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.

In January 1965, a Reaction Control System (RCS) pressure vessel (titanium alloy Ti-6Al-4V) on an Apollo spacecraft cracked in six adjacent locations. It used N2O4 for vehicle attitude control through roll, pitch, and yaw engines, and was protected from the N2O4 by a Teflon positive expulsion bladder. Investigation (visual inspection, pressure testing of 10 similar vessels, and chemical testing of the N2O4) supported the conclusion that the failure was due to stress corrosion from the N2O4, and specifically from a specification change in the military specification MIL-P-26539. Recommendations included revising the specification to require a minimum NO content of 0.6%.

Graphitization, the formation of graphite nodules in carbon and low alloy steels, contributes to many failures in high-temperature environments. Three such failures in power-generating systems were analyzed to demonstrate the unpredictable nature of this failure mechanism and its effect on material properties and structures. In general, the more randomly distributed the nodules, the less effect they have on structural integrity. In the cases examined, the nodules were found to be organized in planar arrays, indicating they might have an effect on material properties. Closer inspection, however, revealed that the magnitude of the effect depends on the relative orientation of the planar arrangement and principle tensile stress. For normal orientation, the effect of embrittlement tends to be most severe. Conversely, when the orientation is parallel, the nodules have little or no effect. The cases examined show that knowledge is incomplete in regard to graphitization, and the prediction of its occurrence is not yet possible.

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.

A portable propane container with a name-plate soldered onto it exploded in service. When the vessel was inspected afterwards, it was found to have developed a crack in the top end plate. A portion of the end plate cut out to include the midlength and one termination of the crack was examined microscopically. This revealed that the crack was associated with intergranular penetration by molten metal. The microstructure in general was indicative of a good-quality mild steel. It was evident from that solder that was responsible for the penetration and that fused brass from the hand wheel had not played any part. Tensile stress was present at the time of the failure sufficiently high to enable solder penetration to take place. The use of soft solder as a medium for attaching name-plates directly on to stressed steel parts is not recommended. It would be preferable to use a welded-on patch plate or to employ one of the high-strength, non-metallic adhesives.

A carbon steel (ASTM A515 grade 70) pressure vessel failed by brittle fracture while being hydro tested in the fabricating shop. The fracture origin was a small crack at a welding arc strike associated with the toe of a nozzle weld. A fracture mechanics calculation indicated that this imperfection, although small, initiated fracture because of the local geometry and stress conditions and the low toughness of the steel. It was recommended that (1) the probability of flaws be reduced by welding over or grinding out arc strikes, (2) the local stresses be lowered by post weld stress relief and improved weld toe geometry, and (3) toughness be improved by specifying fine-grain steel and/ or by normalizing.

A Ti-6Al-4V alloy pressure vessel failed during a proof-pressure test, fracturing along the center girth weld. The girth joints were welded with the automatic gas tungsten arc process utilizing an auxiliary trailing shield attached to the welding torch to provide inert-gas shielding for the exterior surface of the weld. A segmented backup ring with a gas channel was used inside the vessel to shield the weld root. The pressure vessel failed due to contamination of the fusion zone by oxygen, which resulted when the gas shielding the root face of the weld was diluted by air that leaked into the gas channel. Thermal stresses cracked the embrittled weld, exposing the crack surfaces to oxidation before cooling. One of these cracks caused a stress concentration so severe that failure of the vessel wall during the proof test was inevitable. A sealing system at the split-line region of the segmented backup ring was provided, and a fine-mesh stainless steel screen diffuser was incorporated in the channel section of the backup ring to prevent air from leaking in. A titanium alloy color chart was furnished to permit correlation of weld-zone discoloration with the degree of atmospheric contamination.

A forged pressure vessel made from high temperature austenitic steel X8Cr-Ni-MoVNb 16 13 K (DIN 1.4988) failed. The widest part of the burst had fine cracks on the internal wall running longitudinally. When the internal wall was cleaned, numerous even finer cracks were exposed. On the fracture surfaces in this region an irregularly formed zone was visible in the direction of the internal wall and a fibrous oriented fracture zone towards the external wall. The fracture was typical of stress-corrosion cracking in austenitic steels. Vanadium trichloride was present and tensile stresses were of necessity set up by the internal pressure. Stress-corrosion cracking does not occur if one of the basic requirements is lacking. Because the chloride agent and tensile stresses were inevitably present, the only possible way to prevent future reoccurrence is to forge the entire pressure vessel from a material immune to stress-corrosion cracking or to use interchangeable linings of such a material. A nickel alloy could be considered.

