Impact of repeated nitriding cycles on extrusion die life - Some statistical and metallurgical observations

This paper presents some observations about die failure patterns in commercial aluminum extrusions. A total of 97 dies were analyzed to observe the nature of scatter in the life of these commonly used dies. Each die in its complete life cycle passes through a number of nitriding treatments. The number of these nitriding treatments generally ranges from 5 to 10 in most cases. Each nitriding event is equivalent to die surface renewal, and a new time between failures is calculated after each renewal. This article analyzes the performance of both solid and hollow dies to first failure and to subsequent failures after nitriding. The Weibull model describes the time to first failure and cumulative time to n th failure of this group of dies and indicates an overall wearout pattern. The median time to failure, scatter in cumulative life, and progress of die death rate are discussed. A die is considered to fail when the surface must be nitrided. The interarrival time between failures (nitriding events) is analyzed for both solid and hollow dies. It is shown that interarrival time is also Weibull distributed. Based on these observations, it is concluded that if die is treated as a repairable system and the die surface nitriding makes the die as good as new, the failure can essentially be modeled as a nonhomogeneous Poisson process with a nonlinear intensity of failure. The results presented are of the first phase of an ongoing project dealing with life improvement of extrusion dies. The analyses presented provide a starting point for subsequent in-depth study of die-life reliability, optimal nitriding interval determination, and optimal cumulative time before die replacement.