A steam line drip leg was found to be leaking during service. The drip leg is a steel pot used to collect condensate from a steam system. The expected operating conditions for the pot were 1050°F and 450 psig. After approximately 7 years in service, a cracked weld seam was discovered and repaired. Three years later, multiple cracks and a large bulge were reported in the drip leg and a failure analysis was conducted. The objectives of the evaluation were to determine the failure mechanism(s) for the pot and to identify any manufacturing, material or service conditions that could have contributed to the failure. The evaluation included visual inspection, light microscopy, scanning electron microscopy, and metallographic examination.
Multiple cracks were observed on the outer surface of the drip leg. Cracks at the ports and welds consisted primarily of a singular, isolated crack. There were multiple cracks at the bulge that had a crosshatched appearance on both surfaces.
Features on each of the crack surfaces included rachet marks and beach marks. These features are consistent with a progressive fracture mechanism with multiple origins, such as fatigue fracture. In a metallographic section, the cracks had a dagger-shaped profile, and were filled with nonmetallic scale. Some of the cracks had penetrated the entire wall thickness. The appearance of the cracking pattern, crack profile, and fracture surfaces indicated that the mechanism for the cracking was thermal fatigue in each location.
The microstructure of the drip leg material both near and away from the cracks was primarily bainite with fine evenly dispersed