Environmental Fracture Mechanics Testing of Dissimilar Metal Welds
TWI Industrial Member Report 1211-2025
By Mike Dodge and Huan Wu
Industrial Need
Dissimilar metal welds (DMWs) between low alloy steels and corrosion-resistant alloys are widely used across industries such as energy, chemical processing, and in subsea applications. However, their performance under hydrogen-charging conditions, such as exposure to gaseous hydrogen or cathodic protection (CP), is less predictable, because they are subject to a significant risk of hydrogen embrittlement (HE).
Qualitative fracture mechanics-based ranking tests have provided, historically, valuable insights and have proved successful in replicating failure modes seen in service. They have also offered a means of screening materials for embrittlement susceptibility. However, the lack of standardised guidelines for such tests on DMWs has led to unknown levels of conservatism, which limits their use in design and fitness-for-service assessments.
This study employed a combined experimental and modelling approach to investigate HE in DMWs. The experimental phase involved single edge notched bend and tension (SENB and SENT) fracture toughness testing under cathodic polarisation in a sodium chloride solution. Specimens were notched at the fusion boundary between F22 (2.25Cr-1Mo) low alloy steel and Alloy 625 (Ni-Cr-Mo-Nb) weld metal. Tests were conducted across a temperature range of 4 to 80°C to evaluate the temperature effect on environmental fracture resistance.
Coupled hydrogen diffusion-mechanical numerical models were developed to elucidate the differences in apparent toughness in terms of hydrogen diffusion towards highly-strained regions of the joint. Whilst understanding the contributory factors in the embrittlement of these joints is inherently compelling from an integrity management standpoint, HE of DMWs also serves as an ideal platform for developing numerical models of this kind, due to the significantly different mechanical properties and hydrogen diffusion behaviour expected in the materials either side of the fusion boundary. This challenge was therefore particularly appealing in terms of achieving one of the project’s main objectives: to develop modelling techniques that can be used to understand the interaction between strain and hydrogen diffusion behaviour.
Key Findings
- The microstructures observed within DMW C4 were typical of arc-welded F22-Alloy 625 joints. The dissimilar interface comprised thin bands of non-equilibrium microstructures, including interfacial martensite, the volume fraction of which was greater at bead-overlap positions, where weld metal swirls were present. These complex interfacial microstructures were found to be susceptible to hydrogen embrittlement during environmental SENB and SENT testing.
- Both the SENT and SENB tests revealed a positive correlation between testing temperature and resistance to hydrogen-assisted crack propagation. However, comparison between sets of environmental J R-curves from SENB and SENT specimens, tested at temperatures ranging from 4 to 80°C, revealed a broader data range for the SENT tests. In particular, the SENT tests conducted at lower temperatures (4 and 20°C) produced lower toughness values than the equivalent SENB tests.
- Despite exhibiting lower constraint, the SENT specimens did not show the expected increase in fracture toughness, particularly at lower temperatures. The numerical modelling results suggest that factors such as hydrogen diffusion behaviour, potentially influenced by the strain field’s shape, size and magnitude, plays a significant role in the material's response to HE during testing.
Impact
The testing programme and models developed throughout this project have revealed a complex relationship between the strains generated during fracture toughness testing and hydrogen diffusion to the fracture process zone, influenced by temperature and constraint. Notably, they underscore the importance of further investigation into the interaction between hydrogen uptake and its impact on embrittlement, which, as demonstrated in this study, can lead to unexpected outcomes.
The numerical models developed during this project offer potential for identifying the most suitable testing methods based on the conditions and loading scenarios that in-field components are likely to encounter. They could potentially be used to understand which testing methods should be deployed and the quantity of those tests required, depending on the conditions and loading to which in-field components will be exposed. Furthermore, the same experimental and modelling approach could be applied to simulate the behaviour of in-service components operating under hydrogen-charging conditions, thereby assessing whether such components are at increased risk of hydrogen embrittlement in the field.