A type 316 stainless steel pipe reducer section failed in service of bleached pulp stock transfer within 2 years in a pulp and paper mill. The reducer section fractured in the heat-affected zone of the flange-to-pipe weld on the flange side. The pipe reducer section consisted of 250 and 200 mm (10 and 8 in.) diam flanges welded to a tapered pipe section. The tapered pipe section was 3.3 mm (0.13 in.) thick type 316 stainless steel sheet, and the flanges were 5 mm (0.2 in.) thick CF8M (type 316) stainless steel castings. Visual and metallographic analysis indicated that the fracture was caused by intergranular corrosion/stress-corrosion cracks that initiated from the external surface of the pipe reducer section. Contributory factors were the sensitized condition of the flange and the concentration of corrosive elements from the bleach stock plant environment on the external surface. In the absence of the sensitized condition of the flange, the service of the pipe reducer section was acceptable. A type 316L stainless steel reducer section was recommended to replace the 316 component because of its superior resistance to sensitization.

Two suction rolls at the first press section of a 25 ft. wide paper machine developed cracks within two years of service. The rolls were austenitic stainless steel castings made of ASTM A 351 Grade CF8M alloy containing molybdenum. The rolls were exposed to slightly acidic white water (pH approximately 4.7) containing chlorides (45 ppm). Visual and liquid penetrant inspections of the rolls revealed extensive cracking at the roll inside surface. The cracks penetrated more than 30 percent of the wall thickness and a few cracks were several inches long. The cracks were preferentially oriented along the roll length and primarily at the roll inside surface. Field metallographic examination showed significant grain boundary chromium-carbide precipitation and intergranular corrosion. The roll failures were attributed to chromium depletion along the grain boundaries (sensitization) resulting from slow cooling of the casting to avoid large residual stresses. The roll manufacturer recommended a proprietary ferritic/austenitic stainless steel as the replacement material for the rolls.

A desuperheater diffuser nozzle in the steam supply line failed within nine months of service in an 8.25 MN/sq m (1200 psig) steam line. The nozzle was an austenitic stainless steel casting in conformance to material. The nozzle had numerous cracks on the inside and outside surfaces, and the cracks had penetrated through the wall thickness in several areas. The fracture surfaces had distinct beach markings delineating the crack front, representative of crack propagation stages. The cracks were transgranular and, unlike classical corrosion-fatigue cracks, exhibited branching, characteristic of chloride-induced SCC in austenitic stainless steels. The failure resulted from chloride-induced SCC, possibly assisted by cyclic stress. The recommendation for alternate material for the desuperheater nozzle included nickel base alloys per ASTM B 564, Grades 600 or 800 titanium alloy per ASTM B 367, Grades C3/C4, or ferritic stainless steel alloy per ASTM 182, Grade FXM27.

Failure analysis was performed on a fractured impeller from a boiler feed pump of a fossil fuel power plant. The impeller was a 12% Cr martensitic stainless steel casting. The failure occurred near the outside diameter of the shroud in the vicinity of a section change at the shroud/vane junction. Sections cut from the impeller were examined visually and by SEM fractography. Microstructural, chemical, and surface analyses and surface hardness tests were conducted on the impeller segments. The results indicated that the impeller failed in fatigue with casting defects increasing stress and initiating fracture. In addition, the composition and hardness of the impeller did not meet specifications. Revision of the casting process and institution of quality assurance methods were recommended.

To ensure no malfunctions and although there were no apparent problems, a main fuel control was returned to the factory for examination after service on a test aircraft engine that had experienced high vibrations. When the fuel control was disassembled, a lever, cast from AMS 5350 (AISI type 410) stainless steel that was through-hardened to 26 to 32 HRC and passivated, was shown to be cracked. The crack initiated at the sharp corner of the elongated milled slot and propagated across to the outer wall. The sections around the crack were spread about 30 deg apart, showing the fracture surface under investigation had beach marks initiating at the sharp corner along the milled slot. Changes in frequency or amplitude of vibration caused different rates of propagation, resulting in a change in pattern. This evidence supported the conclusion that the lever failed in fatigue as a result of excessive vibration of the fuel control on the test engine. Recommendations included redesign of the lever with a large radius in the corner where cracking originated. This would reduce the stress-concentration factor significantly, thus minimizing the susceptibility of the lever to fatigue.

