Effect of Acicular Ferrite and Bainite in API X70 Steel Obtained After Applying a Heat Treatment on Corrosion and Cracking Behaviour

The effect of acicular ferrite (AF) and bainite (B) obtained by heat treatment (HT) applied to an API X70 steel on corrosion resistance and stress corrosion cracking (SCC) susceptibility was evaluated through polarization curves and slow strain rate tests respectively, in NS4 solution and congenital water (CW). The HT was carried out at 1050 °C for 15 and 30 min followed by water quenching. Change of ferrite–perlite to AF and B was observed as result of heat treatment applied to X70 steel. Effect of HT time in terms of microstructure obtained was analysed. The mechanical properties obtained by the HT are higher than X100 steel. HT-30 min improve the corrosion resistance of X70 steel exposed to both solutions, which are related to coarse and homogeneous microstructure of AF and B. Localized corrosion was observed in the steel exposed to both solutions. HT-15 min improve mechanical properties and higher content of fine AF, but decreases SCC resistance. Steel with higher content of fine AF microstructure is more susceptible to SCC. SCC indexes revealed that the heat-treated steel could be susceptible to SCC in CW confirmed by the secondary cracks observed. The SCC mechanism was hydrogen embrittlement. Meanwhile, steel HT by 15 min exposed to NS4 solution could be susceptible to SCC. Crack growth rate was higher in steel HT by 15 min, which are related to more fine and brittle AF microstructure.