Joseph Allen Ronevich

Assessing Gaseous Hydrogen Assisted Fatigue Crack Growth Susceptibility of Pipeline Steel Weld Fusion Zones and Heat Affected Zones.

The objective of this work was twofold: (1) measure reliable fatigue crack growth relationships for X65 steel and its girth weld in high-pressure hydrogen gas to enable structural integrity assessments of hydrogen pipelines, and (2) evaluate the hydrogen accelerated fatigue crack growth susceptibility of the weld fusion zone and heat-affected zone relative to the base metal. Fatigue crack growth relationships (da/dN versus ΔK) were measured for girth welded X65 pipeline steel in high pressure hydrogen gas (21 MPa) and in air. Hydrogen assisted fatigue crack growth was observed for the base metal (BM), fusion zone (FZ), and heat-affected zone (HAZ), and was manifested through crack growth rates reaching nearly an order of magnitude acceleration over rates in air. At higher ΔK values, crack growth rates of BM, FZ, and HAZ were coincident; however, at lower ΔK, the fatigue crack growth relationships exhibited some divergence with the fusion zone having the highest crack growth rates. These relative fatigue crack growth rates in the BM, FZ, and HAZ were provisional, however, since both crack closure and residual stress contributed to the crack-tip driving force in specimens extracted from the HAZ. Despite the relatively high applied R-ratio (R = 0.5), crack closure was detected in the heat affected zone tests, in contrast to the absence of crack closure in the base metal tests. Crack closure corrections were performed using the adjusted compliance ratio method and the effect of residual stress on Kmax was determined by the crack-compliance method. Crack-tip driving forces that account for closure and residual stress effects were quantified as a weighted function of ΔK and Kmax (i.e., Knorm), and the resulting da/dN versus Knorm relationships showed that the HAZ exhibited higher hydrogen accelerated fatigue crack growth rates than the BM at lower Knorm values.

SNL/SRNL Joint Project on degradation of mechanical properties in structural metals and welds for GTS reservoirs.

This project was intended to enable SNL-CA to produce appropriate specimens of relevant stainless steels for testing and perform baseline testing of weld heat-affected zone and weld fusion zone. One of the key deliverables in this project was to establish a procedure for fracture testing stainless steel weld fusion zone and heat affected zones that were pre-charged with hydrogen. Following the establishment of the procedure, a round robin was planned between SNL-CA and SRNL to ensure testing consistency between laboratories. SNL-CA and SRNL would then develop a comprehensive test plan, which would include tritium exposures of several years at SRNL on samples delivered by SNL-CA. Testing would follow the procedures developed at SNL-CA. SRNL will also purchase tritium charging vessels to perform the tritium exposures. Although comprehensive understanding of isotope-induced fracture in GTS reservoir materials is a several year effort, the FY15 work would enabled us to jump-start the tests and initiate long-term tritium exposures to aid comprehensive future investigations. Development of a procedure and laboratory testing consistency between SNL-CA and SNRL ensures reliability in results as future evaluations are performed on aluminum alloys and potentially additively-manufactured components.

Fracture Threshold Measurements of Hydrogen Precharged Stainless Steel Weld Fusion Zones and Heat Affected Zones

Austenitic stainless steels are used in hydrogen environments because of their generally accepted resistance to hydrogen embrittlement; however, hydrogen-assisted cracking can occur depending on the microstructures or composition of the stainless steel. One area that has not been well researched is welds and in particular heat affected zones. The goal of this work was to measure the subcritical cracking susceptibility of hydrogen precharged gas tungsten arc (GTA) welds in forged stainless steels (21Cr-6Ni-9Mn and 304L). Welds were fabricated using 308L filler metal to form 21-6-9/308L and 304L/308L weld rings, and subsequently three-point bend specimens were extracted from the fusion zone and heat affected zone and precharged in high-pressure hydrogen gas. Crack growth resistance curves were measured in air for the hydrogen precharged fusion zones and heat affected zones under rising-displacement loading, revealing significant susceptibility to subcritical cracking. Fracture thresholds of 304L/308L welds were lower than 21-6-9/308L welds which was attributed to higher ferrite fractions in 304L/308L since this phase governed the crack path. Fracture thresholds for the heat affected zone were greater than the fusion zone in 21-6-9/308L which is likely due to negligible ferrite in the heat affected zone. Modifications to the weld joint geometry through use of a single-J design were implemented to allow consistent testing of the heat affected zones by propagating the crack parallel to the fusion zone boundary. Despite low hydrogen diffusivity in the austenitic stainless steels, effects of displacement rates were observed and a critical rate was defined to yield lower-bound fracture thresholds.Copyright