Electric heating – Inductive heating – With power supply system
Reexamination Certificate
2002-07-29
2003-05-20
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
With power supply system
C219S663000, C324S203000
Reexamination Certificate
active
06566636
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to the field of heat treatment of metals, and more specifically to a method and system of providing real-time, closed-loop control of an induction heating machine by using a miniature magnetic sensor to measure the local changes in magnetic field close to a workpiece during induction heating.
Induction heating is a well-known process for efficiently applying energy directly to metals and other conductive materials for heat treating, melting, welding, brazing, tempering, normalizing, aging, or pre-heating prior to hot working.
Induction heating can also be used in non-metal applications, including adhesive bonding, graphitizing carbon, drying, curing, and superheating glass. In induction heating, alternating electric current is passed through an induction heating coil that is positioned closely to a workpiece. Where the lines of magnetic flux produced by the induction heating coil enter the workpiece, the alternating magnetic fields induce an alternating electric potential (e.g. voltage) in the workpiece. The alternating electric potential drives eddy currents in a thin surface layer. These eddy currents dissipate some of their energy within the surface layer by resistive Joule heating losses. The depth of resistive heating (e.g. skin depth) is inversely proportional to the square root of the product of three parameters: applied induction frequency, magnetic permeability, and electrical conductivity. The resultant temperature rise in the resistively heated surface layer is related to the specific heat, density, thermal conductivity, power level, and duration of heating. Magnetic coupling of the induction heating coil to the workpiece depends strongly on the geometrical arrangement, among other properties.
A common use of induction heating is case hardening of medium-carbon steel parts, such as gears, axles, and driveshafts. Many industrial applications require a steel part having a hardened outer surface (e.g. “case”) and an interior region of higher toughness to provide improved strength, wear resistance, fatigue life, and toughness. Other applications include induction hardening of crankshafts, valve seats, railroad rails, rolling-mill rolls, and hand tools. Induction heating rapidly heats the outer surface layer of the steel workpiece in a short period of time (e.g. 5 seconds). Above a critical transition temperature (about 760 C for 1050M steel with 0.45% C) the initial ferrite-pearlite microstructure (BCC) transforms into the austenite phase (FCC).
Upon continued heating of the part, the transformed austenite layer thickens and extends deeper from the surface. Optimum peak surface temperatures can be 870-925 C, depending on the carbon concentration, and the desired depth of hardening. For some applications, the peak surface temperature can be as high as 1200° C. Final hardening of the outer layer occurs when the heating power is shut off and the part is quenched (e.g. rapidly cooled from the outside to less than 200-400° C. in 10-20 seconds). This converts the austenitic layer into a hard, metastable martensitic phase with a Rockwell hardness of R
c
=50-60. An optional tempering step can follow the quench cycle, which can further improve the metallurgical properties.
Induction hardened steel parts are designed to have a case hardened layer with a specific desired depth. For example, a 25 mm diameter 1050 M steel automobile axle may be designed with a hardened layer from 4-5 mm thick, as defined by a Rockwell hardness of at least R
c
=50. Should the layer be too thin, the axle would wear too quickly or have insufficient strength; should the layer be too thick, the axle would be too brittle. During mass production, the measured case depth should be repeatable to within +/−0.1 mm. This requires close control of the induction heating process, as well as tight control of material properties, chemistry, workpiece alignment, etc.
Closed-loop control of the induction heating and hardening processes has been an elusive goal of the industry for many years. Existing induction hardening equipment is typically operated with open-loop process controllers, wherein an operator manually selects power and time (e.g. heating duration). Production users of this equipment monitor the process by destructively sectioning finished parts and inspecting the results; i.e., a finished part is cut apart and the case depth is directly measured radially across the cross-section by using a Rockwell hardness indentor, metallographic inspection, or chemical analysis of the carbon concentration profile. Process development for new parts is accomplished by time-consuming and expensive trial-and-error; for a given coil and part design, heating and quenching parameters are varied until destructive analysis reveals that the desired hardness profile is being produced.
These parameters are then utilized in the production run and the hardened parts are sampled and analyzed at regular intervals for quality control and assurance. If the tested part is bad, the production run from the previously tested good part is sampled to determine where the process failed. Production equipment may be taken out of service until subsequent parts test satisfactory. Since each test can take a minimum of several minutes by a trained technician, this process is quite inefficient for mass production. Unfortunately, small variations in the steel's chemistry and microstructure can produce unacceptably large variations in the measured case depths, even for nominally acceptable material specifications. The cause of these variations is not well understood.
Other sources of variability include improper part positioning (e.g. misalignment relative to the heating coil), defects in the part (e.g. cracks), and damaged or aged heating coils. Low hardness values measured on a finished part may be caused by: surface decarburization; lower carbon content than specified; inadequate austenitizing temperature; prior structure; retained austenite (mostly in high-carbon steels); and unsatisfactory quenching.
Eddy current testing is a commonly used non-destructive method in the automotive manufacturing industry for measuring case depth. Eddy current testing can measure case depths in hardened shafts over a range from 0.2 mm to 9 mm, with an accuracy of about 0.15 mm RMS error. See Automotive Application of Eddy Current Testing, in
Electromagnetic Testing
, Vol. 4, 2
nd
ed.,
Nondestructive Testing Handbook
, American Society of Nondestructive Testing, Inc., 1986, p 424-426. However, to achieve this level of accuracy requires the use of a master shaft for calibration purposes. This requires destructive measurement of the case depth in the master shaft by conventional hardness scans. All eddy current responses for the batch of test shafts are then normalized to the response for the master. A computer uses the responses at a few different frequencies to estimate the case depth using multiple linear regression fits. However, this type of eddy-current test is only performed after the part has been induction hardened (e.g. on finished parts); it is not used to provide real-time process control.
What is needed is a real-time, non-destructive, non-contact diagnostic technique that can respond quickly to the temperature changes and phase transformations in the workpiece during the induction heating process. The diagnostic should be small enough to provide sufficient spatial resolution, and robust enough to withstand the hostile environment (high temperatures, high fields, large volumes of quenching fluids, etc.). Use of an active feedback of process information measured directly from the part, coupled with closed-loop control of the heating process, would greatly improve the efficiency of induction hardening systems, while increasing accuracy and reducing part rework.
Direct measurement of the workpiece's surface temperature during induction heating could provide a useful signal for closed-loop feedback control. However, use of contact thermocouples is impractical fo
Bentley Anthony E.
Kelley John Bruce
Zutavern Fred J.
Leung Philip H.
Sandia Corporation
Watson Robert D.
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