Induction hardening is a type of
surface hardening in which a metal part is
induction-heated and then
quenched. The quenched metal undergoes a
martensitic transformation
A diffusionless transformation, commonly known as displacive transformation, denotes solid-state alterations in crystal structures that do not hinge on the diffusion of atoms across extensive distances. Rather, these transformations manifest a ...
, increasing the
hardness
In materials science, hardness (antonym: softness) is a measure of the resistance to plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...
and brittleness of the part. Induction hardening is used to selectively harden areas of a part or assembly without affecting the properties of the part as a whole.
Process
Induction heating is a non contact heating process which uses the principle of
electromagnetic
In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...
induction to produce heat inside the surface layer of a work-piece. By placing a
conductive
In physics and electrical engineering, a conductor is an object or type of material that allows the flow of Electric charge, charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. The flow ...
material into a strong alternating
magnetic field
A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
, electric current can be made to flow in the material thereby creating heat due to the I
2R losses in the material. In magnetic materials, further heat is generated below the
curie point due to
hysteresis
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
losses. The current generated flows predominantly in the surface layer, the depth of this layer being dictated by the frequency of the alternating field, the surface power density, the
permeability of the material, the heat time and the diameter of the bar or material thickness. By
quenching
In materials science, quenching is the rapid cooling of a workpiece in water, gas, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, suc ...
this heated layer in water, oil, or a
polymer
A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
based quench, the surface layer is altered to form a
martensitic structure which is harder than the base metal.
Definition
A widely used process for the
surface hardening of steel. The components are heated by means of an alternating magnetic field to a temperature within or above the transformation range followed by immediate quenching. The core of the component remains unaffected by the treatment and its physical properties are those of the bar from which it was machined, whilst the hardness of the case can be within the range 37/58
HRC.
Carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
and alloy steels with an
equivalent carbon content
The equivalent carbon content concept is used on ferrous materials, typically steel and cast iron, to determine various properties of the alloy when more than just carbon is used as an wikt:alloyant, alloyant, which is typical. The idea is to conve ...
in the range 0.40/0.45% are most suitable for this process.
A large alternating current is driven through a coil, generating a very intense and rapidly changing magnetic field in the space within. The workpiece to be heated is placed within this alternating magnetic field where eddy currents are generated within the workpiece and resistance leads to
Joule heating
Joule heating (also known as resistive heating, resistance heating, or Ohmic heating) is the process by which the passage of an electric current through a conductor (material), conductor produces heat.
Joule's first law (also just Joule's law), ...
of the metal.
Many mechanical parts, such as shafts, gears, and springs, are subjected to surface treatments after machining in order to improve wear behavior. The effectiveness of these treatments depends both on surface materials properties modification and on the introduction of
residual stress
In materials science and solid mechanics, residual stresses are stresses that remain in a solid material after the original cause of the stresses has been removed. Residual stress may be desirable or undesirable. For example, laser peening im ...
. Among these treatments, induction hardening is one of the most widely employed to improve component
durability
Durability is the ability of a physical product to remain functional, without requiring excessive maintenance or repair, when faced with the challenges of normal operation over its design lifetime. There are several measures of durability in us ...
. It determines in the work-piece a tough core with tensile residual stresses and a hard surface layer with
compressive stress
Compressive stresses are generated in objects when they are subjected to forces that push inward, causing the material to shorten or compress. These stresses occur when an object is squeezed or pressed from opposite directions. In everyday life, ...
, which have proved to be very effective in extending the component
fatigue
Fatigue is a state of tiredness (which is not sleepiness), exhaustion or loss of energy. It is a signs and symptoms, symptom of any of various diseases; it is not a disease in itself.
Fatigue (in the medical sense) is sometimes associated wit ...
life and
wear resistance.
Induction surface hardened low alloyed medium carbon steels are widely used for critical automotive and machine applications which require high wear resistance. Wear resistance behavior of induction hardened parts depends on hardening depth and the magnitude and distribution of residual compressive stress in the surface layer.
History
The basis of all induction heating systems was discovered in 1831 by
Michael Faraday
Michael Faraday (; 22 September 1791 – 25 August 1867) was an English chemist and physicist who contributed to the study of electrochemistry and electromagnetism. His main discoveries include the principles underlying electromagnetic inducti ...
