Failure Investigation and Analyses of Ni-Based Super Alloy Barrel Used in Twin Screw Extruder

Abstract

Barrel cooling system is very critical for the extruder application such as polymer compounding, reactive extrusion, food processing and plasticizing etc., in order to achieve the required quality of the output material. One of the typical failures of barrel cooling system is water leakage due to poor weld integrity. A few published investigations address the root cause(s) for such failure and provide robust corrective actions to enhance the durability of barrel cooling system from design and manufacturing points of view.However, water leakage from the heater hole of the Inconel 625 barrel from one of our customers has been unique. Macro- examination of the failed barrel reveals multiple cracks inside the heater hole on the barrel top, but no such cracks found on the barrel bottom. With detailed metallographic analysis we observe precipitation of Nickel-Niobium rich gamma prime phase near the heater hole causing increased hardness from 220 to 400HV0.5. Barrels of the other region conforms to precipitation free, austenitic Ni-Matrix typical of Inconel 625 solution annealed condition. Fractography analysis conducted on the cut section clearly shows crack originating from cooling hole and propagated towards the heater hole. Various aspects of the investigation findings were analyzed in detail, arrived at the comprehensive failure mechanism, and concluded that the failure is primarily due to Thermal Fatigue. Also, recommendation to prevent such failures during service has been provided as part of this analysis.