A deformed steel tube was received for failure analysis after buckling during a heat-treat operation. The tube was subjected to various metallurgical tests as well as nondestructive testing to confirm the presence of residual stresses. The microstructure of the tube was found to be homogenous and had no banded structure. However, x-ray diffraction analysis confirmed the presence of up to 6% retained austenite which likely caused the tube to buckle during the 910 °C heat treating procedure.

An investigation was conducted to determine the factors responsible for the occasional formation of cracks on the parting lines of medium plain carbon and low-alloy medium-carbon steel forgings. The cracks were present on as-forged parts and grew during heat treatment. Examination revealed that areas near the parting line exhibited a large grain structure not present in the forged stock. High-temperature scale was also found in the cracks. It was concluded that the cracks were caused by material being folded over the parting line. The folding occurred because of a mismatch in the forgings and from material flow during trimming and/or material flow during forging.

Tubes in heat exchangers, made of copper alloy C44300 and used for cooling air failed after 5 to six years of service. Air passed over the shell-side surface of the tubes and was cooled by water flowing through the tubes. Water vapor in the air was condensed (pH 4.5) on the tube surfaces during the cooling process. Air flow over the tubes reversed direction every 585 mm as a result of baffling placed in the heat exchangers. An uneven ridgelike thinning and perforation of the tube wall on the leeward side of the tube was revealed by visual examination. Undercut pits on the outer surface of the tube were revealed by metallographic examination of a cross section of the failed area. Impingement attack which led to perforation was revealed by both the ridgelike appearance of the damaged area and the undercut pitting. The heat exchanger was retubed with tubes made of aluminum bronze (copper alloy C61400).

Fusing of the switch contacts of a boiler feed pump drive motor led to the failure of a turbine. After rubbing of most of the Ni-Cr steel LP wheels had occurred, due to the admission of water carried over with the steam, a copper-rich alloy from the interstage gland rings melted, penetrated the wheel material, and gave rise to radial and circumferential cracking in four of the LP wheels. It was concluded that when the rotor moved axially and the wheels came into contact with the diaphragms there was a tendency for the former to dish, with the development of both radial and circumferential tensile stresses on the side in contact with the adjacent diaphragm. In the presence of the molten copper-rich alloy, these stresses gave rise to severe hot cracking.

This article explains the main types and characteristic causes of failures in boilers and other equipment in stationary and marine power plants that use steam as the working fluid with examples. It focuses on the distinctive features of each type that enable the failure analyst to determine the cause and suggest corrective action. The causes of failures include tube rupture, corrosion or scaling, fatigue, erosion, and stress-corrosion cracking. The article also describes the procedures for conducting a failure analysis.

A root cause failure analysis was performed on a vaporizer coil removed from a horizontal forced circulation vaporizer. The carbon steel coil was wound in a right-hand helix with a coil centerline diameter of about 2 m. The vaporizer was gas fired and used Dowtherm A as the heat transfer fluid. Design conditions are based on annular fluid flow to cool the coil wall. NDE, metallographic and fractographic examinations were performed. Numerous, circumferentially oriented, OD initiating cracks were found near the crown for two coils near the non-fired end of the vaporizer. The cracking was confined to the inner diameter of the vaporizer coil at positions from 4:00 to 7:00. The cracking was characterized as transgranular and the fracture surface had beach marks. The failure mechanism was thermal fatigue. The heat transfer calculation predicted that dryout of the coil would occur for coils at the non-fired end of the vaporizer during low flow transients. Dryout results in rapid increase in the tube wall temperature. Thermal cycling of the coil is completed by liquid quenching resulting from resumption of normal flow rates and the return to annular flow. The probable root cause of failure was low flow transient operation.

This case study demonstrates that Alloy 601 (UNS N06601) is susceptible to strain-age cracking. The observation illustrates the potential importance of post weld heat treatment to the successful utilization of this alloy in certain applications.

