Effect of thermomechanical parameters on mechanical properties of base metal and heat affected zone of X65 pipeline steel weld in the presence of hydrogen

In this study, the effect of different microstructures resulting from thermochemical processes of X65 pipeline steel on mechanical properties of welding in the presence of hydrogen was investigated. According to studies conducted, due to the charge of hydrogen, the hardness of base metal (BM) increased by 10%, the hardness of heat affected zone (HAZ) samples increased by about 13%, the yield strength of base metal increased by an average of 5% and the yield strength of HAZ increased by about 7%, indicating hardening. Elongation was reduced by 32-70% and the percentage of ductile fracture decreased by an average 46% which shows that the fracture of samples in the presence of hydrogen is brittle. In the study of hydrogen defect and concentration of hydrogen permeated to the material it was determined that, the ferrite-pearlite banded microstructure of base metal has a higher hydrogen defect sensitivity than HAZ with an acicular ferrite microstructure and hard phase. In base metal samples under various rolling conditions, the two-pass rolled and the final rolling temperature of 800 degrees C sample (2BM800), with a smaller grain microstructure and higher interface, had the most diffusive hydrogen content and therefore had the highest hydrogen defects. In the case of HAZ samples, the effect of hydrogen on the increase in hardness and strength properties is greater than that of the base metal samples due to the different microstructure (of the acicular ferrite) and the variable grain size (i.e. the presence of two coarse-grained and fine grained areas). Overall, the results of this study showed that the microstructure has a significant effect on mechanical properties, the amount of diffusive hydrogen and hydrogen defects in welds.