Root cause analysis and Resolution of a brass extrusion mandrel failure

Abstract

Mandrels made of DIN 1.2367 (AISI H10) hot-work tool steel are critical components in the hot extrusion of seamless C27000/C38500 brass tubes. In this case, after a change in the tooling supplier, mandrel life dropped to ~ 10% of the historical benchmark, with premature brittle fractures occurring during routine production. A comprehensive failure investigation was per-formed combining chemical composition verification (ASTM E415), tensile testing (ASTM E8), hardness mapping (ASTM E18), surface roughness measurements, Charpy impact testing (ASTM E23), fractographic examination of fracture surfaces, and finite-element simulations of the extrusion stage. The steel chemistry complied with DIN 1.2367; however, the hardness in the upper region exceeded the specified range, consistent with reduced toughness. Fractography showed crack initiation at the inner surface of the cooling channel, where drilling-induced surface damage (e.g., high roughness, burrs, and potential microcracking) served as critical stress con-centrators within a region exhibiting ardness gradients. FEM confirmed that this bore is subjected to high axial tensile stresses during extrusion, particularly for the current die geometry. Corrective actions targeted the channel surface quality by replacing conventional twist drilling with carbide-tipped gun drilling, thereby removing critical defects and producing a uniform in-ternal finish. After implementation, mandrel service life returned to the original level (~250 pressings), restoring process reliability and reducing unplanned downtime.