Evaluation of Hydrogen Assisted Cracking Susceptibility of Dissimilar Metal Welds for Subsea Service Under Cathodic Protection

James Rule The Ohio State University/Welding Engineering

1248 Arthur E Adams Dr Columbus, OH 43221

U.S.A.

Boian Alexandrov The Ohio State University/Welding Engineering

1248 Arthur E Adams Dr Columbus, OH 43221

U.S.A.

Ryan Buntain The Ohio State University/Welding Engineering

1248 Arthur E Adams Dr Columbus, OH 43221

U.S.A.

ABSTRACT The recently developed delayed hydrogen cracking test (DHCT) has been successfully utilized to qualitatively

rank the susceptibility of various steel (ASTM A182 F22, API 5L X65, AISI 8630, ASTM A694 F65) to nickel alloy (Alloy 625) dissimilar metal welds (DMWs) to hydrogen assisted cracking (HAC). This test consists of accelerated electrolytic charging with hydrogen of unnotched samples subject to a constant tensile load to replicate cathodic charging conditions that may be experienced in subsea service. The test method has proved to be sensitive to effects of welding procedure, weld sequence, and post weld heat treatment replicating the fracture morphologies of in-service DMW failures.

The current state of the test has established a methodology to develop a pass-fail criterion on which to provide a consistent quantitative measure of material/weld susceptibility to HAC. This methodology provides a link between microstructure, as influenced by material selection/combination, welding procedure, and post-weld heat treatment (PWHT), and diffusible hydrogen content.

Key words: [A list of relevant key words should be included after the abstract to facilitate searches. No trade names may appear in the key words.] "[Click here and type a list of key words.]"

INTRODUCTION

Development of the DHCT method was in direct response to HAC failures experienced in oil and gas subsea production systems (SPS) and has shown ability to replicate the failure mechanism and morphology, Figure 1, as well proven sensitivity to materials selection, weld procedure, and PWHT [1] [2]. The failures experienced in such welds are consistently limited to the interface/transition region between the steel base metal and nickel-base alloy filler metal. This transition region is usually narrow leading to steep compositional and microstructural gradient. Previous research has shown fresh martensite in the as-welded condition or a combination of fresh martensite and brittle M7C3 carbides in the PWHT condition to be the leading cause for HAC susceptibility of DMWs [1-9].

The Delayed Hydrogen Cracking Test Method The DHCT method was developed to allow for simultaneous hydrogen charging under a constant tensile load of the test specimen concept illustrated in Figure 1. This methodology simulates the external service conditions experienced in SPS where coating failure exposes bare metal which becomes susceptible to hydrogen absorption due electrolysis of the surrounding seawater by the sacrificial anode cathodic protection system. DMWs are particularly prone to HAC due to the composition and microstructure gradients which provide for relatively fast hydrogen diffusion through

Figure 1: Example of fracture morphologies experienced in failed subsea manifold butter weld.

the steel (ferritic) side and hydrogen pileup along the fusion boundary due to the much slower hydrogen diffusion through the nickel-base alloy (austenitic) side.

Figure 2: Illustration of the delayed hydrogen cracking test concept [1].

Previous work used the DHCT method to compare the performance of the DMW interface of Alloy 625 butter welds on two different steel substrates, AISI 8630 and ASTM A182 F22. Each material combination utilized two weld buttering procedures and were tested in the as-welded and PWHT conditions, illustrated in Figure 2. From this work, a qualitative ranking was established based on time to failure as a function of applied load which demonstrated the ASTM A182 F22|Alloy 625 bead sequence 3 (BS3) weld in the PWHT condition to be the most resistant whereas the AISI 8630|Alloy 625 bead sequence 1 (BS1) weld in the as-welded condition was the least resistant to HAC, shown graphically in Figure 3.

The DHCT

Figure 3: Illustration of butter weld procedure differences. Bead sequence 1 (left) offsets each weld pass along the axis of the pipe creating layers perpendicular to the pipe axis. Bead sequence 3 (right) offsets each weld pass perpendicular to the pipe axis creating layers parallel to the pipe axis.

Although the qualitative ranking agrees well with service experience, the test method does not provide a complete view of fitness for service of DMWs in hydrogen containing environments. Previous work by Bourgeois established a tentative time based criterion based on published upper and lower bound hydrogen diffusion rates in ferritic and austenitic materials [2]. The concept of this criterion is calculation of time required to saturate the DMW with hydrogen. The criterion attempted to account for the various features encountered in DMWs using average length ratios of microstructural constituents which established a global condition for all DMWs within the study. Within the work performed by Bourgeois it was well recognized the each DMW did not contain the same resultant microstructures. As such, it should be recognized that a global pass/fail criterion may not be representative of all DMWs leading to the need to develop a methodology for determining a pass/fail criterion specific to each DMW. This paper focuses on establishing and application of a specific pass/fail criterion for materials subjected to DHCT.

