Persistent cracking in a forged 1080 steel turntable rail in a wind tunnel test section was investigated. All cracks were oriented transverse to the axis of the rail, and some had propagated through the flange into the web. Through-flange cracks had been repair welded. A section of the flange containing one through-flange crack was examined using various methods. Results indicated that the cracks had initiated from intergranular quench cracks caused by the use of water as the quenching medium. Brittle propagation of the cracks was promoted by high residual stresses acting in conjunction with applied loads. Repair welding was discontinued to prevent the introduction of additional residual stress., Finite-element analysis was used to show that the rail could tolerate existing cracks. Periodic inspection to monitor the degree of cracking was recommended.

Hydraulic cylinder housings were being fabricated from 4140 grade seamless steel tubing. During production, magnetic-particle inspection indicated the presence of circumferential and longitudinal cracks in a large number of cylinders. Analysis (visual inspection, dye penetrant inspection, 50x/90x/400x SEM micrographs, and metallographic analysis) supports the conclusion that the cracking problem in these components was identified as quench cracks due to their brittle, intergranular nature and the characteristic temper oxide on the fracture surfaces. Although the steel met the compositional requirements of SAE 4140, the sulfur level was 0.022% and would account for the formation of the sulfide stringers observed. Apparently, the combination of the clustered, stringer-type inclusions and the quenching conditions were too severe for this component geometry. The result was a high incidence of quench cracks that rendered the parts useless. Recommendations included changing the specification, requiring the steel to have lower sulfur concentrations. Magnetic-particle cleanliness standards should be imposed that will exclude material with harmful clusters of sulfide stringers, for example, modified AMS 2301.

An AISI 4340 threaded steel connecting rod that was part of a connecting linkage used between a parachute and an instrumented drop test assembly fractured under high dynamic loading when the assembly was dropped from an airplane. A large flaw that originated from the root of a machined thread groove was visible on the fracture surface. Heavy oxidation at elevated temperatures was indicated as most of the surface of the flaw was black. Fine secondary cracks aligned transverse to the growth direction was revealed by scanning electron microscopy. It was established that intergranular cracking observed in this alloy was caused during heat treating as the thread root served as an effective stress concentration and induced quench cracking. It was found that fracture in the overload region occurred by a ductile void growth and coalescence process. Premature failure of the threaded rod was thus attributed to the presence of the quench crack flaw caused by an improper machining sequence and heat treatment practice.

A rocket-motor case made of consumable-electrode vacuum arc remelted D-6ac alloy steel failed during hydrostatic proof-pressure testing. Close visual examination, magnetic-particle inspection, and hardness tests showed cracks that appeared to have occurred after austenitizing but before tempering. Microscopic examinations of ethereal picral etched sections indicated that the cracks appeared before or during the final tempering phase of the heat treatment and that cracking had occurred while the steel was in the as-quenched condition, before its 315 deg C (600 deg F) snap temper. Chemical analysis of the cracked metal showed a slightly higher level of carbon than in the component that did not crack. X-ray diffraction studies of material from the fractured dome showed a very low level of retained austenite, and chemical analysis showed a slightly higher content of carbon in the metal of the three cracked components. Bend tests verified the conclusion that the most likely mechanism of delayed quench cracking was isothermal transformation of retained austenite to martensite under the influence of residual quenching stresses. Recommendations included modifying the quenching portion of the heat-treating cycle and tempering in the salt pot used for quenching, immediately after quenching.

The heads of two AISI 8740 steel bolts severed while being installed into an Army tank recoil mechanism. Both broke into two pieces at the head-to-shank radius and the required torque value had not been attained nor exceeded prior to the failure. A total of 69 bolts from inventory and the field were tested by magnetic particle inspection. One inventory bolt failed because of a transverse crack near the head-to-shank radius. It was deduced that either a 100% magnetic particle inspection had not been conducted during bolt manufacturing, or the crack went undetected during the original inspection. Optical and electron microscopy of the broken bolts revealed topographies and the presence of black oxide consistent with quench cracking. The two bolts failed during installation due to the presence of pre-existing quench cracks. Recommendations to prevent future failures include: ensuring that 100% magnetic particle inspections are conducted after bolts are tempered; using dull cadmium plate or an alternative to the electrode position process, such as vacuum cadmium plate or ion-plate or ion-plated aluminum, to mitigate the potential for delayed failures due to hydrogen embrittlement or stress-corrosion cracking; ensuring that the radius at the shoulder/shank interface conforms to specifications; and replacing all existing bolts with new or reinspected inventory bolts.

A bolt manufacturer observed that products made from certain shipments of steel 41 Cr4 wire were prone to the formation of quench cracks in their rolled threads. The affected wire was tested and found to be highly sensitive to overheating because of the metallurgical method by which it was produced. A stronger decarburization of the case was a contributing factor that could not be prevented by working because the thread was rolled. Hardening tests conducted by the bolt manufacturer showed that quench cracks did not occur in specimens that were turned down before hardening and when notches were machined instead of beaten with a chisel.

A heat-treated, cadmium-plated AISI 8740 steel bolt broke through the head-to-shank fillet while being handled during assembly. Fractographic and metallographic examination of the bolt traced the cause of failure to quench cracking, which occurred when the part was water cooled following hot heading and prior to the production run. The process chart for hot heading was changed from water quenching to air cooling following the forming operation.

Routine magnetic-particle inspection revealed crack indications in a number of shafts produced from hot-rolled 4130 steel bar. A pronounced indication of this size is cause for rejection if the defect is not eliminated during subsequent machining. A microstructural analysis of the shaft cross section revealed that the crack was approximately 0.5 mm (0.020 in.) deep and oriented in a radial direction. Furthermore, no stringer-type nonmetallic inclusions were observed in the vicinity of the flaw, which did not display the intergranular characteristics of a quench crack. The defect did, however, contain substantial amounts of oxide, which evidently resulted from the hot-working operation. This evidence supports the conclusion that the appearance of this discontinuity, with the long axis parallel to the working direction and radial orientation with regard to depth, strongly suggests a seam produced during rolling. Use of components with surface-defect indications as small as 0.5 mm (0.02 in.) can be risky in certain circumstances. Depending on the orientation of the flaw with respect to applied loads, the nature of the applied forces (for example, cyclic), and the operating environment, such a surface flaw can become the initiating site for a fatigue crack or a corrosion-related failure.

An electric transport vehicle, similar to an electric trolley or subway rail car, experienced frequent breakdowns due to in-service fractures of torsion springs that support the weight of an overhead electric pickup assembly. Scanning electron microscopy and metallographic examinations determined that the fractures stemmed from electric arc damage. Intergranular quench cracks in the transformed untempered martensite on the surface of the spring provided crack initiations that propagated during operation causing fatigue fracture.

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.

Two 25 x 40 mm (1 x 1.5 in.) AISI 4150 hot-rolled steel bars that cracked during heat treatment were examined to determine whether the heat treating procedure had contributed to the failure. Metallographic examination of a cross section taken through the fracture revealed an oxide coating on both sides of the fracture surface. The oxide was also found on the top and bottom sides of the sample. Sawcut sides of the bar did not exhibit the oxide layer The presence of the oxide in the fracture, combined with its absence on all exterior surfaces, indicated that the fracture occurred as a result of an oxide seam in the original material rather than from oxide from heat treating. Nondestructive testing prior to machining and heat treatment was recommended.

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.

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.