Failure Analysis

Manufacturing Defects

Failures due to manufacturing defects are rare and seldom occur. Manufacturing defects are easily recognized and it is important that you understand what these defects look like if you are to file accurate claims for warranty reimbursement. No manufacturer is excluded from the possibility of defects in material or workmanship and the following failure examples include defects from all manufactures.

Figure 26 is an example of mill defects. Mill defects occur along one side of the rod body and these discontinuities normally have longitudinally tapered, sharp "V" shaped bottoms with indications of the longitudinal seam in the base. The example on the far left in Figure 26 is an example of a sliver. The rod body third from the left in Figure 26 is also an example of a sliver. When fishing the rod failure, the sliver folded against the fracture surface. The rod body second from right in Figure 26 is an example of a scab. A sliver is a small loose or torn segment and a scab is a large loose or torn segment of material longitudinally rolled into the surface of the bar. One end of the sliver or scab is normally metallurgically bonded into the rod body while the remaining end is rolled into the surface and physically attached. Fatigue failures, which result from slivers or scabs, will have a piece of loose material protruding over the fatigue portion of the fracture surface. The rod body second from the left in Figure 26 is an example of rolled-in-scale. Rolled-in-scale is a surface discontinuity caused when scale (metal oxide), formed during a prior heat, has not been removed prior to bar rolling. The rod body sample on the far right in Figure 21 is an example of a rolling lap. Rolling laps are longitudinal surface discontinuities that have the appearance of a seam from rolling, with sharp corners folded over and rolled into the bar surface without metallurgical bonding.

Figure 27 is an example of forging defects. The fracture begins internally below a forging crack in the upset area and is brittle or granular in appearance. A crack initiation site may or may not be visible and a fatigue portion may not be present on the fatigue fracture surface. The examples on the left and in the middle in Figure 27 occur as a result of low forging temperatures. The example on the left in Figure 27 is a failure from cold-shut and the example in the middle is a failure from a forging crack. The fracture on the right in Figure 21 is a failure caused by a subsurface longitudinal seam located near the end of the raw bar. During the forging process the orientation of this discontinuity was changed transversely.

Figure 28 is an example of incipient grain boundary melting, an extremely rare manufacturing defect. This condition is caused by forging the upset end of the rod at too high a temperature for the steel. Unfortunately, no inspection exists that will catch this before the rod is shipped. Fortunately, these brittle pins usually snap off during makeup. No crack initiation point is visible and no fatigue portion will be present on the fracture surface. Optical pyrometers on forging equipment will virtually eliminate this problem.

Figure 29 is an example of processing defects. The lower example in Figure 29 is a casehardened sucker rod and the upper example in Figure 29 is a coupling that has been processed through a grinding operation to reduce the diameter. In both examples, a difference in the material hardness has resulted in preferential corrosion attack.

Figure 30 is an example of a mill defect and a machining defect. The lower example in Figure 30 is a failure due to a large, internal, nonmetallic inclusion in the pin. The fracture began internally and is brittle or granular in appearance. A crack initiation site may or may not be visible and a fatigue portion may not be present on the fatigue fracture surface. The upper example in Figure 30 is from rolling the pin threads twice. Rolling twice has flattened the pin thread crest and will not be capable of achieving the correct friction load required for makeup.

Your initial investment in sucker rods is substantial. Moreover, the costs related to replacing damaged sucker rods generally out weigh the original cost of the new rod string. Protecting your investment and getting the maximum service life out of your rod string just makes good sense. It is important to diagnose rod failures accurately and to implement corrective action measures to prevent future failure occurrences. This photo essay is intended for use as a reference guide in sucker rod failure analysis. It explains how rod failures occur and expounds methods for identifying the characteristics of the several failure mechanisms. Where sucker rod failures are concerned, there are no absolutes and no two fractures look exactly alike in appearance. But, by recognizing the visual clues and identifying characteristics of the different failure mechanisms, corrective action measures can be taken to prevent sucker rod failures, thus allowing the operator to produce marginally profitable wells more cost effectively.