A segment from a premium-quality H13 tool steel die for die casting of aluminum failed after only 700 shots. The segment was subjected to visual, macroscopic, hardness, and metallographic testing. The investigation revealed that failure occurred as a result of fatigue at an electrical-discharge-machined surface where the resulting rehardened layer had not been removed. This rehardened layer had cracked, providing a source for fatigue initiation.

The A2 tool steel mandrel, part of a rolling tool used for mechanically joining two tubes was fractured after making five rolled joints. A 6.4 mm diam hole was drilled by EDM through the square end of the hardened mandrel due to difficulty was experienced in withdrawing the tool. The fracture progressed into the threaded section and formed a pyramid-shape fragment after it was initiated at approximately 45 deg through the hole in the square end. An irregular zone of untempered martensite with cracks radiating from the surface of the hole (result of melting around hole) was revealed by metallographic examination. A microstructure of fine tempered martensite containing some carbide particles was exhibited by the core material away from the hole. Brittle fracture characteristics with beach marks were exhibited by the fracture surfaces which is characteristic of a torsional fatigue fracture. As a corrective measure, the hole through the square end of the mandrel was incorporated into the design of the tool and was drilled and reamed before heat treatment and specified hardness of the threaded portion and square end of the mandrel was reduced.

Ball bearings made of type 440C stainless steel hardened to 60 HRC and suspected as the source of intermittent noise in an office machine were examined. A number of spots on the inner-ring raceway were revealed by scanning electron microscopy. The metal in the area around the spot was evidenced to have been melted and welded to the inner-ring raceway. It was revealed by randomly spaced welded areas on the raceways that the welding was the result of short electrical discharges between the bearing raceways and the balls. The use of an electrically nonconductive lubricant in the bearings was suspected to have caused the electric discharge by accumulation and discharge of static charge. The electrical resistance between the rotor and the motor frame lubricated with electrically conductive grease and the grease used in the current case was measured and compared to confirm the fact the currently used grease was nonconductive. It was concluded that the pits were formed by momentary welding between the ball and ring surfaces. The lubricant was replaced by electrically conductive grease as a corrective measure.

A 52000 bearing steel valve guide component operating in the fuel supply system of a transport aircraft broke into two pieces after 26 h of flight. The valve guide fractured through a set of elongated holes that had been electrodischarge machined into the component. Analysis indicated that the part failed by low cycle fatigue. The fracture was brittle in nature and had originated at a severely eroded zone of craters in a hard, deep white layer that was the result of remelting during electrodischarge machining. It was recommended that the remaining parts be inspected using a stereoscopic microscope and/or a borescope.

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.

A tri-lobe cylindrical roller bearing was submitted for investigation to determine the cause of uniformly spaced axial fluting damages on its rollers and outer raceway surfaces. The rollers and raceways were made from premium-melted M50 and M50NiL, aircraft quality steels often used in bearings to minimize the effects of orbital slippage and rolling-contact fatigue. The damaged areas were examined under a scanning electron microscope, which revealed a high density of microcraters, characteristic of local melting and material removal associated with bearing currents. Investigators also examined the effect of electrical discharge on crater dimensions and density and the role that thermoelectric voltage potentials may have played.

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 is a compilation of abbreviations of terms, techniques, standards, compounds, and properties of materials that are relevant to the characterization and failure analysis of plastics.

Two cases of failure of centrifuge baskets were investigated. The first involved a centrifuge running at approximately 1000 rpm. The basket was constructed from a perforated sheet of stainless steel rolled into a cylinder and joined by a single vee longitudinal weld. Detailed examination showed the weld had not completely penetrated the full depth of the section. The fracture faces showed a gradually progressing fatigue crack developing from a notch, formed by the lack of penetration, at the root of the weld. Microscopic examination of the parent plate showed it was a typical titanium stabilized austenitic steel. It is probable that had the basket been subjected to a periodic inspection by a competent person, this failure would not have occurred. The second case concerned a continuous duty centrifuge operating at 2200 rpm. Fracture had occurred at the circumferential weld attaching the stainless steel skirt to the basket rim and also in the region of the vertical weld which was made when the skirt was formed into a cone. Stress-corrosion cracking of the skirt material, which contained residual stresses due to cold-rolling, had been caused by the presence of sodium chloride.

This article addresses electrical testing and characterization of plastics and presents a number of techniques for evaluating the electrical properties of insulating materials, with a special focus on plastics, accompanied by a list of the electrical properties of different types of plastics. It provides the reader with sufficient information to select the appropriate electrical test(s) for a specific application. The tests covered in this article are widely used in industry to determine the electrical properties of insulating materials, particularly plastics. The article lists and defines terms used in connection with testing and specification of plastics for electrical applications.

Incorrect grounding of an electric motor resulted in electric current passing through a 52100 steel ball bearing and caused multiple arcing between the rolling elements. The multiple arcing developed a pattern on the outer race known as ‘fluting’. A section of ball race outer showed the distinct banding (fluting) resulting from spark discharges while the bearing was rotating. The severe distress of the surface resulted in unacceptable levels of vibration. An SEM photograph of the banded regions showed smoothing of the asperities from continued operation is evident. In the craters the residue of partial melting was seen.