Aircraft missile launcher attachment bolts fabricated from cadmium-coated Hy-tuf steel were found broken. Subsequent analysis of the broken bolts indicated three causes of failure. First, the bolts had been carburized, which was not in conformance with the heat treating requirements. Second, macroetching showed that the bolts has been machined from stock rather than forged, and the threads cut rather than rolled. It was also determined that hydrogen-assisted stress-corrosion cracking also played a part in the failure of the high-strength bolts.

Failures in boilers and other equipment taking place in power plants that use steam as the working fluid are discussed in this article. The discussion is mainly concerned with failures in Rankine cycle systems that use fossil fuels as the primary heat source. The general procedure and techniques followed in failure investigation of boilers and related equipment are discussed. The article is framed with an objective to provide systematic information on various damage mechanisms leading to the failure of boiler tubes, headers, and drums, supplemented by representative case studies for a greater understanding of the respective damage mechanism.

Occasional failures were experienced in spool-type valves used in a hydraulic system. When a valve would fail, the close-fitting rotary valve would seize, causing loss of flow control of the hydraulic oil. The rotating spool in the valve was made of 8620 steel and was gas carburized. The cylinder in which the spool fitted was made of 1117 steel, also gas carburized. Investigation (visual inspection, low magnification images, 400x images, metallographic exam, and hardness testing) supported the conclusion that momentary sliding contact between the spool and the cylinder wall caused unstable retained austenite in the failed cylinder to transform to martensite. The increase in volume resulted in sufficient size distortion to cause interference between the cylinder and the spool, seizing, and loss of flow control. The failed parts had been carburized in a process in which the carbon potential was too high, which resulted in a microstructure having excessive retained austenite after heat treatment. Recommendations included modifying the composition of the carburizing atmosphere to yield carburized parts that did not retain significant amounts of austenite when they were heat treated.

A failure analysis was conducted on a flow-sensing device that had cracked while in service. The polysulfone sensor body cracked radially, adjacent to a molded-in steel insert. This article describes the investigative methods used to conduct the failure analysis. The techniques utilized included scanning electron microscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, thermomechanical analysis, and melt flow rate determination. It was the conclusion of the investigation that the part failed via brittle fracture, with evidence also indicating low cycle fatigue associated with cyclic temperature changes from normal service. The design of the part and the material selection were significant contributing factors because of stresses induced during molding, physical aging of the amorphous polysulfone resin, and the substantial differential in coefficients of thermal expansion between the polysulfone and the mating steel insert.

A heat transport pump in a heavy water reactor failed (exhibiting excessive vibration) during a restart following a brief interruption in coolant flow due to a faulty valve. The pump had developed a large crack across the entire length of a bearing journal. An investigation to establish the root cause of the failure included chemical and metallurgical analysis, scanning electron fractography, mechanical property testing, finite element analysis of the shrink fitted journal, and a design review of the assembly fits. The journal failure was attributed to corrosion fatigue. Corrective actions to make the journals less susceptible to future failures were implemented and the process by which they were developed is described.

Jet pumps, which have no moving parts, provide a continuous circulation path for a major portion of the core coolant flow in a boiling water reactor. Part of the pump is held in place by a beam-and-bolt assembly, wherein the beam is preloaded by the bolt. The Alloy X-750 beams had been heat treated by heating at 885 deg C (1625 deg F) for 24 h and aging at 705 deg C (1300 deg F) for 20 h. Jet pump beams were found to have failed in two nuclear reactors, and other beams were found to be cracked. Investigation (visual inspection, metallurgical examination, tension testing, and simulated service testing in oxygenated water) supported the conclusion that intergranular SCC under sustained bending loading was responsible for the failure. The location of the cracking was consistent with the results of stress analysis of the part. Recommendations included either replacing the beams, reheat treatment, or preload reduction.

A missile detached from a Navy fighter jet during a routine landing on an aircraft carrier deck because of a faulty missile launcher detent spring. Visual inspection of Inconel 718 detent spring assembly revealed that four of the nine spring leafs comprising the assembly were plastically deformed while two of the deformed leafs did not meet minimal hardness or tensile requirements. Liquid penetrant testing revealed no cracks or other surface discontinuities on the leaf springs. Material sectioned from the soft spring leafs was heat-treated according to specifications in the laboratory. The resultant increase in mechanical properties of the re-heat-treated material indicated that the original heat treatment was not performed correctly. The failure was attributed to improper heat treatment. Recommendations focused on more stringent quality control of the heat-treat operations.

