A falling film black liquor evaporator consisted of flat twin plate heat exchangers and was used to increase black liquor solids content prior to its burning in the recovery boiler. Several plate heat exchangers were fabricated of AISI type 316L stainless steel by electric resistance welding. Cracks initiated at the inside surface of the welded areas and penetrated through the wall thickness. In several locations, the weld fractured and the plates separated with significant spring back, indicative of high residual stresses attributed to fabrication and weld procedures. The cracks had extended radially from the electric resistant weld into the base metal. Metallographic examination revealed the cracks were transgranular and branching, characteristic of SCC in austenitic stainless steels. The fracture surfaces had a brittle cleavage-like appearance, typical of SCC in austenitic stainless steels. Chlorides in the service environment were a contributory factor. The primary factor causing SCC localized at the electric resistant welds was substantial residual stresses as a result of fabrication procedures. It was recommended that the heat exchanger plates be subjected to stress-relief heat treatment following fabrication and welding.

Following disruption of the austenitic stainless steel basket of a hydro-extractor used for the separation of crystals of salt (sodium chloride) from glycerin, samples of the broken parts were analyzed. Examination revealed that the fish-plates joining the reinforcing hoops had broken, the shell had split from top to bottom adjacent to the weld, the top and bottom cover plates had become loose, all the rivets having pulled out, and the shaft was also found to be bent. Fracture took place in an irregular manner and was of the shear type towards both ends; it occurred immediately adjacent to the weld or a short distance from it and on alternate sides. Microscopical examination did not reveal any intergranular carbide precipitation, such as is well known to result in the weld-decay mode of failure. It was concluded that the primary cause of failure was stress-corrosion cracking arising from the combined effect of residual stresses and the corrosive effect of the material being centrifuged. If the shell had been stress-relieved after fabrication, the failure likely would not have occurred.

Personnel responsible for laboratory protection at some plants are required to participate in exercises simulating a breach of security at the site. This document reports a metallurgical investigation of blank firing adapters (BFA), one of which exploded during such a training exercise. Determination of the cause of the explosion was the primary objective of the examination. Metallographic studies included the examination of BFAs fabricated from two different types of alloys that were tested for shock reaction. Optical microscopy supported by electron microscopy and analytical methods were used. Our investigation supports the supposition that a live round of ammunition was inadvertently fired.

A sleeve-shaped fire shield that operates inside one of two burner trains in an oil and gas processing unit ruptured after 15 y of service. A detailed analysis was conducted to determine how and why the sleeve failed. The investigation included visual inspection, chemical and gas analysis, mechanical property testing, stereomicroscopy, and metallographic examination. The fire sleeves are fabricated from 3-mm thick plate made of Incoloy 800 rolled into 540-mm diam sections welded along the seam. Three such sections are joined together by circumferential welds to form a single 2.8 m sleeve. The findings from the investigation indicated that internal oxidation corrosion, driven by high temperatures, was the primary cause of failure. Prolonged exposure to temperatures up to 760 °C resulted in sensitization of the material, making it vulnerable to grain boundary attack. This led to significant deterioration of the grain boundaries, causing extensive grain loss (grain dropping) and the subsequent thinning of sleeve walls. Prior to failure, some portions of the sleeve were only 1.6 mm thick, nearly half their original thickness.

Four truck cross members intended for use in heavy-duty transport trucks were investigated. Two of the members had cracked on a prototype vehicle and two had been fatigue tested in the laboratory. The cross members were fabricated from SAE 950X plate and consisted of a formed channel section and an internal fillet-welded diaphragm. Sections from each of the cross members were subjected to a complete analysis, including chemical analysis, magnetic particle testing, mechanical testing, scanning electron microscope/fractography, and metallography. The primary mode of failure was found to be fatigue cracking that initiated at the toes of the fillet welds. Secondary fatigue cracking occurred at the torque rod mounting holes. Failure was attributed to cyclic stresses at the weld toes that exceeded the lowered fatigue strength at this location. A design change that eliminated the fillet welds alleviated the problem.

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.

A 5000-gal (20,000-L) hot-water holding tank fractured at a large automotive manufacturing plant. The tank was made from Type 304 austenitic stainless steel. The inner diameter of the tank displayed a macroscopic, web-like network of cracks that deceptively suggested intergranular stress-corrosion cracking. The problem, however, originated on the outside surface of the tank where a tensile stress (due to low applied stress and fabrication-induced residual stresses) accelerated the growth of numerous stress corrosion cracks that eventually broke through to the inner surface, causing leakage and ultimately failure.

