The effect of melt temperature on the mechanical properties of cast ASTM F75 CoCrMo alloy as explained by nitrogen and oxygen content

Abstract

Investment cast ASTM F75 Cobalt Chromium Molybdenum (CoCrMo) alloy is commonly used for orthopaedic implants due to its biocompatibility and wear characteristics. Apart from the values listed in the standard, the relationship between individual processing parameters and the resulting physio-mechanical properties are not well documented. This study characterises the effect of melt temperature on the physical properties and chemical composition of investment cast CoCrMo alloy. Furnace loads of ∼23−34 kg of alloy were inductively melted using 125 kW of power and cast into tensile test bars and chemistry coupons. The bars were ground to create samples conforming to ASTM E8M and tested per ISO 6892-1. The oxygen and nitrogen content of the chemistry coupons were assessed using Inert Gas Fusion (IGF) per ASTM E1019. It was found that increasing the melt temperature had the effect of removing nitrogen from the melt and adding oxygen. It was also determined that there was a strong positive correlation between nitrogen content and strength values. If desired, there is scope to minimise both oxygen pickup and nitrogen loss, nitrogen is correlated to increased mechanical properties, of this alloy system by targeting lower melt temperatures. However, this would be at the expense of melt viscosity and mould filling.

Keywords

• Investment casting
• Cobalt chrome molybdenum
• ASTM F75 - Standard specification for Cobalt-28 Chromium 6 Molybdenum Alloy Castings and Casting Alloy for Surgical Implants
• Orthopaedic devices
• Induction heating
• ASTM E8M - Standard Test Methods for Tension Testing of Metallic Materials (Metric)