Evaluating Weld Metal Strength Mismatch in X100 Girth Welds

Proceedings of IPC 2004, International Pipeline Conference, October 4 - 8, 2004 Calgary, Alberta, Canada

Abstract

A relatively simple method based on standard fracture mechanics flaw assessment procedures, such as BS 7910, but modified using published mismatch limit load solutions is described. It is used to illustrate the effects of weld width and strength mismatch on CTOD requirements for girth welds in Grade X100 strength pipeline material subjected to axial stress. It is shown that fracture toughness requirements based on standard analyses not allowing for mismatch effects can be unnecessarily conservative when either undermatched or overmatched welds are present. Adverse effects of undermatching, in reducing the allowable stress, can be mitigated by reducing weld width. It is shown that even small amounts of overmatching (e.g. 10%) can be beneficial by allowing axial stress to exceed the SMYS of the parent pipe and reducing CTOD requirements.

Introduction

Use of Grade X100 strength pipeline steels for transmitting gas in harsh and environmentally sensitive regions, such as Northern Canada and Alaska, poses particular challenges for weld metal design. Welding consumable design for high strength steels is more highly constrained than is typical for more conventional pipe steels (e.g. X70 and below) in several ways.

The usual means of increasing strength is through increasing alloy levels. As strength levels increase, weld metal properties become far more sensitive to cooling rate variations than typical 70 or 80 series weld metals. Furthermore, alloying tends to increase the risk of weld metal hydrogen cracking and tends to limit the fracture toughness achievable. The tendency to higher toughness requirements for high strength steel applications is a furtherfactor influencing welding consumable development. Since the cooling rate sensitivity of weld metal properties increases with strength level, welding consumable development requires parallel development of corresponding welding processes and procedures to ensure the mechanical performance required. The practical result of such material design criteria is welding processes and procedures that are less user friendly necessitating far more care in implementation on the part of fabricators. Adherence to preheat/interpass temperature requirements and heat input limitations become far more critical to ensure the necessary weld properties and quality are achieved.

In general, it is far more difficult to achieve the desired balance of weld metal strength and toughness and nearly impossible to do so without reducing weldability and limiting usable procedure ranges in practice. Consequently, it is important to take a more holistic view of specification requirements in order to contain costs and minimize risks while ensuring that failure by fracture or plastic collapse will not occur under envisaged loading conditions. These would include not only internal pressure but external axial loading due to environmental effects such as ice, subsidence, and earthquake. However, in achieving the optimum mix of factors to achieve good weldability, freedom from significant flaws and good fracture toughness properties, and a high level of weld metal strength overmatch may not always be possible. For example, it may be more cost effective to redesign weld joint geometries and welding processes to ensure higher levels of weld quality than to elevate fracture toughness requirements. Furthermore, in order to avoid unnecessary delays and to maintain an economic welding process, it is unrealistic to repair every flaw indication reported by non-destructive testing.

This paper is intended as an example of how fracture mechanics can be used to examine the effects of such competing constraints on girth weld performance. Fracture mechanics provides relatively simple procedures for setting rational flaw acceptance criteria under various circumstances. Specifically, this paper examines the effects of strength mismatch on fracture resistance of girth welds in a Grade X100 pipe. Although a number of papers have been published to assess these effects, most are based on advanced methods which rely on finite element analysis that may be difficult to reproduce and where it is difficult for users to gauge what effect changes in key parameters have on the outcome without having to repeat the analyses. ^[1,2] An alternative is to evaluate effects by experiment using large-scale tests such as curved wide plates. ^[3] A limitation to such an approach is that, in practice, only a limited set of parameters can be tested and material/test variability can disguise important trends. The method described here is relatively simple to use as it is based on generally recognised standard methods and published solutions. It can be used to rapidly investigate expected trends as a result of changes in key parameters. In this paper the effect on fracture toughness requirements forgirth welds (weld metal and HAZ) in Grade X100 strength pipeline due to weld metal strength under and overmatching and interplay with weld width are explored for different applied axial stress regimes.

