The effect of microstructure on the stress-strain behaviour of pipeline steels was studied. Slow strain rate ( 2 × 1 0 ？ 6 ？s？1) tests were conducted on grade X65 and X100 steels in silicone oil and hydrogen carbonate/carbonate solution. The as-received grade X100 steel at 7 5 ° C showed serrated stress-strain curves. The magnitude of the serrations depended upon the strain rate and test temperature. Annealing at 6 0 0 ° C or above removes the serrations, but this increased the susceptibility to transgranular cracking in hydrogen carbonate/carbonate solution at potentials below ？800？mV (sce). The removal and reformation of banding in pipeline steels were also studied. Ferrite/pearlite becomes aligned during the hot rolling stage of pipe manufacture and causes directionality in crack propagation and mechanical properties. Heat treatments were carried out which show that banding in grade X65 and X100 can be removed above 9 0 0 ° C. This depends on homogenisation of carbon which also depends on temperature, time, and cooling rate. 1. Introduction The strength and toughness requirements of pipeline steels may be achieved by appropriate steel production techniques. Different strengthening methods are used by manufacturers to satisfy codes. Mechanical performance and susceptibility to environment-assisted cracking depend on microstructure, and so heat-treated pipeline steels with uniform fine-grained microstructure such as bainite are generally more resistant to stress corrosion cracking, SCC, than those consisting of pearlite and ferrite [1]. Some cracks that are hydrogen-induced have been shown to be associated with the boundary between ferrite and pearlite [2]. Such work suggests that homogenisation of composition and phases in steels improve their resistance to cracking. Pearlite-ferrite bands in carbon steels are associated with the segregation of substitutional alloying elements which raise or lower the temperature (Ar3) for the formation of proeutectoid ferrite upon cooling [3]. The segregation occurs during solidification of steel and gives rise to longitudinal bands during hot rolling [3–5]. If the Ar3？？is raised by the solute, then proeutectoid ferrite nucleates first in the solute-rich regions. On the other hand, if the Ar3 temperature is lowered by the solute, then the nucleation of the proeutectoid ferrite begins in the solute lean regions. In either case, carbon atoms, which diffuse rapidly, are rejected from the proeutectoid ferrite, thus producing carbon-rich regions of austenite, which transform finally to pearlite [3, 4]. In pipeline steels,
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