Miscellaneous, special processing techniques and heat treatments

2 Miscellaneous, special processing techniques and heat treatments In the preparation of intermetallic alloys, both in massive quantities for commercial purposes, or as small specimens for laboratory investigations, very often the alloys must be subjected to selected and well-defined heat treatments, in some cases in addition to mechanical treatments, in order to have their full characterization and/or optimal performance. 

Characteristics and implementation of the treatments depend on the expected results and on the properties of the material considered a variety of processes are employed. In ferrous alloys, in steels, a eutectoid transformation plays a prominent role, and aspects described by time-temperature-transformation diagrams and martensite formation are of relevant interest. See a short presentation of these points in 5.10.4.5. Titanium alloys are an example of the formation of structures in which two phases may be present in comparable quantities. A few remarks about a and (3 Ti alloys and the relevant heat treatments have been made in 5.6.4.1.1. More generally, for the various metals, the existence of different crystal forms, their transformation temperatures, and the extension of solid-solution ranges with other metals are preliminary points in the definition of convenient heat treatments and of their effects. In the evaluation and planning of the treatments, due consideration must be given to the heating and/or cooling rate and to the diffusion processes (in pure metals and in alloys). 

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 

Cementation The introduction, by diffusion at high temperature, of elements in the outer portion of a metal piece. 

Hardening A hardness increase obtained by a convenient treatment (often by heating and cooling). 

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