Search Results for graphitizing annealing

Cast iron bearing caps in tractor engines fractured repeatedly after only short operating periods. The fracture originated in a cast-in groove and ran approximately radially to the shaft axis. The smallest cross section was at the point of fracture. The core structure of the caps consisted of graphite in pearlitic-ferritic matrix. Casting stresses did not play a decisive role because of the simple shape of the pieces that were without substantial cross sectional variations. Two factors exerted an unfavorable effect in addition to comparatively low strength. First, the operating stress was raised locally by the sharp-edged groove, and second, the fracture resistance of the cast iron was lowered at this critical point by the existence of a ferritic bright border. To avoid such damage in the future it was recommended to observe one or more of the following precautions: 1) Eliminate the grooves; 2) Remove the ferritic bright border; 3) Avoid undercooling in the mold and therefore the formation of granular graphite; 4) Inoculate with finely powdered ferrosilicon into the melt for the same purpose; and, 5) Anneal at lower temperature or eliminate subsequent treatment in consideration of the uncomplicated shape of the castings.

An experimental high-temperature rotary valve was found stuck due to growth and distortion after approximately 100 h. Gas temperatures were suspected to have been high due to overfueled conditions. Both the rotor and housing in which it was stuck were annealed ferritic ductile iron similar to ASTM A395. Visual examination of the rotor revealed unusually heavy oxidation and thermal fatigue cracking along the edge of the gas passage. Material properties, including microstructure, composition, and hardness, of both the rotor and housing were evaluated to determine the cause of failure. The microstructure of the rotor was examined in three regions. The shaft material, the heavy section next to the gas passage and the thin edge of the rotor adjacent to the gas passage. The excessive gas temperatures were responsible for the expansion and distortion that prevented rotation of the rotor. Actual operating temperatures exceeded those intended for this application. The presence of transformation products in the brake-rotor edge indicated that the lower critical temperature had been exceeded during operation.

A carbon-molybdenum (ASTM A209 Grade T1) steel superheater tube section in an 8.6 MPa (1250 psig) boiler cracked because of long-term overheating damage that resulted from prolonged exposure to metal temperatures between 482 deg C (900 deg F) and 551 deg C (1025 deg F). The outer diameter of the tube exhibited a crack (fissure) oriented approximately 45 deg to the longitudinal axis and 3.8 cm (1.5 in.) long. The inner diameter surface showed a fissure in the same location and orientation. Microstructure at the failure near the outer diameter surface exhibited evidence of creep cracking and creep void formation at the fissure. A nearly continuous band of graphite nodules was observed on the surface of the fissure. In addition to the graphite band formation, the microstructure near the failure exhibited carbide spheroidization from long-term overheating in all the tube regions examined. It was concluded that preferential nucleations of graphite nodules in a series of bands weakened the steel locally, producing preferred fracture paths. Formation of these graphite bands probably expedited the creep failure of the tube. Future failures may be avoided by using low-alloy steels with chromium additions such as ASTM A213 Grade T11 or T22, which are resistant to graphitization damage.

Engineering plastics, as a general class of materials, are prone to the development of internal stresses which arise during processing or during servicing when parts are exposed to environments that impose deformation and/or temperature extremes. Thermal stresses are largely a consequence of high coefficients of thermal expansion and low thermal diffusivities. Although time-consuming techniques can be used to analyze thermal stresses, several useful qualitative tests are described in this article. The classification of internal stresses in plastic parts is covered. The article describes the effects of low thermal diffusivity and high thermal expansion properties, and the variation of mechanical properties with temperature. It discusses the combined effects of thermal stresses and orientation that result from processing conditions. The article also describes the effect of aging on properties of plastics. It explains the use of high-modulus graphite fibers in amorphous polymers.

A ductile iron T-hook hook was reported to have fractured in service. It was further reported that the hook had been subjected to a load that did not exceed 5900 kg (13,000 lb) at the time of fracture. No information was provided regarding the type of metal used to manufacture the hook. A failure analysis was requested to determine the cause of fracture. Two hooks were submitted for examination. Analysis (visual inspection, 2.7x light fractography, chemical analysis, 110x SEM fractography, 27x/110x/215x nital-etched micrographs) supported the conclusions that this component fractured in service as a consequence of ductile tensile overload. Evidence indicates that the fractured region was subjected to a load exceeding the capacity of the material. Because the information available from the service application indicated that the component had not been subjected to a stress that exceeded 5900 kg (13,000 lb), the observations made in this investigation suggested that either the load was underestimated or that the indicated load was applied at a more rapid rate (perhaps with a jerk), which would tend to increase the effective force of the load.

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.

The information provided in this article is intended for those individuals who want to determine why a casting component failed to perform its intended purpose. It is also intended to provide insights for potential casting applications so that the likelihood of failure to perform the intended function is decreased. The article addresses factors that may cause failures in castings for each metal type, starting with gray iron and progressing to ductile iron, steel, aluminum, and copper-base alloys. It describes the general root causes of failure attributed to the casting material, production method, and/or design. The article also addresses conditions related to the casting process but not specific to any metal group, including misruns, pour shorts, broken cores, and foundry expertise. The discussion in each casting metal group includes factors concerning defects that can occur specific to the metal group and progress from melting to solidification, casting processing, and finally how the removal of the mold material can affect performance.

