Thermal spray cladding optimization for steel drill pipe

Abstract

Thermal spraying is commonly used to apply a hard and wear resistant coating to the outside of drill pipe. These coatings, however, are susceptible to impact damage and brittle fracture. A fundamental understanding is needed for the relationship among wire composition, spray parameters, and developed microstructure to improve coating toughness and bonding strength to the substrate. Characterization of the coating’s main features was determined by optical microscopy (OM) and scanning electron microscopy (SEM) analysis. Effect of process parameter voltage was evaluated by four-point bend testing with digital image correlation and Vickers and nano-indentation testing across the coating-substrate interface. It was found that as the voltage increased, the number of defects, primarily oxides, decreased. The decrease in oxide content led to increased coating strength at the higher voltage conditions. The oxide concentration did not exhibit a linear relationship with voltage, indicating that no significant change would occur above 36.5V. A new composition coating was proposed to increase strength and toughness while still maintaining wear properties. The new multi principle element alloy (MPEA) coating composition (Fe0.32Co0.08Cr0.2Ni0.32Mo0.07Ta0.01) was based on favorable findings reported for the FeCoCrNiMo0.2 MPEA. Two wires of different material were arced together to form the final coating. The effect of spray pressure on the mechanical properties of the coatings was evaluated by four-point bend testing, Vickers hardness, and dry sand rubber wheel wear testing. OM and SEM characterizations were performed on the new coatings. It was found that the anode and cathode wires did not mix within the arc, during flight, or upon deposition resulting in a coating with two different splat compositions. An increase in spray pressure from 172 to 310 kPa, resulted in the dramatic increase in oxide concentration from 21% to 64% but an 8% decrease in outer-fiber stress. The maximum new coating strength was found to be two times that of the HH1 coating. The wear rate was over three times greater than that of the HH1 coating. The two spray pressure conditions tested exhibited virtually the same wear rate suggesting that the spinel oxides responsible for the exceptional wear behavior of the FeCoCrNiMo0.2 MPEA did not form due to the lack of mixing.