Elucidating the Process of Environmentally-Assisted Cracking in Duplex Stainless Steel Weld Microstructures
By Q Lu, M Gittos, P Reccagni* and D Engelberg*
*University of Manchester
The demand for use of duplex stainless steels (DSSs) in the energy and petrochemical sectors has increased significantly in recent years due to several advantages of the material over austenitic stainless steel (ASS) grades, including their high mechanical strength and localised corrosion and stress corrosion cracking (SCC) resistance in chloride containing environments.
However, better understanding of the role of different phases in parent/weld metals and heat affected zone (HAZ) of DSSs on localised corrosion and environmentally-assisted cracking (EAC) in chloride-containing environments, is needed. The current project has focused on understanding the relationship between localised corrosion, EAC and microstructure susceptibility, especially HAZ microstructural features. Application of tools such as Gleeble simulation and scanning Kelvin probe force microscopy (SKPFM) were employed to simulate HAZ microstructures and to identify the inherent anodic and cathodic behaviour of different precipitates in the weld/HAZ, and the change of the electrochemical behaviour with environmental exposure time.
- The simulation of high-temperature HAZs (HT-HAZs) corresponding to different heat inputs revealed the progressive evolution of the microstructure, with the higher heat input promoting coarsening of the austenite plates and reduction in intragranular austenite content.
- Chromium nitrides were found in the simulated HT-HAZ, although their appearance is normally associated with low heat input and reduced austenite content. Despite the advantage of a coarse austenite morphology, the corrosion properties could be compromised by Cr2N, potentially offsetting any benefit.
- Arc welding causes microstructural changes and chemical partitioning in the HT-HAZ that have an effect on the electrochemical properties of the alloy. In particular, the reduction of galvanic activity measured suggests that the effectiveness of the electrochemical protection of ferrite by austenite is reduced.
- Hydrogen-induced stress cracking in DSSs is strongly driven by the ferrite content and morphology, with intragranular austenite and grain boundary austenite offering limited cracking resistance in the regions where the austenite fraction is heavily reduced.
- Coarser austenite grains are beneficial to HISC resistance but austenite can also become embrittled in environments in which hydrogen recombination poisons are present and/or high charging currents are employed.
- Different experimental techniques are available to evaluate the environmental resistance of DSSs and the methodology used strongly influences the observations, which are not always transferable between different methods.
Chromium (II) nitrides at the ferrite-to-ferrite phase boundaries. Overlapped, nitrogen elemental mapping with windowless EDX.
Quasi in-situ SEM straining of specimen taken from a TIG-welded pipe. Detail of the HAZ with crack nucleation at 6% strain.