The gear of a spiral bevel gear set broke into three pieces after about two years of service. The gear (made of 4817 steel) broke along the root of a tooth intersected by three of the six 22-mm diam holes used to mount the gear to a hub. Fatigue progression for about 6.4 mm at the acute-angle intersections of three mounting holes with the root fillets of three teeth was revealed by examination of gear. Cracks at the intersections of the remaining three mounting holes and the adjacent tooth-root fillets were revealed by magnetic-particle inspection. Through hardening at the acute-angle intersections of the mounting holes and tooth-root fillets was revealed by metallographic examination. Design of the gear and placement of the mounting holes, which resulted in through hardening, were concluded to be the contributing factors to the fatigue failure of the gear.

This article provides an overview of the effects of various material- and process-related parameters on residual stress, distortion control, cracking, and microstructure/property relationships as they relate to various types of failure. It discusses phase transformations that occur during heat treating and describes the metallurgical sources of stress and distortion during heating and cooling. The article summarizes the effect of materials and the quench-process design on distortion and cracking and details the effect of cooling characteristics on residual stress and distortion. It also provides information on the methods of minimizing distortion and tempering. The article concludes with a discussion on the effect of heat treatment processes on microstructure/property-related failures.

Screens made of stainless steel X5 Cr-Ni-Mo 18 10 (Material No. 1.4401), which were exposed to cooling water from the mouth of a river, became unserviceable after a few months because of the breaking out of parts of the bars. The multiple fracturing of the screen bars in the brackish water of the mouth of the river was attributed to stress corrosion and pitting. The steel used, which contained molybdenum, would have withstood the severe corrosive conditions in the heat-treated condition, i.e. quenched after high temperature anneal. However, the stresses caused by deformation and welding, as well as the intensification of corrosive conditions brought about by design, i.e. creation of corrosion currents in the poorly aerated gaps (Evans elements), made this impossible.

The fractured end of a piston rod of a hydraulic press failed in line with the leading face of the piston retaining nut. Although the nut apparently had been seated uniformly, the face was polished, indicating that relative movement between it and the piston had taken place. Failure resulted from the culmination of two principal fatigue cracks which developed on approximately parallel planes from the roots of adjacent threads. A longitudinal section through the screw thread on the piston rod showed it had been carburized but not hardened, and that subsequent surface de-carburization to a depth of approximately 0.001 in. had occurred. It was concluded that insufficient tightening, as evidenced by the polish markings, was the main reason for failure, the portion of the rod therefore being subjected to a greater variation of cyclic stress during operation. The presence of the de-carburized layer lowered its resistance to the initiation of a fatigue crack to that of iron, considerably less than the resistance of the mild steel from which the rod was made and well below that shown by the carburized layer.

Textile-machine crankshafts forged from 4140 steel fractured transversely on one cheek during one to three years of service. The cause of failure for two forgings (one complete fractured forging and second a section that contained the shorter shaft fracture cheek) was determined. Indication of fatigue failure was revealed by visual examination of the fracture surfaces. Rough grooves from hot trimming of the flash were visible on the surface of the cheeks. The outer face of one cheek of the throw on the forging contained shallow surface folds. Slightly decarburized forged surface was identified around one of the folds and a fatigue crack initiated in the fold and propagated across the cheek. Properties representative of 4140 steel, quenched and tempered to a hardness of 20 to 22 HRC, were observed. Tempered bainite was revealed in the general microstructure. As a corrective measure, the forgings were normalized, hardened and tempered to 28 to 32 HRC before being machined to increase fatigue strength and extremely rough surfaces were removed by careful grinding.

