The Problem

The experts were contacted by a client who had detected a suspected crack in one of the welds in a large six-inch thick, heavy walled pressure vessel. Since the vessel (or tank) had an operating pressure of 1500 psi and an internal temperature of 400°F, catastrophic failure of this weld during plant operations could have resulted in significant property damage as well as possible serious personnel injury or even fatalities.

Client found the evidence of the cracking at the girth weld, a seam between the vessel’s tubular shell body and its lower hemispherical end. This was discovered during a routine, scheduled vessel inspection, using liquid dye penetrant for the examination. Since the vessel was steel, a Magnetic Particle (MP) examination could also have been used. The suspected crack was noted in the weld metal close to the shell body side of the weld. Upon closer examination, the client’s liquid dye test showed the existence of two separate indications of cracking.

 

The Solution

The experts were quickly contacted and engaged to verify and quantify the extent of the cracking, and to perform a failure analysis.

Our consultants examined the two locations identified by the client using the Ultrasonic Examination Technique (UT). This technique passes a beam of ultrasound into the metal, and the extent of cracking is then quantified by sound waves that bounce back to the UT test probe. Crack depths of 1.0” and 1.5” were verified, and the information was immediately relayed to the client. 

The use of X-ray, or Radiography (RT), was also considered in order to establish the depth of the cracking. The use of RT, however, is expensive since the wall thickness of this vessel would require a cobalt source emitting a high level of radiation, which would have necessitated a large area of the plant be cordoned off in order to prevent radiation exposure to the plant workers. As this vessel was an intrinsic part of client’s manufacturing process, the experts also had to use their tools to rapidly evaluate the situation, and quickly take corrective action. The use of RT (X-ray) however, is an excellent examination technique for hardware, and is similar to what is used extensively in the medical profession to detect and quantify bone fracture and other physical injury.

To determine the nature and the cause of the cracking, a sample of the cracked material was removed for metallographic examination. The sample in this case was a “Boat” sample, so named because of its boat-like shape. The sample was removed using a disc grinder, penetrated into the vessel’s surface on either side of the crack at an angle of about 30 degrees.

The Boat sample was then sent to a metallography laboratory where it was immediately sectioned further, perpendicular to the crack, to establish if the crack was singular or branched, if it was transgranular (passing through the metal grains) or intergranular (passing around the metal grains), or possibly a combination of both. Examination was performed on a highly polished surface using an optical microscope. A separate piece of the Boat sample was cut and broken open to examine the actual fracture surface. This was performed using an Optical Binocular Microscope. When higher magnifications are required, a Scanning Electron Microscope (SEM) would normally be used; in this case it was not necessary.

Using the selected metallographic techniques, the experts could establish the type or cause of the cracking, which might be:

- Brittle Cracking

- Ductile Tearing

- Fatigue Cracking

- Creep Rupture

- Trans- or Intergranular Corrosion Cracking

In some cases, the use of various chemical analysis techniques may also be used, especially if the cracking is caused by corrosion.

In this case the cracking was determined to be caused by fatigue, wherein the metal was stressed and relaxed repeatedly over a period of time, to the point that the fatigue strength of the metal was exceeded and cracking was initiated. Fatigue cracking can be initiated or exacerbated by a surface notch such as a scratch, a pre-existing flaw, or an abrupt change in the metal’s cross section. 

In this case, the experts ascertained the root cause to be ineffective cleaning/polishing of the weld surface after welding, which led to the premature fatigue cracking. 

 

Benefits Realized

Once the nature and cause of cracking was established, the cracking was removed and the weld was repaired.

The client was able to put the vessel back into service after the complete girth weld (original and repaired) was first dressed to remove any scratches or other discontinuities that might have provided a second initiation site of further fatigue cracking.

 

This article was written by Lead Consultant, Peter Habicht, and edited by Lead Consultant, Tom Weisgerber. Peter can be reached at 317-536-7044 or via email at PeterH@KevinKennedyAssociates.com. Tom can be reached at 317-536-7009 or via email at TomW@KevinKennedyAssociates.com.

 

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Peter Habicht, Lead Consultant
Peter specializes in welding and metallurgical engineer with 40 years industry experience in commercial nuclear power plant construction.

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