Surface engineering and hardfacings
A loss due to wear of mechanical parts is estimated to be in the range 1.3‒1.6 % of a country's gross domestic product (GDP), therefore wear protection is crucible for improving the economical efficiency of a manufacturing process. Application of a hard wear resistant overlay (called hardfacing) to a metal surface by welding or thermal spraying is a widely acknowledged way to achieve this goal. In addition to that, hardfacings are also used for restoration of worn machine parts.
The research in TTÜ has been focused on composite hardfacings with a nickel- or iron-based matrix, reinforced by carbide (WC/W2C) and carbide-based (WC-Co, Cr3C2-Ni, TiC-NiMo) particles, for work under different wear types (abrasion, erosion, abrasive-impact wear) in a broad range of temperatures (20‒650 ºC). The main objective of the research has been to tailor the wear resistance of the hardfacings according to the working conditions and, on the other hand, to lower the final price. High velocity oxy-fuel spraying (HVOFS) and plasma transferred arc welding (PTAW) have been mostly applied as hardfacing technologies. HVOFS technology is principally used for hardfacings with thickness up to 1 mm and PTAW one – for hardfacings with thickness over 1 mm. The advantage of the HVOFS process is a much lower heat input to the substrate, what provides its lower distortion and neglidgible or absent loss of mechanical properties due to the annealing effect. In their turn, PTAW hardfacings demonstrate a much higher adhesion and resistance to impact loading, but the thermal input to the substrate is relatively high.