Schedule 80 low-carbon steel pipes used to transfer kraft liquor in a Kamyr continuous pulp digester failed within 18 months after installation. Visual and metallographic examinations established that the cracking initiated on the internal surfaces of the equalizer pipes in the welds and heat-affected zones (HAZs). Fracture/crack morphology was brittle and primarily intergranular and deposits at crack tips were primarily iron oxides with significant amounts of sodium compounds. On these bases, the cracking was characterized as intergranular stress-corrosion cracking (IGSCC). Corrosion-related deterioration was not found, indicating that the material was generally suitable for the intended service. High residual tensile stresses in the welds and HAZS, resulting from field welding under highly constrained conditions using inadequate weld procedures, were the most probable cause of the failures. Minimizing residual stresses through use of welding procedures that include appropriate preweld and interpass temperatures and postweld stress relief heat treatment at 650 deg C (1200 deg F) was recommended to prevent further failures.

Two AISI type 316 stainless steel dished ends failed through the formation of intergranular stress-corrosion cracks (IGSCC) within a few months of service. The dished ends failed in the straight portions near the circumferential welds that joined the ends to the cylindrical portions of the vessel. Both dished ends were manufactured from the same batch and were supplied by the same manufacturer One of the dished ends had been exposed to sodium at 550 deg C (1020 deg F) for 500 h before failure due to sodium leakage was detected. The other dished end was used to fabricate a second vessel that was kept in storage for 1 year Clear evidence of sensitization was found in areas where IGSCC occurred. Sensitization was extensive in the dished end that had been exposed to sodium at high temperature, and it occurred in a narrow band similar to that typical of weld decay in the dished end that had been kept in storage. Solution annealing was recommended to relieve residual stress, thereby reducing the probability of failure. It was also recommended that the carbon content of the steel be lowered, i.e., that a 316L grade be used.

An intergranular stress-corrosion cracking failure of 304 stainless steel pipe in 2000 ppm B as H3BO3 + H2O at 100 deg C was investigated. Constant extension rate testing produced an intergranular type failure in material in air. Chemical analysis was performed on both the base metal and weld material, in addition to fractography, EPR testing and optical microscopy in discerning the mode of failure. Various effects of Cl-, O2 and MnS are discussed. Results indicated that the cause of failure was the severe sensitization coupled with probable contamination by S and possibly by Cl ions.

Two tanks made of AISI type 304 stainless steel exhibited cracking in the heat-affected zone (HAZ) of the weld that joined the dished end and the shell. The dished ends had been produced by cold deformation. Hardness measurement and simulation tests showed that the deformation was equivalent to a 30% reduction in thickness. Residual stresses were measured at about 135 MPa (20 ksi). The HAZ was found to be sensitized. The tanks had been stored in a coastal atmosphere for about 4 years before installation. The failure was attributed to intergranular stress-corrosion cracking in a sensitized HAZ due to chloride from the environment. Use of low-carbon type AISI 304L was recommended. Minimization of fit-up stresses and covering with polyethylene sheets during storage were also suggested.

Several of the springs, made of 1.1 mm diam Inconel X-750 wire and used for tightening the interstage packing ring in a high-pressure turbine, were found broken after approximately seven years of operation. Intergranular cracks about 1.3 mm in depth and oriented at an angle of 45 deg to the axis of the wire were revealed by metallographic examination. A light-gray phase, which had the appearance of liquid-metal corrosion, was observed to have penetrated the grains on the fracture surfaces. The spring wires were found to fracture in a brittle manner characteristic of fracture from torsional loading (along a plane 45 deg to the wire axis). Liquid-metal embrittlement was expected to have been caused by metals (Sn, Zn, Pb) which melt much below maximum service temperature of the turbine. The springs were concluded to have fractured by intergranular stress-corrosion cracking promoted by the action of liquid zinc and tin in combination with static and torsional stresses on the spring wire. As a corrective measure, Na, Sn, and Zn which were present in pigmented oil used as a lubricant during spring winding was cleaned thoroughly by the spring manufacturer before shipment to remove all contaminants.

The presence of secondary, branching intergranular stress-corrosion cracking in a type 440C stainless bearing caused the analyst to overlook the real culprit, which was a mechanically-initiated, primary transgranular crack that propagated through the steel's hard chromium carbide. Failure was actually caused by overload. Had the original conclusion been accepted, a relatively exotic alloy would have been specified. In another case, brass heat exchanger tube failure was automatically attributed to attack by an acidic cleaner, and a decision was made to stop using the solution. A more thorough analysis showed failure was caused by tube vibration. In a third case, a type 304 stainless steel bellows in a test loop was thought to have failed because of chloride stress corrosion. The report concluded with a recommendation that carbon steel be used as an alternative bellows material. Caustic, not chloride, stress corrosion was the culprit. Had material substitutions been made on the original premise of countering chloride stress corrosion, most of the loop's highly stressed components would have eventually failed.

