Nanocrystalline diamond coatings on the interior of WC–Co dies for drawing carbon steel tubes: Enhancement of tube properties

• Diamond coatings were deposited on the interior of WC–Co drawing dies using HFCVD.
• A field trial was conducted on the production line for drawing carbon steel tubes.
• Surface roughness of tubes drawn through the NCD coated die was decreased by 28%.
• Residual stress of tubes drawn through the NCD coated die was lowered by 25%.
• NCD coated dies would have commercial ramifications in metal forming industries.

Tungsten carbide (WC–Co) dies are commercially used for the tube drawing process. However they wear out progressively and are unable to meet the high demands required by the industry. In this study, the effect of nanocrystalline diamond (NCD) coatings on the interior of WC–Co drawing dies using a hot filament chemical vapour deposition technique is reported. A field trial was conducted on the production line for drawing AISI 1541 steel tubes to investigate the quality of the drawn tubes. The surface roughness of the tubes drawn through the NCD coated die was lower (Ra = 381 nm) when compared to the tubes drawn through a regular carbide die (Ra = 527 nm). The average residual stress of tubes drawn through the NCD coated drawing die was lowered by 25%. A pin-on-disc sliding wear test, carried out to estimate the coefficient of friction, showed that the coefficient of friction in the case of the NCD coated die was almost half that of the regular WC–Co dies. The excellent thermal conductivity and lower friction coefficient of NCD coatings also helped to decrease the working temperature of the tube drawing process, thereby resulting in a superior product.