Cemre Ablay, 030060150, 9th week

HARDFACING: (09.04.2011, 22:18)

Hardfacing is defined as the process of obtaining desired properties or dimensions by applying, using oxyfuel or arc welding an integral layer of metal of one composition. The hardfacing operation makes the surface highly resistant to abrasion.
There are various techniques of hardfacing. Some apply a hard surface coating by fusion welding. In other techniques, no material is added but the surface metal is changed by heat treatment of by contact with other materials.

(Jeffus L.F, Welding: Principles and Applications, 5th Edition p.708)
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THE SEEBECK EFFECT: (09.04.2011, 22:40)

In 1826, Seebeck discovered the thermoelectric effect, which after his name is also known as Seebeck effect. The phenomenon of production of an e.m.f. causing an electric current to flow in a thermocouple, when its two junctions are kept at two different temperatures, is known as Seebeck effect. To study this effect, two wires of different materials, say copper and iron are joined at their ends so as to form two junctions. A sensitive galvanometer is included in the circuit. This arrangement is called a copper-iron thermocouple. When one junction of the thermocouple is kept hot and the other cold, the galvanometer gives deflection indicating the production of current in the arrangement. The current so produced is called thermoelectric current. Further the continuous flow of current in the thermocouple indicates that there must be a source of e.m.f. in the circuit, which is causing flow of current. This e.m.f. is called thermoelectric e.m.f.

(Sharma H.C., Kharb R., Sharma R., Comprehensive Physics for Engineers, p.161)
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GRAPHITIZATION: (09.04.2011, 22:54)

Graphitization is a solid-state transformation of thermodynamically unstable non-graphitic carbon into graphite by thermal treatment. The degree of graphitization is a measure of the extent of long-range 3D crystallographic order, as determined by diffraction studies alone. The degree of graphitization significantly affects many properties, such as thermal conductivity, electrical conductivity, strength, and stiffness.
A common, but incorrect, use of the term graphitization is to indicate a process of thermal treatment of carbon materials above 2200 C regardless of any resultant crystallinity. The use of the term graphitization without reporting confirmation of long-range 3D crystallographic order determined by diffraction, studies should be avoided, as it can be misleading.

(Scheffler M., Colombo P., Cellular Ceramics: Structure, Manufacturing, Properties and Applications, p.139)

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TTT DIAGRAMS: (09.04.2011, 23:08)

A TTT (Time-Temperature-Transformation) diagram is also called an S-curve, C-curve isothermal (decomposition of austenite) diagram and Bain’s curve. TTT diagrams are extensively used in the assessment of the decomposition of austenite in heat-treatable steels. As the iron-carbon phase diagram does not show time as variable the effects of different cooling rates on the structures of steels are not revealed. Secondly, equilibrium conditions are not maintained in heat treatment. The iron-carbon equilibrium diagram reveals on the phases and corresponding microstructures under equilibrium conditions but many useful properties of the steels are obtained under non-equilibrium conditions such as variable rates of cooling as produced during quenching and better transformation of austenite into pearlite and martensite.

(Narula G.K., Narula K.S., Gupta V.K., Materials Science, p.172)