Low alloy steel/nickel base alloy dissimilar metal welds (DMWs) have been widely used in the oil and gas industries to join tubular components internally clad with Ni-base alloys and stainless steels. However, susceptibility to hydrogen assisted cracking (HAC) has been reported in DMWs exposed to hydrogen containing environments. The fusion boundary in such DMWs is highly susceptible to HAC due to formation of susceptible microstructural constituents which form during welding and post weld heat treatment (PWHT). Migration of carbon from low alloy steel towards the weld metal, formation of martensite and high level residual stresses in the heat affected zone (HAZ) all contribute to the increased HAC susceptibility along the fusion boundary. This study compares two test methods HAC susceptibility evaluation by testing a DMW between Grade F22 steel and Alloy 625 filler metal. These are the constant tensile load delayed hydrogen cracking test (DHCT) and the slow strain rate test (SSRT).
DMW of Grade F22 steel (ASTM A182) forging with Alloy 625 filler metal (ER-NiCrMo-3) was used in this study. The DMW was subjected to the PWHT at 923 K for 10 hours. Two test methods of DHCT and SSRT were employed to evaluate the HAC susceptibility. DHCT was performed under a constant load corresponding to the 90% of the base metal yield strength and SSRT was conducted at strain rate of 10-5 s-1. Hydrogen was introduced to specimens during tests with at current density of 10 mA/cm2. Optical microscopy and scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD) and energy-dispersive X-ray spectroscopy (EDS) were used to examine the fusion boundary microstructure and generate compositional EBSD maps before and after tests. Fracture surface of failed specimens was observed by SEM with an acceleration voltage of 20 kV.
Results of DHCT showed that the DMW is not susceptible to hydrogen assisted cracking and has not failed for more than 1200 h under sustained tensile load corresponding to the 90% of the base metal yield strength. In contrast, the same DMW failed in the SSRT and stress of 537 MPa, which is 20 % higher than the base metal yield strength. Fractography of the SSRT failed specimen exhibited brittle features on the fracture surface. Post-mortem microstructural characterization of the hydrogen fractured surface revealed surface cracking within the partially mixed zones and along carbides decorated planar solidification boundaries. Extensive network of finely distributed HACs was also found in the hydrogen charging exposed portion of the Alloy 625 weld metal and in the F22 steel HAZ.
The differences in the test results are related to different failure mechanisms introduced by the DHCT and the SSRT. In the DHCT, the hydrogen absorption in the tested specimen is controlled by gradient in the hydrogen concentration driven diffusion. The crack nucleation and propagation are controlled by local strass-strain field driven hydrogen accumulation. In contrast, hydrogen absorption in the SSRT is driven by transport with dislocations towards microstructural constituents acting as stress risers and strain concentrators. Therefore, the SSRT is thought to simulate an overload condition (loading beyond yield), which is not a typical service condition. Overload may also aid to explain the recent service failures in F22 steel/Alloy 625 DMW failures which cannot be replicated in DHCT. Therefore, selection of a suitable test as a means to replicate actual in-service conditions is important for accurate evaluation of the HAC susceptibility of DMWs.
In this study, the susceptibility to hydrogen assisted cracking of F22/Alloy 625 DMW was investigated using two test methods of DHCT and SSRT. Specimen tested under DHCT showed resistance to HAC while hydrogen significantly deteriorated the total elongation and ultimate tensile strength of the DMW in the SSRT. Hydrogen transport with dislocations under significant plastic strains introduced by the SSRT resulted in failure along the dissimilar fusion boundary and extensive microcracking in the exposed to hydrogen charging Alloy 625 weld metal and F22 steel HAZ.