To permit bolting of a 90 lb/yd. flat-bottomed rail to a steel structure, rectangular slots 2 in. wide x 1 in. deep were flame-cut in the base of the rail at 2 ft intervals to suit existing bolt holes. During subsequent handling, one of the rails (which were about 25 ft long) was dropped from a height of approximately 6 ft on to a concrete floor and it fractured into 11 pieces, each break occurring at a slot. The sample piece submitted for examination showed a wholly brittle fracture at each end, the fractures having originated at the sharp corners of the slots. During flame-cutting, a narrow band of material on each side of the cut was raised above the hardening temperature. When the torch had passed the rate of abstraction of heat from this zone by conduction into the cold mass of the rail was sufficiently rapid to amount to a quench and thus cause local hardening. The steel in the regions of the slots possessed little capacity for deformation, and fracturing of the martensitic layer, under cooling or impact stresses, would be likely to occur. The slots should have been cut mechanically.

The rim of a gear wheel of 420 mm width and 3100 mm in diam broke after four years of operation time in a sheet bar three-high rolling mill. The rim was forged from steel with about 0.4C, 0.8Si and 1.1Mn. The rim started to break in the tooth bottom from a fatigue fracture which extended from the gear side to more than half the rim width. A second incipient failure commenced from the opposite tooth bottom. Both fractures joined below the tooth of the rim. Both incipient cracks were fatigue fractures with several starting points, all located in the transition between tooth flank and tooth bottom. The remaining failure was a fine-grained ductile fracture. It was found that the teeth were not supported uniformly over the entire width and were thus overloaded on one side. The transition from the tooth flanks to the tooth bottom was sharp-edged, causing a tension peak there. The tooth bottom was machined only roughly. Also, the yield point was a little bit too low.

A high-speed pinion gear shaft, part of a system that compresses natural gas, was analyzed to determine why it failed. An abnormal wear pattern was observed on the shaft surface beneath the inner race of the support bearings. Material from the shaft had transferred to the bearing races, creating an imbalance (enough to cause noise and fumes) that operators noted two days before the failure. Macrofeatures of the fracture surface resembled those of fatigue, but electron microscopy revealed brittle, mostly intergranular fracture. Classic fatigue features such as striations were not found. To resolve the discrepancy, investigators created and tested uniaxial fatigue samples, and the microfeatures were nearly identical to those found on the failed shaft. The root cause of failure was determined to be fatigue, and it was concluded that cracks on the pinion shaft beneath the bearings led to the transfer of material.

Failure analysis results were employed to identify a better alloy. Chipper knives used in the field to chip logs failed frequently. The knives were made of alloys with a composition of Fe-0.48C-0.30Mn-0.90Si-8.50Cr-1.35Mo-1.20W-0.30V. The development of tougher alloy steel with superior properties was initiated. The nominal composition of Fe-0.50C-0.30Mn-0.40Si-5.00Cr-2.00Mo was developed which achieved the goals of edge retention, resistance to softening under frictional heating, wear resistance, ease of heat treatment, dimensional stability in heat treatment, grindability, and low alloy cost. A chip harvester made from this composition was tested in field with older composition knives. It was found that the new knives outperformed the older knives. The key to the development was interpreted to be careful study of a number of failed knives with different problems used in different types of operations.

This article briefly reviews the analysis methods for spalling of striking tools with emphasis on field tests conducted by A.H. Burn and on the laboratory tests of H.O. McIntire and G.K. Manning and of J.W. Lodge. It focuses on the metallography and fractography of spalling. The macrostructure and microstructure of spall cavities are described, along with some aspects of the numerous specifications for striking/struck tools. The article also describes the availability of spall-resistant metals and the safety aspects of striking/struck tools in railway applications.

This article describes the characteristics of tools and dies and the causes of their failures. It discusses the failure mechanisms in tool and die materials that are important to nearly all manufacturing processes, but is primarily devoted to failures of tool steels used in cold-working and hot-working applications. It reviews problems introduced during mechanical design, materials selection, machining, heat treating, finish grinding, and tool and die operation. The brittle fracture of rehardened high-speed steels is also considered. Finally, failures due to seams or laps, unconsolidated interiors, and carbide segregation and poor carbide morphology are reviewed with illustrations.

