gives peak values of t oughness. While a low value of toughness in the underaged condition is attributed to the planar slip mode of deformation, the formation of r everted austenite and coarsening of precipitates were responsible for the poor toughness in the overaged condition. The strengthening in til e present steel obeys the modified Orowan equation.