On the relationship between microstructure mechanical properties and weld metal hydrogen assisted cold cracking

Abstract

Hydrogen introduced during shielded metal arc welding with cellulosic welding consumables can severely degrade the fracture resistance of the deposited weld metal and promote Weld Metal Hydrogen Assisted Cold Cracking (WM HACC), which is a particular type of weld defect with distinctive characteristics. Failure typically occurs after the deposited weld has cooled down to temperatures below 200°C and can initiate within minutes to even days after welding. Due to its time-delayed nature the onset of WM HACC may be undetected and can result in catastrophic failure. Many important properties of weld metal such as strength, toughness and the resistance to WM HACC are a function of its microstructure, comprised of diverse constituents with characteristic features and different mechanical properties, which co-exist and interact at the smallest microstructural dimensions. Hence, conventional test methods used to determine the bulk material’s properties are not suitable to evaluate the intrinsic properties of its individual microstructural constituents. Because of these experimental limitations, there is a lack of understanding of microstructural aspects that control the mechanical properties and the resistance to HACC at the micro-scale. Therefore, a major objective of the current work was to address these limitations by employing advanced characterisation and micro-mechanical testing techniques to evaluate the fundamental link between microstructure, mechanical properties and HACC susceptibility for individual weld metal microstructural constituents. This first part of the work examined the microstructure and mechanical properties of acicular ferrite and upper bainite in weld metal. Two localised microstructural regions, one acicular ferrite and the other one upper bainite, were first selected and then characterised using a high-resolution Scanning Electron Microscope (SEM) and Electron Backscattered Diffraction (EBSD). Semi-empirical models, based on microstructural aspects and physical principles, were used to determine the theoretical yield strengths of both microstructures. Different micromechanical tests were then conducted within each of the initially selected microstructural regions to characterise their intrinsic mechanical properties. Conventional nanoindentation and an advanced characterisation procedure were employed to obtain the yield strength, hardness, elastic modulus and strain hardening exponent. Micro-fracture tests in combination with linear and non-linear approaches of fracture mechanics were used to evaluate the deformation behaviour, fracture behaviour and fracture resistance. This study provided experimental evidence for a direct link between the microstructure and yield strength of acicular ferrite and upper bainite. It was thereby possible to identify the individual contributions of particular microstructural features. Furthermore, the relationship between strength, hardness and toughness was evaluated for both microstructures and the elastic and plastic components of their CTOD’s, could be identified. The results also showed that the fracture toughness values measured in microscopic regions of acicular ferrite and upper bainite were at least by an order of magnitude lower than the typical range for the fracture toughness of steels, obtained from conventional fracture tests. This may result from the fact that micro-fracture tests imply small specimen dimensions, which could cause a confinement of plastic deformations that contribute significantly to the fracture resistance. The results may also indicate that not all fracture toughening mechanisms are activated at the micro-scale. Nevertheless, it is worth noting that the fracture toughness values measured in this work, for a relatively ductile material, are higher than those reported for brittle and semi-brittle materials, tested with similar methods at the micro-scale. In the current literature, at such small scales, no empirical data on the fracture toughness of specific microstructural constituents in weld metal is available. Hence, it was not possible to directly verify the results. The second part of the work examined the microstructure and HACC propagation resistance of acicular ferrite and upper bainite in weld metal. A modified version of the Welding Institute of Canada (WIC) weldability test was employed to generate WM HACC under controlled conditions. The hydrogen crack propagation through selected microstructural regions of acicular ferrite and upper bainite was then characterised using EBSD. Where The Unit Crack Path (UCP) was utilised as a parameter to evaluate the HACC propagation resistance of both microstructures. Fractographic observations were conducted with a high resolution SEM, to characterise the fracture behaviour in the selected microstructural regions. The investigations showed that HACC propagates along a path of least resistance through the surrounding microstructure, where the UCP was significantly shorter for acicular ferrite than for upper bainite, thereby implying more frequent changes in direction and thus increased dissipation of energy from the crack driving force. The results indicate that acicular ferrite, increases the localised resistance to HACC propagation more than upper bainite, despite its higher strength, hardness and lower fracture toughness, which are all properties usually considered to be detrimental for the HACC resistance of the bulk material. The outcomes of this study suggest that macroscopic observations of the correlation between mechanical properties and HACC susceptibility are not necessarily applicable at the micro-scale. Which also implies that mechanical properties per se are not a good indicator of absolute HACC susceptibility and in fact may be misleading in terms of the intrinsic susceptibility of particular microstructural constituents. The third part of the work examined the microstructure and HACC initiation resistance of acicular ferrite in weld metal. A selected microscopic region of acicular ferrite was characterised using a high resolution SEM in combination with EBSD. Micro-fracture tests were then conducted at different loads on a hydrogen pre-charged specimen, that was fabricated into the selected region with a Focused Ion Beam (FIB). Linear Elastic Fracture Mechanics (LEFM) was applied to determine the range for the threshold stress intensity factor, Kth, to initiate HACC. The microstructure, deformation behaviour as well as the fracture behaviour were examined and compared with the data obtained from first part of the work, where a micro fracture test was conducted on an uncharged specimen, that was fabricated into the same region of acicular ferrite. This study showed that the obtained range for the threshold stress intensity factor of acicular ferrite was well below the threshold values for weld metal as well as low and medium carbon steels with similar yield strengths. This indicates that, at the micro-scale, hydrogen cracks can grow at stress intensity factors well below the threshold values measured with conventional tests at the macro-scale. It seems that also in this case critical fracture toughening mechanisms may not be activated if the specimen dimensions are very small. The crack growth rate in acicular ferrite, also appeared to be significantly lower than typically observed for the bulk material. Besides plastic deformations during fracture and the threshold stress intensity to initiate fracture, the yield strength and the Young’s modulus also decreased due to the presence of hydrogen, which correlates well with previous observations and proposed hydrogen embrittlement models.