An Incoloy 800H (UNS N08810) transfer line on the outlet of an ethane-cracking furnace failed during decoking of the furnace tubes after nine years in service. A metallographic examination using optical and scanning electron microscopy as well as energy-dispersive x-ray spectroscopy revealed that the failure was due to sulfidation. The source of the sulfur in the furnace effluent was either dimethyl disulfide, injected into the furnace feed to prevent coke formation and carburization of the furnace tubes, or contamination of the feed with sulfur bearing oil.

A Nicrofer 3718 sinter belt used in a sinter furnace operated at 965 deg C (1770 deg F) for the curing of nickel briquettes stretched and fractured after only 6 months in service. Macrofractographic, metallographic, and chemical analyses of several broken links of the woven belt and an unused section of new wire showed that the fracture resulted from sulfur attack and overheating during service. It was recommended that the sinter belt material be changed to Nicrofer 3220-H (alloy 800H).

A tie rod, nut, and bellows from a failed 610 mm (24 in.) diam tied universal expansion joint that carried tail gases consisting of N 2 + O 2 with slight traces of nitrogen oxides and water were examined. The materials were SA 193-B7 (AISI 4140), SA 194–214, and Incoloy 800H, respectively. Visual examination of the bellows revealed cracks in heavily cold-worked areas (both inside and outside) and considerable corrosion. SEM analysis showed a classical intergranular failure pattern with microcracking. The threaded tie rod microstructure contained spheroidized carbide that was more pronounced at the tie rod end of the failure. Energy-dispersive X-ray analysis of fracture surfaces from the bellows showed the presence of chlorine and sulfur. Failure of the bellows was attributed to stress-corrosion cracking, with chlorine and sulfur being the corroding agents. The rod damage was the result of failure of the bellows, which allowed escaping hot gases to impinge on the tie rods and heat them to approximately 595 deg C (1100 deg F). It was recommended that the insulation be analyzed to determine the origin of the chlorine and sulfur and that it be replaced if necessary.

A 150 mm (6 in.) diam, 1.6 mm (0.065 in.) thick alloy 800 1iner from an internal bypass line in a hydrogen reformer was removed from a waste heat boiler because of severe metal loss. Visual and metallographic examinations of the liner indicated severe metal wastage on the inner surface, along with sooty residue. Patterns similar to those associated with erosion/corrosion damage were observed. Microstructural examination of wasted areas revealed a bulk matrix composed of massive carbides, indicating that gross carburization and metal dusting had occurred. X-ray diffraction analysis showed that the carbides were primarily chromium based (Cr 23 C 7 and Cr 7 C 3 ). The sooty substance was identified as graphite. Wasted areas were ferromagnetic and the degree of ferromagnetism was directly related to the degree of wastage. Three actions were recommended: (1) inspection of the waste heat boiler to determine the extent of metal damage in other areas by measuring the degree of ferromagnetism, (2) replacement of metal determined to be magnetic, and (3) closer monitoring of temperatures in the region of the reformer furnace outlet.

During 5.7 years of service, dye penetrant inspection of Inconel 800H pigtail connections regularly showed cracks at weld toes. Weld repairs were not able to prevent reoccurrence but often aggravated the condition. Samples containing small, but detectable, reducer-to-pigtail cracks showed intergranular cracks originating at weld toes and filled with oxidation product, which precluded determination of the cracking mechanism. All weldments exhibited high degrees of secondary precipitates, with original fabrication welds exhibiting higher apparent levels than repair welds. SEM/EDS analysis showed base metal grain boundary precipitates to be primarily chromium carbides, but some titanium carbides were also observed. Failure was believed to result from the synergism of thermally driven tube distortion, which resulted in over-stress, and from the intergranular oxidation products and intergranular carbides which contributed to cracking. It was recommended that stresses be reduced and /or that materials and components be changed. Refinements in welding procedures and implementation of preweld/postweld heat treatments were recommended also.

An SB407 alloy 800H tube failed at a 100 deg bend shortly after startup of a new steam superheater. Three bends failed and one bend remote from the failure area was examined. Visual examination showed that the fracture started on the outside surface along the inside radius of the bend and propagated in a brittle, intergranular fashion. Chemical analysis revealed that lead contamination was a significant factor in the failure and phosphorus may have contributed. The localized nature of the cracks and minimum secondary cracking suggested a distinct, synergistic effect of applied tensile stress with the contamination. Stress analysis found that stress alone was not enough to cause failure; however the operating stresses in the 100 deg bends were higher than at most other locations in the superheater Reduced creep ductility may be another possible cause of failure. Remedial actions included reducing the tube temperature, replacing the Schedule 40 100 deg bends with Schedule 80 pipe, and solution annealing the pipe after bending.

The failure of a reformer tube furnace manifold has been examined using metallography. It has been shown that the cause of failure was thermal fatigue; the damage was characterized by the presence of voids produced by creep mechanisms operating during the high temperature cycle under high local stress. The study indicates that standard metallographic procedures can be used to identify failure modes in high temperature petrochemical plants.

The curved parts of exit pigtails made of wrought Incoloy 800H tubing used in steam reforming furnaces failed by performance after a period of service shorter than that predicted by the designers. Examination of a set of tubes consisting of both curved (perforated) and straight parts revealed that the cracks initiated at the outer surface by a combined mechanism of creep and intergranular embrittlement. A smaller grain size resulting from cold bending fabrication procedures for the curved parts was responsible for accelerating the embrittlement. It was recommended that hot bending be used for fabrication of the curved parts. A change of alloy to a low-alloy chromium-molybdenum allay to protect against heat was also suggested.

During a planned shut-down in 1990 it appeared that the bottom manifold parts made of wrought Incoloy 800H had undergone diametrical expansion of up to 2% due to creep. Further, cracking at the outer diam was found. It was decided to replace these parts. Microscopical investigations showed that the cracking could not be caused by creep. It was found that the cracking was confined to a 4-mm deep coarse-grained zone (ASTM 0-1) at the outer diameter. The cracking appeared to be caused by strain-induced intergranular oxidation. When the cracks reached the fine-grained material, the oxidation-cracks stopped. To determine the residual creep life of the sound (non-cracked) bottom manifold material, iso-stress creep tests were performed. It was found that tertiary creep started at 7% strain. The time-to-rupture was greater than 100,000 h. It was concluded that the bottom manifold (and thus the furnace) could be used safely during the foreseen production period.

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.