The shafts on two centrifugal pumps failed during use in a petroleum refinery. Light optical microscopy and scanning electron microscopy were used to analyze the damaged materials to determine the cause of failure. The results showed that one shaft, made of duplex stainless steel, failed by fatigue fracture, and the other, made of 316 austenitic stainless steel, experienced a similar fracture, which was promoted by the presence of nonmetallic inclusion particles.

The drive shaft on a centrifugal pump fractured after five months of operation. The fracture occurred at a packing gland inside a sleeve against which the packing material sealed. The shaft and sleeve were of duplex stainless steel. In contrast to a previous conclusion that heating had caused property deterioration resulting in embrittlement and fracture, it was concluded that the shaft must have fractured (most probably by fatigue cracking originating at the change of section) and that heating had then taken place from friction between the rotating input shaft and the remaining part attached to the pump. High temperature was thus a result, not the cause, of the failure.

Several type 304L (UNS S30400) stainless steel seamless tubes in a high-pressure synthesis gas cooler condensing ammonia in a fertilizer plant leaked in an unexpectedly short time. Representative samples of the tubes were subjected to chemical analysis, hardness tests, and optical microscopy examination. The tests revealed that the tubes conformed to specification. Crack morphology indicated stress-corrosion cracking by chlorides present in the cooling water. Use of a duplex stainless steel (for example, UNS S32304 S31803) as a tube material was recommended.

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.

Small paddles used to mix pulp had experienced a high incidence of breakage through the shafts. In some of the shanks, shrinkage was found relatively close to the surface where threads had been cut all the length of the shaft. Chemistries were within normal CF-8M ranges. Metallography showed the parts to be correctly heat treated. Cross sections of several of the parts showed pitting corrosion, and beneath the pits, stress-corrosion cracks in areas where the shafts had been bent during use. All the samples showed deep SCC in the areas where bending had occurred. In several cases, centerline shrinkage from inadequate risering had decreased life by reducing the cross-sectional area. Type CF-8M is not resistant to chloride SCC where the chloride concentration is considerable. The biggest problem was the bending of these parts. Deformed material with high residual stresses would always be susceptible to SCC. Redesign to lower stresses was essential. In addition, change to a high-strength duplex stainless steel with its higher strength and greater resistance to chlorides was recommended. Finally, the part must be adequately risered to produce solid shanks free from shrinkage.

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.

An E-Brite /Ferralium explosively bonded tube sheet in a nitric acid condenser was removed from service because of corrosion. Visual and metallographic examination of tube sheet samples revealed severe cracking in the heat-affected zone between the outer tubes and the weld joining the tube sheet to the floating skirt. Cracks penetrated deep into the tube sheet, and occasionally into the tube walls. The microstructures of both alloys and of the weld appeared normal. Intergranular corrosion characteristic of end-grain attack was apparent. A low dead spot at the skirt / tube sheet joint allowed the Nox to condense and subsequently reboil. This, coupled with repeated repair welding in the area, reduced resistance to acid attack. Intergranular corrosion continued until failure. Recommendations included changing operating parameter inlet to prevent HNO3 condensation outside the inlet and replacement of the floating skirt with virgin material (i.e., material unaffected by weld repairs).

The welds joining the liner and shell of a fluid catalytic cracking unit failed. The shell was made of ASTM A515 carbon steel welded with E7018 filler metal. The liner was made of type 405 stainless steel and was plug welded to the shell using ER309 and ER310 stainless steel filler metal. Fine cracks starting inside the weld zone and spreading outward through the weld and toward the surface were observed during examination. Decarburization and graphitization of the carbon steel at the interface was noted. The high carbon level was found to allow martensite to form eventually. The structure was found to be austenitic in the area where the grain-boundary precipitates appeared heaviest. The composition of the precipitates was analyzed using an electron microprobe to reveal presence of sulfur. Microstructural changes in the weld alloy at the interface were interpreted to be caused by dilution of the alloy and the presence of sulfur caused hot shortness. The necessary internal stress to produce extensive cracking was produced by the differential thermal expansion of the carbon and stainless steels. Periodic careful gouging of the affected areas followed by repair welding was recommended.

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.

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.

