Cracking of aluminum cast pistons of fuel gas reciprocating compressors

Several aluminum cast pistons used in fuel gas reciprocating compressors suffered from cracking during operation after a short time of service. The pistons were obtained from two manufacturing sources and the failure time was different. Metallurgical investigation was made on the failed pistons to identify the root cause of cracking. The investigation revealed that the cracks primarily existed at the top surfaces of the pistons and joined screwed plugs. The investigation also showed that the cracks had originated as fatigue cracks starting from the roots of broken threads in the body of the pistons. The root cause of failure was found to be the improper screwing of the plugs which resulted in the shear deformation of the threads and development of incipient microcracks. The difference in failure time was attributed to differences in materials properties and the amount of casting defects.