MATERIALS AND EXPERIMENTAL PROCEDURE Two steel base materials, AISI 8630 and ASTM A182 F22, were butter welded with Alloy 625 filler metal, compositions in Table 1. The welding procedure is limited to bead sequence 3 type as previous work has shown bead sequence 1 welds were prone to lack of fusion defects. The investigation includes welds in both the as-welded and PWHT conditions. PWHT was performed at 650°C for 10 hours. These welds were the same as utilized in previous work [2,3]. Table 1: Material compositions.

Material Compositions (wt.%) Material C Cr Fe Mn Mo Nb+Ta Ni P S Si

AISI 8630 0.32 0.97 Bal 0.93 0.41 0.004 0.82 0.008 0.003 0.35 ASTM A182 F22 0.13 2.37 Bal 0.53 1.02 0.019 0.09 0.007 0.004 0.26

Figure 4: DHCT results from previous work showing relative susceptibility to HAC of DMWs [2].

Alloy 625 0.02 21.7 0.4 0.1 8.5 3.8 Bal 0 0 0.14 Notes: Bal. : remaining balance to achieve 100%

Delayed Hydrogen Cracking Testing DHCT specimens were extracted normal to the weld fusion boundary as illustrated in Figure 4 using wire electrical discharge machining. The dimensions of the specimens are also provided in Figure 5. After machining, each specimen was power brushed with stainless steel wire brush using a rotary tool with the direction of brushing along the length of the specimen (perpendicular to the fusion boundary). After wire brushing, the gauge section of each specimen was ground using a series of SiC paper to a final 600 grit finish. Following grinding, the gauge sections were polished with 15μm diamond paste using a rotary tool until the surface exhibited uniformly bright metal. After final polishing, samples were cleaned and degreased and subject to coating/masking exposing only a 10mm band of the gauge section on the steel side of the fusion boundary. All fixtures and clevises were coated as well to isolate them from participation in the electrolytic charging. The coatings were allowed to cure for at least 24 hours prior to exposure to the environment.

Figure 5: Illustration of DHCT specimen extraction from DMW butter.

After coating, each DHCT specimen was assembled into the environmental cell. The power supply was then turned on and attached such that the DHCT specimen is the cathode and a spiral wound platinum wire was the anode. This ensures the cathodic current is supplied at all times during filling and testing such that corrosion of the exposed metal is avoided. The load was then applied to the specimens followed by filling the cell with the electrolyte, composition provided in Table 2. The test recording was started as soon as the electrolyte contacted the exposed metal surface of the DHCT specimen. The electrolyte was continuously circulated throughout the test duration at a pumping rate of 12mL/min. Time to failure was recorded using a continuously monitored linear variable displacement transducer recording position once per minute. Upon a fracture event, the test cell was drained and samples were cleaned using acetone and subjected to scanning electron microscopy (SEM) to characterize fracture morphology. Table 2: Dilute acid electrolyte composition. Total Solution Volume

H2SO4 Content Na2S2O3 content pH Charging Condition

10 L 35mL/L 0.1g/L 1.15-1.25 10mA/cm2 Hydrogen Charging and Measurement From the DHCT specimen machining, remnant material adjacent to the gauge section, shown in Figure 6, was utilized as hydrogen charging and measurement samples. These remnant samples were further sectioned down to replicate the exposed area of a typical DHCT specimen. Samples were cut with a water-cooled abrasive saw to reduce dimensions to nominally 3mm x 13mm x 25mm leaving a 1mm band of Alloy 625 weld metal with the remainder being the steel. Once sectioned, the edges of the samples were rounded using a water-cooled belt sander. The samples surfaces were prepared using the same procedure as DHCT surface preparation above. Upon reaching the final polished condition, each sample was weighed using an analytical scale and all dimensions measured using digital calipers for calculation of volume and surface area.

Figure 6: DHCT specimen dimensions.