Helicopter rotor blade components that included the horizontal hinge pin, the associated nut, and the locking washer were examined. Visual examination of the submitted parts revealed that the hinge pin, fabricated from 4340 steel, was broken and that the fracture face showed a flat beach mark pattern indicative of a preexisting crack. The threaded area of the pin had an embedded thread that did not appear to come from the pin. A chemical analysis was conducted on the embedded thread and on an associated attachment to determine the origin of the thread. Analysis showed that the thread and nut were 4140 steel. Scanning electron fractographic examination of the fracture initiation site strongly suggested that the fracture progressed by fatigue. It was concluded that the failure of the horizontal hinge pin initiated at areas of localized corrosion pits. The pits in turn initiated fatigue cracks, resulting in a failure mode of corrosion fatigue. It was recommended that all of the horizontal hinge pins be inspected. Those pins determined to be satisfactory for further use should be stripped of cadmium, shot peened, and coated with cadmium to a minimum thickness of 0.0127 mm (0.0005 in.).

Six cracked A36 steel gusset plates that formed part of the roof trusses of a large manufacturing facility were discovered during a routine final inspection of a new building construction. Two different-size plates from different locations in the building were removed and tested. It was determined that the gusset plates failed in the heat-affected zone via an intergranular microcracking mode due to hydrogen-assisted underbead and toe-weld cracking. Proper nondestructive testing techniques for magnetic particle and radiographic inspection of the plate-weld gusset areas were recommended.

Two Z-shape impeller vanes failed. The vane material was 14-hard type 301 stainless steel. The vanes were of two-piece construction, with a longitudinal weld. Analyses indicated that the vanes had not been solution annealed after welding, leaving the heat-affected zone above the welds in a highly sensitized state. The sensitized material lost corrosion resistance, became embrittled along the grain boundaries, and finally failed by intergranular cracking. Use of type 410 martensitic stainless steel was recommended.

The secondary cooling water system pressure boundary of Savannah River Site reactors includes expansion joints utilizing a thin-wall bellows. While successfully used for over thirty years, an occasional replacement has been required because of the development of small, circumferential fatigue cracks in a bellows convolute. One such crack was recently shown to have initiated from a weld heat-affected zone liquation microcrack. The crack, initially open to the outer surface of the rolled and seam welded cylindrical bellows section, was closed when cold forming of the convolutes placed the outer surface in residual compression. However, the bellows was placed in tension when installed, and the tensile stresses reopened the microcrack. This five to eight grain diameter microcrack was extended by ductile fatigue processes. Initial extension was by relatively rapid propagation through the large-grained weld metal, followed by slower extension through the fine-grained base metal. A significant through-wall crack was not developed until the crack extended into the base metal on both sides of the weld. Leakage of cooling water was subsequently detected and the bellows removed and a replacement installed.

A portion of the roof of a single story building collapsed during a thunder storm. A failure analysis was conducted to determine whether this structural failure was due to improper design, substandard construction materials, faulty erection, or extreme weather conditions. The failure analysis consisted of an onsite inspection, macrofractographic examination of the fractures where the girders were welded to the columns, macrofractographic examination of the fractured trusses, metallographic examination of the girder and truss materials, chemical analysis of the low-carbon steel girder and truss materials, and mechanical testing of the truss material. It was concluded that substandard structural components in combination with faulty construction was responsible for this service failure.

The US Army Research Laboratory performed a failure investigation on a broken main landing gear mount from an AH-64 Apache attack helicopter. A component had failed in flight, and initially prevented the helicopter from safely landing. In order to avoid a catastrophe, the pilot had to perform a low hover maneuver to the maintenance facility, where ground crews assembled concrete blocks at the appropriate height to allow the aircraft to safely touch down. The failed part was fabricated from maraging 300 grade steel (2,068 MPa [300 ksi] ultimate tensile strength), and was subjected to visual inspection/light optical microscopy, metallography, electron microscopy, energy dispersive spectroscopy, chemical analysis, and mechanical testing. It was observed that the vacuum cadmium coating adjacent to the fracture plane had worn off and corroded in service, thus allowing pitting corrosion to occur. The failure was hydrogen-assisted and was attributed to stress corrosion cracking (SCC) and/or corrosion fatigue (CF). Contributing to the failure was the fact that the material grain size was approximately double the required size, most likely caused from higher than nominal temperatures during thermal treatment. These large grains offered less resistance to fatigue and SCC. In addition, evidence of titanium-carbo-nitrides was detected at the grain boundaries of this material that was prohibited according to the governing specification. This phase is formed at higher thermal treatment temperatures (consistent with the large grains) and tends to embrittle the alloy. It is possible that this phase may have contributed to the intergranular attack. Recommendations were offered with respect to the use of a dry film lubricant over the cadmium coated region, and the possibility of choosing an alternative material with a lower notch sensitivity. In addition, the temperature at which this alloy is treated must be monitored to prevent coarse grain growth. As a result of this investigation and in an effort to eliminate future failures, ARL assisted in developing a cadmium brush plating procedure, and qualified two Army maintenance facilities for field repair of these components.

