The defects observed along weldings of stainless steel pipelines employed in marine environments were evidenced by metallographic and electrochemical examination. A compilation of cases on the effect of defective weldings, in addition to improper choice of stainless steel for water pipelines, lead to the conclusion that intercrystalline corrosion in steels involved precipitation of a surplus phase at grain boundaries. Intercrystalline corrosion in austenitic stainless steels due to precipitation of chromium carbides during conditions generated due to welding and ways to avoid the precipitation (including reduction of carbon content, appropriate heat treatment, cold work of steel, reduction of austenitic grain size and stabilizing elements) were described. The presence of microcracks due to highly localized heat concentrations with consequent thermal expansion and considerable shrinkages during cooling was investigated. The specimens were taken from various sources including transverse and longitudinal welding seam, sensitized areas and it was concluded appropriate material selection with respect to medium could control some corrosion processes.

Waspaloy (AMS 5586) fabricated inner ring of a spray-manifold assembly failed transversely through the manifold tubing at the edge of the tube and support sleeve brazed joint. The assembly was brazed with AWS BAu-4 filler metal (AMS 4787). Fatigue beach marks propagating from extremities of a granular gold-tinted surface region adjacent to the tube-to-sleeve brazed joint and extending circumferentially were revealed by microscopic examination. Embrittlement of the tube caused by molten braze metal penetration along grain boundaries was evidenced by micrographs of a granular portion of the fracture. It was revealed by the initial fracture profile that fatigue cracks begun as an intergranular separation and subsequently became transgranular. It was concluded that failure of the tube was caused by excessive alloying between the braze metal and the Waspaloy. Reduced temperatures during torch debrazing or rebrazing were recommended to minimize molten braze metal penetration.

An investigation was conducted to determine why 16 out of 139 pipe bends cracked during hot induction bending. The pipe conformed to API 5L X65 PSL2 line pipe standards and measured 1016 mm (40 in.) in diam with a wall thickness of 18.5 mm. A metallurgical cross section was removed along a crack on the extrados to document the crack morphology using optical microscopy. In addition to cracking, golden-yellow streaks were visible at the extrados, and the composition was examined using scanning electron microscopy with energy dispersive spectroscopy. Based on the results, investigators concluded the pipe was contaminated with copper at the mill were it was produced.

A locomotive type boiler was fitted with a copper firebox of orthodox construction. Flanged tube- and firehole-plates were attached to a wrapper plate by means of copper rivets. Shortly after it was put into service the fireside heads of a number of rivets broke off at different parts of the seams. By the time the investigation was begun a total of fifty heads had broken off. Repairs had been effected from time to time by fitting screwed rivets, none of which gave trouble in service. Microscopic examination confirmed the fracture path to be wholly intergranular. In the region of the fracture the grain boundaries were delineated as a near-continuous network of cavities and films of oxide. It was evident that the failure of the rivets in service was attributable to intergranular weakness in the material due to gassing.

A fire in a storage yard engulfed several propane delivery trucks, causing one of them to explode. A series of elevated-temperature stress-rupture tears developed along the top of the truck-mounted tank as it was heated by the fire. Unstable fracture then occurred suddenly along the length of the tank and around both end caps, following the girth welds that connect them to the center portion of the tank. The remaining contents of the tank were suddenly released, aerosolized, and combusted, creating a powerful boiling liquid expanding vapor explosion (BLEVE). Based on the metallography of the tank pieces, the approximate tank temperature at the onset of explosion was determined. Metallurgical analysis provided additional insights as well as a framework for making tanks less susceptible to this destructive failure mechanism.

Microstructural examinations on transverse cross sections of a steam reformer tube, showed the presence of large macrovoids elongated in the radial direction and emanating from the internal surface of the tube. The macrovoids were located at the interdendritic regions, and were partially filled by a Mn-Fe bearing chromium oxide film. The areas adjacent to the oxide film were chemically depleted in C, Cr and Mn and rich in Fe and Ni. Associated with this depletion were a large concentration of microvoids. It was suggested that the dissolution of carbides in areas surrounding the macrovoids and the concentration of stresses at their tips, caused extensive localized plastic deformation which led to the formation of microvoids and subsequently to the spalling of the oxide film. The non-protective character of the film induced a progressive deterioration of the grain boundaries properties. Grain boundary sliding and dislocation motion were enhanced, causing a local increase in the steady state strain rate and the premature failure of the tube.

