Wear and corrosion resistant laser coatings for hydraulic piston rods

Hydraulic piston rods on oil & gas drilling platforms, hydro-power stations, chemical plants and underground mines are exposed to severe tribo-corrosive conditions under static and dynamic mechanical loads. Piston rods made of carbon, quenched & tempered (QT) and stainless steels are frequently surface coated with methods such as thermal spraying, hard chrome plating and overlay welding. Unfortunately, several premature failures have been reported particularly in marine applications due to insufficient coating properties. Laser cladding has recently drawn lot of attention in this field due to high coating quality and significant improvements in productivity. In this study, several potential Fe-, Ni- and Co-based alloys were laser clad on carbon and QT steels. Their corrosion and mechanical performances were explored in long-term salt spray, immersion, hardness, abrasive wear and 4-point bending fatigue tests. Most of the laser coatings outperformed hard chrome in corrosion properties but hardness values were somewhat lower. In single point abrasion tests (scratch tests), the hardest laser coatings, however, outperformed hard chrome due to brittle nature of hard chrome layers. Post-machining induced significant superficial hardness increase in laser coatings, which was the main reason for good wear performance. Fatigue performance was strongly dependent on material pairs, presence of cladding defects and applied loads.