Assessment method

Equivalent material stress-strain curves

The procedures are based on the engineering critical assessment methods described in BS 7190 ^[4] but modified for strength mismatch effects in accordance with R6. ^[5] These modifications affect the shape of the failure assessment diagram (FAD) and the definition of L _r which accounts for plasticity effects. For simplicity, the material specific (Level 2B) FAD was employed which requires single values of fracture toughness. The procedure is also applicable to the Level 3B Fad, which requires input of a fracture toughness resistance curve and is appropriate when material is on the upper shelf and there is no risk of brittle fracture.

The main modification to the Level 2B FAD is the introduction of an 'equivalent' stress-strain curve to generate the FAD and definition of yield strength for the L _r axis. The equivalent stress-strain curve is derived from a weighted average of the weld metal and parent pipe stress-strain curves. The weighting is provided by the ratio of the mismatch limit load to the limit load for homogeneous material (i.e. no strength mismatch).

In this paper the mismatch ratio M is defined as:

M _x = σ _Yw / σ _Yb     (1)

Where σ _Yw or σ _Yb are the stresses corresponding to the same amount of plastic strain of the weld metal and base metal, respectively. The subscript x in M defines the plastic strain at which mismatch is defined.

Since the weld metal and base metal will, in general, work harden differently, the degree of mismatch will vary with plastic strain.

The equivalent stress-strain curve is given by ^[5] :

F _Lmis is the mismatch limit load. F _Lb is the limit load for a homogeneous component made of material with yield stress σ _Yb . The ratio ( F _Lmis / F _Lb is defined for each value of mismatch ratio M _x , at different values of plastic strain

The equivalent yield strength at 0.2% strain is given by:

An 'equivalent' flow strength,

, for the mismatched geometry is defined as: 

Where the term σ _b

is the flow strength of the material with the lower plastic strain at flow. Here, flow strength is defined as the average of the yield stress and ultimate tensile strength. 

Mismatch limit loads

Mismatch limit loads are given in R6 ^[5] for fully circumferential internal cracks in cylinders; these are taken from work by Schwalbe et al. ^[6] (As far as is known, there are currently no general mismatch limit load solutions for finite length flaws in girth welds in the public domain.) This geometry was considered representative of a long surface flaw in a pipelinegirth weld. The mismatch limit load solutions consider parallel sided welds and are for weld metal centreline flaws and flaws located at the interface (in this paper, the interface is considered to be representative of the HAZ/fusionline). The choice of limit load depends on the expected deformation pattern. For undermatched welds (M<1) two deformation patterns are possible. Deformation pattern A: deformation is confined to the weld metal. Deformation patternB: deformation penetrates into the base metal, as illustrated in Fig.1.

Fig.1. Possible deformation patterns in undermatched welds

For overmatched welds (M>1), again two deformation patterns are possible. Deformation pattern C: deformation from the crack tip penetrates through weld metal into the base metal (like pattern B in Fig.1). Deformation pattern D: deformation takes place in the lower strength base metal, away from the crack; gross deformation does not occur in the weld metal.

The governing equations are as follows.

Weld centreline flaw

Deformation pattern A (undermatching, see Fig.1):

The ratio ψ is given by:

where W is pipe wall thickness, a is crack depth and H is half the weld width.

Deformation pattern B (undermatching, see Fig.1):

Deformation pattern C (overmatching):

where

Deformation pattern D (overmatching): 

Interface Flaw

Deformation pattern A (undermatching weld metal):

Deformation pattern B (undermatching weld metal):

Interface flaw in overmatched welds: 

The superscripts 'con' and 'pen' denote deformation confined to or penetrating into base metal, respectively.

The limit load solution for homogeneous material is given by:

Where R _o and R _I are the external and internal radii of the cylinder, respectively, and a is flaw depth.