Alloy 430 stainless steel tube-to-header welds failed in a heat recovery steam generator (HRSG) within one year of commissioning. The HRSG was in a combined cycle, gas-fired, combustion turbine electric power plant. Alloy 430, a 17% Cr ferritic stainless steel, was selected because of its resistance to chloride and sulfuric acid dewpoint corrosion under conditions potentially present in the HRSG low-pressure feedwater economizer. Intergranular corrosion and cracking were found in the weld metal and heat-affected zones. The hardness in these regions was up to 35 HRC, and the weld had received a postweld heat treatment (PWHT). Metallographic examination revealed that the corroded areas contained undertempered martensite. Fully tempered weld areas with a hardness of 93 HRB were not attacked. No evidence of corrosion fatigue was found. Uneven temperature control during PWHT was the most likely cause of failure.

This article commences with a discussion on the characteristics of stress-corrosion cracking (SCC) and describes crack initiation and propagation during SCC. It reviews the various mechanisms of SCC and addresses electrochemical and stress-sorption theories. The article explains the SCC, which occurs due to welding, metalworking process, and stress concentration, including options for investigation and corrective measures. It describes the sources of stresses in service and the effect of composition and metal structure on the susceptibility of SCC. The article provides information on specific ions and substances, service environments, and preservice environments responsible for SCC. It details the analysis of SCC failures, which include on-site examination, sampling, observation of fracture surface characteristics, macroscopic examination, microscopic examination, chemical analysis, metallographic analysis, and simulated-service tests. It provides case studies for the analysis of SCC service failures and their occurrence in steels, stainless steels, and commercial alloys of aluminum, copper, magnesium, and titanium.

Stress-corrosion cracking (SCC) is a form of corrosion and produces wastage in that the stress-corrosion cracks penetrate the cross-sectional thickness of a component over time and deteriorate its mechanical strength. Although there are factors common among the different forms of environmentally induced cracking, this article deals only with SCC of metallic components. It begins by presenting terminology and background of SCC. Then, the general characteristics of SCC and the development of conditions for SCC as well as the stages of SCC are covered. The article provides a brief overview of proposed SCC propagation mechanisms. It discusses the processes involved in diagnosing SCC and the prevention and mitigation of SCC. Several engineering alloys are discussed with respect to their susceptibility to SCC. This includes a description of some of the environmental and metallurgical conditions commonly associated with the development of SCC, although not all, and numerous case studies.

Nineteen out of 26 bolts in a multistage water pump corroded and cracked after a short time in a severe working environment containing saline water, CO 2 , and H 2 S. The failed bolts and intact nuts were to be made from a special type of stainless steel as per ASTM A 193 B8S and A 194. However, the investigation (which included visual, macroscopic, metallographic, SEM, and chemical analysis) showed that austenitic stainless steel and a nickel-base alloy were used instead. The unspecified materials are more prone to corrosion, particularly galvanic corrosion, which proved to be the primary failure mechanism in the areas of the bolts directly exposed to the working environment. Corrosion damage on surfaces facing away from the work environment was caused primarily by chloride stress-corrosion cracking, aided by loose fitting threads. Thread gaps constitute a crevice where an aggressive chemistry is allowed to develop and attack local surfaces.

Three blades from 45,000 kW, 3,000 rpm turbine were received for examination, comprising the root of blade 28, blade 89 showing a crack in one of the root teeth, and blade 106 which was free from defects. Microscopic examination of the blade material showed it to be a ferritic stainless steel of the type commonly used for turbine blades. A number of non-metallic inclusions were present which had been drawn into threads in rolling; these appeared to consist largely of duplex silicates. The failure of blade 28 was the result of the development of a creeping crack. Magnetic crack examination of blade 89 revealed a crack in a tooth in an identical position to the start of the crack in blade 28 but on the opposite, i.e., steam inlet, side of the blade. Similar examination of blade 106 did not reveal any cracks. Cracking was associated with unsatisfactory bedding of the blade teeth on the faces of the wheel grooves. It was concluded that the blade failures were due primarily to over-loading of the individual blade teeth due to incorrect fitting in the wheel. Vibration was an important contributory factor, as it resulted in the imposition of fluctuating stresses on the overloaded teeth. Non-metallic inclusions in the blade material playing a minor part.