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Identification of alloys using quantitative chemical analysis is an essential step during a metallurgical failure analysis process. There are several methods available for quantitative analysis of metal alloys, and the analyst should carefully approach selection of the method used. The choice of appropriate analytical techniques is determined by the specific chemical information required, the condition of the sample, and any limitations imposed by interested parties. This article discusses some of the commonly used quantitative chemical analysis techniques for metals. The discussion covers the operating principles, applications, advantages, and disadvantages of optical emission spectroscopy (OES), inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray spectroscopy, and ion chromatography (IC). In addition, information on combustion analysis and inert gas fusion analysis is provided.

A failure analysis investigation was conducted on a fractured aluminum tailwheel fork which failed moments after the landing of a privately owned, 1955 twin-engine airplane. Nondestructive evaluation via dye-penetrant inspection revealed no discernible surface cracks. The chemical composition of the sand-cast component was identified via optical emission spectroscopy and is comparable to an aluminum sand-cast alloy, AA 712.0. Metallographic evaluation via optical microscopy and scanning electron microscopy revealed a high degree of porosity in the microstructure as well as the presence of deleterious intermetallic compounds within interdendritic regions. Macrohardness testing produced hardness values which are noticeably higher than standard hardness values for 712.0. The primary fracture surfaces indicate evidence of mixed-mode fracture, via intergranular cracking, cleaved intermetallic particles, and dimpled cellular regions in the matrix. The secondary fracture surface demonstrates similar features of intergranular fracture.

Catastrophic pitting corrosion occurred in type 304L stainless steel pipe flange assemblies in an industrial food processor. During regular service the pumped medium was pureed vegetables. In situ maintenance procedures included cleaning of the assemblies with a sodium hypochlorite solution. It was determined that the assemblies failed due to an austenite-martensite galvanic couple activated by a chlorine bearing electrolyte. The martensitic areas resulted from a transformation during cold-forming operations. Solution annealing after forming, revision of the design of the pipe flange assemblies to eliminate the forming operation, and removal of the source of chlorine were recommended.

High failure rates in the drive shafts of 40 newly acquired articulated buses was investigated. The drive shafts were fabricated from a low-carbon (0.45%) steel similar to AISI 5046. Investigators examined all 40 buses, discovering six different drive shaft designs across the fleet. All of the failures, a total of 14, were of the same type of design, which according to finite-element analysis, produces a significantly higher level of stress. SEM examination of the fracture surface of one of the failed drive shafts revealed fatigue striations near the OD and ductile dimpling near the ID, evidence of high-cycle fatigue. Based on the failure rate and fatigue life predictions, it was recommended to discontinue the use of drive shafts with the inferior design.

A controllable pitch propeller (CPP) on a dynamic positioning ship failed after eight months of operation. The CPP design consists of a hollow propeller shaft and a concentrically located pipe that operates inside. The pitch of the propeller blades is controlled hydraulically through the longitudinal displacement of the inner (concentric) pipe. Fractography, microstructural, microhardness, and chemical analyses revealed that the concentric pipe failed due to fatigue. Fatigue cracks initiated along longitudinal welds where wire spacers attach to the external surface of the pipe. The effect of crack-like defects, stress concentration at the weld toe, residual tensile stress, and lack of penetration contributed to a shorter fatigue crack initiation phase and premature failure.

A cast silicon bronze (UNS C86700) impeller that had been severely corroded was submitted for failure analysis. The failed part was used to pump potable water, but service life and chlorine content of the water were unknown. The impeller displayed a Cu-rich red phase on its surfaces and showed a pattern very similar to dezincification. Further investigation to determine the cause of damage using light microscopy and SEM-EDS techniques revealed that the microstructure consisted of multiple phases and that a Si-rich phase was being preferentially attacked, leading to increased porosity. After a thorough examination, it was concluded that the part had failed due to dealloying via desiliconification.

An A-470 steel rotor disk was removed from the high-pressure portion of a steam turbine-powered compressor after nondestructive testing revealed cracks in the shoulder of the disk during a scheduled outage. Samples containing cracks were examined using various methods. Multiple cracks, primarily intergranular were found on the inlet and outlet faces along prior-austenite grain boundaries. The cracks initiated at the surface and propagated inward. Multiple crack branching was observed. Many of the cracks were filled with iron oxide. X-ray photoelectron spectroscopy indicated the presence of sodium on crack surfaces, which is indicative of NaOH-induced stress-corrosion cracking. Failure was attributed to superheater problems that resulted in caustic carryover from the boiler. Two options for disk repair, installing a shrink-fit disk or applying weld buildup, were recommended. Weld repair was chosen, and the rotor was returned to service; it has performed for more than 1 year without further incident.

Chemical analysis is a critical part of any failure investigation. With the right planning and proper analytical equipment, a myriad of information can be obtained from a sample. This article presents a high-level introduction to techniques often used for chemical analysis during failure analysis. It describes the general considerations for bulk and microscale chemical analysis in failure analysis, the most effective techniques to use for organic or inorganic materials, and examples of using these techniques. The article discusses the processes involved in the chemical analysis of nonmetallics. Advances in chemical analysis methods for failure analysis are also covered.

