Manufacturing Processes Leading to Improved High pH SCC Resistance in Pipes

Abstract

Stress Corrosion Cracking (SCC) is a damaging form of environmental corrosion found in pipeline steels that forms in the presence of a high pH environment. SCC forms at the surface of gas pipes under damaged coatings and propagates towards the inner wall of the pipe until a critical depth is reached and the internal pressure ruptures the pipe. Prior to considering the role that crystallographic texture had on SCC, the accepted SCC mechanism was unable to explain why adjacent sections of pipe suffered differing levels of SCC damage. When comparing the crystallographic textures of adjacent pipes that suffered differing levels of SCC damage, it was observed that crystallographic texture, produced during manufacturing, was markedly different. The primary aim of this thesis was to analyse the influence that manufacturing has on the crystallographic texture development in pipeline steels and to investigate the effect that has on the subsequent SCC susceptibility. The project was executed in three main phases; the first phase involved laboratory rolling pipeline steel under different rolling schedules and characterising the resulting mechanical and microstructural properties. The second phase consisted of characterising the crystallographic texture of the produced steels. The final stage involved testing the SCC susceptibility of the produced steels and determining which material properties influenced the susceptibility. During laboratory scale rolling, recrystallized, hot and warm rolling processes were undertaken with the influence these manufacturing processes had on the steel investigated. Of the three rolling schedules, only recrystallized rolling met the minimum specifications necessary for an API 5L X70 grade steel whilst the other two resulted in suboptimal mechanical properties. All three rolling schedules produced bainitic structures with upper bainite dominant in the recrystallized rolling whilst granular bainite and polygonal ferrite mixtures, typical of X70 steels, were observed in the hot and warm rolled steels. When investigating the crystallographic texture of the produced steels, all three rolling schedules produced crystallographic textures reminiscent of those identified in steel rolling literature. Recrystallized rolling resulted in (001)[110], (001)[110], (110)[110] and (110)[001] textures in the quarter plane. Hot rolling resulted in (112)[110] textures joined along the y-fibre to (554)[225] along the quarter plane. Warm rolling produced textures between recrystallized rolling and hot rolling. SCC testing was undertaken in a cyclic LIST apparatus where samples were loaded in a 1NNa2CO3 + NaHCO3 solution for 10 cycles with the resulting cracks investigated. It was observed that recrystallized rolling had the worst SCC susceptibility with a crack velocity of 2:27x10-7 mm/s followed by hot rolling with a crack velocity of 1:16x10-7 mm/s and finally warm rolling with the lowest susceptibility of 8:70x10-8 mm/s. It was identified that SCC susceptibility was strongly linked to the grain size and yield strength with larger grains and higher yield stresses resulting in higher crack velocities. With regards to crystallographic texture, increases in f111g<112>, (332)[113], (554)[225], (112)[110] and (113)[110] textures and decreasing the proportion of (110)[110] corresponded to lower crack velocities.