Machinery's Handbook, 31st Edition
Hard Facing 1607 This property is an asset in impact wear situations. Machining is difficult with ordinary tools and equipment; finished surfaces are usually ground. Austenitic High-Chromium Irons.— These metals are available in rod (RFeCr-A) and electrode (EFeCr-A) form and are used for facing agricultural machinery parts, coke chutes, steel mill guides, sand-blasting equipment, and brick-making machinery. Hardness: The as-welded deposit ranges in hardness from 51 to 62 Rockwell C. Under impact, the deposit work hardens somewhat, but the resulting deformation also leads to cracking and impact service is therefore avoided. Hot hardness decreases slowly at tem peratures up to 800 and 900 ° F (427 and 482 ° C). At 900 ° F (482 ° C), the instantaneous hardness is 43 Rockwell C. In 3 minutes under load, the hardness drops to 37 Rockwell C. At 1200 ° F (649 ° C), the instantaneous hardness is 5 Rockwell C. The decrease in hardness during hot testing is practically recovered on cooling to ambient temperatures. Resistance Properties: Deposits will withstand only light impact without cracking. Dynamic compression stresses above 60,000 psi (414 MPa) should be avoided. These metals exhibit good oxidation resistance up to 1800 ° F (982 ° C) and can be considered for hot wear applications where hot plasticity is not objectionable. They are not very resistant to corrosion from liquids and will rust in moist air, but are more stable than ordinary iron and steel. Resistance to low-stress scratching is outstanding and is related to the amount of hard carbides present. However, under high-stress grinding abrasion, performance is only mediocre and they are not deemed suitable for such service. Other Properties or Characteristics: The deposited metals have a yield strength (0.1 percent offset) of between 80,000 and 140,000 pounds per square inch (552 and 965 MPa) in compression and an ultimate strength of from 150,000 to 280,000 psi(1034–1930 MPa). Their tensile strength is low and therefore tension uses are avoided in design. These deposits are considered to be commercially unmachinable and are also very difficult to grind. When ground, a grinding wheel of aluminum oxide abrasive with a 24-grit size and a hard (Q) and medium-spaced resinoid bond is recommended for off-hand high-speed work and a slightly softer (P) vitrified bond for off-hand low-speed work. Cobalt-Base Alloys.— These metals are available in both rod (RCoCr) and electrode (ECoCr) form and are frequently used to surface the contact surfaces of exhaust valves in aircraft, truck, and bus engines. Other uses include parts such as valve trim in steam engines, and on pump shafts, where conditions of corrosion and erosion are encountered. Several metals with a greater carbon content are available (CoCr-B, CoCr-C) and are used in applications requiring greater hardness and abrasion resistance but where impact resis tance is not mandatory or expected to be a factor. Hardness: Hardness ranges on the Rockwell C scale for gas-welded deposits are as follows: CoCr-A, 38 to 47; CoCr-B, 45 to 49; and CoCr-C, 48 to 58. For arc-welded deposits, hardness ranges (Rockwell C) as follows: CoCr-A, 23 to 47; CoCr-B, 34 to 47; and CoCr-C, 43 to 58. The values for arc-weld deposits depend for the most part on the base metal dilution. The greater the dilution, the lower the hardness. Many surfacing alloys are softened permanently by heating to elevated temperatures, however, these metals are exceptional. They do exhibit lower hardness values when hot but return to their approximate original hardness values upon cooling. Elevated-temperature strength and hardness are outstanding properties of this group. Their use at 1200 ° F (649 ° C) and above is considered advantageous but between 1000 and 1200 ° F (538–649 ° C), their advantages are not definitely established, and at temperatures below 1000 ° F (538 ° C), other surfacing metals may prove better. Resistance Properties: In the temperature range from 1000 to 1200 degrees F (538– 649 ° C), weld deposits of these metals have a great resistance to creep. Tough martensitic steel deposits are considered superior to cobalt-base deposits in both flow resistance and toughness. The chromium in the deposited metal promotes the formation of a thin, tightly adherent scale that provides a scaling resistance to combustion products of internal
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