Of the above mismatch limit load equations, the actual limit load is the smaller of and . The appropriate mismatch corrected limit load solution is then used to derive the equivalent stress-strain (Eq.(2)) and the assessment line in the FAD and for deriving L _r .

Material properties

Engineering stress-strain curves were obtained from a Grade X100 pipe with an outside diameter of 915mm and wall thickness of 19mm using 8.9mm diameter specimens. All weld metal stress-strain curves were obtained from welds made in the Grade X100 pipe in a V preparation with 60° included angle using two welding processes employed to fabricate girth welds. These were a GMAW process with 95% Ar 5% CO ₂ shielding gas at a heat input of 0.94J/mm, and a FCAW process with 75% Ar 25% CO ₂ shielding gas at a heat input of 2.2kJ/mm. The consumables and welding procedures were designed to provide tensile properties that closely matched and undermatched the SMYS of the pipe. The result was weld metal that undermatched the actual strength of the pipe. The tensile properties are summarised in Table 1 and stress-strain curves are compared in Fig.2. Based on yield strength, the levels of undermatching achieved were M = 0.83 and 0.685 in the two welds.

Table 1 Tensile properties

* Artificially created

Fig.2. Experimental stress-strain curves for parent pipe, GMAW and FCAW girth welds

10% overmatching (M = 1.1) was artificially created for the purposes of analysis by assuming the weld metal has the same stress-strain curve as the parent pipe but uplifted by 10%.

Fracture mechanics analyses

The mismatch corrected or equivalent stress-strain curves were derived from the experimental stress-strain curves using the procedures described previously. The flaw cases considered were an internal circumferential flaw located in a girth weld in a Grade X100 pipe with a diameter of 915mm and wall thickness of 19mm. For the purposes of assessment, the girth weld was assumed to be parallel sided and weld widths (2H) of 5, 10 and 20mm were considered. A flaw, 3mmhigh (intended to represent a welding flaw equivalent to a typical weld bead height) was located at the weld centreline and fusion line. With this information material specific FADs to BS 7910 ^[4] (Level 2B) were generated for each mismatch case. The L _rmax cut-off was set to the ratio of equivalent tensile strength to yield strength, as suggested in DNV OS-F101:2000 ^[7] . This enables the L _rmax cut-off to be extended beyond that recommended in BS 7910 where it is set to the material flow strength defined as the average of yield and tensile strengths. Analyses were conducted using TWI software Crackwise 3 which automates many of the procedures in BS 7910. They were conducted to establish the relationship between the driving force or required fracture toughness (in this case CTOD) and applied stresses. For simplicity, the analyses were conducted assuming that no residual stresses are present. In practice residual stresses would need to be considered. However, further work is necessary to quantify appropriate transverse residual stresses in Grade X100pipe girth welds for the cases considered here.

Results of fracture assessment

Figures 3 and 4 show how the allowable axial stresses (represented as the ratio of allowable stress to the SMYS of Grade X100 strength pipe; i.e. 690MPa) changes with mismatch level in different weld widths. In these figures the mismatch ratio represents actual mismatch at the yield stress (M _0.2 ). The assessments were conducted assuming that a minimum CTOD of 0.25mm at the minimum design temperature is achieved in both weld metal and HAZ. An initial analysis was conducted assuming that the flaw is inhomogeneous material (no strength mismatch) with tensile properties equal to the lowest strength present. This is the recommended procedure in BS 7910 when mismatch limit load solutions are not available. As can be seen, this provides,as intended, a conservative estimate of allowable axial stress. However, if mismatch and weld width effects are specifically considered in the analyses, benefits to the allowable stress are realised as the weld width is reduced from 20to 5mm. Although the allowable stress is increased for undermatched welds (M<1) it always remains below that for matched and overmatched welds (M ≥1). When overmatching is present (M>1) and the flaw is located in weld metal, increasing the weld width permits the allowable stresses to beincreased. Consequently, when the differences in weld metal and parent pipe strengths are small (M close to 1), the best strategy is to use a narrow weld. If slight undermatching occurs, the reduction in allowable stresses is minimised, but there is still a benefit in allowable stress (> pipe SMYS) if overmatching occurs.

