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The effect of inclusions on the fracture toughness of line pipe

Fracture toughness test results.
The objective of the project was to measure the fracture toughness of local brittle zones (LBZs) in susceptible heat affected zones (HAZ) from gas metal arc welded (GMAW) and shielded metal arc welded (SMAW) girth welds in X70 pipe material, and to identify the cause(s) of these low toughness regions.

Background

Girth welds in modern line pipe steels generally exhibit excellent HAZ toughness, when tested at ordinary service temperatures. Offshore pipelines are usually designed down to temperatures of around -20°C, while land pipelines can be designed down to -40°C. At lower temperatures (for example, land pipelines in Arctic conditions), isolated cases where material fails to meet the required toughness specification may be recorded, related to the presence of local brittle zones in the HAZ. Frequency of these failures is often very low, and standard testing schemes, based on impact toughness testing during welding procedure qualification (WPQ) only, may be unable to detect this behaviour. Pipe girth welds were provided to TWI fabricated from material that was potentially susceptible to occasional incidence of brittle fracture in the HAZ at low temperatures. A research programme was proposed to capture and measure potential local brittle zones in the HAZs of these girth welds.

Techniques involved

A programme of testing and research was carried out to deliberately capture and measure potential LBZs in the HAZ of girth welds in a 20mm thick X70 grade pipe. This involved determining the most susceptible test temperature and notch location within the HAZ of two different welds by Charpy testing to define the T40J. Weld W01 was welded using GMAW and the other, W02, using SMAW. The Charpy notch position with the highest 40J transition temperature was determined to be FL+1mm for the GMAW weld (giving a T40J of -85°C), and the FL+0.5mm location for the SMAW weld (giving a T40J of -55°C).

Subsequent fracture toughness testing of a large set of fracture mechanics test specimens notched into these regions and tested at a temperature of T40J + 10°C was intended to produce occasional low fracture toughness results, which could then be investigated to identify the cause(s) of LBZs in girth weld HAZs. The fracture toughness test results were successfully able to achieve occasional low fracture toughness results (figure 1), including some pop-ins (figure 2). After post-test metallography it could be confirmed that the lowest toughness results coincided with the target weld regions in each weld.
Fracture surfaces of two specimens.
Fig 2: Fracture surfaces of two specimens from W02 showing oval-shaped pop-ins where the fracture morphology is shiny and facetted and outlined by a thin line of dull grey stable tearing. Arrows indicate the initiation positions. Scale is mm.
The fracture toughness results for the GMAW weld tested at -75°C showed that even where relatively high values of CTOD were obtained (greater than 0.2mm CTOD), the load versus displacement traces did not show much ductility, and most specimens failed to reach maximum load. The higher test temperature of the W02 specimens meant that more of the specimens showed fully ductile results, particularly when the actual notch position was more than half a millimetre from the fusion line. The HAZs of both the SMAW and GMAW welds showed occasional pop-ins and low fracture toughness results at test temperatures of -45 and -75°C respectively.

Microstructure and inclusions

Post-test metallography also identified that the parent metal contained a large number of clustered and significantly sized inclusions (figure 3). From examination of the fracture toughness test specimens that showed pop-ins, the HAZ region gave low toughness due to the parent material inclusion distribution rather than any other microstructural features. The large and clustered inclusions act as LBZs and/or fracture initiation sites. Regions were observed containing many similar inclusions rather than there being an even distribution of different types of inclusions in the material. This would indicate that there may be concerns over the temperature of the melt being too low to dissolve certain inclusions, or with the mixing of the melt prior to casting. Energy dispersive X-ray analysis of the inclusions identified the presence of manganese sulphide, titanium and niobium carbo-nitrides, aluminium and calcium. These inclusions were significant microstructural features, up to 10µm. The parent material composition and fine grain size were both typical of X70 steel and gave no indication that local brittle effects may occur in the HAZ when welded.
Clusters of inclusions in the parent X70 steel
Fig 3: Clusters of inclusions in the parent X70 steel
The parent metal microstructure has a fine effective grain size, however, grain coarsening in the HAZ may have occurred due to a localised reduction in grain pinning by inclusions from the intended level. Local coarsening of the inclusions would have resulted in an increase in the mean distance between the inclusions. Clustering of inclusions during processing would also have resulted in some relatively large localised inclusion free zones. Either mechanism would have resulted in a reduction in the grain pinning effect and an increase in the local grain size near the HAZ. The coarse grain region in the HAZ could also act as a low toughness fracture initiation region. The relative effect of these two mechanisms will depend on the welding procedure, the heat input of the welding process and hence the amount of grain coarsened HAZ in the welded joint. Hence the wide HAZ of the SMAW weld was associated with occasional fracture events mainly from the coarse grained HAZ, whereas the narrow HAZ of the GMAW weld showed occasional fracture events only associated with clustered inclusions.

Conclusions and recommendations

The following conclusions were drawn from the results of this work:

1. The Charpy impact tests on the girth weld HAZs in both welds were very good, with impact toughness in excess of 40J at temperatures as low as -100°C, with even the worst location giving T40J of -55°C.
2. The notch position with the highest 40J transition temperature was determined to be FL+1mm for the GMAW weld (-85°C) and FL+0.5mm for the SMAW weld (-55°C).
3. The parent material composition and grain size were both typical of X70 steel and gave no indication that local brittle effects may occur in the HAZ when welded.
4. Most of the fracture toughness tests gave reasonably high values of CTOD (above 0.2mm for the GMAW welds tested at -75°C, and above 0.45mm for the SMAW welds tested at -45°C). However, a number of specimens did give very low fracture toughness values (below 0.1mm) in both welds; either due to pop-ins or critical fracture events.
5. The low values of fracture toughness were attributed to the size and distribution of inclusions in the steel, and to grain coarsening in the HAZ microstructures.
6. Inclusions which were relatively large in size and/or clustered or aligned within the microstructure acted as planes of weakness to allow brittle crack initiation and propagation.
7. The size and distribution of inclusions was also ineffective at grain boundary pinning, such that they did not prevent grain growth within the HAZs.

TWI recommended that modifications be made to the steel making process to reduce the number of large and elongated inclusions in the steel, and to ensure they are well distributed throughout the steel. In addition, carrying out fracture toughness testing of specimens notched into the HAZ during development of new steels should be performed, so that the effectiveness of any changes in inclusion shape, size and distribution on HAZ fracture toughness can be quantified.

Reference

Philippa Moore & Joanna Nicholas 'The effects of inclusions on the fracture toughness of line pipe', OMAE 2013, 32nd International Conference on Ocean, Offshore and Arctic Engineering, Nantes, France, June 9-14 2013.

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