Turbine buckets in a 37.5-MW gas turbine made of Udimet 500 superalloy failed in service. The power plant was located 1 km (0.6 miles) from the Pacific Ocean and operated on No. 2 diesel fuel, which was supplied by tanker ship. Turbine bucket failures occurred on three units after 2500 to 6400 h of operation. Investigation (visual inspection, metallographic examination, and stress analysis) supported the conclusion that the differing microstructure of the airfoil resulted in changes in mechanical properties. Because normal operation includes cycling of loads and temperatures, the shroud tip fractured due to thermomechanical fatigue in its degraded state. Recommendations included special chromium or silicon-rich coating to minimize corrosion in gas turbines operating in a marine environment with operating temperatures in the range of type 2 corrosion (650 to 750 deg C, or 1200 to 1380 deg F). Additionally, it was suggested that fuel delivery, handling, and treatment be high quality, to maintain fuel contamination within design limits, and inlet air filtration must be designed for the coastal site. Also, changing the bucket tip by increasing its thickness and changing the casting technique would reduce the stress and make the design more tolerant of corrosion.

A spiral heat exchanger made from 316L stainless steel developed a leak after eight years of service as a condenser on a distillation tower. Examination identified the leak as being located on the cooling water side in the heat affected zone (HAZ) of a weld joining two plates. Cooling water deposits were observed in a V-shaped corner formed by the weld. A metallurgical examination identified the presence of transgranular cracks in the HAZ on the cooling water side. Analysis of the cooling water revealed the presence of chlorides. Based on the metallurgical analysis and other findings, it was determined that the cracks and associated leak were the result of chloride stress-corrosion cracking.

A gas-fired, ASTM A-106 Grade B carbon steel vaporizer failed on three different occasions during attempts to bring the vaporizer on line. Dye penetrant examination indicated the presence of multiple packets of ductile cracks on the inside of the coil radius at the bottom of the horizontal axis coils. Visual examination of the inside of the tubing indicated the presence of a carbonaceous deposit resulting from decomposition of the heat-exchanging fluid. Subsequent metallographic examination and microhardness testing indicated that the steel was heated to a temperature above the allowable operating temperature for the fluid. The probable cause for failure is thermal fatigue due to the localized overheating. Flow conditions inside the tubing should be reexamined to ensure suitable conditions for annular fluid flow.

The horizontal heat-exchanger tubes made of copper alloy C70600, in one of two hydraulic-oil coolers in an electric power plant, leaked after 18 months of service. River water was used as the coolant in the heat-exchanger tubes. Several nodules on the inner surface and holes through the tube wall, which appeared to have formed by pitting under the nodules, were revealed by visual examination. Steep sidewalls, which indicated a high rate of attack, were revealed by microscopic examination of a section through the pit which had penetrated the tube wall. The major constituent of reddish deposit on the inner surfaces of the tubes was revealed to be iron oxide and slight manganese dioxide. Effluent from steel mills upstream was indicated by the presence of these and other constituents to be the source of most of the solids found in the tubes. It was concluded that the tubing failed by crevice corrosion. The tubing in the cooler was replaced, and cooling-water supply was changed from river to city water, which contained no dirt to deposit on the tube surfaces. An alternate solution of installing replacement tubes in the vertical position to make deposition of solids from river water less likely was suggested.

This article describes the general root causes of failure associated with wrought metals and metalworking. This includes a brief review of the discontinuities or imperfections that may be the common sources of failure-inducing defects in bulk working of wrought products. The article discusses the types of imperfections that can be traced to the original ingot product. These include chemical segregation; ingot pipe, porosity, and centerline shrinkage; high hydrogen content; nonmetallic inclusions; unmelted electrodes and shelf; and cracks, laminations, seams, pits, blisters, and scabs. The article provides a discussion on the imperfections found in steel forgings. The problems encountered in sheet metal forming are also discussed. The article concludes with information on the causes of failure in cold formed parts.