METALLOGRAPHY AND HEAT TREATMENT BASIC INFORMATION


The great advantage of steel as an engineering material is its versatility, which arises from the fact that its properties can be controlled and changed by heat treatment.Thus, if steel is to be formed into some intricate shape, it can be made very soft and ductile by heat treatment; on the other hand, heat treatment can also impart high strength.

The physical and mechanical properties of steel depend on its constitution, that is, the nature, distribution, and amounts of its metallographic constituents as distinct from its chemical composition.

The amount and distribution of iron and iron carbide determine the properties, although most plain carbon steels also contain manganese, silicon, phosphorus, sulfur, oxygen, and traces of nitrogen, hydrogen, and other chemical elements such as aluminum and copper.

These elements may modify, to a certain extent, the main effects of iron and iron carbide, but the influence of iron carbide always predominates. This is true even of medium-alloy steels, which may contain considerable
amounts of nickel, chromium, and molybdenum.

The iron in steel is called ferrite. In pure iron-carbon alloys, the ferrite consists of iron with a trace of carbon in solution, but in steels it may also contain alloying elements such as manganese, silicon, or nickel. The atomic arrangement in crystals of the allotrophic forms of iron is shown in Fig. 2.1.


Cementite, the term for iron carbide in steel, is the form in which carbon appears in steels. It has the formula Fe3C, and consists of 6.67% carbon and 93.33% iron. Little is known about its properties, except that it is very hard and brittle.

As the hardest constituent of plain carbon steel, it scratches glass and feldspar but not quartz. It exhibits about two-thirds the induction of pure iron in a strong magnetic field.

Austenite is the high-temperature phase of steel. Upon cooling, it gives ferrite and cementite. Austenite is a homogeneous phase, consisting of a solid solution of carbon in the y form of iron. It forms when steel is heated above 79O0C.

The limiting temperatures for its formation vary with composition and are discussed below. The atomic structure of austenite is that of y iron, fee; the atomic spacing varies with the carbon content.

When a plain carbon steel of ~ 0.80% carbon content is cooled slowly from the temperature range at which austenite is stable, ferrite and cementite precipitate together in a characteristically lamellar structure known as pearlite. It is similar in its characteristics to a eutectic structure but, since it is formed from a solid solution rather than from a liquid phase, it is known as a eutectoid structure.

At carbon contents above and below 0.80%, pearlite of ~ 0.80% carbon is likewise formed on slow cooling, but excess ferrite or cementite precipitates first, usually as a grain-boundary network, but occasionally also along the cleavage planes of austenite.

The excess ferrite or cementite rejected by the cooling austenite is known as a proeutectoid constituent. The carbon content of a slowly cooled steel can be estimated from the relative amounts of pearlite and proeutectoid constituents in the microstructure.

Bainite is a decomposition product of austenite consisting of an aggregate of ferrite and cementite. It forms at temperatures lower than those where very fine pearlite forms and higher than those at which martensite begins to form on cooling.

Metallographically, its appearance is feathery if formed in the upper part of the temperature range, or acicular (needlelike) and resembling tempered martensite if formed in the lower part.

Martensite in steel is a metastable phase formed by the transformation of austenite below the temperature called the Ms temperature, where martensite begins to form as austenite is cooled continuously.

Martensite is an interstitial supersaturated solid solution of carbon in iron with a bodycentred tetragonal lattice. Its microstructure is acicular.

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