Effect of trace niobium alloying on drilling conditions influencing quality of vermicular graphite cast iron

To enhance the mechanical properties of compacted graphite iron (CGI), a novel as-cast niobium-microalloyed compacted graphite iron (L-Nb-CGI) was developed via the introduction of trace Nb. It was observed that Nb addition reduces graphite area and vermicularity in CGI, promotes pearlite formation, refines its lamellar spacing, and significantly improves the tensile strength and hardness of the material. Through comparative experiments involving multiple drilling parameter sets, along with analysis using ultra-depth-of-field and scanning electron microscopy, it was found that L-Nb-CGI demonstrates an average reduction of 16% in hole tearing factor and 8% in hole wall surface roughness under identical machining conditions compared to conventional RuTSi ₄ Mo compacted graphite iron. Optimization of drilling parameters further inhibits hole surface damage, with the underlying mechanism explained as follows: increasing drill diameter to 10 mm enhances chip removal capacity; elevating spindle speed to 3000 r/min facilitates material thermal softening; and reducing feed per revolution to 0.1 mm/r effectively suppresses brittle fracture. The incorporation of Nb element effectively enhances the drilling quality of CGI, and drilling performance is improved through microstructure refinement and processing parameter optimization. The findings of this study offer theoretical and technological guidance for high-performance drilling applications of L-Nb-CGI.