Metallurgical Investigation on Embrittlement of Copper Cable of an Electric Motor

Copper wires used in the cable of an electric motor were found to be embrittled after a burn up. A metallurgical investigation (microstructural examination, fractography and microhardness measurement) was done to find out the root cause of the embrittlement. The microstructure of the embrittled wire showed cavities and cracks along the grain boundaries. The fracture surface was also completely intergranular. It was concluded that the embrittlement of the copper wire was caused by hydrogen present in the moist atmosphere diffusing in copper wire and reacting with oxygen at the grain boundaries to form water vapor bubbles. The water vapor bubbles at elevated temperatures (above 400 °C) during burn up of the motor generated pressures high enough to cause cracking along the grain boundaries leading to embrittlement of the copper wires.