Interesting & Intriguing Case Studies
New Castle, Delaware-based Lehigh Testing Laboratories has taken on some pretty fascinating projects for clients over the years. The following case studies are a few of the highlights.
Detonator Cap Fusion Failure
A manufacturer of detonator caps was failing to achieve 100 percent fusion during welding with components supplied during a recent manufacturing period. No such difficulties were observed in material from previous runs. Assembled caps and individual components were supplied for comparative metallurgical evaluation.
The recent plug was observed to have an oxidized surface. Oxidation was sufficient to hide machining marks observed on previous plugs. Microscopic examination of fabricated welded caps revealed incomplete fusion in the weld (as evidenced by the presence of a crack at the weld root) and oxidized thermal cracks along the length of the recent plug surfaces.
These features were not found in the previously examined cap. The welding difficulties were attributed to oxidized plugs supplied in the recent manufacturing period.
Gymnasium Roof Collapse
A high school gymnasium roof in Virginia collapsed under a load of over 20 inches of snow. The vertical support in a roof truss had buckled. Cell phone pictures taken while the gymnasium was evacuated showed the top of a roof truss pulling to the side just before roof failure.
Evaluation of truss structural members revealed that the collapse occurred when the truss vertical support buckled, with the vertical member bent below its base support. At that point, the support fractured and the now unsupported roof collapsed.
The evaluation identified the following three critical deficiencies in construction and design, which contributed to the failure:
1. Truss support members did not contain gussets. Gussets are oriented perpendicular to the support member and allow supports to sustain lateral loading without movement in that direction.
2. The failed vertical structural member was deficient in strength. Yield strength measured 32.8 ksi (kilopounds per square inch), 9 percent below the minimum specified yield strength of 36 ksi.
3. Lack of fusion was observed along fracture surfaces due to a lack of penetration through the cross-sections. ANSI/AWS D1.1 requires full penetration, double butt welds for these truss structural supports.
Impeller Shaft Failure
Condenser cooling tower impeller shafts exhibited evident corrosion pitting damage on seats adjacent to impellers during a maintenance outage. The system used three pumps. One pump was operational while the other two were dormant. Switching between pumps occurred infrequently. The shaft had been made with Type 416 resulfurized martensitic steel.
Evaluation of the porous pits using energy dispersive spectroscopy (EDS) in a scanning electron microscope found the unexpected presence of significant phosphorus. Unetched cross-sections taken through the pitted region revealed that the pits were surrounded by porous material. The porous regions were bordered by the unaffected matrix.
The structure after etching was observed to be a combination of alternating ferrite (light) and martensite plates, a morphology indicating the shaft had been annealed. Preferential attack was observed along the martensitic plates.
The evaluation indicated corrosion occurred as a result of microbiologically-induced corrosion (MIC) with the high phosphorus content from the residue of anaerobic bacteria, which had attacked the structure. MIC occurred because shafts sat in dormant water for long periods of time. The customer has increased frequency of operation of the replacement shafts to prevent the long-term stagnant conditions that lead to MIC.
Refinery Pump Stud Failure
Tee 7/8"-diameter studs used in a refinery ALKY acid regeneration pump fractured at the first thread loaded by the nut. The studs were used in HF (hydrofluoric acid) service. Studs were specified to be made from Monel 400 nickel-based alloy.
Optically, the fracture surface showed no evidence of progressive fracture from a source such as fatigue. However, copper had plated onto the rim of the fracture surface. The fracture, as viewed in a scanning electron microscope, had a rock-candy appearance, which identified the fracture mode as intergranular.
The evaluation identified the following three critical deficiencies in construction and design which contributed to the failure:
1. The presence of copper around the rim of the fracture indicates migration had occurred from cathodic protection.
2. Cathodic protection is typical and expected in this application. However, Monel K-500 is prone to hydrogen embrittlement where Monel 400 is not.
3. The cause of fracture occurred due to the use of incorrect material for the environment.