Root Cause Analysis of Failed AISI 440C High Carbon Stainless Steel Used in Aerospace Actuator Systems

AISI 440C is a high carbon martensitic stainless steel with superior wear and abrasion resistance among all grades of stainless steel. Hydraulic amplifier valve (HAV) spools, made of AISI 440C stainless steel, are used in flow control devices of launch vehicle actuator systems. In the present case, a failure occurred near the undercut region of the HAV spool during the vibration test. The failure was observed specifically at M5 threaded region of the spool. A detailed investigation has been carried out to understand the root cause of the failure. 612 µm deep pit was observed with corrosion products on the fracture surface near to the undercut region of M5 thread. Elemental chemical composition analysis via energy-dispersive X-ray spectroscopy (EDS) confirmed the presence of oxygen and chlorine in the pit location. Primary carbides exposed to the rough machined surface at the undercut region lead to the pit initiation and propagation because of the galvanic potential difference between matrix and primary carbides. Failure of the HAV spool is attributed to corrosion-assisted overload.