Search Results for Case-hardened fasteners

Several case-hardened and zinc-plated carbon-manganese steel wheel studs fractured in a brittle manner after very limited service life. The fracture surfaces of both front and rear studs showed no sign of fatigue beach marks or deformation in the form of shear lips that would indicate either a fatigue mechanism or ductile overload failure. SEM analysis revealed that the mode of fracture was intergranular decohesion, which indicates an environmental influence in the fracture mechanism. The primary fracture initiated at a thread root and propagated by environmentally-assisted slow crack growth until final fracture. The natural stress concentration at the thread root, when tightened to the required clamp load concomitant with the presence of cracks in the carburized case, was sufficient to exceed the critical stress intensity for hydrogen-assisted stress cracking (HASC). The zinc plating exacerbated the situation by providing a strong local corrosion cell in the form of a sacrificial anode region adjacent to the cracked thread. The enhanced generation of hydrogen in a corrosive environment subsequently lead to HASC of the wheel studs.

A spindle made of hardenable 13% chromium steel X40 Cr13 (Material No. 1.4034) that was fastened to a superheated steam push rod made of high temperature structural steel 13Cr-Mo44 (Material No. 1.7335) by means of a convex fillet weld, fractured at the first operation of the rod directly next to the weld bead. Investigation showed that the fracture of the superheated steam push rod spindle was caused by hardening and hardening crack formation in the weld seams and adjoining areas. It would have been preferable to avoid welding near the cross sectional transitions altogether in consideration of the crack sensitivity of high hardenability steels. If for some reason this was not possible, then all precautions should have been taken that are applicable to the particular steel, such as preheating, slow cooling and stress relief tempering after welding. The selection of an austenitic additive material should have been considered because it could have equalized stresses due to its high elongation. Most probably, however, a material of lower hardenability should have been selected for the spindle if high operating properties were of paramount importance.

One percent Cr-Mo low alloy constructional steel is widely used for high tensile applications, e.g., for manufacture of high tensile fasteners, heat treated shafts and axles, for automobile applications such as track pins for high duty tracked vehicles etc. The steel is fairly through hardening and heat treatment does not present any serious difficulty. Care is still required in processing to avoid decarburization. In an application of track pins for tracked vehicles, bars about 22 mm diam were required in heat treated and centerless-ground condition prior to induction hardening of the surface. Indifferent results were obtained in induction hardening; cracks were noticed, and patchy hardness figures were obtained on the final product in several batches. Metallographic examination of transverse sections through the defective areas showed decarburization to varying degrees, i.e., from partial to total decarburization. Observations suggested the defects originated at the stages of ingot making and rolling. This was apparently the reason for complete decarburization of the area with original surface defect which opened up further in the oxidizing atmosphere of the furnace with low melting clinkers from scale and furnace lining filling up the crevice of the original defect.

This article discusses the failure of cylinder clamping rods in single cylinder diesel engines. The AISI 4140 hardened and tempered steel clamping rods were failing after 200 to 250 h of operation. The fatigue failures initiated at the root of the last thread on the clamping rod that was engaged in a blind hole in the cylinder block. The failures were caused by loose tolerances on the threads that resulted in a non-uniform distribution of load. The load was concentrated on the last threads to engage, thus causing fatigue crack nucleation at the thread root and propagation until the rod broke by overload. Changing the tolerance on the threads virtually eliminated the fatigue problem.

Cracks were discovered between interference-fit fasteners (MoS2-coated Ti-6Al-4V) that had been incorporated into a fighter aircraft primary structural frame (D6ac steel) to enhance structural fatigue life. Examination of sections cut from the cracked frame established that the cracks propagated by stress-corrosion cracking. The cause of cracking was twofold: use of interference-fit fasteners exposed to moisture intrusion from a marine environment and poor hole quality. Failure was intensified by dissimilar-metal contact in the presence of weak acidic electrolyte (dissociated MoS2). Control of machining parameters to prevent formation of brittle martensite, use of galvanically compatible fasteners, and use of an alternate lubricant were recommended.

