JOURNAL OF MATERIALS SCIENCE 35 (2 0 0 0 ) 1379 – 1388 Strength differential effect in four commercial steels A. P. SINGH, K. A. PADMANABHAN ∗ , G. N. PANDEY † , G. M. D. MURTY, S. JHA Reserch and Development Centre for Iron and Steel, Steel Authority of India Limited, Ranchi-834 002, India The difference between compressive and tensile flow stress of a material at a given strain termed as strength differential (S-D) effect, has been evaluated in case of four commercial steels via a series of heat treated conditions. The results have unequivocally established that the magnitude of S-D was maximum in the as quenched condition and tempering of the quenched structure led to a decrease in S-D. Spheroidised and/or annealed structures exhibited the lowest value of S-D. A linear relationship of S-D value with hardness and mean stress for each case has been established. Attempts have been made to explain the observed S-D effect in terms of models based on atomic mechanism and of continuum mechanics. C  2000 Kluwer Academic Publishers 1. Introduction The difference between compressive and tensile strength of a material at a given strain is termed as the strength differential (S-D). Theoretically the com- pressive and tensile strength of a material should be equal assuming that only shear stresses are responsi- ble for plastic deformation and hydrostatic component of the stress system does not affect plastic flow [1]. However, in real materials like steels [2–36], zircaloy- 2 [37–40], α (plutonium [41], dispersion strength- ened alloys [42, 43], titanium alloys [44, 45], plastics [46], metal matrix compsosites [47, 48] and more re- cently in Ni Ti shape memory alloys [49, 50], a dif- ference between the compressive and tensile strength is observed. Experimental evidence proves beyond doubt that the difference is genuine and cannot be ac- counted for merely as due to friction in the compression tests. This phenomenon is gaining importance, as apart from providing theoretical curiosity, the S-D effect also has commercial relevance. In constructional steels, like beams, angles, channels etc., the members may often be subjected to high compressive loads. The choice of right microstructure to derive the maximum benefit from the S-D effect will reduce the quantity of metal used and thus reduce costs. In the aerospace industry, optimal exploitation of phenomenon can result in considerable savings in material costs. Another area is in nuclear re- actor design to predict analytically the cladding creep collapse time during fuel densification for light water reactors [38, 39]. With reference to steels, most of the steels so far stud- ied contain in addition to carbon a number of alloying additions. From a fundamental point of view it would ∗ Present Address: Indian Institute of Technology, Kanpur, India. † Present Address: Heavy Engineering Corporation, Ranchi, India. be difficult to isolate the contribution to the total effect S-D from each of the alloying additions, in addition to the role of morphology and phase distribution. In the present investigations, two plain carbon steels, differing in carbon content and two low alloy steels, essentially differing in nickel content have been con- sidered. The study is mainly to conclude the role of carbon and nickel on the magnitude of S-D in identi- cal experimental conditions. Contrary to most of earlier studies, the S-D effect has been evaluated at relatively larger strains of 1% offset and 10% true strain for dif- ferent experimental conditions in case of all the four steels. Experimentally observed S-D values obtained under different experimental conditions have been anal- ysed in terms of existing theories to explain the S-D phenomenon. 2. Experimental procedure 2.1. Materials, specimen preparation and heat treatment The studied steels are two plain carbon and two low alloy steels, respectively named as A, B, C and D. The chemical compositions of these steels are given in Table I. These steels were received in the form of bars of 25.40 mm diameter. Cylindrical specimens of a constant diameter of 7.62 mm and different diam- eter to height ratios ( D 0 / H 0 ) of 1.00, 0.80 and 0.67 were machind from the bars for compression testing. Specimens of 25 mm gauge length and 6.25 mm di- ameter were machined from received bars for tensile tests. Different heat treatments like brine quenching (Quenched in Sodium Chloride Solution), tempering, 0022–2461 C  2000 Kluwer Academic Publishers 1379