SURFACE DAMAGE OF STAINLESS STEEL EN3358 SUBJECT TO FRICTION, IMPULSIVE ACCELERATION AND CORROSIVE ENVIRONMENT

The present paper describe the failure analysis of an aircraft component subjected to several episodes of in service failure, resulted in loss of the aircraft safety. Modern aircrafts are provided with mechanical systems which have the task to open not pressurized hatches during landing. The components of such systems are subject to considerable mechanical stresses in harsh environment (presence of moisture and pollutants, significant and sudden temperature variations). The system is constituted by a sliding piston, and related nipple, assisted in the movement by 4 steel balls which act as a ball bearing, the whole system is activated by a preloaded spring. The machined parts, nipple and piston, are made of N3358 steel (X3CrNiMo13-8-2), an austenitic stainless steel with very low content of carbon often used in the aerospace. The samples provided by the manufacturer present different types of damage all referable to phenomena relative to the sliding of the piston inside the nipple. Particularly, on the cylindrical sample (piston) were detected circular impressions -probably due to the ball-bearing- and various kinds of longitudinal and helicoidal grooves, due both to the original machining and to the spring. The nipple presented longitudinal grooves and residual of lubricant containing metal residues resulting from the relative sliding. The present paper describes the different damage observed and the microstructure of the material, then are reported the results obtained from the characterization of the material of the samples by means of optical and electronic microscopy, carried out to define the mechanisms involved in the system seizure. In order to define the primary cause of failure and to propose solutions to be adopted, the results of different tests were compared with system design and working data.