Machinery's Handbook, 31st Edition
Heat Treatment of Steel 467 temperature takes place is called the critical cooling rate . When a piece of carbon steel is quenched at this rate or faster, a new structure is formed. The austenite is transformed into martensite , which is characterized by an angular needle-like structure and a very high hardness. If carbon steel is subjected to a severe quench or to extremely rapid cooling, a small percentage of the austenite, instead of being transformed into martensite during the quenching operation, may be retained. Over a period of time, however, this remaining austenite tends to be gradually transformed into martensite even though the steel is not subjected to further heating or cooling. Martensite has a lower density than austenite, and such a change, or “aging” as it is called, often results in an appreciable increase in volume or “growth” and the setting up of new internal stresses in the steel. Steel Heat Treating Furnaces.— Various types of furnaces heated by gas, oil, or elec tricity are used for the heat treatment of steel. These furnaces include the oven or box type in various modifications for “in-and-out” or for continuous loading and unloading; the retort type; the pit type; the pot type; and the salt-bath electrode type. Oven or Box Furnaces: This type of furnace has a box or oven-shaped heating chamber. The “in-and-out” oven furnaces are loaded by hand or by a track-mounted car that, when rolled into the furnace, forms the bottom of the heating chamber. The car type is used where heavy or bulky pieces must be handled. Some oven-type furnaces are provided with a full muffle or a semimuffle, which is an enclosed refractory chamber into which the parts to be heated are placed. The full-muffle, being fully enclosed, prevents any flames or burning gases from coming in contact with the work and permits a special atmosphere to be used to protect or condition the work. The semimuffle, which is open at the top, protects the work from direct impingement of the flame although it does not shut off the work from the hot gases. In the direct-heat-type oven furnace, the work is open to the flame. In the electric oven furnace, a retort is provided when gas atmospheres are to be employed to confine the gas and prevent it from attacking the heating elements. Where muffles are used, they must be replaced periodically, and a greater amount of fuel is required than in a direct-heat type of oven furnace. For continuous loading and unloading, there are several types of furnaces, such as rotary hearth car; roller-, furnace belt-, walking-beam, or pusher-conveyor; and a continuous- kiln-type through which track-mounted cars are run. In the continuous type of furnace, the work may pass through several zones that are maintained at different temperatures for preheating, heating, soaking, and cooling. Retort Furnace: This is a vertical type of furnace provided with a cylindrical metal retort into which the parts to be heat treated are suspended either individually, if large enough, or in a container of some sort. The use of a retort permits special gas atmospheres to be employed for carburizing, nitriding, etc. Pit-Type Furnace: This is a vertical furnace arranged for the loading of parts in a metal basket. The parts within the basket are heated by convection, and when the basket is low ered into place, it fits into the furnace chamber in such a way as to provide a dead-air space to prevent direct heating. Pot-Type Furnace: This furnace is used for the immersion method of heat treating small parts. A cast-alloy pot is employed to hold a bath of molten lead or salt in which the parts are placed for heating. Salt Bath Electrode Furnace: In this type of electric furnace, heating is accomplished by means of electrodes suspended directly in the salt bath. The patented grouping and design of electrodes provide an electromagnetic action that results in an automatic stirring action. This stirring tends to produce an even temperature throughout the bath.
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