Failure Investigation & Analyses of AISI D3 Dies Used in Tableting Machines for Pharmaceutical Application

Abstract

AISI D3 material finds wide use as Dies in Tableting Machines for Pharmaceutical applications owing to its good strength, toughness and moderate corrosion resistance combined with cost effectiveness. However, 10-15% of AISI D3 Dies used in rotary Tableting Machines were found to fail within 4-6 months of service resulting in frequent disruption of regular production and loss in overall productivity besides having potential long-term risks such as metal contamination in the output product. Visual examination of the failed Die showed 180o crack across the top Die face with rust marks, but no such crack was found on the bottom face. Also, the crack on the top Die face had a jagged morphology and those cracks on the Outer Diameter (OD) surface oriented in the axial direction were found to be straight and sharp. Macro-examination of Die sectioned diametrically and perpendicular to the crack on the Die face revealed jagged axial cracks on the Internal Diameter (ID) surface. Cluster pitting marks centered around mid-height of the Die were observed on the ID surface and the cut open sections of the Die along the crack revealed extensive corrosion. While microstructural examination of samples taken on the ID surface showed presence of banded carbides oriented axially in tempered martensite matrix, the same examination carried out on samples taken on Die face revealed fine needle like carbides in tempered martensite matrix. Fractography analysis conducted in the cluster pitting zone on the ID surface clearly showed presence of micro-cracks having multiple points of origin with clear indications of propagation into the cross section of the Die. Scanning Electron Microscopy (SEM) carried out in the aforementioned zone near ID surface also showed the presence of multiple micro-cracks. The SEM-EDX analysis in the referred zone at specific locations in the vicinity of microcracks confirmed inadequate Chromium oxide content leading to lack of corrosion resistance in the material. Based on the investigation & analyses and the detailed study of the Tableting Process in rotary Tableting Machines that revealed the nature of applied load to be cyclic, a comprehensive failure mechanism was arrived at, and it was inferred that the predominant mode of AISI D3 Die failure was Corrosion Fatigue. Finally, recommendation has been proposed to prevent such failures during service.