Effects of temperature and strain rate on tensile stress–strain and workhardening behaviour of P92 ferritic steel G. Sainath, B. K. Choudhary*, J. Christopher, E. Isaac Samuel and M. D. Mathew Detailed analysis on true stress s–true plastic strain e data indicated that the tensile flow and workhardening behaviour of P92 ferritic steel can be described most accurately by the combination of Ludwigson and Hollomon relations at strain rates ranging from 3?16610 25 to 1?26610 23 s 21 over the temperature range of 300–923 K. At room and intermediate temperatures, the Ludwigson equation follows the s–e data closely, whereas at high temperatures, the Ludwigson equation reduces to the Hollomon relation. The variations in s–e, workhardening parameters and h–s with temperature exhibited three distinct temperature regimes. At intermediate temperatures, anomalous variations in s–e, workhardening parameters and h–s with respect to temperature and strain rate have been observed. At high temperatures, the dominance of recovery is reflected in the rapid decrease in flow stress and workhardening parameters associated with Ludwigson/Hollomon relations with increasing temperature and decreasing strain rate. Keywords: P92 steel, True stress–true strain behaviour, Ludwigson equation, Hollomon equation, Workhardening behaviour, Dynamic strain aging, Dynamic recovery Introduction Appropriate description of true stress s–true plastic strain e behaviour of metals and alloys is important for useful engineering applications. Several constitu- tive relationships, namely Hollomon, 1 Ludwik, 2 Swift 3 , Ludwigson 4 and Voce, 5,6 have been proposed in the literature to describe the tensile flow and workhardening behaviour of metals and alloys. Hollomon 1 proposed a simple power law relation interrelating true stress–true plastic strain in the uniform plastic strain regime as s~K H e nH (1) where n H is the strain hardening exponent, and K H is the strain hardening coefficient. The Hollomon relation provides a measure of uniform plastic elongation and ultimate tensile strength through n H and K H respec- tively. In order to account for the mechanical history in the flow relationship, Ludwik 2 proposed a relationship with the addition of a stress term s 0 describing the positive stress deviation due to yielding at low strains as s~s 0 zK L e n L (2) where n L is the strain hardening exponent, and K L is the strain hardening coefficient. Similarly, for the prestrain left in the material, Swift 3 suggested a relationship with the addition of a strain term e 0 as s~K S e 0 ze ð Þ nS (3) where n S and K S are the strain hardening exponent and coefficient respectively. It has been demonstrated that the stress–strain behaviour of many face centred cubic metals and alloys having low stacking fault energy cannot be described by the Holloman equation due to the large positive stress deviation at low strains. 4 It was proposed that the positive stress deviation at low strains from the Hollomon equation can be accounted in the flow curve by an additional term as s~K 1 e n 1 zexp (K 2 zn 2 e) (4) where K 1 and n 1 are the same as K H and n H in equation (1) respectively, and K 2 and n 2 are the additional constants. K 2 in the Ludwigson relation corresponds to a finite stress at zero strain, and the value of ‘exp (K 2 )’ represents a proportional limit of the material. For materials showing saturation in stress at high stress/strain levels, Voce 5,6 proposed a flow relationship as s~s S { s S {s I ð Þ exp { e{e I ð Þ e c   (5) where s S is the saturation stress, s I and e I are true stress and true plastic strain at the onset of plastic deformation respectively and e c is a constant. For the condition e I 50, equation (5) reduces to s~s S { s S {s I ð Þ exp n V e ð Þ (6) Materials Science and Technology mst10833.3d 15/7/13 14:02:13 The Charlesworth Group, Wakefield +44(0)1924 369598 - Rev 7.51n/W (Jan 20 2003) Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603102, India *Corresponding author, email bkc@igcar.gov.in ß 2013 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 3 May 2013; accepted 1 July 2013 DOI 10.1179/1743284713Y.0000000349 Materials Science and Technology 2013 VOL 000 NO 000 1