Fracture Resistance of Weld and Heat-Affected Zones in Line Pipe Steels in High Pressure H2 Gas: Microstructure-Property Correlation

Weld heat-affected zone (HAZ) microstructures were generated on an X65 steel in a Gleeble 3500® using numerically simulated thermal profiles of a seven-pass girth weld. The HAZ microstructures were then studied using scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), X-ray diffraction (XRD), and Vickers hardness measurements. Simultaneously, four fracture toughness (in 21 MPa H2 gas) samples from weld and HAZ regions of X65 and X70 steel pipes were studied for fractography and microstructures and subsequently, compared with the simulated HAZ microstructures. The X70 steel pipe HAZ condition had a lower fracture resistance than its weld, due to a microstructure of banded ferrite-pearlite compared to a fine microstructure of grain boundary ferrite, quasi-polygonal ferrite and acicular ferrite in the weld. One of the two weld conditions from the X65 steel pipe with a ferrite-tempered martensite showed a better fracture resistance than the other one with ferrite and lower transformation temperature products (bainite, martensite, martensite-austenite islands). The former weld condition may have experienced a slower cooling rate (auto-tempering) or tempered during subsequent welding passes. These observations indicate the influence of weld-HAZ phase transformation characteristics during welding and the resultant microstructures on the fracture resistance in high-pressure H2 gas.