. Faraday proved that by winding two coils of wire around a common magnetic core it was possible to create a momentary
electromotive force
In electromagnetism and electronics, electromotive force (also electromotance, abbreviated emf, denoted \mathcal) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical ''transducer ...
in the second winding by switching the
electric current
An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...
in the first winding on and off. He further observed that if the current was kept constant, no EMF was induced in the second winding and that this current flowed in opposite directions subject to whether the current was increasing or decreasing in the circuit.
Faraday concluded that an electric current can be produced by a changing magnetic field. As there was no physical connection between the primary and secondary windings, the emf in the secondary coil was said to be ''induced'' and so
Faraday's law of induction was born. Once discovered, these principles were employed over the next century or so in the design of
dynamo
"Dynamo Electric Machine" (end view, partly section, )
A dynamo is an electrical generator that creates direct current using a commutator. Dynamos employed electromagnets for self-starting by using residual magnetic field left in the iron cores ...
s (
electrical generator
In electricity generation, a generator, also called an ''electric generator'', ''electrical generator'', and ''electromagnetic generator'' is an electromechanical device that converts mechanical energy to electrical energy for use in an extern ...
s and
electric motor
An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a electromagnetic coil, wire winding to gene ...
s, which are variants of the same thing) and in forms of electrical
transformer
In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...
s. In these applications, any heat generated in either the electrical or magnetic circuits was felt to be undesirable. Engineers went to great lengths and used
laminated cores and other methods to minimise the effects.
Early last century the principles were explored as a means to melt steel, and the motor generator was developed to provide the power required for the
induction furnace
An induction furnace is an electrical furnace in which the heat is applied by induction heating of metal.
Induction furnace capacities range from less than one kilogram to one hundred tons, and are used to melt iron and steel, copper, aluminu ...
. After general acceptance of the methodology for melting steel, engineers began to explore other possibilities for the use of the process. It was already understood that the depth of current penetration in steel was a function of its magnetic permeability, resistivity and the frequency of the applied field. Engineers at
Midvale Steel and The Ohio Crankshaft Company drew on this knowledge to develop the first surface hardening induction heating systems using motor generators.
The need for rapid easily automated systems led to massive advances in the understanding and use of the induction hardening process and by the late 1950s many systems using motor generators and
thermionic emission
Thermionic emission is the liberation of charged particles from a hot electrode whose thermal energy gives some particles enough kinetic energy to escape the material's surface. The particles, sometimes called ''thermions'' in early literature, a ...
triode
A triode is an electronic amplifier, amplifying vacuum tube (or ''thermionic valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated Electrical filament, filament or cathode, a control grid, grid ...
oscillators were in regular use in a vast array of industries. Modern day induction heating units use the latest in semiconductor technology and digital control systems to develop a range of powers from 1 kW to many
megawatts
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named in honor o ...
.
Principal methods
Single shot hardening
In single shot systems the component is held statically or rotated in the coil and the whole area to be treated is heated simultaneously for a pre-set time followed by either a flood quench or a drop quench system. Single shot is often used in cases where no other method will achieve the desired result for example for flat face hardening of hammers, edge hardening complex shaped tools or the production of small gears.
In the case of shaft hardening a further advantage of the single shot methodology is the production time compared with progressive traverse hardening methods. In addition the ability to use coils which can create longitudinal current flow in the component rather than diametric flow can be an advantage with certain complex geometry.
There are disadvantages with the single shot approach. The coil design can be an extremely complex and involved process. Often the use of
ferrite or laminated loading materials is required to influence the magnetic field concentrations in given areas thereby to refine the heat pattern produced. Another drawback is that much more power is required due to the increased surface area being heated compared with a traverse approach.
Traverse hardening
In traverse hardening systems the work piece is passed through the
induction coil progressively and a following quench spray or ring is used. Traverse hardening is used extensively in the production of shaft type components such as axle shafts, excavator bucket pins, steering components, power tool shafts and drive shafts. The component is fed through a ring type inductor which normally features a single turn. The width of the turn is dictated by the traverse speed, the available power and frequency of the generator. This creates a moving band of heat which when quenched creates the hardened surface layer. The quench ring can be either integral a following arrangement or a combination of both subject to the requirements of the application. By varying speed and power it is possible to create a shaft which is hardened along its whole length or just in specific areas and also to harden shafts with steps in diameter or splines. It is normal when hardening round shafts to rotate the part during the process to ensure any variations due to
concentricity of the coil and the component are removed.