A roll assembly consisting of a forged AISI type 440A stainless steel sleeve shrink fitted over a 4340 steel shaft and further secured with tapered keys on opposite ends was crated and shipped by air. Upon arrival, the sleeve was found to have cracked longitudinally between the keyways. A roll manufacturer had successfully used the above procedure for many years to make them. Analysis (visual inspection; 150x micrograph of sections etched with a mixture of 2 parts HNO3, 2 parts acetic acid, and 3 parts HCI; electron microscopy; and stress testing) supported the conclusion that superficial working of the metal, probably insufficient hot working, produced a microstructure in which the carbide particles were not broken up and evenly distributed. As a result, the grains were totally surrounded with brittle carbide particles. This facilitated the formation of a crack at a fillet in the keyway. Crack growth was rapid once the crack had initiated, causing brittle fracture to occur.

This case study describes the failure analysis of a steel nozzle in which cracking was observed after a circumferential welding process. The nozzle assembly was made from low-carbon CrMoV alloy steel that was subsequently single-pass butt welded using gas tungsten arc welding. Although no cracks were found when the welds were visually inspected, X-ray radiography showed small discontinuous surface cracks adjacent to the weld bead in the heat affected zone. Further investigation, including optical microscopy, microhardness testing, and residual stress measurements, revealed that the cracks were caused primarily by the presence of coarse untempered martensite in the heat affected zone due to localized heating. The localized heating was caused by high welding heat input or low welding speed and resulted in high transformation stresses. These transformation stresses, working in combination with thermal stresses and constraint conditions, resulted in intergranular brittle fracture.

The failure of a boiler operating at 540 °C and 9.4 MPa was investigated by examining material samples from the near-failure region and by thermodynamic analysis. A scanning Auger microprobe, SEM, and commercial thermodynamic software codes were used in the investigation. Results indicated that the boiler failure was caused by grain-boundary segregation of phosphorous, tin, and nitrogen and the in-service formation of carbide films and granules on the grain boundaries.

Following a freight train derailment, part of a fractured side frame was retained for study because a portion of its fracture surface exhibited a rock candy appearance and black scale. It was suspected of having failed, thereby precipitating the derailment. Metallography, scanning electron microscopy, EDXA, and x-ray mapping were used to study the steel in the vicinity of this part of the fracture surface. It was found to be contaminated with copper. Debye-Scherrer x-ray diffraction patterns obtained from the scale showed that it consisted of magnetite and hematite. It was concluded that some copper was accidentally left in the mold when the casting was poured. Liquid copper, carrying with it oxygen in solution, penetrated the austenite grain boundaries as the steel cooled. The oxygen reacted with the steel producing a network of scale outlining the austenite grain structure. When the casting fractured as a result of the derailment, the fracture followed the scale in the contaminated region thus creating the “rock candy” fracture.

An exhaust diffuser assembly failed prematurely in service. The failure occurred near the intake end of the assembly and involved fracture in the diffuser cone (Corten), diffuser in take flange (type 310 stainless steel), diffuser exit flange (type 405 stainless steel), expansion bellows (Inconel 600), and bellows intake flange (Corten). Individual segments of the failed subassemblies were examined using various methods. The analysis indicated that the weld joint in the diffuser intake flange (type 310 stainless steel to Corten steel) contained diffusion-zone solidification cracks. The joints had been produced using the mechanized gas-metal arc welding process. Cracking was attributed to improper control of welding parameters, and failure was attributed to weld defects.

A water tube boiler with two headers and 15.5 atm working pressure became leaky in the lower part due to the formation of cracks in the rivet-hole edges. The boiler plate of 20 mm thickness was a rimming steel with 0.05% C, traces of Si, 0.38% Mn, 0.027% P, 0.035% S, and 0.08% Cu. The mean value of the yield point was 24 (24) kg/sq mm, the tensile strength 39 (38) kg/sq mm, the elongation at fracture, d10, 26 (24)%, the necking at fracture 71 (66)% and notch impact value 11.5 (9.4) kgm/sq cm (the values in brackets are for the transverse direction). The specimen from inside surface of the boiler was polished and etched with Fry-solution, which revealed parallel striations formed due to the cold bending of the plate. The zones of slip were concentrated around the rivet holes. The cracks were formed here. The structure examination proved that the cracks had taken an exactly intercrystalline path, which is characteristic for caustic corrosion cracks. It was recommended that the internal stresses be removed through annealing or alternatively lye-resistant steel should be used.

