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Incomplete Weld in a Carbon Steel Gas Main Pipe

ENVIRONMENT:

Underground

EQUIPMENT:

Gas Main Pipe

MATERIAL:

Carbon Steel
SERVICE TIME: approximately fifty years

FAILURE:

Incomplete Weld

 

Summary

A section of 8-inch diameter steel lap-welded gas pipeline, that leaked, was evaluated.  No gross defects or fractures were found, however the plane of the lap weld contained numerous areas of lack-of-fusion defects as well as excessive amounts of non-metallic inclusions.  These lack-of fusion defects were believed to be interconnected and to be the primary cause of the gas leakage.  Impact fracture tests made on this pipe indicated that the pipe will fracture in a brittle fashion at room temperature or below.

 

Background

A routine leak detection survey discovered a natural gas leak in an underground pipeline. After excavation of the leak area, the leak site was pinpointed using soap and water bubble indication. A three-foot section of the pipe containing the leak was cut out for evaluation, as were three other nearby sections, for comparison.  The bitumastic coating was removed from the pipe sections in the field.  The four pipe sections were brought to Corrosion Testing Laboratories for analysis and determination of the cause of the leak.

 

Description of Pipe

The as-received pipe section samples were numbered 1 through 4.  Each pipe section was about 3 feet long.  The location of the leak was marked on pipe section #2.  Pipe section #s 2 and 3 were removed from the same pipe joint.  Pipe section #s 1 and 4 were removed from, respectively, the pipe joints located to the west and to the east of the leaking joint.  This piping was steel lap welded and 8-inches in diameter.  Most of the bitumastic coating had been removed from sample #2. Less of the coating had been removed from the other samples. The exterior surfaces of the pipe did not appear to be significantly corroded.

The steel pipe was purchased as API 5L and was lap welded and was believed to have been in the ground for nearly fifty years. It was originally coated, and in recent years had an impressed current cathodic protection system. The operating internal gas pressure was 380 psi.

 

Findings

Visual & Macro-Examination

The pipe sections are similar in surface appearance. The pipes are superficially rusty on the outer surface and have considerable coating remaining on the surface. The interior surfaces of the pipe sections are rusty, but not excessively so.  The lap welded pipe and the location of the lap weld is indicated by a line of cross hatching marks made by the pressure rolls applied during the lap welding process.  There are intermittent longitudinal grooves along each edge of the lap weld about 1 inch apart, see Figure 1.

 

Figure 1. Close-up of leak area on pipe section #2. Note also cross-hatching marks from pressure rolls during welding.

 

The area on pipe sample #2 indicated to be the leak zone, Figure 1, was scrutinized under the binocular microscope at magnifications up to 40X but no visible crack or metal discontinuity was observed in the leak zone.  This area was further subjected to dye penetrant testing, but no cracks or discontinuities were found using this technique.  The indicated leak zone of pipe sample #2 was cleaned using ultrasound and citric acid and when it was reexamined under the binocular microscope, no cracks or discontinuities were observed.

Metallography

A section from pipe #2 containing the leak area was cut out and macro etched to reveal the location and profile of the lap weld.   Then this piece was mounted and polished for microscopic examination. The unetched weld cross section was observed, Figure 2. Several areas of lack-of-complete-weld-fusion were visible along the lap weld plane.  The entire lap weld cross section was seen.  Approximately 20% of the lap weld was not fused.  Figure 3 showed the same area after etching to reveal the metallographic structure.  In addition to the voids along the weld plane resulting from lack of weld fusion, there was an almost continuous line of heavy sulfide inclusions all along the weld fusion line. 

 

Figure 2. Pipe section #2.  Unetched transverse cross section through weld showing lack of fusion and sulfide inclusions. (25X Original Magnification)

Figure 3. Pipe section #2: Transverse cross-section through weld.   Microstructure revealed by etching with 2% nital.  Note string of gray sulfide inclusions along fusion line. (100X Original Magnification)

 

Metallographic structure of the same pipe sample from the side opposite the weld. The unwelded side of the pipe shows a typical ferritic microstructure with patches of pearlite. This is the normal microstructure of annealed low carbon steel.  Heating to the welding temperature along the weld lap allows some decarburization of the pipe material in the weld zone, and thus there is less pearlite present in the microstructure and the ferritic grain size is somewhat larger.

Microscopic examination of the as-polished longitudinal section of the pipe permits an assessment of the quantity of non-metallic inclusions present in the steel.  This is fairly dirty steel, with lots of manganese sulfides and alumina inclusions and some oxides.  However it is probably typical for the period in which this steel was manufactured.

Chemical Analysis

Samples from the pipe sections were chemically analyzed and were determined to be in compliance with API 5L requirements.

Mechanical Properties

Longitudinal samples were cut from pipe sections #1, #2 and #4 at the lap weld line and opposite the weld line. The purpose of testing both the weld zone and the non-welded zone was to define any property differences in the two areas.  Standard tensile tests were made in accordance with ASTM A370, from which the ultimate tensile strength, the 0.2% yield strength, the reduction in area, and elongation in 2 inches were measured. 

API 5L standard for pipe has two levels of strength, grades A and B.  This pipe barely meets the requirements of the lower strength A grade, and in fact the #2 pipe, which leaked, has a yield strength slightly below the specified minimum.  Otherwise the mechanical properties look normal for this grade of pipe.

Notched Impact Tests

Samples were taken from the lap weld area and from the side opposite the lap weld.  These slotted samples were fractured by impact at varying test temperatures.  The fracture appearance of the broken pieces was recorded in terms of the percent of the fracture area exhibiting ductile fracture characteristics. 

The test results show that all three pipe samples exhibit predominately brittle fracture characteristics under impact loading at room temperature and below.  Above room temperature the fractures become increasingly ductile and at 150F are predominantly ductile. Figure 4 illustrates the separation of the lap weld in the impact sample from the lap weld area of pipe.

 

Figure 4. Impact fracture of pipe section #2 at the lap weld line.  Note weld separation starting at OD. surface  and extending part of the way through the pipe wall. (4X Original Magnification)
 

Bend Test

An extra impact specimen prepared from pipe section #2 was left un-notched and subjected to a bend test in which it was slowly bent through a 45 angle.  Examination of the bent specimen showed nearly complete separation of the edges of the plate in the weld zone, indicating lack of fusion in the weld.  Figure 5 shows the bend test specimen after bending.

 

Figure 5. Bend specimen taken from lap weld on pipe section #2.  Bending of specimen caused separation along pipe edges in weld zone (arrows), indicating lack of fusion.  Pipe OD. is at bottom.  Left arrow points to transverse cross-sectional face.  (2X Original Magnification)

 

Discussion

Visual and macroscopic examination failed to reveal any gross defect or fracture in pipe section #2 that might account for the leakage.  It was also noted that corrosion, either external or internal, did not appear to be a factor.

The metallography of the lap weld cross-sections showed that the weld fusion in the lap weld zone was incomplete, and further that the lap weld plane was infused with an excess of non-metallic inclusions.  This condition, while not leading to fracture of the pipe, probably provided minute passages for gas leakage.

Continuous lap welded pipe is a process which involves rolling the edges of the pipe scalp to a taper, then rounding up the cylinder, superheating the thinned edges to the welding temperature and applying pressure to fuse the tapered edges together.  It is a form of forge welding no longer used in pipe production.

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