Traverse methods also feature in the production of edge components, such as paper knives, leather knives, lawnmower bottom blades, and hacksaw blades. These types of application normally use a hairpin coil or a transverse flux coil which sits over the edge of the component. The component is progressed through the coil and a following spray quench consisting of nozzles or drilled blocks.
Many methods are used to provide the progressive movement through the coil and both vertical and horizontal systems are used. These normally employ a
digital
Digital usually refers to something using discrete digits, often binary digits.
Businesses
*Digital bank, a form of financial institution
*Digital Equipment Corporation (DEC) or Digital, a computer company
*Digital Research (DR or DRI), a software ...
encoder and
programmable logic controller
A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity that ...
for the positional control, switching, monitoring, and setting. In all cases the speed of traverse needs to be closely controlled and consistent as variation in speed will have an effect on the depth of hardness and the hardness value achieved.
Equipment
Power required
Power supplies for induction hardening vary in power from a few kilowatts to hundreds of kilowatts depending on the size of the component to be heated and the production method employed i.e. single shot hardening, traverse hardening or submerged hardening.
In order to select the correct power supply it is first necessary to calculate the surface area of the component to be heated. Once this has been established then a variety of methods can be used to calculate the power density required, heat time and generator operating frequency. Traditionally this was done using a series of graphs, complex
empirical
Empirical evidence is evidence obtained through sense experience or experimental procedure. It is of central importance to the sciences and plays a role in various other fields, like epistemology and law.
There is no general agreement on how t ...
calculations and experience. Modern techniques typically use
finite element analysis
Finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical models, mathematical modeling. Typical problem areas of interest include the traditional fields of structural ...
and
computer-aided manufacturing
Computer-aided manufacturing (CAM) also known as computer-aided modeling or computer-aided machining is the use of software to control machine tools in the manufacturing of work pieces. This is not the only definition for CAM, but it is the most ...
techniques, however as with all such methods a thorough working knowledge of the induction heating process is still required.
For single shot applications the total area to be heated needs to be calculated. In the case of traverse hardening the circumference of the component is multiplied by the face width of the coil. Care must be exercised when selecting a coil face width that it is practical to construct the coil of the chosen width and that it will live at the power required for the application.
Frequency
Induction heating systems for hardening are available in a variety of different operating frequencies typically from 1 kHz to 400 kHz. Higher and lower frequencies are available but typically these will be used for specialist applications. The relationship between operating frequency and current penetration depth and therefore hardness depth is inversely proportional. i.e. the lower the frequency the deeper the case.
The above table is purely illustrative, good results can be obtained outside these ranges by balancing power densities, frequency and other practical considerations including cost which may influence the final selection, heat time and coil width. As well as the power density and frequency, the time the material is heated for will influence the depth to which the heat will flow by conduction. The time in the coil can be influenced by the traverse speed and the coil width, however this will also have an effect on the overall power requirement or the equipment throughput.
It can be seen from the above table that the selection of the correct equipment for any application can be extremely complex as more than one combination of power, frequency and speed can be used for a given result. However in practice many selections are immediately obvious based on previous experience and practicality.
Advantages
* Fast process, no holding time is required, hence more production rate
* No scaling or decarburizing
* More case depth, up to 8 mm
* Selective hardening
* High wear and fatigue resistance
Applications
The process is applicable for electrically conductive magnetic materials such as steel.
Long work pieces such as axles can be processed.
See also
*
Case hardening
*
Induction forging
*
Induction heater
*
Induction shrink fitting
References
Notes
Bibliography
*.
*.
*.
External links
Frequently Asked Questions About The Induction Hardening Process with examples of Induction Heating ApplicationsThe National Metals Centre offering Design, Modeling & Simulation (DMS) technologies relating to Induction Hardening processes - NAMTEC
{{Iron and steel production
Metal heat treatments
it:Tempra#Tempra ad induzione