Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1999-07-09
2001-05-22
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S908000, C148S566000, C148S320000, C148S333000, C266S249000
Reexamination Certificate
active
06235131
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a system for heat treating coiled springs, such as steel automotive and truck suspension springs, brake springs, automotive power springs, and the like. The present invention also relates to a method and apparatus for resistance hardening coiled steel springs.
2. Discussion of the Related Art
Steel brake springs, suspension springs, and other automotive springs may be manufactured according to a “hot coil” process, an “annealed wire” process, or a “pre-tempered wire” process. In the hot coil process, straight steel bars are heated by natural gas or induction to a temperature in the range of from 1,600 to 1,900 degrees Fahrenheit (° F.). The hot bars are then coiled into the desired shape, then quenched in oil, and then tempered. The hot coil process may be used to produce straight-sided springs; it has not been used effectively to produce variable body diameter springs.
In the annealed wire process, steel springs are first cold formed and then austenitized, quenched and tempered. The austenitizing step may be performed in either a batch furnace or a continuous furnace. The steel may be heated to a temperature in the range of from 1,500 to 1,620° F. Unlike the hot coil process, the annealed wire process may be used to produce variable body diameter springs. The annealed wire process involves difficult material handling steps, however, and it may be subject to quality control problems. In particular, springs produced according to the annealed wire process may be subject to lot-to-lot inconsistency, decarburization, hardness non-uniformity, and distortion.
In the pre-tempered wire process, steel springs are cold coiled from pre-tempered wire. After coiling, the springs are stress relieved at a temperature in the range of from 700 to 800° F. In the pre-tempered wire process, the steel material is hardened before it is coiled. The pre-tempered wire process is not economical and has other disadvantages.
SUMMARY OF THE INVENTION
The disadvantages of the prior art are overcome to a great extent by the present invention. The present invention relates to a method of heat treating a coiled steel spring. According to one aspect of the invention, steel springs are cold coiled from annealed wire, then hardened by resistance heating, and then quenched.
According to another aspect of the invention, a spring is resistance heat hardened by applying electrical current through its opposite ends, and then the spring is quenched. Although the invention is described herein with reference to helical coiled springs, the invention is also applicable to spiral springs, torsion springs, and other structures.
The spring may be resistance heated to an austenitization temperature of at least 1,500° F. In a preferred embodiment, the spring is resistance heated to at least 1,600° F. The temperature employed may depend on the particular alloy composition of the spring and the microstructure and other characteristics desired for the finished product. In a preferred embodiment of the invention, the spring includes chromium and silicon, and the finished product has a ductile martensite fine grain microstructure.
An advantage of the invention is that the spring may be resistance hardened in a very short period of time. For example, the heating cycle may be completed in less than one hundred seconds. In a preferred embodiment of the invention, electrical current is applied to the spring for no more than about forty five seconds. The amount of applied electrical current may vary during the heating cycle. For example, the current may be reduced at the end of the heating cycle, after the spring achieves the desired high temperature.
According to another aspect of the invention, an axial force may be applied during the heating step to control the spring's free length. The axial force may be applied through the conductive end connectors. The connectors may be fixed at a desired spacing. Alternatively, the connectors may be moved axially by suitable actuators.
In a preferred embodiment of the invention, the quenching step is performed in an oil bath. The invention is not limited, however, to the preferred embodiment. Thus, other suitable quenching mediums, such as water, molten salt, etc., may be used if desired.
The present invention also relates to a resistance hardened steel spring. The spring may have a coiled section and opposite ends. The coiled section may be harder than the ends, especially where the ends of the spring are not subjected to as much active stress in use as the coiled section. In a preferred embodiment of the invention, the ends of the spring have hardnesses, measured on the Rockwell “C” (Rc) scale, in the range of from about 30 to 50 Rc. The coiled section may have a hardness greater than about 50 Rc.
The present invention may be used to heat harden a wide variety of springs, including springs with round cross sections, variable body diameters, and/or variable pitches.
In a preferred embodiment of the invention, a digital timer is used to control the flow of electrical current through the spring. If desired, the timer may apply variable amounts of current during each heating cycle. For example, the current may be reduced as the spring approaches or reaches the desired austenitizing temperature.
An object of the invention is to provide a manufacturing system that meets or exceeds the quality characteristics associated with the pre-tempered wire process and that is at least as economical as the annealed wire process.
Another object of the invention is to provide steel springs with improved material and mechanical characteristics, including but not limited to fine grain size and high fatigue resistance.
Another object of the invention is to provide a method of making coil springs with minimal distortion, uniform lengths, and uniform response to load characteristics.
Another object of the invention is to provide an uncomplicated heat hardening system that requires minimal set-up and processing time.
Another object of the invention is to provide a heat treatment process that can be easily controlled.
Another object of the invention is to provide a heat treatment system that has a compact construction and that occupies less factory floor space than prior art systems.
These and other features and advantages will become apparent from the following detailed description of preferred embodiments of the invention.
REFERENCES:
patent: 2261878 (1941-11-01), Hathaway
Keihle Scott C.
Orr Ronald D.
Schnettler Kenneth J.
Sebastian Daniel R.
Dickstein , Shapiro, Morin & Oshinsky, LLP
Mathew Warren Industries, Inc.
Yee Deborah
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