Microstructural examinations on transverse cross sections of a steam reformer tube, showed the presence of large macrovoids elongated in the radial direction and emanating from the internal surface of the tube. The macrovoids were located at the interdendritic regions, and were partially filled by a Mn-Fe bearing chromium oxide film. The areas adjacent to the oxide film were chemically depleted in C, Cr and Mn and rich in Fe and Ni. Associated with this depletion were a large concentration of microvoids. It was suggested that the dissolution of carbides in areas surrounding the macrovoids and the concentration of stresses at their tips, caused extensive localized plastic deformation which led to the formation of microvoids and subsequently to the spalling of the oxide film. The non-protective character of the film induced a progressive deterioration of the grain boundaries properties. Grain boundary sliding and dislocation motion were enhanced, causing a local increase in the steady state strain rate and the premature failure of the tube.

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.

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.

The outlet-piping system of a steam-reformer unit failed by extensive cracking at four weld locations. The welded system consisted of Incoloy 800 (Fe-32Ni-21Cr-0.05C) pipe and fittings. The exterior surfaces of the system were insulated with rock wool that did not contain weatherproofing. On-site visual examination and magnetic testing indicated severe external corrosion of most of the piping. The system showed extensive cracking in weld HAZ. One specimen indicated that corrosion extended to a depth of 3.2 mm and cracks were seen at the edge of the cover bead and in the HAZ of the weld. Metallographic examination showed that cracking was intergranular and that adjacent grain boundaries had undergone deep intergranular attack. Examination at higher magnification revealed heavy carbide precipitation, primarily at grain boundaries, indicating that the alloy had been sensitized, which resulted from heating during welding. Electron probe x-ray microanalysis showed the outside surface of the tube did not have the protective chromium oxide scale normally found on Incoloy 800. The inside surface of the tube had a thin chromium oxide protective scale. This evidence supported the conclusions that the deep oxidation greatly decreased the strength of the weld HAZ and cracking followed.

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.

This case study demonstrates that Alloy 601 (UNS N06601) is susceptible to strain-age cracking. The observation illustrates the potential importance of post weld heat treatment to the successful utilization of this alloy in certain applications.

A tubular heat exchanger in a refinery reformer unit leaked after one month of service. The exchanger contained 167 type 304 stainless steel U-bent integral-finned tubes. Cracks in the tube wall were revealed during examination. Hardness of the tube was found to be 30 HRC at the inside surface and up to 40 HRC at the base of the fin midway between the roots which indicated that the fins were cold formed and not subsequently annealed thus susceptible to SCC because of a high residual stress level. It was revealed by metallographic examination that the fracture was predominantly by transgranular branched cracking and had originated from the inside surface. It was concluded that the tubes failed in SCC caused by chlorides in the presence of high residual stresses. The finned tubes were ordered in the annealed condition as a corrective measure.

Two steam-methane reformer furnaces were subjected to short-time heat excursions because of a power outage, which resulted in creep bulging in the Incoloy 800 outlet pigtails, requiring complete replacement. Each furnace had three cells, consisting of 112 vertical tubes per cell, each filled with a nickel catalyst. The tubes were centrifugally cast from ASTM A297, grade HK-40 (Fe-25Cr-20Ni-0.40C), heat-resistant alloy. The tube was concluded after metallurgical inspection to have failed from creep rupture (i.e., stress rupture). A project for detecting midwall creep fissuring was instigated as a result of the failure. It was concluded after laboratory radiography and macroexamination that if the fissure were large enough to show on a radiograph, either with or without the catalyst, the tube could be expected to fail within one year. The set up for in-service radiograph examination was described. The tubes of the furnace were radiographed during shut down and twenty-four tubes in the first furnace and 53 in the second furnace showed significant fissuring. Although, radiography was concluded to be a practical technique to provide advance information, it was limited to detecting fissures caused by third-stage creep in tubes because of the cost involved in removing the catalysts.