Figure 7: Photograph showing the remnant material from DHCT specimen machining utilized for hydrogen charging experiments. Each sample was subject to electrolytic hydrogen charging using the same conditions for DHCT above (Table 2). Each unique DMW condition underwent hydrogen charging for various times across a logarithmic time scale to facilitate capture of the hydrogen saturation event using one test sample per target charging time. Once the charging duration was reached, samples were removed from the electrolyte and rinsed in acetone and dried. Samples were then placed in the stainless-steel desorption chamber within the hydrogen measurement device. The timing between the end of charging and the sample being placed in the measurement device was less than one minute to minimize initial unmeasured desorption. Diffusible hydrogen measurements utilized a thermal conductivity. The calibration curves for this work were established using a 100μL gas syringe with 2μL delineations (VICI050025). Calibration was performed for 10-100μL at 10μL increments prior to measurement of hydrogen for each sample. The temperature of the sample chamber and ambient pressures were recorded for each reading to allow for conversion to standard temperature and pressure (STP) conditions using the combined gas law, Equation 1. Flow rates were maintained at 30+/-5μL/min for each gas stream. Equation 1: Combined gas law showing the relationship of pressure (P), volume (V), and temperature (T) for a fixed mass of a substance. For conversion to standard temperature and pressure, P2 is replaced by 101.325kPa and T2 is replaced with 273.15K. P1, V1, and T1 come from those recorded during hydrogen measurement

Desorption of hydrogen was performed at 150°C (+/-10°C) over a 6-hour period in accordance with guidance from AWS A4.3 [10]. It is recognized that this methodology only provides a measure of diffusible hydrogen and does not account for hydrogen that may be trapped. During the desorption process, measurements were taken at fixed nominal intervals of 15th minute, 30th minute, 1st hour, 2nd hour, 3rd hour, 4th hour, and 6th hour. When the desorbed hydrogen is not being measured, the

desorption chamber is blocked in by a cross-over valve prohibiting the exit of hydrogen gas from the chamber. The process of collecting the volume of desorbed hydrogen for each interval reading involved allowing only reference gas to flow for the first minute to collect a consistent background signal of reference nitrogen only. The cross-over valve is then opened to allow the blocked-in hydrogen from the hydrogen desorption chamber to enter the sampling stream of the thermal conductivity detector and is left in this state for 320 seconds to provide a full volumetric release of the chamber. After 320 seconds the cross-over valve is closed to allow only reference gas flow where a background signal is collected for an additional 280 seconds. This methodology provided consistent time intervals of integration from the voltage-time signatures. To determine the saturation time, the total diffusible hydrogen of each charge duration was divided by the maximum diffusible hydrogen measurement of the set. These values were then plotted as a function of charging time. Based on a methodology established by Crank for adsorption/desorption from a plane sheet, an estimation of saturation was calculated using Equation 2 [11]. This calculation was performed for both a fitted logarithmic curve as well as from an interpolated fitted spline curve (direct fit to data points). These measurements were then plotted against DHCT results to test validity as a pass/fail criterion. Equation 2: Equation set for defining the diffusible hydrogen saturation time [11]. Resultant measurement curves correlate the time, t, it takes for the value of Cx,t/Cx=λ,∞=0.8920. The saturation condition is satisfied when the diffusion coefficient, D, multiplied by time divided by the length over which diffusion has occurred (λ) is unity.

Hardness Measurement and Categorization Hardness surveys were performed on a specimen with the same geometry as the diffusion specimen for each DMW combination and condition. The surveys were performed with a Vickers diamond indenter utilizing a 100-gram load. Each survey was setup as a 400-indent map of the surface as depicted in Figure 7. The spacing between indents in the area of the fusion boundary and heat-affected zone (HAZ) was 100 micrometers to capture possible locally hard spots generated during welding and PWHT. Outside of the HAZ into the unaffected base metal, spacing was increased to 300 micrometers and number of rows limited to 3 (total of 90 indents). The hardness data was then broken down as histograms binned every 25 Vickers hardness number (VHN).

RESULTS AND DISCUSSION Hydrogen Measurement Results

Figure 8: Illustration of hardness survey layouts.

