The effect of microstructure on the susceptibility of pipeline steels to environment-assisted cracking

Abstract

Little has been reported regarding the susceptibility of grade X100 pipeline steel to environment-assisted cracking (EAC). The relative susceptibility of as-received bulk, welded and simulated heat-affected zone (HAZ) microstructures were compared using slow strain rate techniques and smooth tensile specimens. The susceptibility was also compared with that of X80 steel. Specimens were tested in a 1M sodium hydrogen carbonate / 0.5M sodium carbonate solution with a pH of ~ 9.5 at a temperature of 75oC within an applied potential range of – 500 to – 1100 mV (sce). For comparison a 4M NaNO3 environment was also used. The as-received bulk grade X100 was found to be less susceptible than a Widmanstätten microstructure, or a pearlite/ferrite, or a “spheroidised” structure. Welding has a significant effect on the microstructure and hence resistance to EAC. Tests on the as-received steel gave serrated load-extension curves and the serrations occur at particular combinations of strain rate and temperature. The serrations at 75oC and strain rate of 2 x 10-6/s were removed by annealing at 600oC but this increases the susceptibility to secondary cracking at potentials more negative than – 800 mV (sce). The grade X100 is less susceptible to stress corrosion cracking (SCC) than to hydrogen embrittlement. The role of carbon and manganese solute atoms in microstructural segregation (banding) was investigated. Banding induced directionality in the growth of stress corrosion cracks and caused fracture surface ovality. EDX microanalysis confirmed that the concentration of manganese is higher in the bands of pearlite. Band removal depends on homogenisation of either carbon or manganese which also depends on temperature, time and diffusion rate. Carbon diffusion may be restricted by cooling austenitised steel at a fast rate. Rapid cooling removes carbon segregation but does not remove manganese segregation. Homogenisation of manganese at 1175oC takes more than five hours.