Several 304H stainless steel superheater tubes fractured in stressed areas within hours of a severe caustic upset in the boiler feedwater system. Tests performed on a longitudinal weld joint, which connected two adjacent tubes in the tertiary superheater bank, confirmed caustic-induced stress-corrosion cracking, promoted by the presence of residual welding stresses. Improved maintenance of check valves and routine inspection of critical monitoring systems (conductivity alarms, sodium analyzers, etc.) were recommended to help avoid future occurrences of severe boiler feedwater contamination. Additional recommendations were to eliminate these short longitudinal weld joints by using a bracket assembly joint between the tubes, use a post-weld heat treatment to relieve residual welding stress or select a more stress-corrosion cracking resistant alloy for this particular application.

A carbon-molybdenum (ASTM A209 Grade T1) steel superheater tube section in an 8.6 MPa (1250 psig) boiler cracked because of long-term overheating damage that resulted from prolonged exposure to metal temperatures between 482 deg C (900 deg F) and 551 deg C (1025 deg F). The outer diameter of the tube exhibited a crack (fissure) oriented approximately 45 deg to the longitudinal axis and 3.8 cm (1.5 in.) long. The inner diameter surface showed a fissure in the same location and orientation. Microstructure at the failure near the outer diameter surface exhibited evidence of creep cracking and creep void formation at the fissure. A nearly continuous band of graphite nodules was observed on the surface of the fissure. In addition to the graphite band formation, the microstructure near the failure exhibited carbide spheroidization from long-term overheating in all the tube regions examined. It was concluded that preferential nucleations of graphite nodules in a series of bands weakened the steel locally, producing preferred fracture paths. Formation of these graphite bands probably expedited the creep failure of the tube. Future failures may be avoided by using low-alloy steels with chromium additions such as ASTM A213 Grade T11 or T22, which are resistant to graphitization damage.

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.

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.

Failure of AISI 1015 steel brake discs used in power transmissions in emergency winches was investigated using various testing methods. The failed discs were stampings that had replaced cast discs. Residual stresses in the fillets of new cast and new stamped brake discs were measured by x-ray diffraction. The results indicated that the stamped brake discs had failed by fatigue caused by a tensile residual stress pattern in the fillet. The residual stress pattern was attributed to the change in manufacturing process from casting to stamping. Use of a manufacturing process that yields a compressive residual stress in the fillet, appropriate heat treatment of stamped discs, or redesign of the disc and/or transmission assembly was recommended.

Several heavy truck Cr-Mo steel steering arms in service less than three years fractured during stationary or low-speed turning maneuvers that required power-assisted steering. Metallographic examination of the cracked AISI 4135 arms, heat treated to a hardness of 285 to 341 HB, revealed that fatigue crack initiation occurred from the tip of oxide scale inclusions forged into the U-shaped arm at the inside radius. Corrective action involved redesigning the steering arm to increase the minimum forging radius and reduce the stress level at the inner-bend radius, and reducing the level of power assistance to the wheels to encourage the driver to put the vehicle in motion prior to turning.

A ring-type joint in a reactor pipeline for a hydrocracker unit had failed. Cracks were observed on the flange and the associated ring gasket during an inspection following a periodic shutdown of the unit. The components were manufactured from stabilized grades of austenitic stainless steel; the flange from type 321, and the ring gasket from 347. Examination revealed that the failure occurred by transgranular stress-corrosion cracking, initiated by the presence of polythionic acid. Detailed metallurgical investigation was subsequently conducted to identify what may have caused the formation of polythionic acid in the process gas.

New type 321 corrosion-resistant steel heat shields were cracking during welding operations. A failure analysis was performed. The cause was found to be chloride induced stress-corrosion cracking. Packaging was suspected and confirmed to be the cause of the chloride contamination. A contributing factor was the length of time spent in the packaging, 21 years.

Failure analysis of a mobile harbor crane wheel hub that included SEM and EDS analyses demonstrated that the mechanism of failure was fatigue. The wheel hub was a ductile cast iron component that had been subjected to cyclic loading during a ten-year service period. The fracture surface of the fatigue failure also contained corrosion deposit, suggesting that cracking occurred over a period of time sufficient to allow corrosion of the cracked surfaces. Replacement and alignment of the failed wheel hub was recommended along with inspection of the nonfailed wheel hubs that remained on the crane.

The drive shaft in a marine propulsion system broke, stranding a large vessel along the Canadian seacoast. The shaft was made from quenched and tempered low-alloy steel. Fractographic investigation revealed that the shaft failed under low rotating-bending variable stress. Fatigue propagation occurred on about 95% of the total cross section of the shaft, under both low-cycle and high-cycle fatigue mechanisms. It was found that the fillet radius at the fracture’s origin was smaller than the one provisioned by design. As a result, the stresses at this location exceeded the values used in the design calculations, thus causing the initiation of the cracking. Moreover, although the shaft had been quenched and tempered, its actual hardness did not have the optimal value for long-term fatigue strength.