Failure of AISI 1015 steel brake discs used in power transmissions in emergency winches was investigated using various testing methods. The failed discs were stampings that had replaced cast discs. Residual stresses in the fillets of new cast and new stamped brake discs were measured by x-ray diffraction. The results indicated that the stamped brake discs had failed by fatigue caused by a tensile residual stress pattern in the fillet. The residual stress pattern was attributed to the change in manufacturing process from casting to stamping. Use of a manufacturing process that yields a compressive residual stress in the fillet, appropriate heat treatment of stamped discs, or redesign of the disc and/or transmission assembly was recommended.

This article aims to identify and illustrate the types of overload failures, which are categorized as failures due to insufficient material strength and underdesign, failures due to stress concentration and material defects, and failures due to material alteration. It describes the general aspects of fracture modes and mechanisms. The article briefly reviews some mechanistic aspects of ductile and brittle crack propagation, including discussion on mixed-mode cracking. Factors associated with overload failures are discussed, and, where appropriate, preventive steps for reducing the likelihood of overload fractures are included. The article focuses primarily on the contribution of embrittlement to overload failure. The embrittling phenomena are described and differentiated by their causes, effects, and remedial methods, so that failure characteristics can be directly compared during practical failure investigation. The article describes the effects of mechanical loading on a part in service and provides information on laboratory fracture examination.

This article focuses on the general root causes of failure attributed to the casting process, casting material, and design with examples. The casting processes discussed include gravity die casting, pressure die casting, semisolid casting, squeeze casting, and centrifugal casting. Cast iron, gray cast iron, malleable irons, ductile iron, low-alloy steel castings, austenitic steels, corrosion-resistant castings, and cast aluminum alloys are the materials discussed. The article describes the general types of discontinuities or imperfections for traditional casting with sand molds. It presents the international classification of common casting defects in a tabular form.

Overload failures refer to the ductile or brittle fracture of a material when stresses exceed the load-bearing capacity of a material. This article reviews some mechanistic aspects of ductile and brittle crack propagation, including a discussion on mixed-mode cracking, which may also occur when an overload failure is caused by a combination of ductile and brittle cracking mechanisms. It describes the general aspects of fracture modes and mechanisms. The article discusses some of the material, mechanical, and environmental factors that may be involved in determining the root cause of an overload failure. It also presents examples of thermally and environmentally induced embrittlement effects that can alter the overload fracture behavior of metals.

Failures of 10Cr-Mo9-10 and X 20Cr-Mo-V12-1 superheated pipes during service in steam power generation plants are described. Through micrographic and fractographic analysis, creep and overheating were identified as the cause of failure. The Larson-Miller parameter is computed, as a function of oxidation thickness, temperature and time, confirming the creep failure diagnostic.

Practical examples of stress-corrosion cracking (SCC) and methods for its prevention were presented. Cracks in chloride-sensitive austenitic steels were very branched and transcrystalline. Etched cross sections of molybdenum-free samples showed chloride-induced cracks running out of the pitted areas. Alternatively polishing and etching micro-sections for viewing at high magnification made crack detail more visible. Optical and scanning electron micrographs showed cracking in austenitic cast steel and cast iron due to both internal tensile and critical residual stresses; the latter causes flake-like spalling. Measures to prevent SCC include stress reduction, use of austenitic steels or nickel alloys not susceptible to grain boundary attack, use of ferritic chromium steels, surface slag removal, control of temperature and chloride concentration, and cathodic protection.

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.

This article discusses pressure vessels, piping, and associated pressure-boundary items of the types used in nuclear and conventional power plants, refineries, and chemical-processing plants. It begins by explaining the necessity of conducting a failure analysis, followed by the objectives of a failure analysis. Then, the article discusses the processes involved in failure analysis, including codes and standards. Next, fabrication flaws that can develop into failures of in-service pressure vessels and piping are covered. This is followed by sections discussing in-service mechanical and metallurgical failures, environment-assisted cracking failures, and other damage mechanisms that induce cracking failures. Finally, the article provides information on inspection practices.

This article reviews the applied aspects of creep and stress-rupture failures. It discusses the microstructural changes and bulk mechanical behavior of classical and nonclassical creep behavior. The article provides a description of microstructural changes and damage from creep deformation, including stress-rupture fractures. It also describes metallurgical instabilities, such as aging and carbide reactions, and evaluates the complex effects of creep-fatigue interaction. The article concludes with a discussion on thermal fatigue and creep fatigue failures.

The principal types of elevated-temperature mechanical failure are creep and stress rupture, stress relaxation, low- and high-cycle fatigue, thermal fatigue, tension overload, and combinations of these, as modified by environment. This article briefly reviews the applied aspects of creep-related failures, where the mechanical strength of a material becomes limited by creep rather than by its elastic limit. The majority of information provided is applicable to metallic materials, and only general information regarding creep-related failures of polymeric materials is given. The article also reviews various factors related to creep behavior and associated failures of materials used in high-temperature applications. The complex effects of creep-fatigue interaction, microstructural changes during classical creep, and nondestructive creep damage assessment of metallic materials are also discussed. The article describes the fracture characteristics of stress rupture. Information on various metallurgical instabilities is also provided. The article presents a description of thermal-fatigue cracks, as distinguished from creep-rupture cracks.

The first part of this article focuses on two major forms of machining-related failures, namely machining workpiece (in-process) failures and machined part (in-service) failures. Discussion centers on machining conditions and metallurgical factors contributing to (in-process) workpiece failures, and undesired surface layers and metallurgical factors contributing to (in-service) machined part failures. The second part of the article discusses the effects of microstructure on machining failures and their preventive measures.