Fig.3. Ratio of allowable axial stress (P _m ) to SMYS of Grade X100 pipe for circumferential weld metal flaws 3mm high at different levels of strength mismatch and weld widths (2H)

Fig.4. Ratio of allowable stress (P _m ) to SMYS of Grade X100 pipe for circumferential HAZ flaws 3mm high at different levels of strength mismatch and weld widths (2H)

According to the limit load solutions currently available, weld metal overmatching confers no benefit for interface (HAZ) flaws. However, because the HAZ usually has a higher strength than the base metal, there will be some benefit. It may be noted that for a given level of undermatching, HAZ flaws are more sensitive to weld width effects than are weld metal flaws. Nevertheless, the analyses show a clear benefit of using narrow welds if undermatching is likely.

Figures 5 and 6 show how the CTOD requirement (CTOD driving force) varies with applied axial stress for different weld widths and mismatch levels. Also included are analyses for homogeneous material, assuming no mismatch effects. These provide bounds to the analyses and illustrate the benefits of allowing for mismatch effects.

Fig.5. Relation between CTOD requirement and ratio of axial stress (P _m ) to Grade X100 SMYS for weld metal flaws with different mismatch levels (M) and weld widths (2H). Note, curves for M=0.85, 2H=10 and M=0.685, 2H=5 are nearly coincident

Fig.6. Relation between CTOD requirement and ratio of axial stress (P _m ) to Grade X100 SMYS for HAZ flaws (3mm high) with different mismatch levels (M) and weld widths (2H)

The figures show that CTOD driving force (or fracture toughness requirements) increases more rapidly than the increase in allowable stress. However, a limit appears to be reached when further increases in fracture toughness have no benefit on allowable stress. This condition is obtained when failure is predicted to occur by plastic collapse rather than fracture. However, in practice crack growth by stable ductile tearing will occur before plastic collapse and this will affect the analyses at higher CTOD values (say above about 0.3mm when initiation by ductile tearing is likely to have taken place). To assess this condition a tearing (Level 3B) analysis ^[4] is required, but this is beyond the scope of this paper. However, a tearing analysis would be beneficial because a CTOD-resistance curve with a positive slope would enable higher axial stresses to be tolerated, provided that limited ductile tearing is accepted in the pipeline girth weld.

The analyses show the clear benefit of just small amounts of weld strength overmatch, in this case just 10% actual strength overmatch, on the allowable stress when considering weld metal flaws. Indeed, for the case considered, axial stresses above the pipe SMYS are acceptable even with a 3mm high flaw present in the weld metal provided that CTOD is greater than about 0.1mm.

The analyses illustrate how weld metal (but not necessarily HAZ) fracture toughness requirements can be reduced if specific allowance is made of the benefits of over matching. Most specifications are based on analyses that do not consider mismatch effects. Figure 5 shows, for example, that for an axial stress equal to base metal SMYS, the CTOD requirement based on parent pipe strength (M=1), would be 0.14mm to avoid failure from a 3mm high flaw. However, even with 10%overmatching, the CTOD requirement can be reduced to between 0.09 to 0.06mm (depending on weld width) if specific allowance is made for mismatch effects.