This article first provides an overview of the types of mechanical fasteners. This is followed by sections providing information on fastener quality and counterfeit fasteners, as well as fastener loads. Then, the article discusses common causes of fastener failures, namely environmental effects, manufacturing discrepancies, improper use, or incorrect installation. Next, it describes fastener failure origins and fretting. Types of corrosion in threaded fasteners and their preventive measures are then covered. The performance of fasteners at elevated temperatures is addressed. Further, the article discusses the types of rivet, blind fastener, and pin fastener failures. Finally, it provides information on the mechanism of fastener failures in composites.

This article discusses different types of mechanical fasteners, including threaded fasteners, rivets, blind fasteners, pin fasteners, special-purpose fasteners, and fasteners used with composite materials. It describes the origins and causes of fastener failures and with illustrative examples. Fatigue fracture in threaded fasteners and fretting in bolted machine parts are also discussed. The article provides a description of the different types of corrosion, such as atmospheric corrosion and liquid-immersion corrosion, in threaded fasteners. It also provides information on stress-corrosion cracking, hydrogen embrittlement, and liquid-metal embrittlement of bolts and nuts. The article explains the most commonly used protective metal coatings for ferrous metal fasteners. Zinc, cadmium, and aluminum are commonly used for such coatings. The article also illustrates the performance of the fasteners at elevated temperatures and concludes with a discussion on fastener failures in composites.

Two type 316L stainless steel orthopedic screws broke approximately 6 weeks after surgical implant. The screws had been used to fasten a seven-hole narrow dynamic compression plate to a patient's spine. The broken screws and screws of the same vintage and source were examined using macrofractography, SEM fractography, and hardness testing. Fractography established that fracture was by fatigue and that the fatigue cracking originated at corrosion pits. Hardness while below specification, still indicated that the screws were in the cold-worked condition and notch sensitive during fatigue loading. Use of a steel with a higher molybdenum content (317L) in the annealed condition was recommended.

Distortion failure occurs when a structure or component is deformed so that it can no longer support the load it was intended to carry. Every structure has a load limit beyond which it is considered unsafe or unreliable. Estimation of load limits is an important aspect of design and is commonly computed by classical design or limit analysis. This article discusses the common aspects of failure by distortion with suitable examples. Analysis of a distortion failure often must be thorough and rigorous to determine the root cause of failure and to specify proper corrective action. The article summarizes the general process of distortion failure analysis. It also discusses three types of distortion failures that provide useful insights into the problems of analyzing unusual mechanisms of distortion. These include elastic distortion, ratcheting, and inelastic cyclic buckling.

Distortion often is observed in the analysis of other types of failures, and consideration of the distortion can be an important part of the analysis. This article first considers that true distortion occurs when it was unexpected and in which the distortion is associated with a functional failure. Then, a more general consideration of distortion in failure analysis is introduced. Several common aspects of failure by distortion are discussed and suitable examples of distortion failures are presented for illustration. The article provides information on methods to compute load limits, errors in the specification of the material, and faulty process and their corrective measures to meet specifications. It discusses the general process of material failure analysis and special types of distortion and deformation failure.

A cylindrical spiral gear, part of a locomotive axle assembly, cracked ten days after it had been press-fit onto a shaft, after which it sat in place as other repairs were made. Workers at the locomotive shop reported hearing a sound, and upon inspecting the gear, found a crack extending radially from the bore to the surface of one of the tooth flanks. The crack runs the entire width of the bore, passing through an oil hole in the hub, across the spoke plate and out to the tip of one of the teeth. Design requirements call for the gear teeth to be carburized, while the remaining surfaces, protected by an anti-carburizing coating, stay unchanged. Based on extensive testing, including metallographic examination, microstructural analysis, microhardness testing, and spectroscopy, the oil hole was not protected as required, evidenced by the presence of a case layer. This oversight combined with the observation of intergranular fracture surfaces and the presence of secondary microcracks in the case layer point to hydrogen embrittlement as the primary cause of failure. It is likely that hydrogen absorption occurred during the gas carburizing process.