A sample tube was removed from a reformer furnace for life assessment after 69,000 h of service. Sections were cut from the tube, which was a spindle cast A297 Grade HK 40 (25 Cr, 20 Ni, 0.4 C) austenitic steel of 122.5 mm OD and 10.5 mm nominal wall thickness. They were examined metallographically on transverse sections and on longitudinal sections through the butt welds joining the separate cast segments of the tube. Creep damage was mainly concentrated within the inner one third of the wall thickness. The use of damage assessment parameters in evaluating the reformer tube remaining life showed the welds to be inadequate, and to have a strength and creep resistance below those of the base metal.

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.

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.

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.

This article describes the characteristics of tubing of heat exchangers with respect to general corrosion, stress-corrosion cracking, selective leaching, and oxygen-cell attack, with examples. It illustrates the examination of failed parts of heat exchangers by using sample selection, visual examination, microscopic examination, chemical analysis, and mechanical tests. The article explains corrosion fatigue of tubing of heat exchangers caused by aggressive environment and cyclic stress. It also discusses the effects of design, welding practices, and elevated temperatures on the failures of heat exchangers.

Heat exchangers are devices used to transfer thermal energy between two or more fluids, between a solid surface and a fluid, or between a solid particulate and a fluid at different temperatures. This article first addresses the causes of failures in heat exchangers. It then provides a description of heat-transfer surface area, discussing the design of the tubular heat exchanger. Next, the article discusses the processes involved in the examination of failed parts. Finally, it describes the most important types of corrosion, including uniform, galvanic, pitting, stress, and erosion corrosion.

Radiant tubes that failed prematurely in an ethylene cracking furnace were analyzed to determine the cause of their early demise. The tubes were made from austenitic heat-resistant steel and cracked along their longitudinal axis. New and used tubes were compared using scanning electron microscopy, energy dispersive x-ray spectrometry, and mechanical property testing. This provided critical information and revealed that improper coking and decoking had removed the protective oxide layer (Cr 2 O 3 ) that normally prevents coke deposits from forming on exposed surfaces. Without this layer, coke readily accumulates on the surface of the tubes, fueling carbon diffusion into the metal and a corresponding degradation in microstructure and loss of ductility at high temperatures.

This article discusses the effect of using unsuitable alloys, metallurgical discontinuities, fabrication practices, and stress raisers on the failure of a pressure vessel. It provides information on pressure vessels made of composite materials and their welding practices. The article explains the failure of pressure vessels with emphasis on stress-corrosion cracking, hydrogen embrittlement, brittle and ductile fractures, creep and stress rupture, and fatigue with examples.

A 76 mm (3 in.) type 304 stainless steel tube that was used as a heat shield and water nozzle support in a hydrogen gas plant quench pot failed in a brittle manner. Visual examination of a sample from the failed tube showed that one lip of the section was eroded from service failure, whereas the opposite side exhibited a planar-type fracture. Sections were removed from the eroded area and from the opposite lip for microscopic studies and chemical analysis. The eroded edges exhibited river bed ditching, indicative of thermal fatigue. Microstructural analysis showed massive carbide formations in a martensite matrix and outlining of prior-austenite grains by a network of fine, white lines. These features indicated that the material had been transformed by carburization by the impinging gas. The outer surface exhibited a heavy scale deposit and numerous cracks that originated at the surface of the tube. The cracks were covered with scale, indicating that thermal fatigue (heat cracking) had occurred. Chemical analysis confirmed that the original material was type 304 stainless steel that had been through-carburized by the formation of an endothermic gas mixture. It was recommended that plant startup and shutdown procedures be modified to reduce or eliminate the presence of the carburizing gas mixture.

High temperature corrosion may occur in numerous environments and is affected by factors such as temperature, alloy or protective coating composition, time, and gas composition. This article explains a number of potential degradation processes, namely, oxidation, carburization and metal dusting, sulfidation, hot corrosion, chloridation, hydrogen interactions, molten metals, molten salts, and aging reactions including sensitization, stress-corrosion cracking, and corrosion fatigue. It concludes with a discussion on various protective coatings, such as aluminide coatings, overlay coatings, thermal barrier coatings, and ceramic coatings.