Hydrogen embrittlement of 8630M/625 subsea dissimilar joints: Factors that influence the performance

Viviane C M Beaugrand, Lee S Smith and Mike F Gittos

Paper presented at ASME 28th International Conference on Ocean, Offshore and Arctic Engineering, OMAE 2009, Honolulu, Hawaii, 31 May - 5 June 2009. Paper # OMAE 2009 - 80030.

Abstract

Following the failure of a small number of subsea dissimilar joints, there is significant interest in understanding the fracture mechanism(s) and in qualifying future items for avoidance of further failures. Subsea dissimilar joints typically comprise a butter weld deposit onto the joint face of an alloy steel forged hub/tee/elbow, postweld heat treated, and completed with a closure weld to a pipeline steel. The case examined in this paper, consists of a build up of alloy 625 onto an 8630M forging. Hydrogen embrittlement test methods are described and the results from the tests interpreted with respect to their implications for fabrication and service. The microstructure and chemistry across the dissimilar interface is found to be of fundamental importance, and this is illustrated by the relative performance of gas-tungsten arc (GTA), hot-wire GTA and friction welds. Guidance for fabrication, installation and service is given for avoidance of further hydrogen embrittlement failures.

1. Introduction

1.1 Background:

A common subsea dissimilar joint comprises a shop fabricated, postweld heat treated hot wire GTA nickel alloy butter weld deposit on a low alloy forging steel fitting, and an as-welded field GTA/manual metal arc nickel alloy closure weld between the butter and, typically, a micro-alloyed steel pipeline. Thus, the heat affected zone (HAZ) of the completion field weld lies wholly in the nickel alloy butter layers and no postweld heat treatment (PWHT) of the field weld is required. As for any fusion welded dissimilar joint, a narrow, partially-mixed zone (PMZ) is observed at the dissimilar interface, where the composition is graded between that of the parent forging and the 'bulk' chemistry of the first layer buttering runs. In order to prevent corrosion of piping systems subsea, cathodic protection is applied. This does inhibit corrosion, but can result in a certain amount of hydrogen charging. Whilst the vast majority of subsea joints have given successful service, a small number has failed at the dissimilar interface, with significant consequence.

The work presented in this paper is part of a larger programme of work conducted at TWI on a range of subsea dissimilar joints. The microstructure and failure modes in commercially produced hot-wire GTA buttered 8630M dissimilar joints, has been detailed elsewhere,^[1] but is briefly summarized in the following text to establish the terminology and highlight the critical zones in the present paper.

1.2 Microstructure of 8630M/625 dissimilar joints:

The interface can be divided into six different microstructural zones, which are summarized in Figure 1 moving from low alloy steel to butter deposit:

1. Parent material of the 8630M forging. This zone exhibited a body centred cubic (BCC) ferritic microstructure, with a small volume fraction of inclusions and carbides;
   
   
2. A fine decarburised zone within the 8630M grain coarsened HAZ (GCHAZ) close to the dissimilar interface, with fingers of fused material penetrating into prior austenite grain boundaries (labelled Zone Δ). Again, this zone was mostly ferritic and BCC;
   
   
3. An iron-rich martensite region mostly at inter-pass positions (where 'swirls' of trapped steel extend a few tens of microns into the butter weld - see schematic drawing in Figure 1) extending from the dissimilar interface into thePMZ (labelled Zone M). This zone exhibited a body centred tetragonal (BCT) martensitic microstructure;
   
   
4. A hard, particle-free, supersaturated carbon solid solution in an 'austenitic' region of the PMZ that has probably solidified in a planar manner (a 'featureless zone', labelled Zone π would remain). Rotary friction welding of a solid alloy 625 directly to the steel forging achieves a more abrupt interface with no real PMZ, even though a mixed region (MR) was still observed. Thus, the effect of the carbon diffusion on the performance can be investigated isolated from any interaction with the chemistry of Zone Φ that might affect the carbon activity. Tests performed demonstrated that friction welds performance was similar to commercially produced welds that had been given a similar PWHT. The performance of the friction welds were significantly affected by the PWHT, demonstrating the final means of minimising susceptibility is through optimisation of the PWHT. Optimisation of heat treatment to result in lower carbon contents in the critical regions, whilst achieving the softening required in the steel HAZ would decrease susceptibility of the joints to hydrogen embrittlement. Work performed on clad steels for high temperature hydrogen plant^[3] has demonstrated the potential improvements to be had from modification of PWHT procedure. Again, reducing the carbon content of the forging would be of benefit by reducing the peak carbon content in the PMZ (by lowering the activity gradient). The effect of PWHT on commercially welds is currently being investigated.

    

   Appropriate qualification of fabrication procedures (on environmental performance) is necessary to give sufficient confidence in MIG or friction welding processes, since the relationship between chemistry, fabrication parameters, microstructure and performance is complex. Work is ongoing^[1] to define acceptance criteria and provide guidance on demonstrated solutions and enable confident design of robust dissimilar joints for subsea service.

8. Concluding remarks

The following conclusions are drawn:

1. The hydrogen embrittlement resistance of subsea dissimilar joints depends on three primary factors: (i) material selection, (ii) butter welding procedure, (iii) postweld heat treatment. Each of these governs the microstructure andchemistry across the dissimilar interface and unless specifically modified to take hydrogen embrittlement into account, can lead to a joint susceptible to environmental failure.
2. Two microstructural zones dominate fracture path in environmental tests: Zone Φ; a hard, carbon-supersaturated, austenitic, solid solution immediatelyadjacent to the fusion boundary, which fails by cleavage; and zone M, which contains martensite in highly-diluted weld material particularly at swirls of diluted steel penetrating into the butter deposit.
3. The present work demonstrated the difficulty of eliminating both Zone Φ and Zone M, simultaneously in arc welded butter deposits. Zone Φ could be eliminated by employing a low arc energy. However, Zone M, could only be limited by employing high arc energy. Thus, the elimination of a susceptiblemicrostructure at the dissimilar interface, at least between 8630M and alloy 625 butter deposits, could not be prevented by modifying welding parameters alone.
4. Hydrogen embrittlement resistance was improved by using low temperature PWHT, limiting the carbon-supersaturation of Zone Φ.
5. The best environmental performance was obtained for a friction welded dissimilar joint given a PWHT of 10 hours at 1100F, achieved by a combination of the elimination of Zones Φ and M, and the mitigation of carbon diffusion during PWHT.

10. Acknowledgments

The work was funded by Industrial Members of TWI as part of the Core Research Programme. The authors acknowledge the contribution of Peter Sketchley, David Seaman, Jerry Godden, Sheila Stevens, Briony Holmes and Henryk Pisarski.

11. References

1. Beaugrand VCM, Smith LS and Gittos MF, 22-26 March, Atlanta, Corrosion 2009, NACE International.
2. Perng T and Altstetter C, ASTM STP 962, 1988.
3. Gittos MF, 16-20 March 2008, New Orleans, Corrosion 2008, NACE International.
4. Asami K and Sakai T, Trans. of the Iron and Steel Institute of Japan, Vol 21, issue 6, 1981.
5. ASTM STP 668.
6. ASTM STP 527.
7. Anderson TL, Fracture mechanics 3d ed., 2005, CRC press, Taylor and Francis group.
8. Wang Z, Xu B and Ye C, 1993, Welding Journal Research Supplement, August, p.398-s.
9. Gnriss G, 2001, Conference IIW/IIS Ljubljana, July.
10. Baboian R, Corrosion tests and standards, 2nd ed., ASTM, 2005.