Uncoated high-strength alloy steel cap screws retaining a cast aluminum (356.0) diffuser assembly in a centrifugal refrigerant compressor failed in a brittle manner a short time after the system was placed in operation. Evidence obtained during the failure analysis indicated that the failures were the result of hydrogen embrittlement produced by galvanic corrosion and attendant evolution of hydrogen at the dissimilar junction, which was also the site of the highest tensile stress. Suggested measures for minimizing recurrences included use of lower-strength, galvanically-compatible fasteners and appropriately-applied and treated compatible coatings.

Fasteners, made in high-production progressive dies from 0.7 mm thick cold-rolled 1060 steel, were used to secure plastic fabric or webbing to the aluminum framework of outdoor furniture. It was found that approximately 30% of the fasteners cracked and fractured as they were compressed to clamp onto the framework prior to springback. The heat treatment cycle of the fasteners consisted of austenitizing, quenching, tempering to obtain a tempered martensite microstructure, acid cleaning, zinc electroplating, coating with a clear dichromate and thereafter baking to remove the nascent hydrogen. It was revealed that fasteners treated in this manner were brittle due to hydrogen embrittlement as the baking process was found to not be able to remove all the nascent hydrogen which had induced during acid cleaning and electroplating. The heat treatment cycle was modified to produce a bainitic structure and the method of plating the fastener with zinc was changed from electroplating to a mechanical deposition process to thus avoid hydrogen embrittlement.

Visual examination, using the unaided eye or a low-power optical magnifier, is typically one of the first steps in a failure investigation. This article presents the guidelines for selecting samples for scanning electron microscope examination and optical metallography and for cleaning fracture surfaces. It discusses damage characterization of metals, covering various factors that influence the damage, namely stress, aggressive environment, temperature, and discontinuities.

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.

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.

This article introduces some of the general sources of heat treating problems with particular emphasis on problems caused by the actual heat treating process and the significant thermal and transformation stresses within a heat treated part. It addresses the design and material factors that cause a part to fail during heat treatment. The article discusses the problems associated with heating and furnaces, quenching media, quenching stresses, hardenability, tempering, carburizing, carbonitriding, and nitriding as well as potential stainless steel problems and problems associated with nonferrous heat treatments. The processes involved in cold working of certain ferrous and nonferrous alloys are also covered.

Metal-induced embrittlement is a phenomenon in which the ductility or fracture stress of a solid metal is reduced by surface contact with another metal in either liquid or solid form. This article summarizes the characteristics of solid metal induced embrittlement (SMIE) and liquid metal induced embrittlement (LMIE). It describes the unique features that assist in arriving at a clear conclusion whether SMIE or LMIE is the most probable cause of the problem. The article briefly reviews some commercial alloy systems where LMIE or SMIE has been documented. It also provides some examples of cracking due to these phenomena, either in manufacturing or in service.

A maintenance worker was injured when his 3/4 in. (19 mm) open-ended wrench failed, fracturing in overload fashion along the jaw. The failed wrench was unavailable for testing, but an identical one that failed in the same manner was acquired and subjected to hardness, chemistry, SEM, and metallurgical analyses. SEM imaging revealed microvoid coalescence within the fracture zone. The microvoids were flat and smooth edged indicating insufficient bonding. In addition, a cross sectional sample, mounted and etched using alkaline chromate, revealed an oxygen-rich zone in the jaw. It was concluded that the failures stemmed from forging laps in the jaw that broaching failed to remove.

Metal-induced embrittlement is a phenomenon in which the ductility or the fracture stress of a solid metal is reduced by surface contact with another metal in either the liquid or solid form. This article summarizes some of the characteristics of liquid-metal- and solid-metal-induced embrittlement. This phenomenon shares many of these characteristics with other modes of environmentally induced cracking, such as hydrogen embrittlement and stress-corrosion cracking. The discussion covers the occurrence, failure analysis, and service failures of the embrittlement. The article also briefly reviews some commercial alloy systems in which liquid-metal-induced embrittlement or solid-metal-induced embrittlement has been documented and describes some examples of cracking due to these phenomena, either in manufacturing or in service.