A cast stainless steel femoral head replacement prosthesis fractured midway down the stem within 13 months of implantation. Visual examination showed severe “orange peel” around the fracture on the concave side. This effect was not observed on the convex side, which suggested fatigue fracture. Metallographic examination of samples revealed an extremely large grain size and corroborated fatigue fracture. Chemical analysis indicated that the material conformed to the requirements for type 316L stainless steel. Substandard-size tensile bars machined from another prosthesis from the same manufacturer showing identical grain sizes were used for mechanical testing. Tensile tests indicated that the material did not meet the manufacturer's stated strength criteria in the portion of the stem that fractured. The failure was attributed to low strength, which resulted in fatigue. The extremely coarse grain size was considered a major factor in strength reduction.

An investigation was conducted to determine the cause of numerous cracks and other defects on the surface of a cast ASTM A743 grade CA-15 stainless steel main boiler feed pump impeller. The surface was examined using a stereomicroscope, and macrofractography was conducted on several cross sections removed from the impeller body. Areas that appeared to have the most severe surface damage were sectioned, fractured open, and examined using SEM. The chemistry of the impeller and an apparent repair weld were also analyzed. The examination indicated that the cracks were shrinkage voids from the original casting process. Surface repair welds had been used to fill in or cover over larger shrinkage cavities. It was recommended that more stringent visual and nondestructive examination criteria be established for the castings.

A lever (machined from a casting made of AISI type 410 stainless steel, then surface hardened by nitriding) that was a component of the main fuel-control linkage of an aircraft engine fractured in flight after a service life of less than 50 h. Investigation (radiographic inspection) supported the conclusions that the lever broke at a cold shut extending through approximately 95% of the cross section. The normally applied load constituted an overload of the remainder of the lever. Recommendations included adding magnetic-particle inspection to the inspection procedures for this cast lever.

A heavily worked 304 stainless steel wire basket recrystallized and distorted while in service at 650 deg C (1200 deg F). This case study demonstrates that heavily cold worked austenitic stainless steel components can experience large losses in creep strength, and potentially structural collapse, under elevated temperature service, even at temperatures more than 300 deg C (540 deg F) below the normal solution annealing temperature. The creep strength of the recrystallized 304/304L steel was more than 1000 times less than that achievable with solution annealed 304H. These observations are consistent with limitations (2000 Addendum to ASME Boiler and Pressure Vessel Code) on the use of cold worked austenitic stainless steels for elevated temperature service.

A CF-8M (cast type 316) neck liner or manway was removed from the top of a digester vessel. Repeated attempts to repair the part in the field during its life cycle of many years had failed to keep the unit from leaking. The casting was a CF-8M modified with the molybdenum level at the top end of the range. The plate was standard 317L material. The filler metal was type 316, although marginal in molybdenum content. Investigation (visual inspection, chemical analysis, micrographs, and metallographic examination) supported the conclusion that the damage to the neck liner was due to Cl-SCC in an area of debris buildup. It appeared the original casting suffered SCC in a low-oxygen area high in chlorides from repeated wet/dry cycles where there was a buildup of debris. Recommendations included redesigning the neck liner to eliminate the abrupt change where there was debris buildup. If redesign was impossible, an alloy more resistant to Cl-SCC, such as a duplex stainless steel or a high-molybdenum (4 to 6%) austenitic stainless steel, should be used.

Several large-diameter type 304L stainless steel impeller/propeller blades in a circulating water pump failed after approximately 8 months of operation. The impeller was a single casting that had been modified with a fillet weld buildup at the blade root. Visual examination indicated that the fracture originated near the blade-to-hub attachment in the area of the weld buildup. Specimens from four failed castings and from an impeller that had developed cracks prior to design modification were subjected to a complete analysis. A number of finite-element-method computer models were also constructed. It was determined that the blades failed by fatigue that had been accelerated by stress-corrosion cracking. The mechanism of failure was flow-induced vibration, in which the vortex-shedding frequencies of the blades were attuned to the natural frequency of the blade/hub configuration. A number of solutions involving material selection and impeller redesign were recommended.

