Investigation of Surface Intergranular Attack and Its Effects on the FatigueProperties of AM355 Stainless Steel

摘要

A critical flight safety component fabricated from AM355, a semiausteniticprecipitation hardenable stainless steel, from an Army attack helicopter failed catastrophically in service. The U.S. Army Research Laboratory (ARL) performed an in-depth metallurgical analysis of the broken component, which revealed that the premature failure was attributable to fatigue. Further analysis of material taken from various stages of processing revealed an intergranular surface attack that ARL determined to be caused by acid pickling during primary processing. ARL hypothesized that the surface intergranular attack may have led to premature crack initiation due to the breakdown of the protective passive layer or the stress concentration effect of the attack. This study was conducted to quantify the effects of varying degrees of surface intergranular attack on the fatigue properties of the material. Fatigue specimens were machined from actual components taken from inventory and from fielded components and were subsequently categorized into four groups which described the degree or severity of attack based on appearance and depth measurement: none, light, moderate, and heavy. Fatigue test data showed a direct relationship between the number of cycles to failure and the severity of surface intergranular attack. ARL recommended to control the amount of surface intergranular attack or to remove it altogether with a light sanding operation.