High strength bolts are generally used in critical applications such as in bridges, vehicle engines, aircraft and Furnaces, where a fastener failure can have catastrophic consequences. In this paper, we report a failure analysis on a carbon steel bolt have been failed during operation in melting furnace. The failed samples was first investigated by visual examinations to record any obvious failure remarks. Different samples from different parts of the specimen were selected for metallographic, electron microscopy and hardness examinations. Results showed that structure consists of tempered martensite and also the hardened layer was observed. Investigating the fracture surfaces showed that in the outer area to the morphology of fracture was brittle and inter-granular, and the fatigue effects were evident. In core of sample the failure morphology is softly and ductile. During the treatment the atomic hydrogen formed on the surface of the sample and these hydrogen diffused inside of the sample and gathering around high energy side like grain boundary. Based on the failure analysis results and production history it can be concluded that due to the presence of tensile loads, cyclic loads due to cooling and heating of the furnace, and the presence of carburized layer of cracking initiate.