Resultant diffusible hydrogen content measurement at each charging interval for each DMW is shown in Figure 8. From these measurements, the diffusible hydrogen saturation time is estimated using the methodology described previously, results summarized in Figure 9. It is clear that the as-welded condition for both DMWs result in higher peak diffusible hydrogen measurements. This behavior is likely due to the higher instances of untempered martensite as indicated by the hardness histograms shown in Figure 10. The trends of lower measured diffusible hydrogen in the PWHT condition disagrees with previous work by Olden which showed that in general as PWHT time or temperature increased, hydrogen content increased [12]. This difference may be due to Olden utilizing melt extraction which provides the total hydrogen content without delineation of trapped and diffusible (mobile) hydrogen whereas the current study only measures diffusible hydrogen extracted up to 150°C. The measurements performed by Olden were also limited to the fusion boundary area on the nickel filler metal side of the DMW interface providing no insight to the content that may exist on the base metal steel side of the interface. It is interesting that the F22|Alloy 625 bead sequence 3 DMW in the as-welded condition showed the highest diffusible hydrogen content. Again, this agrees with the hardness histograms where this DMW combination/condition recorded the hardness values corresponding with untempered martensite more frequently than all other welds. On the contrary, the F22 base metal contains a significant amount of strong carbide formers (Cr, Mo) which should increase total solubility of hydrogen but reduce the diffusible hydrogen due to the associated carbides forming trap sites [13-17]. This measured diffusible hydrogen may be due to performing desorption at 150°C where there is sufficient thermal energy for weak traps to release hydrogen. Lee et al., have found Cr-rich carbides are not as effective hydrogen traps as other carbides (V-, Mo-) showing Cr-rich type carbides to release most of the total absorbed hydrogen at temperatures below 150°C whereas the V-, Mo- maintain a significant amount of trapped hydrogen up to 500-600°C [15].

Figure 9: Diffusible hydrogen content measurements showing logarithmic curve fitting (top) and direct curve fitting (bottom).

Figure 10: Measured diffusible hydrogen saturation times and contents for each DMW.

Figure 11: Histogram of measured hardness of each DMW. The as-welded conditions demonstrate more instances of

high hardness values commensurate with untempered martensite. Correlation of Diffusible Hydrogen Measurements with DHCT DHCT results were compiled along with the hydrogen measurement results to gauge the usefulness of the diffusible hydrogen measurements towards establishing a pass/fail criterion. The DHCT datapoints utilize both historical data from studies by Bourgeois and Fink and Alexandrov, as well as new data from this study. Figure 11 shows the DHCT results (bars) against the calculated diffusible hydrogen saturation time (lines) for the the as-welded condition of the two DMWs. It is quite clear from this chart that the samples fail before the saturation time is reached in either case. Extending this to the PWHT conditions, Figures 12 & 13, there is evidence that the samples that do not fail due to HAC extend well beyond the diffusible hydrogen saturation time. The 8630|Alloy 625 BS3 PWHT testing, Figure 12, provides unique insight showing samples which do fail due to HAC, do so before or right up to the measured diffusible hydrogen saturation time.

CONCLUSIONS

Figure 12: Results of as-welded DMW testing showing HAC failure occurring before the diffusible hydrogen saturation time.

Figure 13: DHCT results for 8630|625 BS3 in PWHT condition (a). Failures beyond the diffusible hydrogen saturation time did not fail at the fusion boundary and were not HAC type failures exhibiting corrosion thinning and ductile overload fracture morphology (b). Failures occurring before or right up to the diffusible hydrogen saturation time exhibit brittle morphologies associated with HAC such as quasi-cleavage (c) and interfacial disbonding (d).

From the above, it is reasonable to establish a pass/fail criterion for DHCT based on diffusible hydrogen saturation time. From results, welds which regularly exhibit HAC failure did so prior to the diffusible hydrogen saturation time. For situations where HAC failure occurred beyond the saturation time, the time to failure was within about ~2x the saturation time. This implies that following exactly the procedure performed in this study, application of a safety factor of 2 to the measured diffusible hydrogen saturation time would provide confidence of the pass/fail criterion ability to predict performance in-service. Using this information, a summary of results table was constructed and color coded to provide a heat map of susceptibility the DMWs to HAC, Table 3. In doing so, the relative and absolute HAC susceptibility becomes readily apparent as well as the sensitivity of the DHCT method to material conditions/combinations. Although timing of the saturation event is highly dependent on service environment, the ability to withstand service loading beyond hydrogen saturation is material property dependent. As such, the use of the saturation event as the pass/fail criterion for DHCT should also serve as confidence of expected performance in actual service. Table 3: DHCT results summary showing load as a function of yield stress as well as the pass/fail criterion as established by the unique diffusible hydrogen saturation time.

DMW

2x Diffusible Hydrogen Saturation Time (hrs)

Applied Relative Load (% of Yield Stress)

40 50 60 70 80 90 92.5 95 ≥100

8630|Alloy 625 BS3 AW 483.7 - - - - - F - - -

8630|Alloy 625 BS3 PWHT 102.4 - NF NF F F F F F F

Figure 14: DHCT results for F22|Alloy 625 BS3 in PWHT condition. This DMW condition did not experience failures during testing.

F22|Alloy 625 BS3 AW 98.3 - - - F - F - - -

F22|Alloy 625 BS3 PWHT 228.6 - - - - - NF - - NF

Notes: -: Not Tested, NF: No Failure, F: Failure

CONCULSIONS

1. The delayed hydrogen cracking test has been successfully utilized to evaluate the susceptibility of DMWs to HAC. The test method has proven sensitivity to show the relative difference in HAC susceptibility based on materials selection, welding procedure, and PWHT conditions.

