Failure Analysis of Open End Wrenches Containing Forging Defects

Abstract

A maintenance worker was injured when his 3/4-inch (19 mm) open-ended wrench failed. The jaw of the wrench failed in an overload fashion. Wrenches were tested according to American National Standards Institute (ANSI) specification B107.6-1978. All wrenches passed ANSI standards. An additional wrench of the same size and manufacturer failed in the same fashion as the first wrench. Due to pending litigation the first failed wrench was not available for analysis; however, no injuries were sustained in the second failure allowing for a detailed analysis. Hardness, chemistry, scanning electron microscopy (SEM), and metallurgical evaluations were performed on the second failure. SEM analysis revealed microvoid coalescence within the fracture zone. The microvoids were flat and smooth edged indicating insufficient bonding. A cross section of the wrench was mounted and etched using alkaline chromate to reveal an oxygen-rich zone in the jaw of the wrench. Due to the impending litigation, a joint failure analysis was performed on the first failure using both metallographic and SEM analysis. It was concluded that the failures were due to forging laps that remained present in the jaw due to improper or insufficient material having been removed during the broaching process of the wrenches.