FURNACES & OVENS
HEAT TREATING FURNACES written by: John Bouley of Furnace Brokers Inc.
What is the difference between furnaces and ovens? The simple answer is an oven is sheet metal construction with insulation between its inner and outer walls and is usually operated below 1300 F. A furnace is heavy duty steel construction with the inner wall being refractory (brick or lightweight fiber) and is operated above 1000 F. If you backed a forklift into an oven, you would probably damage it. Probably not so with a furnace. Every piece of metal you touch has been through at least one and in most cases many different furnaces. From a melting furnace in the mill that will cast a slab of metal, to a forging furnace prior to hammering out a product, to a harden-quench-temper line. Eyeglass frames need to go through an annealing furnace so you can bend the metal without breaking the frame. Nitriding furnaces make wear surfaces on steel tougher. Hardening furnaces make metal hard but usually build up stresses when quenched so the metal would then need to go through a temper furnace (sometimes called a stress relieving or draw furnace) to relieve the built up stress in the metal. Each furnace type is constructed differently. Some may be capable of doing more than one process. Vacuum furnaces are the most expensive and technical. The interior of a vacuum furnace can be constructed of graphite or exotic (expensive) metal because of the high temperature (usually 2000 F and above). The operating temperature will determine the material used in the internal or “hot zone.” Vacuum furnaces can do many types of heat treating. In general, the way they work is the chamber is pumped down by external vacuum pumps and the heating elements are energized in the vacuum atmosphere. At the completion of the heat cycle, some vacuum furnaces can be backfilled with inert atmosphere, such as nitrogen, argon, etc. A fan circulates the heated atmosphere through heat exchangers and the workload is quickly cooled. Furnaces that are rated as “positive pressure quench” cool quicker. The early heat treaters were blacksmiths. What they were doing was forging (heating the metal and with force making it change shape) and then hardening the metal by heating and quenching it. When buying a furnace, the age is not the most important factor. Many furnaces will last 40 years or more with proper maintenance. Temperature controls can be easily updated. Asbestos has not been used in furnaces since 1976. If you are buying a furnace that is older than that but has been completely rebricked, chances are asbestos will not be present. About the Author:
John Bouley jbouley@aol.com and his partner Clarence Sherman founded Furnace Brokers Inc. in 1983. Together they have over 75 years combined experience in the used equipment business, specializing in heat treat furnaces and industrial baking ovens. Phone: 860-875-3712 12
March 2015
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