A main steam pipe was found to be leaking due to a large circumferential crack in a pipe-to-fitting weld in one of two steam leads between the superheater outlet nozzles and the turbine stop valves (a line made of SA335-P22 material). The main crack surface was found to be rough, oriented about normal to the outside surface, and had a dark oxidized appearance. The cracking was found to be predominantly intergranular. Distinct shiny bands that etched slower than the remainder of the sample at the top of each individual weld bead were revealed by microscopic examination. These bands were found contain small cracks and microvoids. A mechanism of intergranular creep rupture at elevated temperature was identified as a result of a series of stress-rupture and tensile tests. It was revealed by the crack shape that cracking initiated on the pipe exterior, then propagated inward and in the circumferential direction in response to a bending moment load. It was concluded that the primary cause of failure was the occurrence of bending stresses that exceeded the stress levels predicted by design calculations and that were higher than the maximum allowable primary membrane stress.

In Nov. 1998, the west superheater outlet header at an electricity generating plant began to leak steam. Subsequent investigation revealed the presence of a crack that extended for 360 deg around the full circumference of the header and through the full cross-sectional thickness. The subsequent inspection of this header and the east superheater header revealed the presence of extremely severe ligament cracking. They operated at 2400 psi (16.5 MPa) and a temperature of 540deg C (1005 deg F). Both were fabricated from seamless pipe produced in accordance with ASME Specification SA-335, and the steel was Grade P22, a 2.25Cr-1Mo alloy steel. Visual and metallurgical evaluations showed the cracking in the west superheater outlet header was caused by thermal fatigue. Tube holes had served as a preferential site for thermal fatigue cracking.

The maximum life of base-loaded headers and piping is not possible to be predicted until they develop microcracking. The typical elements of a periodic inspection program after the occurrence of the crack was described extensively. Cracks caused by creep swelling in the stub-to-header welds in the secondary superheater outlet headers (constructed of SA335-P11 material) of a major boiler were described as an example. The OD of the header was measured to detect the amount of swelling and found to have increased 1.6% since its installation. Ligament cracks extending from tube seat to tube seat were revealed by surface inspection. Cracks were found to originate from inside the header, extend axially in the tube penetrations and radially from those holes into the ligaments. Cracks in 94 locations, ranging from small radial cracks to full 360Ý cracks were revealed by dye-penetrant inspection. The unit was operated under reduced-temperature conditions and with less load cycling than previously until a redesigned SA335-P22 header was installed.

This article discusses the effect of using unsuitable alloys, metallurgical discontinuities, fabrication practices, and stress raisers on the failure of a pressure vessel. It provides information on pressure vessels made of composite materials and their welding practices. The article explains the failure of pressure vessels with emphasis on stress-corrosion cracking, hydrogen embrittlement, brittle and ductile fractures, creep and stress rupture, and fatigue with examples.

This article discusses pressure vessels, piping, and associated pressure-boundary items of the types used in nuclear and conventional power plants, refineries, and chemical-processing plants. It begins by explaining the necessity of conducting a failure analysis, followed by the objectives of a failure analysis. Then, the article discusses the processes involved in failure analysis, including codes and standards. Next, fabrication flaws that can develop into failures of in-service pressure vessels and piping are covered. This is followed by sections discussing in-service mechanical and metallurgical failures, environment-assisted cracking failures, and other damage mechanisms that induce cracking failures. Finally, the article provides information on inspection practices.