The effect of tempering on the toughness of hardened steel

The effect of tempering upon the toughness of hardened steel has been investigated. A steel having the nominal analysis: 0.35 % C, 1.5 % Si, 20 % Ni was chosen for this work. The analysis was mainly chosen with the objectives in mind that the Ms temperature should be low enough to ensure that no tempering will occur on quenching, and that the carbides produced on tempering might precipitate each within its own particular tempering temperature range. Test bars made of the investigated steel were austenitized and quenched in liquid nitrogen, with subsequent isothermal tempering at various temperatures in the range Tt = -110 to 450 °C. The structures of the heat-treated specimens were examined by x-rays and by electron microscopy, using both the replica and thin foil technique. Changes occurring in the structures during the tempering process were followed with the aid of a dilatometer. The principal part of the work consists of mechanical tests, for which a torsion testing machine was designed and built. The machine was so designed that the true stress vs. true strain diagram could be readily obtained with its aid. The experimental conditions, including speed of rotation (n) and testing temperature (Tk), were variable within wide limits. In the mechanical tests performed, the speed of rotation was varied within the range of n = 0.00135 to 640 r.p.m. The testing temperatures were: Tk = -110, +25 and +95 °C. The deformed structures were also examined by electron microscopy