Sunday, November 5, 2017

Hard Facing- What Is It?

Today I would like to explore a market that peaked in the 1960s. Like today, two of the major problems causing replacement of parts include wear and corrosion or combinations of the two. In the sixties, the major materials available were mild and low alloy steels. Stainless steels were just starting to attain popularity. However, there were many applications where the steels available were incapable of sustaining wear or corrosion in environments such as  drilling in mines, off shore applications for drilling rigs aircraft repair and shaping of glass bottles to hold liquids.Yes, silicon is abrasive and is the major constituent of most glass. Yes, glass bottles were a major market then because plastics were not yet replacing the cheap glass bottles. However, silicon is abrasive, and therefore something better was needed to extend the life of the part through improved protection of the surfaces used in these applications.
Engineers at small companies in those days, like Metco and Wall Colmonoy, recognized these differences between metals and glass. Therefore it wasn't long before someone decided that if you alloyed metals with silicon and boron  you could create a series of alloys tha,t when heated, became cherry red and did not melt immediately, but slicked up and remained that way for a period of time until the coating became fluid enough to melt and drip from the steel. Furthermore if you alloyed the powder or the wire with elements such as nickel and chromium, and other additives, such as particles of tungsten carbide, you had a surface that was corrosion resistant and had hard wear resistant carbides, borides and nitrides distributed within to provide surfaces that were significantly resistant to corrosion and wear.
Therefore a process developed whereby powders were atomized to these basic compositions with varying amounts of the constituents described that, when fused,  deposits ranging from 40 to 60 HRC hardness protected the surface from wear and corrosion. Tungsten carbide powder was admixed in a fine particle size distribution to further enhance the surface against abrasion.These powders were poured into a canister contained within the spray gun or within an external container, depending on the design of the spray gun. An acetylene oxygen fuel was then activated and the powder introduced within the fuel where it was heated to a temperature that was sufficiently warm to cause the powder to be plastic so that when it emerged from the gun as a spray, it deposited on the substrate as what is known as a splat. This porous deposit was then further heated with a welding torch or placed in a furnace at an appropriate temperature to fuse, similar to a glass with minute porosity. Unfortunately the bond with the substrate is not the most desirable, Therefore the fused coatings had a tendency to spall or flake from the surface, depending on the application, and replaced to renew adequate protection of the substrate. Also much depended on the skill of the operator in making the deposit and fusing it.
Today more ways to deposit coatings on surfaces have emerged. Plasma Spraying, which employs higher temperatures approaching those of the sun,  capable of providing any coating that can be engineered, is available.Also, furnace deposited and diffused coatings are possible that compete with hard faced coatings and do not have the weak interface associated with spalling and fracture that are common with hard faced coatings but have a transitional interface that blends to the microstructure of the alloy substrate.
Moyer, owner of MagnaTech was one of the first people to improvise Hoeganaes hard facing powders to supply most of the hard facing gun manufacturers. More recently he has developed a process that deposits an atomic reactant on the surface of the substrate that reacts with the alloying elements within the substrate to provide a hard wear, corrosion resistant, surface that can compete with hard facing processes in existence today. Many say that like the one horse buggy that hard facing is becoming passe'. However, recently MagnaTech has been provided with an opportunity to quality control plasma sprayed chromium carbide coatings. We look forward to the emerging of this opportunity.