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Thermochemical Treatment

Heat treatment carried out in a medium suitably chosen to produce a change in the chemical composition of the object by exchange with the medium.



(1) In heat treatment, reheating hardened steel or hardened cast iron to some temperature below the eutectoid temperature for the purpose of decreasing hardness and increasing toughness. The process also is sometimes applied to normalized steel. (2) In tool steels, temper is sometimes used, but inadvisedly, to denote the carbon content. (3) In nonferrous alloys and in some ferrous alloys (steels that cannot be hardened by heat treatment), the hardness and strength produced by mechanical or thermal treatment, or both, and characterized by a certain structure, mechanical properties, or reduction in area during cold working.


Temper Color

A thin, tightly adhering oxide skin that forms when steel is tempered at a low temperature, or for a short time, in air or a mildly oxidizing atmosphere. The color, which ranges from straw to blue depending on the thickness of the oxide skin, varies with both tempering time and temperature.


Temper Embrittlement

Embrittlement of alloy steels caused by holding within or cooling slowly through a temperature range just below the transformation range. Embrittlement is the result of the segregation at grain boundaries of impurities such as arsenic, antimony, phosphorus, and tin; it is usually manifested as an upward shift in ductile-to-brittle transition temperature. Temper embrittlement can be reversed by retempering above the critical temperature range, then cooling rapidly.


Tempered Martensite Embrittlement

Embrittlement of ultrahigh-strength steels caused by tempering in the temperature range of 205 to 400 C (400 to 750 F); also called 350 C or 500 F embrittlement. Tempered martensite embrittlement is thought to result from the combined effects of cementite precipitation on prior austenite grain boundaries or interlath boundaries and the segregation of impurities at prioraustenite grain boundaries.


Thermal Analysis

A method for determining transformations in a metal by noting the temperatures at which thermal arrests occur. These arrests are manifested by changes in slope of the plotted or mechanically traced heating and cooling curves. When such data are secured under nearly equilibrium conditions of heating and cooling, the method is commonly used for determining certain critical temperatures required for the construction of equilibrium diagrams.


Tantalum Capacitor

A type of electrolytic capacitor that uses tantalum rather than aluminum. The tantalum can be foil, as is the aluminum in a conventional electrolytic capacitor. It might also take the form of a porous pellet, the irregular surface of which provides a large area in a small volume. An extremely thin oxide layer forms on the tantalum. These capacitors have high reliability and excellent efficiency, and are used in military applications because they have a low failure rate. They can be used in audio and digital circuits in place of aluminum electrolytic capacitors. The main disadvantage of tantalum capacitors is that they are comparatively expensive. Compare CERAMIC CAPACITOR, ELECTROLYTIC CAPACITOR, MICA CAPACITOR, PAPER CAPACITOR, PLASTIC-FILM CAPACITOR.


Tee Antenna

An antenna consisting of a horizontal radiator with a vertical lead-in or feeder connected to its center point.


Telemetric Receiver

A system that selects, amplifies, and demodulates or rectifies a radio signal, and actuates indicating instruments, recorders, or data processors.


Telemetric Transmitter

A specialized transmitter that generates radio-frequency (RF) power, adds to it signals delivered by data transducers, and delivers the modulated power to an antenna for transmission to a distant telemetric receiver.


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