Novel powder modification method for the cold spray of hard steels

Successful cold spray (CS) of tool steels and other hard steels would unlock several opportunities, including the repair of molds as well as ships and heavy industry components. However, the high hardness of typical atomized steel powders strongly limits their cold sprayability and CS of tool steels has not been reported so far. The challenge in cold spraying hard steels can be illustrated by the calculated critical velocities of H13 steel at about 1100m/s, which is well above the current CS system capabilities while using nitrogen as the process gas. In an attempt to alleviate this issue, a novel powder modification method based on the simultaneous softening and agglomeration of steel powders via heat treatment in a rotating furnace has been investigated as a promising way to improve H13 tool steel powder cold sprayability. By adjusting starting powder size and shape as well as heat treatment conditions (maximum temperature, cooling rate and heat treatment atmosphere), CS of H13 powder went from virtually no deposition to the production of dense and thick deposits with a powder deposition efficiency of 70%. Powder agglomeration, surface state, microstructure evolution and softening are key factors determining the powder deposition efficiency and resulting deposit microstructure.