Session: MF-02-09 Materials for Hydrogen Service (Joint with C&S)-9

Paper Number: 155145

155145 - Fracture Resistance of Weld and Heat-Affected Zones in Line Pipe Steels in High Pressure H2 Gas: Microstructure-Property Correlation 

Abstract: 

A critical aspect of the hydrogen infrastructure is a pipeline network for transportation. However, transportation of H₂ gas through steel line pipes remains challenging due to the unknown fracture resistance of these steels in hydrogen service. The standard ASME B31.12 allows low-yield strength steels for H₂ gas transmission pipelines with a mandated minimum fracture toughness (K_1H) of 55 MPa.m^0.5.  Moreover, the standard penalizes using higher-strength line pipe steels (X65 – X80), though using higher-strength steels would enable higher operating pressures for better economic viability. The presence of welding and heat-affected zones (HAZ) in line pipe structures poses a particular challenge because the microstructure can vary substantially from the base metal. It is known that heat-affected zone areas experience varying peak temperatures and cooling rates during the welding process which results in inhomogeneous areas with gradient microstructure over a short length. The present work aims to correlate the microstructural features and fracture response in weld heat-affected zones. The thermal profile in a multi-pass girth weld and associated HAZ in a line pipe steel has been generated using numerical simulations considering the welding geometry, pass configuration, heat input, and welding speed, based on industrial welding procedure specifications. HAZ microstructures have been identified based on thermal profiles in several locations: near 1) the root pass which experiences the effect of the root pass and the subsequent hot pass, 2) the intermediate passes which experience the effect of filling passes after the hot pass, and 3) the final or finishing-pass (also called capping pass). The microstructures in these HAZ were reproduced in an X65 plate using simulated thermal profiles in a Gleeble thermomechanical simulator; these microstructures were characterized with light optical and electron microscopy, Xray diffraction, Vicker’s hardness measurements, and electron back-scatter diffraction to interpret potential regions susceptible to hydrogen embrittlement. Subsequently, these simulated HAZ microstructures will be tested for fracture toughness in high-pressure H₂ gas to enable microstructure-property correlation. Additionally, microstructure characterization is being performed on four fracture toughness samples tested in 21 MPa pure H₂ gas. The samples include X65 and X70 materials extracted from the pipeline girth weld and associated HAZ. Fracture surface features were characterized with scanning electron microscope images and sectioned to correlate the microstructures beneath the fracture surfaces to specific features, e.g. large, flat facets present on the fracture surfaces.

Presenting Author: Santi Gopal Samanta Colorado School of Mines

Presenting Author Biography: PhD (2023), Indian Institute of Technology Kharagpur, Metallurgical and Materials Engineering, Thesis: Hydrogen Embrittlement in High Strength Steels: Assessment and Prevention.

Authors:

Santi Gopal Samanta Colorado School of Mines
Chen Ni Colorado School of Mines
Andrew Leboeuf Colorado School of Mines
Joseph Ronevich Sandia National Laboratories, California
Chris San Marchi Sandia National Laboratories, California
Zhenzhen Yu Colorado School of Mines
Lawrence Cho Colorado School of Mines
Kip Findley Colorado School of Mines