Failure Analysis of ERW Pipe Burst

Abstract:

The catastrophic burst of a piece of LS-125 ERW pipe was attributed to weld defects and low toughness along the fusion line. The fracture initiated at an ERW weld defect on the pipe inner surface. The microstructure was normal for quenched and tempered low alloy steel. Transverse mechanical properties across the ERW weld fusion line showed acceptable yield and tensile strengths but poor ductility.

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Introduction

An order of 7" diameter, 0.5 inch wall thickness LS-125 ERW pipe was processed for a well completion. After mill ultrasonic testing, the order was hydrostatically tested at the mill to 14,000 psi internal pressure.

Another ultrasonic examination of the pipe after failure revealed no defects in the failed pipe joint beyond about one foot from the end of the fracture. However, some defect indications were detected in lengths which had successfully passed a hydrostatic test.

This report details examination of examine failed and intact pieces of this lot of pipe to determine the cause of failure and why the pipe was not rejected on initial ultrasonic inspection.

Visual Examination

Fifteen inch long sections of the two mating halves of the fracture in the center of a burst section of pipe were sent for examination. Considerable corrosion damage had occurred on these sections, obscuring the fracture surface features to some extent. No chevron marks were present to indicate a definite local initiation site.

The fracture showed shear lip on both inside and outside surfaces everywhere except for a small area in the center on the inner surface (Figure 1). Here a smooth, black featureless zone was orientated parallel to the plane of the primary fracture (Figure 2). This zone consisted of two separate regions: a roughly triangular area with an i.d. surface length of 0.5 inch, a total of length of 0.8 inch, and a maximum depth of 0.3 inch, and an embedded 0.25 x 0.2 inch elliptical area centered at mid-wall.

In addition to the mating sections from the center of the fracture, three other sections of pipe were submitted for examination (Figure 3). The smallest section contained unfractured pipe which had survived the hydrostatic tests and yet showed indications on ultrasonic testing (Figure 4). Another section contained in the last six inches of the fracture and approximately two additional feet of unfracture pipe. Ultrasonic testing showed rejectable defects in the first foot of material beyond the end of the fracture. The third section had survived the hydrostatic test and showed no rejectable defects on the ultrasonic testing (Figure 5).

Metallographic Examination

A section through the initial flaw at the center of the fracture was polished and etched for metallographic examination. Figure 6 presents a photomicrograph from this section.

The profile through the initial flaw was flat and non-crystallographic, showing no evidence of welding or subsequent fracture. A thin surface scale was observed on this area. Below a depth of approximately 0.1 inch from the surface, fracture surface features began (Figure 6). In profile the fracture was non-branching with a mixture of transgranular propagation and plasticity on a microscopic scale.

Figure 7 shows a photomicrograph from the area rejected by ultrasonic inspection. A sharp, short mill discontinuity was present near the fusion line. The fusion line itself was visible only as a light-etching band less than 0.4 thousandths of an inch wide. Otherwise the structure through the weld seam was entirely uniform and typical of properly quenched and tempered low alloy steel (Figure 8). This structure was typical of all sections examined except section F, through the tip of the fracture.

Microhardness tests were performed on several of the metallographic sections. The base material well away from the weld showed an average microhardness of KHN_200g 331, which is approximately equivalent to Rockwell C 30. There were a few areas of apparent decarburization on the inner surfaces which showed hardnesses as low as KHN_200g 220. Otherwise, hardness throughout the cross-sections showed little variation except in the light-etching fusion zones. There hardness averaged KHN_200g 257, which is approximately R_C18. This low fusion line hardness was uniform throughout the cross-sections tested, varying little from I.D. to mid-wall.

On Section F, through the tip of the fracture, the crack ran precisely down the center of the fusion line in the center of the wall thickness. A thin (0.06 inch) bridge of unfractured steel still held at inner and outer surfaces. However, a shear lip had started to form on the inner surface, indicating that complete fracture of the remaining ligaments was imminent (Figure 9). The fracture profile was irregular and showed a number of voids and inclusions from the original weld (Figure 10).

Unlike the other sections examined, section F did not have low fusion line hardness. In the two small ligaments remaining, fusion line hardness ranged from KHN_200g 308-334, approximately R_c 27-30.

Scanning Electron Microscopy

Sections from near the fracture origin and from near the tip of the fracture were examined with a scanning electron microscope (SEM). In the fracture initiation zone (the initial flaw shown in Figure 2), the surface was covered with non-metallic inclusions and porosity (Figure 11). Patches of dimpled rupture and quasi-cleavage were also present. Energy-dispersive X-ray (EDAX) analysis of the non-metallic particles revealed no elements other than iron and manganese. This suggests that the particles are oxides. Near the tip of the crack, the primary fracture surface showed a nearly equal mixture of dimpled rupture and quasi-cleavage features (Figure 12). A number of sizeable voids and inclusions were observed on the fracture surface near the tip of the crack. Dimpled rupture covered the shear lips (Figure 13).

Mechanical Properties Tests

Three subsize round tensile specimens with 0.252 inch gage diameter were removed from material approximately 8 to 14 inches from the end of the fracture. The specimens were oriented transverse to the ERW seam with their gage sections centered on the weld, to determine mechanical properties of the weld parallel to the hoop stress direction.

Results of these mechanical properties tests were as follows:

The fracture in specimen 2 was in the base metal just outside the weld. The fractures in specimens 1 and 3 were both in the weld.

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