Cold cracking of structural steel weldments is a well-documented failure mechanism, and extensive work has been done to recognize welding and materials selection parameters associated with it. These efforts, however, have not fully eliminated the occurrence of such failures. This article examines a case of cold cracking failure in the construction industry. Fortunately, the failure was identified prior to final erection of the structural members and the weld was successfully reworked. The article explains how various welding parameters, such as electrode/wire selection, joint design, and pre/postheating, played a role in the failure. Human factors and fabrication practices that contributed to the problem are covered as well.

Service failures have occurred in a number of aircraft parts made of quenched and tempered steel heat treated to ultimate tensile strengths of 260,000 to 280,000 psi. Some of these failures have been attributed to “delayed cracking” as a result of hydrogen embrittlement or to stress-corrosion. Because of the serious nature of the failures and because the mechanism of the fracture initiation is not well understood, unusually complete laboratory investigations have been conducted. Three of these investigations are reviewed to illustrate the methods used in studying failures in aircraft parts. The results of the laboratory studies indicate that unusual care is necessary in the processing and fabrication of ultra-high-strength steel and in the design and maintenance of the structures in which it is used.

High-level radioactive wastes generated during the processing of nuclear materials are kept in large underground storage tanks made of low-carbon steel. The wastes consist primarily of concentrated solutions of sodium nitrate and sodium hydroxide. Each of the tanks is equipped with a purge ventilation system designed to continuously remove hydrogen gas and vapors without letting radionuclides escape. Several intergranular cracks were discovered in the vent pipe of one such system. The pipe, made of galvanized steel sheet, connects to an exhaust fan downstream of high-efficiency particulate air filters. The failure analysis investigation concluded that nitrate-induced stress-corrosion cracking was the cause of the failure.

Numerous cracks observed on the surface of a forged A470 Class 4 alloy steel steam turbine rotor disc from an air compressor in a nitric acid plant were found to be the result of caustic induced stress-corrosion cracking (SCC). No material defects or anomalies were observed in the disc sample that could have contributed to crack initiation or propagation or secondary crack propagation. Chlorides detected in the fracture surface deposits were likely the primary cause for the pitting observed on the disc surfaces and within the turbine blade attachment area. It was recommended that the potential for water carryover or feedwater induction into the turbine be addressed via an engineering evaluation of the plant's water treatment procedures, steam separation equipment, and start-up procedures.

An unacceptable degree of porosity was identified in several closure welds on stainless steel containers for plutonium-bearing materials. The pores developed in the weld tie-in region due to gas trapped by the weld pool during the closure process. This paper describes the efforts to trace the root cause of the porosity to the geometric conditions of the weld joint and establish corrective actions to minimize such porosity.

Leakage was identified around a coupling welded into a stainless steel holding tank that stored condensate water with low impurity content. The tank and fitting were manufactured from type 304 stainless steel. The coupling joint consisted of an internal groove weld and an external fillet weld. Cracking was found to be apparent on the tank surface, adjacent to the coupling weld. Chlorine, carbon, and oxygen in addition to the base metal elements were revealed by energy-dispersive x-ray spectrometric analysis. A great number of secondary, branching cracks were evident in the weld, heat-affected zone, and base metal. The branching and transgranular cracking was found to emanate primarily from the exterior of the tank. It was concluded that the tank failed as a result of stress-corrosion cracking that initiated at the exterior surface as aqueous chlorides, especially within an acidic environment, have been shown to cause SCC in austenitic stainless steels under tensile stress.

Two superheater tubes from a 6.2 MPa (900 psig) boiler failed in service because of creep rupture. One tube was carbon steel and the other was carbon steel welded to ASTM A213 Grade T22 (2.25Cr-1.0Mo) tubing. The failure in the welded tube occurred in the carbon steel section. Portions of the superheater were retubed five years previously with Grade 722 material. The failures indicated that tubes were exposed to long-term overheating conditions. While the carbon steel tube did not experience temperatures above the lower transformation temperature 727 deg C (1340 deg F), the welded tube did experience a temperature peak in excess of 727 deg C (1340 deg F). The long-term overheating conditions could have been the result of excessive heat flux and /or inadequate steam flow. In addition, the entire superheater bank should have been upgraded to Grade 722 material at the time of retubing.