Following a fracture mechanics “fitness-for-purpose” analysis of petroleum industry cold service pressure vessels, using the British Standard PD 6493, it was realized that an analogous approach could be used for the failure analysis of a similar pressure vessel dome which had failed in service some years previously. The failed pressure vessel, with a diam of 2.5 m and several meters tall, had been made of 12 mm thick IZETT steel plate of the same type and heat treatment as used in the earlier fitness-for-purpose already measured. Examination of the fracture surfaces suggested, from fatigue striations manifested by SEM, that the vessel was subject to significant fatigue cracking, which was probably corrosion assisted. From COD measurements at the operating temperature of -130 deg C (-202 deg F), and a finite stress analysis, a fracture mechanics evaluation using BS PD6493 yielded realistic critical flaw sizes (in the range 51 to 150 mm). These sizes were consistent with the limited fracture surface observations and such flaws could well have been present in the vessel dome prior to catastrophic failure. For similar pressure vessels, an inspection program based on a leak-before-break philosophy was consequently regarded as acceptable.

Field metallography and replication were performed on a type 316 stainless steel column in diglycol amine vacuum service to determine the cause of visible OD pitting on the column in several areas above the insulation support rings. The examination revealed transgranular stress-corrosion cracking beneath the pitted areas on the OD. The likely cause of the cracking was chloride stress corrosion, with chlorides deriving from the marine atmosphere and concentrating under the insulation around the support rings. A complete insulation evaluation, including repair or replacement, was recommended to prevent chloride buildup. Painting of the steel surface with an epoxy-phenolic or epoxy-coal tar was also suggested.

During autofrettage of a thick-wall steel pressure vessel, a crack developed through the wall of the component. Certain forged pressure vessels are subjected to autofrettage during their manufacture to induce residual compressive stresses at locations where fatigue cracks may initiate. The results of the autofrettage process, which creates a state of plastic strain in the material, is an increase in the fatigue life of the component. Analysis (visual inspection, 50x/500x unetched micrographs, and electron microprobe analysis) supports the conclusion that the fracture toughness of the steel was exceeded, and failure through the wall occurred because of the following reason: the high level of iron oxide found is highly abnormal in vacuum-degassed steels. Included matter of this nature (exogenous) most likely resulted from scale worked into the surface during forging. Therefore, it is understandable that failure occurred during autofrettage when the section containing these defects was subjected to plastic strains. Because the inclusions were sizable, hard, and extremely irregular, this region would effect substantial stress concentration. No recommendations were made.

A large pressure vessel that had been in service as a hydrogen sulfide (H2S) absorber developed cracks and began leaking at a nozzle. The vessel contained a 20% aqueous solution of potassium hydroxide (KOH), potassium carbonate (K2CO3), and arsenic. The vessel wall was manufactured of ASTM A516, grade 70, low-carbon steel plate. A steel angle had been formed into a ring was continuously welded to the inside wall of the vessel. The groove formed by the junction of the lower tray-support weld and the top part of the weld around the nozzle was found to have a crack. Pits and scale near the crack origin were revealed by microscopic examination and cracking was found to be transgranular. Periods of corrosion alternated with sudden instances of cleavage, under a tensile load, along preferred slip planes were interpreted during examination with a microscope. It was concluded that the combination of the residual plus operating stresses and the amount of KOH present would have caused stress corrosion as a result of caustic embrittlement. It was recommended that the tray support should be installed higher on the vessel wall to prevent coincidence of the lower tray-support weld with the nozzle weld.

A large pressure vessel designed for use in an ammonia plant failed during hydrostatic testing. It was fabricated from ten Mn-Cr-Ni-Mo-V steel plates which were rolled and welded to form ten cylindrical shell sections and three forgings of similar composition. The fracture surfaces were metallographically examined to be typical for brittle steel fracture and associated with the circumferential weld that joined the flange forging to the first shell section. Featureless facets in the HAZ were observed and were revealed to be the fracture-initiation sites. Pronounced banding in the structure of the flange forging was revealed by examination. A greater susceptibility to cracking was interpreted from the higher hardenability found within the bands. Stress relief was concluded to have not been performed at the specified temperature level (by hardness and impact tests) which caused the formation of hard spots. The mode of crack propagation was established by microstructural examination to be transgranular cleavage. It was concluded that failure of the pressure vessel stemmed from the formation of transverse fabrication cracks in the HAZ fostered by the presence of hard spots. It was recommended that normalizing and tempering temperatures be modified and a revised forging practice explored.