2. Each DMW condition displayed a unique diffusible hydrogen saturation time. The diffusible hydrogen saturation event correlates well with DHCT results showing it can be used as a pass/fail criterion as failures due to HAC in test happen before saturation. Failures which occurred beyond the saturation event were not attributable to HAC.

3. Use of the saturation event as a pass/fail criterion should prove useful in gauging fitness-for-service of DMWs in hydrogen environments as beyond saturation, under sustained conditions, there is no further driving force (uptake of hydrogen) towards increasing HAC susceptibility.

4. From the overall testing program, it is realized that the ASTM A182 F22|Alloy 625 BS3 PWHT DMW are more resistant to HAC compared to the 8630|Alloy 625 BS3 PWHT DMW.

5. Establishing recommendations for safe service loads for these DMWs, in terms of avoiding HAC failures during subsea service under cathodic protection, would require creation of extensive database of DHCT results.

ACKNOWLEDGEMENTS

This study has been supported by the NSF IU/CRC Materials and manufacturing joining innovation center (Ma2JIC). Project mentors include Technip-FMC, Shell Global Solutions, and Stress Engineering Services. Additionally, the authors wish to acknowledge the efforts of our undergraduate research assistant, Will Siefert, for sample preparation and assistance in testing.

REFERENCES [1] B. T. Alexandrov, S. Shi, J. M. Rodelas and J. C. Lippold, "A New Test For Evaluation of

Susceptibility to Hydrogen Assisted Cracking In Dissimilar Metal Welds," in Corrosion, Salt Lake City, Utah, 2012.

[2] D. Bourgeois, Hydrogen Assisted Crack in Dissimilar Metal Welds for Subsea Service under Cathodic Protection, PhD Dissertation, The Ohio State University, 2015.

[3] C. Fink and B. T. Alexandrov, "Effect of post-weld heat treatment on fusion boundary microstructure in dissimilar metal welds for subsea service," Materials Testing-Materials and Components Technology and Application, vol. 59, no. 6, pp. 547-554, 2017.

[4] V. Beaugrand, L. Smith and M. Gittos, "Hydrogen Embrittlement of 8630M/625 Subsea Dissimilar Joints: Factors that Influence the Performance," in Ocean, Offshore and Arctic Engineering, Honolulu, Hi, 2009.

[5] V. Beaugrand, L. Smith and M. Gittos, "Subsea Dissimilar Joints: Failure Mechanisms and Opportunities for Mitigation," in NACE Corrosion, San Antonio, TX, 2009.

[6] M. Dodge, H. Dong, M. Milititsky, R. Barnett, V. Marques and M. Gittos, "Environment-Induced Cracking in Weld Joints in Subsea Oil and Gas Systems - Part 1," in Ocean, Offshore and Arctic Engineering, Rio de Janeiro, Brazil, 2012.

[7] M. Dodge, H. Dong, M. Milititsky, R. Barnett and M. Gittos, "Environment-Induced Cracking in Weld Joints in Subsea Oil and Gas Systems - Part II," in Ocean, Offshore and Arctic Engineering, Nantes, France, 2013.

[8] J. Fenske, I. Robertson, A. Ayer, M. Hukle, D. Lillig and B. Newbury, "Microstructure and Hydrogen-Induced Failure Mechanisms in Fe and Ni Alloy Weldments," Metallurgical and Materials Transactions A, vol. 43A, pp. 3011-3022, 2012.

[9] J. Fenske, Microstructure and Hydrogen Induced Failure Mechanisms in Iron-Nickel Weldments, Urbana, IL: University of Illinois at Urbana-Champaign, 2010.

[10] M. Gittos, "Resistance of dissimilar joints between steel and nickel alloys to hydrogen-assisted cracking," in NACE Corrosion, New Orleans, La., 2008.

[11] L. Haldorsen, G. Rorvik, M. Dodge and K. Sotoudeh, "Recent Experiences with Cracking of Load Bearing Dissimilar Metal Welds on Subsea Production Systems," in Ocean, Offshore and Arctic Engineering, Trondheim, Norway, 2017.

[12] V. Olden, P. Kvaale, P. Simensen, S. Aaldstedt and J. Solberg, "The Effect of PWHT on the Material Properties and Micro Structure in Inconel 625 and Inconel 725 Buttered Joints," in Offshore Mechanics & Arctic Engineering, Cancun, Mexico, 2003.