Surface hardening

Generally, the toughness of a material decreases as the hardness increases. There are very many service conditions where the requirement is for a tough material of very high surface hardness, such as shafts and gears. Table 4.9 shows different methods to accomplish surface hardening. 

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P/M steels can be heat treated in the same manner as cast or wrought steels. They may be austenitized, quenched, and tempered. Surface hardening includes pack or gas carburization or nitriding, ie, heating in a nitrogen-containing atmosphere. Because of the greater amount of exposed surface area in the form of porosity, a protective atmosphere is needed (see Metal surface treatments). 

Many steels used for gears and bearings are surface-hardened by carburizing, quenching, and tempering. Molybdenum is frequendy used in carburized steels, and carburized Ni—Mo steels have been shown to provide optimum resistance to fatigue and impact effects (28). 

The addition of surface hardeners or sprinkle finishes (metallic or non-metallic)... 

Sodium silicate and silico fluoride solutions as concrete surface hardeners 9/104... 

Liquid resin-based systems which, like the chemical surface hardeners, penetrate into the surface of a concrete topping or directly finished slab and protect the acid-susceptible cement matrix from attack and, at the same time, strengthen the surface of the concrete are now being increasingly used. These in-surface seals leave the slip resistance of the concrete floor virtually unchanged but the treated floors are easier to clean and are more durable. 

Resin solution penetrating sealers are now available which, for very large warehouse floors, are comparable in applied costs with the concrete surface hardeners and are now being increasingly specified. Experience indicates that certain acrylic resin solutions are proving more durable and offer better protection to chemical and oil spillage than concrete surface hardeners. Acrylic resin solution sealers can markedly improve the abrasion resistance of concrete floors and have rescued a number of poor-quality floors. 

Concrete surface hardeners, two to three coats In-surface seals 1-1.8... 

The Knoop test is a microhardness test. In microhardness testing the indentation dimensions are comparable to microstructural ones. Thus, this testing method becomes useful for assessing the relative hardnesses of various phases or microconstituents in two phase or multiphase alloys. It can also be used to monitor hardness gradients that may exist in a solid, e.g., in a surface hardened part. The Knoop test employs a skewed diamond indentor shaped so that the long and short diagonals of the indentation are approximately in the ratio 7 1. The Knoop hardness number (KHN) is calculated as the force divided by the projected indentation area. The test uses low loads to provide small indentations required for microhardness studies. Since the indentations are very small their dimensions have to be measured under an optical microscope. This implies that the surface of the material is prepared approximately. For those reasons, microhardness assessments are not as often used industrially as are other hardness tests. However, the use of microhardness testing is undisputed in research and development situations. 

The reverse reaction to ammonia synthesis, the decomposition to nitrogen and hydrogen, is used in the nitriding of iron and carried out industrially at temperatures around 800 K and atmospheric pressure to produce surface-hardening. This dissolution reaction must also play a part in the synthesis of ammonia by the industrial process. The attempt to nitride iron by reaction with nitrogen gas is very slow under atmospheric pressure, presumably due to the stability of the nitrogen molecule. 

Polystyrene (PS), a highly rigid and surface-hardened thermoplastic, is glass clear and almost colorless. Its typical slight yellow tinge is easy to compensate for by adding transparent blue colorants to adjust the color. Polystyrene softens between 80 and 100°C. It is processed between about 170 and 280°C, up to a maximum of 300°C, without color change, by any of the methods which are recommended for thermoplastics. The list of products includes extrusion made sheets, profiles, and films, which are often foamed. 

Carburizing One of several thermochemical treatments involving, by heating, carbon absorption from suitable material in contact with the surface layer, and diffusion into solid (ferrous) alloys. Surface hardening processes may then be... 

A second mechanism in the. aging of CTPB propellants also exists and proceeds concurrently with the reactions proposed above. It consists of an attack at the reactive points of unsaturation in the backbone polymer, which causes additional crosslinking and hence an increase in propellant modulus, particularly at the surface. The exposed surface of CTPB propellants changes, as indicated by an increase in hardness. Heavy metal ions are particularly harmful, and it was found that an increase from 10 to 80 p.p.m. of iron caused a significant increase in surface hardening by catalytic attack on the double bonds. Antioxidants in general provide sufficient protection for polymer storage. In CTPB propellants the antioxidant selected to protect the double bond is very important. Amine-type antioxidants have provided better surface stability than phenolic compounds. 

The two basic types are (1) through hardenable, and (2) surface harden-able. Subcategones of surface hardenable alloys include carburizing alloys, flame and induction-hardening alloys, and nitriding alloys. 

CARBONITR1DING. A surface hardening process for steels involving ihe introduction of carbon and nitrogen into steels by healing in a suitable almosphere containing various combinations or hydrocarbons, ammonia, and carbon monoxide followed by a quenching to harden the case. 

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