Visual examination, using the unaided eye or a low-power optical magnifier, is typically one of the first steps in a failure investigation. This article presents the guidelines for selecting samples for scanning electron microscope examination and optical metallography and for cleaning fracture surfaces. It discusses damage characterization of metals, covering various factors that influence the damage, namely stress, aggressive environment, temperature, and discontinuities.

A bent Ni-Cu Monel 400 alloy tube, which operated as part of a pipeline in a petrochemical distillery, failed by through-thickness cracking. The pipeline was used to carry a stream of gaseous hydrocarbons containing hydrochloric acid (HCl) into a reaction tower. The tower provided a caustic solution (NaOH) to remove HCl from the stream, before the latter was directed to a burner. Metallographic examination showed that the cracks were intergranular and were frequently branched. Although nominal chemical composition of the component was found within the specified range, energy dispersive x-ray analysis (EDXA) indicated significant segregation of sulfur and chlorine along the grain boundaries. Failure was attributed to hypochlorous-acid (HClO)-induced stress-corrosion cracking (SCC). The HClO was formed by the reaction of HCl with atmospheric O 2 that entered the tube during shutdowns and startups. Residual stresses, originating from in situ bend forming of the tube during assembly of the line, provided a driving force for crack growth, and the segregation of sulfur on grain boundaries made the material more susceptible to cracking.

A locked coil track rope (LCTR) is essentially composed of wires (round and rail-shaped) laid helically in different layers. These wire ropes are sometimes used in conveyors carrying empty and loaded buckets in mining areas. During service, such wire ropes may fail prematurely due to disintegration/failure of individual groups of wires. To understand the genesis of LCTR wire failures, a detailed metallurgical investigation of failed rope wires was made and included visual examination, optical microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Two types of failed wires were investigated; one is from a 40 mm diameter locked coil track rope and the other from a 53 mm locked coil track rope. Optical microscopy of failed round wires in the 53 mm diameter rope clearly revealed fully decarburized layers at the surface and a few grain-boundary cracks. From the location of the failure, it was clear that apart from static tensile loads, the wire ropes had been subjected to bending and unbending loads near the saddle, as fully loaded or empty buckets traveled access the conveyor. The SEM studies confirmed that the fracture had been caused by initiation of fatigue cracks in the decarburized zone under conditions of repeated bending and unbending stresses superimposed on the static tensile load.

Microstructure, corrosion, and fracture morphologies of prestressed steel wires that failed in service on concrete siphons at the Central Arizona Project (CAP) are discussed. The CAP conveys water for municipal, industrial, and agricultural use through a system of canals, tunnels, and siphons from Lake Havasu to just south of Tucson, AZ. Six siphons were made from prestressed concrete pipe units 6.4 m (21 ft) in diam and 7.7 m long, making them the largest circular precast structures ever built. The pipe was manufactured on site and consisted of a 495-mm thick concrete core, wrapped with ASTM A648 steel prestressing wire. All of the CAP failures evaluated were attributed to corrosion. Longitudinal splits reduced the service life of the pipe significantly by facilitating corrosion and introducing sharp cracks into the microstructure of the wire. A few failures were attributed to general corrosion, where the cross section of the wire is reduced until the strength of the wire is exceeded. Most of the failures evaluated were attributed to stress-corrosion cracking.

A broken exhaust valve from the cylinder of a motor car had a 30-mm disk diam and 8-mm stem diam. The site of the fracture was directly where the valve cone joined the cylindrical stem. Both the cone and the stem were heavily scaled in the vicinity of the fracture; in some parts the scale has flaked off. Furthermore, the rim of the disk was badly damaged by secondary mechanical action. The core of the valve had a very fine austenitic microstructure with precipitations of numerous granular and very fine, mostly rounded carbides and tine segregation bands. A hard alloy facing was welded on to the valve seat. Fracture was a consequence of fatigue corrosion cracking, itself strongly promoted by the presence of sulphur compounds. The origin of these corrosive sulphur compounds could not be explained.

Cracks were discovered in the cast 17-4 PH stainless steel outboard leading edge flap support of an aircraft wing during overhaul inspection. Failure analysis focused on an apparently intergranular area of fracture surface. It was determined that the original mode of crack growth was cleavage, probably caused by cast-in hydrogen. The intergranular appearance resulted from heat treatment of the already cracked part, which caused the formation of grain-boundary “growth figures” on the exposed crack surfaces. It was recommended that the castings be more closely inspected for defects before further processing and that foundry practices be reviewed to correct deficiencies leading to excessive hydrogen absorption during melting and casting.