The analyses also show how fracture toughness requirements need to be increased, to achieve the same flaw tolerance, if undermatching is present. However, the increase would not be as great as predicted by an analysis based on the least strong material. For example, Fig.5 and 6 show that for M = 0.83, an analysis based on the GMAW weld metal would require a CTOD of 0.25mm from the weld metal and HAZ for an axial stress 85% of SMYS. By specifically analysing for the undermatched condition (M =0.83), the CTOD requirement can be reduced to about 0.05mm for both weld metal and HAZ. (Although fracture toughness determination is not considered in this paper, it should be noted that if the HAZ strength undermatches the strength of both weld metal and parent pipe, the likelihood of cleavage can be increased. This can result in very low fracture toughness values being measured in the HAZ. ^[8] )

The analyses of weld metal flaws show that overmatched welds could be used in situations where axial strain rather than stress govern design criteria. To analyse these conditions, a strain rather than stress based fracture mechanics procedure would be preferable. ^[1] Furthermore, in a real life situation, the effects of internal pressure plus axial loading, and their implications on constraint, as well as residual stresses would need to be considered in deriving fracture toughness requirements. Nevertheless, the relatively simple stress-based procedures described here are sufficient to illustrate the effects of mismatch and weld width on fracture toughness requirements to avoid fracture in girth welds.

Concluding remarks

A relatively simple method based on recognised fracture mechanics flaw assessment procedures which utilises published mismatch limit load solutions has been described to illustrate the effects of weld width and strength mismatch on the CTOD requirements for girth welds in Grade X100 strength pipeline material subjected to axial stresses.

It is shown that CTOD requirements derived from analyses based on homogeneous material (i.e. making no allowance for mismatch) are conservative and can be reduced if mismatch effects are specifically considered.

Analyses show that the adverse effects of weld strength undermatching (e.g. reducing allowable stress) can be mitigated by reducing weld width and by specifically considering mismatch in the analyses. The analyses also show the benefits of even small amounts of weld metal overmatching on the maximum axial stress that can be tolerated. For the case considered, 10% weld metal overmatching means that axial strains above actual yield strength can be tolerated in the parent pipe when a 3mm circumferential flaw is present (provided that the CTOD is more than ~0.1mm).

These analyses illustrate the use of fracture mechanics to examine the effects of such competing constraints on girth weld performance. Future applications the same methodology could be used to assess the influence of HAZ softening, flaw acceptance criteria, etc.

References

1. Wang, Y-Y., and Horsley, D,, 2004 'Weld mismatch effects of strain behaviour of flaws', 4th International Conference on Pipeline Technology, 9-13 May 2004, Ostend, Belgium, Vol.1, pp.267-278.
2. Ishikawa, R., Endo, S., Igi, S., Glover, A., Horsley, D., Ohatu and Toyoda, M., 2004, 'Ductile fracture behaviour of girth-welded joints and strain-based design for high strength line pipe', 4th International Pipeline Conference on Pipeline Technology, 9-13 May 2004, Ostend, Belgium, Vol.1, pp.81-98.
3. Denys, R., De Waale, W., Lefevre, A., and De Boets, P., 2004 'Weld strength mismatch effects on plastic straining capacity of axially loaded pipes', 4th International Pipeline Conference on Pipeline Technology, 9-13 May 2004, Ostend, Belgium, Vol.1, pp.209-234.
4. BS 7910:1999 (incorporating Amendment No.1), 'Guide on methods for assessing the acceptability of flaws in metallic structures', British Standards Institute, London, October 2000.
5. R6 Revision 4, 2001, 'Assessment of the integrity of structures containing defects', British Energy Ltd, April 2001.
6. Schwalbe, K-H., Kim, J.J., Hao, S., Cornec, A., and Koçak., 1997, 'EFAM ETM-MM96 - The ETM method for assessing the significance of crack-like defects in joints with mechanical heterogeneity (strength mismatch)', GKSS 97/E/9, GKSS, Geesthacht, Germany.
7. OS-F101, 2000, 'Submarine pipeline systems', Det Norske Veritas, Hovik, Norway.
8. Pisarski, H.G ., and Harrison, P.L., 2002, 'An investigation into the effect of weld strength mismatch on the assessment of HAZ fracture toughness', ECF14 - Fracture Mechanics Beyond 2000, Krakow, Poland, ESIS - ASTM, September 2002, Vol.2, pp.677-686.