金属热处理外文文献及翻译

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Heat treatment of metal

The generally accepted definition for heat treating metals and metal alloys is “heating and cooling a solid metal or alloy in a way so as to obtain specific conditions or properties。”Heating for the sole purpose of hot working (as in forging operations）is excluded from this definition．Likewise，the types of heat treatment that are sometimes used for products such as glass or plastics are also excluded from coverage by this definition．

Transformation Curves

The basis for heat treatment is the time—temperature-transformation curves or TTT curves where，in a single diagram all the three parameters are plotted．Because of the shape of the curves，they are also sometimes called C-curves or S-curves．

To plot TTT curves，the particular steel is held at a given temperature and the structure is examined at predetermined intervals to record the amount of transformation taken place．It is known that the eutectoid steel (T80) under equilibrium conditions contains，all austenite above 723℃，whereas below，it is the pearlite．To form pearlite,the carbon atoms should diffuse to form cementite．The diffusion being a rate process,would require sufficient time for complete transformation of austenite to pearlite．From different samples，it is possible to note the amount of the transformation taking place at any temperature．These points are then plotted on a graph with time and temperature as the axes．Through these points，transformation curves can be plotted as shown in Fig.1 for eutectoid steel．The curve at extreme left represents the time required for the transformation of austenite to pearlite to start at any given temperature．Similarly，the curve at extreme right represents the time required for completing the transformation．Between the two curves are the points representing partial transformation。The horizontal lines Ms and Mf represent the start and finish of martensitic transformation.

Classification of Heat Treating Processes

In some instances,heat treatment procedures are clear—cut in terms of technique and application．whereas in other instances，descriptions or simple explanations are insufficient because the same technique frequently may be used to obtain different objectives．For example, stress relieving and tempering are often accomplished with the same equipment and by use of identical time and temperature cycles．The objectives，however，are different for the two processes．The following descriptions of the principal heat treating processes are generally arranged according to their interrelationships．

Normalizing consists of heating a ferrous alloy to a suitable temperature （usually 50°F to 100°F or 28℃to 56℃) above its specific upper transformation temperature．This is followed by cooling in still air to at least some temperature well below its transformation temperature range．For low-carbon steels，the resulting structure and properties are the same as those achieved by full annealing；for most ferrous alloys, normalizing and annealing are not synonymous.

Normalizing usually is used as a conditioning treatment，notably for refining the grains of steels that have been subjected to high temperatures for forging or other hot working operations. The normalizing process usually is succeeded by another heat treating operation such as austenitizing for hardening, annealing，or tempering.

Annealing is a generic term denoting a heat treatment that consists of heating to and holding at a suitable temperature followed by cooling at a suitable rate。It is used primarily to soften metallic materials, but also to simultaneously produce desired changes in other properties or in microstructure。The purpose of such changes may be, but is not confined to, improvement of machinability, facilitation of cold work （known as in—process annealing)，improvement of mechanical or electrical properties, or to increase dimensional stability。When applied solely to relive stresses, it commonly is called stress-relief annealing, synonymous with stress relieving.

When the term “annealing”is applied to ferrous alloys without qualification, full annealing is applied. This is achieved by heating above the alloy’s transformation temperature, then applying a cooling cycle which provides maximum softness. This cycle may vary widely，depending on composition and characteristics of the specific alloy。

Quenching is a rapid cooling of a steel or alloy from the austenitizing temperature by immersing the work piece in a liquid or gaseous medium. Quenching medium commonly used include water，5％brine, 5％caustic in an aqueous solution，oil, polymer solutions，or gas （usually air or nitrogen)。

Selection of a quenching medium depends largely on the hardenability of material and the mass of the material being treating (principally section thickness)。

The cooling capabilities of the above-listed quenching media vary greatly。In selecting a quenching medium，it is best to avoid a solution that has more cooling power than is needed to achieve the results, thus minimizing the possibility of cracking and warp of the parts being treated. Modifications of the term quenching include direct quenching, fog quenching, hot quenching，interrupted quenching，selective quenching，spray quenching, and time quenching。