A rupture of a thirty-year-old U-tube on a steam generator for a closed-cycle pressurized-water nuclear power plant occurred, resulting in limited release of reactor water. A typical tube bundle can be over 9 m (30 ft) tall and 3 m (10 ft) in diam with over 3,000 22-mm (7/8-in.) diam Inconel Alloy 600 tubes. Tube support plates (TSP) separate the tubes and allow flow of the heating water/steam. Inconel Alloy 600 is susceptible to intergranular stress-corrosion cracking over time, so investigation included review of operational records, maintenance history, and procedures. It also included FEA (thermal gradients, nonlinear material behavior, residual stress, changes in wall thickness during the formation of U-bends, and TSP distortions near the ruptured tube) of three-dimensional solid models of the U-tubes. The conclusion was that distortion of the TSPs and resulting “pinching” of the U-tubes, combined with the operational stresses, caused high stresses at the location where the tube cracked. The stresses were consistent with those required to initiate and propagate a longitudinal crack.

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.

Argonne National Laboratory has conducted analyses of failed components from nuclear power-generating stations since 1974. The considerations involved in working with and analyzing radioactive components are reviewed here, and the decontamination of these components is discussed. Analyses of four failed components from nuclear plants are then described to illustrate the kinds of failures seen in service. The failures discussed are (1) intergranular stress-corrosion cracking of core spray injection piping in a boiling water reactor, (2) failure of canopy seal welds in adapter tube assemblies in the control rod drive head of a pressurized water reactor, (3) thermal fatigue of a recirculation pump shaft in a boiling water reactor, and (4) failure of pump seal wear rings by nickel leaching in a boiling water reactor.

A shrunk-fit 18 Mn-5Cr steel retaining ring failed without warning during normal unit operation of a 380 MW electrical generator. The cause of the ring failure was determined to be intergranular stress-corrosion cracking (IGSCC) because of the high strength of the ring material and the presence of moist hydrogen used to cool the ring. Factors which promoted the failure were higher than normal strength levels in the ring material, lower than normal ring operating temperatures, possible moisture in the lubrication oil system, periodic poor performance of the hydrogen dryers, and a ring design which allowed water to become trapped in a relief groove. These factors caused pitting in the ring in an estimated 100 hours of operation. The ring had been inspected previously 18 months prior to the failure and no defects or pitting were found. Calculations showed that a 0.127-cm (0.050-in.) deep pit could grow to a critical size in 3000 to 4000 hours of operation. To prevent further failures, it was recommended that the ring be replaced with an 18 Mn-18Cr alloy with superior resistance to IGSCC. A program of periodic inspection and replacement of other retaining rings in the system was also recommended.

Some of the admiralty brass tubes were failing in a heat exchanger. The heat exchanger cooled air by passing river water through the inside of the tubes. The wall thickness of all tubes ranged between 1.19 to 1.27 mm (0.047 to 0.050 in.). General intergranular corrosion occurred at the inside surfaces of the tubes. Transgranular stress-corrosion cracking, probably the result of sulphates under basic conditions, and dezincification occurred also as the result of galvanic corrosion under the deposits in the tubes. Recommendations were to use a closed-loop water system to eliminate sulphates, ammonia, etc., and to run trials on one unit with tubes of other alloys such as 80-20 Cu-Ni or 70-30 Cu-Ni to evaluate their performance prior to any large scale retubing operations.

Eddy-current inspection was performed on a leaking absorber bundle in an absorption air-conditioning unit. The inspection revealed crack-like indications in approximately 50% of the tubes. The tube material was phosphorus-deoxidized copper. Investigation (visual inspection, chemical analysis, 0.75x images, 2x macrographs after light acid cleaning to remove corrosion product, and 75x micrographs) supported the conclusion that the absorber tubes failed by SCC initiated by ammonia contamination in the lithium bromide solution. No recommendations were made.

This article briefly reviews the factors that influence the occurrence of intergranular (IG) fractures. Because the appearance of IG fractures is often very similar, the principal focus is placed on the various metallurgical or environmental factors that cause grain boundaries to become the preferred path of crack growth. The article describes in more detail some typical mechanisms that cause IG fracture. It discusses the causes and effects of IG brittle cracking, dimpled IG fracture, IG fatigue, hydrogen embrittlement, and IG stress-corrosion cracking. The article presents a case history on IG fracture of steam generator tubes, where a lowering of the operating temperature was proposed to reduce failures.

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.

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.

A type 410 stainless steel circulating water pump shaft used in a fossil power steam generation plant failed after more than 7 years of service. Visual examination showed the fracture surface to be coated with a thick, spalling, rust-colored scale, along with evidence of pitting. Samples for SEM fractography, EDS analysis, and metallography were taken at the crack initiation site. Hardness testing produced a value of approximately 27 HRC. The examinations clearly established that the shaft failed by fatigue. The fatigue crack originated at a localized region on the outside surface where pitting and intergranular cracking had occurred. The localized nature of the initial damage indicated that a corrosive medium had concentrated on the surface, probably due to a leaky seal. Reduction of hardness to 22 HRC or lower and inspection of seals were recommended to prevent future failures.

Copper electrical feedthrough pins used in a bolting application in a refrigeration compressor had functioned without failure for years of production and thousands of units. When some of the pins began to fail, an investigation was conducted to determine the cause. Visual examination revealed that the observed fractures were mixed brittle intergranular with ductile microvoid dimples. An extensive analysis of failed samples combined with a process of elimination indicated that the fractures were due to stress-corrosion cracking caused by an unidentified chemical species within the sealed compressor chamber. A unique combination of applied stress, residual stress, stress riser, and grain size helped isolate the failure mechanism to a single production lot of material.