This article discusses failure mechanisms in tool and die materials that are very important to nearly all manufacturing processes. It is primarily devoted to failures of tool steels used in cold working and hot working applications. The processes involved in the analysis of tool and die failures are also covered. In addition, the article focuses on a number of factors that are responsible for tool and die failures, including mechanical design, grade selection, steel quality, machining processes, heat treatment operation, and tool and die setup.

This article discusses failures in shafts such as connecting rods, which translate rotary motion to linear motion, and in piston rods, which translate the action of fluid power to linear motion. It describes the process of examining a failed shaft to guide the direction of failure investigation and corrective action. Fatigue failures in shafts, such as bending fatigue, torsional fatigue, contact fatigue, and axial fatigue, are reviewed. The article provides information on the brittle fracture, ductile fracture, distortion, and corrosion of shafts. Abrasive wear and adhesive wear of metal parts are also discussed. The article concludes with a discussion on the influence of metallurgical factors and fabrication practices on the fatigue properties of materials, as well as the effects of surface coatings.

Because of the tough engineering environment of the railroad industry, fatigue is a primary mode of failure. The increased competitiveness in the industry has led to increased loads, reducing the safety factor with respect to fatigue life. Therefore, the existence of corrosion pitting and manufacturing defects has become more important. This article presents case histories that are intended as an overview of the unique types of failures encountered in the freight railroad industry. The discussion covers failures of axle journals, bearings, wheels, couplers, rails and rail welds, and track equipment.

In addition to failures in shafts, this article discusses failures in connecting rods, which translate rotary motion to linear motion (and conversely), and in piston rods, which translate the action of fluid power to linear motion. It begins by discussing the origins of fracture. Next, the article describes the background information about the shaft used for examination. Then, it focuses on various failures in shafts, namely bending fatigue, torsional fatigue, axial fatigue, contact fatigue, wear, brittle fracture, and ductile fracture. Further, the article discusses the effects of distortion and corrosion on shafts. Finally, it discusses the types of stress raisers and the influence of changes in shaft diameter.

This article provides a discussion on the metallographic techniques used for failure analysis, and on fracture examination in materials, with illustrations. It discusses various metallographic specimen preparation techniques, namely, sectioning, mounting, grinding, polishing, and electrolytic polishing. The article also describes the microstructure examination of various materials, with emphasis on failure analysis, and concludes with information on the examination of replicas with light microscopy.

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.

Metallographic examination is one of the most important procedures used by metallurgists in failure analysis. Typically, the light microscope (LM) is used to assess the nature of the material microstructure and its influence on the failure mechanism. Microstructural examination can be performed with the scanning electron microscope (SEM) over the same magnification range as the LM, but examination with the latter is more efficient. This article describes the major operations in the preparation of metallographic specimens, namely sectioning, mounting, grinding, polishing, and etching. The influence of microstructures on the failure of a material is discussed and examples of such work are given to illustrate the value of light microscopy. In addition, information on heat-treatment-related failures, fabrication-/machining-related failures, and service failures is provided, with examples created using light microscopy.

Examination of several fighter aircraft main landing gear legs revealed unusual cracking in the hard chromium plating that covered the sliding section of the inner strut. The cracking was associated with cracks in the 35 NCD 16 steel beneath the plating. A detailed investigation revealed that the cracking was caused by the combination of incorrect grinding procedure, the presence of hydrogen, and fatigue. The grinding damage generated tensile stresses in the steel, which caused intergranular cracking during the plating cycle. The intergranular cracks were initiation sites for fatigue crack growth during service. It was recommended that the damaged undercarriage struts be withdrawn from service pending further analysis and development of a repair technique.

The primary purpose of this article is to describe general root causes of failure that are associated with wrought metals and metalworking. This includes a brief review of the discontinuities or imperfections that may be common sources of failure-inducing defects in the bulk working of wrought products. The article addresses the types of flaws or defects that can be introduced during the steel forging process itself, including defects originating in the ingot-casting process. Defects found in nonferrous forgings—titanium, aluminum, and copper and copper alloys—also are covered.