CASE-HARDENING MATERIALS-carbonitriding-nitrocarburizing
Materials for adding carbon and/or other elements to the surface of low-carbon or medium-carbon steels or to iron so that upon quenching a hardened case is obtained, with the center of the steel remaining soft and ductile. The material may be plain charcoal, raw bone, or mixtures marketed as carburizing compounds. A common mixture is about 60% charcoal and 40 barium carbonate. The latter decomposes, giving carbon dioxide, which is reduced to carbon monoxide in contact with the hot charcoal. If charcoal is used alone, action is slow and spotty. Coal or coke can be used, but action is slow, and the sulfur in these materials is detrimental. Salt is sometimes added to aid the carburizing action. By proper selection of the carburizing material, the carbon content may be varied in the steel from 0.80 to 1.20%. The carburizing temperature for carbon steels typically ranges from 1550 to 1750°F (850 to 950°C) but may be as low as 1450°F (790°C) or as high as 2000°F (1095°C). The articles to be car-burized for case hardening are packed in metallic boxes for heating in a furnace, and the process is called pack hardening, as distinct from the older method of burying the red-hot metal in charcoal.
Steels are also case-hardened by the diffusion of carbon and nitrogen, called carbonitriding, or nitrogen alone, called nitriding. Carbonitriding, also known as dry cyaniding, gas cyaniding, liquid cyaniding, nicarbing, and nitrocarburizing, involves the diffusion of carbon and nitrogen into the case. Nitriding also may be done by gas or liquid methods. In carbonitriding, the steel may be exposed to a carrier gas containing carbon and as much as 10% ammonia, the nitrogen source, or a molten cyanide salt, which provides both elements. Ammonia, from gaseous or liquid salts, is also the nitrogen source for nitriding. Although low- and medium-carbon steels are commonly used for carburizing and carbonitriding, nitrid-ing is usually applied only to alloy steels containing nitride-forming elements, such as aluminum, chromium, molybdenum, and vanadium. In ion nitriding, or glow-discharge nitriding, electric current is used to ionize low-pressure nitrogen gas. The ions are accelerated to the workpiece by the electric potential, and the work-piece is heated by the impinging ions, obviating an additional heat source. All three principal case-hardening methods provide a hard, wear-resistant case. Carburizing, however, which gives the greater case depth, provides the best contact-load capacity. Nitriding provides the best dimensional control, and carbonitriding is intermediate in this respect.
The principal liquid-carburizing material is sodium cyanide, which is melted in a pot that the articles are dipped in, or the cyanide is rubbed on the hot steel. Cyanide hardening gives an extremely hard but superficial case. Nitrogen as well as carbon is added to the steel by this process. Gases rich in carbon, such as methane, may also be used for carburizing, by passing the gas through the box in the furnace. When ammonia gas is used to impart nitrogen to the steel, the process is not called carburizing but is referred to as nitriding. Tufftriding, of Degussa AG of Germany, is a nitriding process using molten potassium cyanate with a small amount of sodium ferro-cyanide in titanium-lined melting pots.
Case-hardening compounds are marketed under a wide variety of trade names. These may have a base of hardwood charcoal or of charred bone, with sodium carbonate, barium carbonate, or calcium carbonate. Char is a carburizing material in which the particles of coal-tar carbon are surrounded by an activator and covered with a carbon coating. Accelerated Salt WS, of Du Pont, for heat-treating baths, has a content of 66% sodium cyanide, with graphite to minimize fuming and radiation losses. For selective case hardening on steel parts, a stiff paste of carburizing material may be applied to the surfaces where a carbon impregnation is desired. Carburit is a car-burizing paste of this kind. Aerocarb and Aerocase, of American Cyanamid Co., are mixtures of sodium and potassium nitrates and nitrides for use in carburizing baths at a temperature up to 1850°F (1010°C).

Chromized steel is steel surface-alloyed with chromium by diffusion from a chromium salt at high temperature. The reaction of the salt produces an alloyed surface containing about 40% chromium. Plasmaplate was a name given by the former Linde Div. of Union Carbide to protective coatings of tungsten or molybdenum, deposited by a plasma torch which gives a concentrated heat to 30,000°F (16,650°C); but the refractory metals can now be deposited at lower temperatures by decomposition of chemical compounds. Molybdenum pentachloride, MoCl5, is a crystalline powder which deposits an adherent coating of molybdenum metal when heated to 1652°F (900°C).
Metalliding is a diffusion coating process involving an electrolytic technique similar to electroplating, but done at higher temperatures [1500 to 2000°F (816 to 1093°C)]. Developed by General Electric, the process uses a molten fluoride salt bath to diffuse metals and metalloids into the surface of other metals and alloys. As many as 25 different metals have been used as diffusing metals, and more than 40 as substrates. For example, boride coatings are applied to steels, nickel-base alloys, and refractory metals. Beryllide coatings can be applied to many different metals by this process. The coatings are pore-free and can be controlled to a tolerance of 0.001 in (0.025 mm).

Additional comments :