Electric heating – Inductive heating – Specific heating application
Reexamination Certificate
2000-04-12
2001-03-06
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
Specific heating application
C219S656000, C219S671000, C219S672000, C219S601000, C148S568000, C266S129000
Reexamination Certificate
active
06198083
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the heat treatment of wire, and more particularly to the heat treatment of high carbon steel spring wire by a combination of inductive heating and conventional heating.
BACKGROUND OF THE INVENTION
Wires of the production of various types of springs, including valve springs, compression springs, torsion springs and extension springs are normally heat treated to produce desired physical properties such as tensile and/or compressive strength, toughness and ductility. In typical processes the wires, which may contain 0.4-8% C, 0.0-1% CR, 0-3%Va, 0-2% Si, balance Fe, are heated to an austenitizing temperature, i.e., a temperature at which part or all of the wire is converted to austenite. This normally requires that the wire be heated to a temperature of about 1,500 to 1,800° F., depending upon the composition of the wire. The wire is then quenched in oil, water or other conventional quenching media to convert the material to martensite, then reheated and held at a temperature such as 600 to 1,200° F. to temper the wire and obtain the desired physical properties.
Furnaces for the continuous heat treatment of steel spring wires have included muffle furnaces, in which the wires pass through pipes or ducts filled with a controlled atmosphere, salt baths, silica fluid beds, and molten lead bath. Unfortunately, due to poor heat transfer characteristics, most of these conventional furnaces, with the exception of molten lead, require prolonged heating times, typically from 2 to 10 minutes, to heat wire to its austenitizing temperature.
Inductive heating has been proposed as a method for reducing the time required to heat these wires. Inductive heating at low to moderate frequencies will not heat this size range of wires to their austenitizing temperature efficiently because steel becomes non-magnetic at its Curie temperature of about 1350° F. The efficiency of conduction heating drops significantly past the wires Curie temperature and continued inductive heating requires large amounts of energy. However, wire can be heated to its austenitizing temperature more economically by heating the wire inductively to its Curie temperature and completing the heating process in a conventional furnace.
U.S. Pat. No. 4,788,394 to Vannese et al and U.S. Pat. No. 5,032,191 to Reiniche disclose processes for inductive heating of steel wire. In each, wires are passed through guides within the inductive heating chamber, with a coil surrounding all of the guides. Other proposed processes for inductive heating of wire also pass a number of wires through a common inductive heating coil. This has certain defects. The inductive heating system must be designed and adjusted for the bundle of wires as a whole, which complicates adjustment for any individual wire. If a wire is missing from the bundle, heating of the other wires can be affected adversely. For example, variations in the number or size of wire changes the conductive coupling properties of the coil, with resultant large and unpredictable variations in wire temperature.
SUMMARY OF THE INVENTION
This invention provides a continuous process for heat treating steel wires in which the wires are passed through an inductive furnace with induction coils connected in series to a common power supply. No more than one wire passes through any one of the induction coils. The wires are heated to their Curie temperature, where the wires become non-magnetic. This makes the process substantially self-regulating. If one or more wires are heated more rapidly than others, little if any heating of the more rapidly heated wires will occur after they reach their Curie temperature, and all wires will leave the inductive heating furnace at or near their Curie temperature.
The system is preferably operated, for reasons of efficiency, with a single wire passing through each induction coil. However, it will operate satisfactorily with one or more wires missing. If this happens the efficiency of the unit degrades, but sufficient excess power remains such that heating of the wires that are passing through the system will be relatively unaffected at the design mass flow rate. Again, the system is largely self-regulating, which provides significant operational advantages.
After leaving the inductive furnace the wires pass through a non-inductive furnace, such as a muffle furnace, which heats the wires to an austenitizing temperature. This combination of inductive and non-inductive heating heats the wires rapidly and efficiently, and provides highly desirable operational flexibility. Other features and advantages of this invention may be seen from the following detailed description.
REFERENCES:
patent: 3619231 (1971-11-01), Johnson
patent: 4427463 (1984-01-01), Spies
patent: 4574604 (1986-03-01), Vogel et al.
patent: 4788394 (1988-11-01), Vanneste et al.
patent: 5032191 (1991-07-01), Reiniche
American Spring Wire Corp.
Calfee Halter & Griswold LLP
Leung Philip H.
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