Microstructural effect on fracture toughness of N26MT2Nb steel in the two-phase state

The aim of investigation was to determine the effect of the Ni26MoTi2Nb alloy (C – 0.02%, Ni – 26.0%, Ti – 2.15%, Mo – 1.15%, Nb – 0.11%) tempering for 40 minutes in the temperature range $A_s-A_f^1$; i.e., within the transformation range of martensite (alpha') into austenite (gamma), on its microstructure and fracture toughness. The initial structure of alloy was composed of martensite ( 70%) and retained austenite ( 30%). During tempering, the alloy underwent ageing. The highest values of fracture toughness J_Ic, four times higher than those in the initial state, were achieved after tempering at 975 K; i.e., at a temperature close to A_f. At this tempering temperature, the austenite content doubled while the tensile strength increased more than threefold. An increase in the martensite content, found on fracture surfaces of specimens held at the upper temperature zone of the alpha'-gamma transformation, amounting to lO-15% as compared with the undeformed region, proves the metastability of reverted austenite and beneficial results of the transformation into martensite during deformation.