Hard facing

Hard facing of various components in the aircraft gas-turbine engine and in industrial apphcations for textile machinery parts, oil and gas machinery parts, paper-shtting knives, etc, is estimated at 1 x 10 in 1995 with an estimated growth rate of 5% annually. The mix is approximately 45% aerospace apphcations, 55% industrial apphcations. Additionally, repair coatings for gas-turbine blades and vanes is estimated at 500 x 10 . These coatings are primarily deposited by plasma spray, arc-wire, HVOF, and detonation gun techniques. 

Includes welding and hard-facing rods as well as materials and nonferrous alloys. 

Weld overlays of stainless steel or cobalt-based wear-resistant and hard-facing alloys such as Stellite may salvage damaged equipment. In addition, weld overlays incorporated into susceptible zones of new equipment may provide cost-effective resistance to cavitation damage. 

Another version of the dewatering screw that has been used successfully in a number of installations (Figure 40) consists of an inclined screw with a box-like back end and an internal overflow weir. The drive is usually a variable-speed motor reducer. Rotary air lock fitted with a variable-speed drive is typically used as shown in Figure 37 in connection with the plain receiving tank and an inclined screw conveyor. Materials of construction can be chrome-plated cast iron or nickel-hard casing. The rotary valve is made of hard faced tips or stainless steel replaceable blades. 

Material Shell carbon steel with 4-5 in. (10-12 cm) thick heavy weight, single-layer, cast-vibrated refractory with needles. Internals 304H stainless steel for temperature >1.200°F (650°C) and Grade H, % chrome for < ,200°F. Internal components exposed to catalyst should be refractory-lined for erosion resistance. Sliding surfaces should be hard-faced, minimum thickness in. (3 mm). 

In recent years, the spraying process has been adapted for hard facing, using the chromium-nickel-boron alloys which have become known as Colmonoy. More recently still, the cobalt-base Stellite alloys have also been used. These materials in powder form are sprayed on to the surface in the usual way. The deposit is afterwards heat treated by a torch, so that fusion takes place. The process is often known as spray-welding. Such coatings are primarily used for hard facing under wear conditions, but as the Anal surface is nickel-chromium or cobalt-chromium they exhibit very high anticorrosive properties. 

Wear and corrosion protection can be provided by the well-established techniques of hard-facing and plating or by surface-modification processes such as bonding, nitriding, carburizing, and ion implantation. The protection these processes afford is adequate in most environments but may fail over a period of time if the conditions are too severe. 

Hardening, 1 528. See also Case hardening of melamine resins, 15 780-788 of steels, 16 196-199 Hard facing, 25 375 Hard fibers, 11 285... 

Figure 10.31 The surface grinding process where a grinding lathe removes excess hard facing material from the outside diameter of the screw (courtesy of Jeffrey A. Kuhman of Glycon Corporation)...

If austenitic stainless steel parts exposed to conditions that promote intergranular corrosion are to be fabricated, hard faced, overlaid, or repaired by welding, these parts shall be made of low-carbon or stabilized grades. 

Case wear rings No Cast iron Bronze Cast iron Ni-resist Cast iron 12% CHR 12% CHR Hard Faced 316 AUS (3) Monel 12% CHR hardened Hard Faced AUS (3) Hard Faced 316 AUS (3) Duplex (3)... 

After a few years of operation there was a significant increase in radiation fields from the primary circuit piping in the Douglas Point generating station. Other water-cooled reactors around the world experienced similar effects. The principal source of the radioactivity was traced to cobalt-60, formed by neutron absorption in the natural cobalt-59 which arose from hard-facing alloys and was also present as an impurity in boiler materials such as HMDnel, and in carbon steel and other structural materials. The mechanism of this radioactivity transport was found to be corrosion of the cobalt bearing materials, transport... 

Case Wear Rings No Cast Iron Bronze Cast Iron Ni-Resist Cast Iron 12%CHR Hardened 12% CHR Hardened Hard-faced 316AUS6 N-Cu Alloy 12% CHR Hardened Hard-faced AUSe Hard-faced 316 AUS e Hard-faced Duplex Hard-faced Super Duplex ... 

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