Examples of metallic inclusions in steels of various types are presented. The structure of an inclusion in an annealed Fe-1C-1.5Cr steel consisted of ferrite with lamellar pearlite. The carbon content of the inclusion was therefore considerably lower than that of the chromium steel and was adapted to the latter by diffusion only at the periphery of the inclusion. In another section of a hardened piece of the same chromium steel, the steel in this case had a structure of martensite with hypereutectic carbide, while the inclusions consisted of a very fine laminated eutectoid of the lower pearlite range (Troostite). In a pipe of 18-8 austenitic stainless steel a weakly magnetizable spot of limited size was found. This inclusion too was probably more alloy-deficient than the austenitic steel, similar to the ones described above. All three cases were casting defects.

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.

Several type 301 half-hard stainless steel clamps used to hold cylindrical galvanized steel covers to galvanized cast iron bases failed in flooded manholes after one to six months of service. Before service, they were treated with antiseize compound containing MoS2. Based on the conditions (the clamp is the cathode of a galvanic cell with zinc) and the brittle nature of the cracks, the failures were diagnosed as hydrogen-stress cracking. Laboratory experiments were conducted to substantiate the above diagnosis and to evaluate the effect of annealing and the hydrogen-stress cracking behavior of type 316 stainless steel. The problem was solved by changing the clamp material from type 301 to type 316 stainless steel and by eliminating the MoS2 antiseize compound.

Three radiant heating element tubes from an aluminum holding furnace failed after a few months of service. One side of each of the tubes had disintegrated, leaving large holes and thinned cross sections. Microstructural analysis showed that the surface of the tube had been oxidized along the grain boundaries and had extensive precipitation inside the grains. Chemical analysis indicated that the steel used for the tubes was AISI type 316 stainless steel Specifications for the tubes had called for AISI type 310S to be used. It was recommended that other tubes made from the same batch of steel sheet be checked.

Type 410 stainless steel bolts were used to hold together galvanized gray cast iron splice case halves. Before installation, the bolts were treated with molybdenum disulfide (MoS 2 ) antiseize compound. Several failures of splice case bolts were discovered in flooded manholes after they were in service for three to four months. Laboratory experiments were conducted to determine if the failure mode was hydrogen-stress cracking, if sulfides accelerate the failure, if heat treatment can improve the resistance against this failure mode, and if the type 305 austenitic stainless steel would serve as a replacement material. Based on test results, the solution to the hydrogen-stress cracking problem consisted of changing the bolt from type 410 to 305 stainless steel, eliminating use of MoS2, and limiting the torque to 60 N·m (540 in.·lb).

This article addresses the forms of corrosion that contribute directly to the failure of metal parts or that render them susceptible to failure by some other mechanism. It describes the mechanisms of corrosive attack for specific forms of corrosion such as galvanic corrosion, uniform corrosion, pitting and crevice corrosion, intergranular corrosion, and velocity-affected corrosion. The article contains a table that lists combinations of alloys and environments subjected to selective leaching and the elements removed by leaching.

Corrosion is the electrochemical reaction of a material and its environment. This article addresses those forms of corrosion that contribute directly to the failure of metal parts or that render them susceptible to failure by some other mechanism. Various forms of corrosion covered are galvanic corrosion, uniform corrosion, pitting, crevice corrosion, intergranular corrosion, selective leaching, and velocity-affected corrosion. In particular, mechanisms of corrosive attack for specific forms of corrosion, as well as evaluation and factors contributing to these forms, are described. These reviews of corrosion forms and mechanisms are intended to assist the reader in developing an understanding of the underlying principles of corrosion; acquiring such an understanding is the first step in recognizing and analyzing corrosion-related failures and in formulating preventive measures.