Failure analysis of AISI 440C strand dies used in twin screw extruders during polymer compounding

Abstract

In view of excellent wear and corrosion resistance, AISI 440C Martensitic Stainless steel finds wide use as Dies in Twin Screw Extruders in Polymer, Food and Pharma applications, especially when either additives, which could be corrosive, are added to aid compounding of polymers or moisture and/or free radicals are present in the processing material. Two AISI 440C Die Plates part of Strand Die Assembly in a φ40 mm size Twin Screw Extruder abruptly cracked during operation in succession disrupting regular production with the second failure occurring after about two years of the Extruder in service. While visual examination of the failed Die Plates showed presence of surface / sub-surface cracks and marks of corrosion, the sectioned Die Plate revealed multiple internal cracks. The hardness survey on the failed Die Plates confirmed values between 30 and 36 HRC, which are about 20–25 HRC lower than the hardness measured in as-supplied condition. Macro-examination of the sectioned Die Plate revealed multiple micro-crack origin locations together with strong evidence of corrosion. The microstructural examination showed carbide network along grain boundaries in a matrix consisting of tempered martensite and revealed intergranular fracture. Fractographic observations using Scanning Electron Microscopy on the cracked Die Plate clearly revealed segregation of fine carbides along crack boundaries. Based on detailed analyses, which included site data collection and investigation, the failure of Die Plates was attributed to Stress Corrosion Cracking. Finally, recommendations were proposed to prevent recurrence of such failures in service.