Induction hardening
Encyclopedia
Induction hardening is a form of heat treatment
in which a metal part is heated by induction heating
and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness 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.
induction to produce heat inside the surface layer of a work-piece. By placing a conductive material into a strong alternating magnetic field
electrical current can be made to flow in the steel thereby creating heat due to the I2R losses in the material. In magnetic materials, further heat is generated below the curie point
due to hysteresis
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 this heated layer in water, oil or a polymer
based quench the surface layer is altered to form a martensitic structure which is harder than the base metal.
. Carbon
and alloy steels with an equivalent carbon content
in the range 0.40/0.45% are most suitable for this process.
A source of high frequency electricity is used to drive a large alternating current through a coil. The passage of current through this coil generates a very intense and rapidly changing magnetic field in the space within the work coil. The workpiece to be heated is placed within this intense alternating magnetic field where eddy currents are generated within the workpiece and resistance leads to Joule heating
of the metal.
This operation is most commonly used in steel alloys. Many mechanical parts, such as shafts, gears, and springs, are subjected to surface treatments, before the delivering, 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
. Among these treatments, induction hardening is one of the most widely employed to improve component durability. It determines in the work-piece a tough core with tensile residual stresses and a hard surface layer with compressive stress, which have proved to be very effective in extending the component fatigue
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.
. Faraday proved that by winding two coils of wire around a common magnetic core it was possible to create a momentary emf
in the second winding by switching the electric current
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
s (electrical generator
s and electric motor
s, which are variants of the same thing) and in forms of electrical transformer
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
. After general acceptance of the methodology for melting steel, engineers began to explore other possibilities for the utilisation 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 utilisation of the induction hardening process and by the late 1950s many systems utilising motor generators and thermionic emission
triode
oscillators were in regular use in a vast array of industries. Modern day induction heating units utilise the latest in semiconductor technology and digital control systems to develop a range of powers from 1 kW to many megawatts.
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.
progressively and a following quench spray or ring is utilised. 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 utilise 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
encoder
and programmable logic controller
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.
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
calculations and experience. Modern techniques typically utilise finite element analysis and Computer-aided manufacturing
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.
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.
Heat treatment
Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...
in which a metal part is heated by induction heating
Induction heating
Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...
and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness 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 utilises the principle of electromagneticElectromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
induction to produce heat inside the surface layer of a work-piece. By placing a conductive material into a strong alternating magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
electrical current can be made to flow in the steel thereby creating heat due to the I2R losses in the material. In magnetic materials, further heat is generated below the curie point
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
due to hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
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
Permeability (electromagnetism)
In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...
of the material, the heat time and the diameter of the bar or material thickness. By quenching this heated layer in water, oil or a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
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 HRCRockwell scale
The Rockwell scale is a hardness scale based on the indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload. There are different scales, denoted by a single...
. Carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and alloy steels with an equivalent carbon content
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 alloyant, which is typical...
in the range 0.40/0.45% are most suitable for this process.
A source of high frequency electricity is used to drive a large alternating current through a coil. The passage of current through this coil generates a very intense and rapidly changing magnetic field in the space within the work coil. The workpiece to be heated is placed within this intense alternating magnetic field where eddy currents are generated within the workpiece and resistance leads to Joule heating
Joule heating
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature...
of the metal.
This operation is most commonly used in steel alloys. Many mechanical parts, such as shafts, gears, and springs, are subjected to surface treatments, before the delivering, 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
Residual stress
Residual stresses are stresses that remain after the original cause of the stresses has been removed. They remain along a cross section of the component, even without the external cause. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment...
. Among these treatments, induction hardening is one of the most widely employed to improve component durability. It determines in the work-piece a tough core with tensile residual stresses and a hard surface layer with compressive stress, which have proved to be very effective in extending the component fatigue
Fatigue (material)
'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
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 FaradayMichael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....
. Faraday proved that by winding two coils of wire around a common magnetic core it was possible to create a momentary emf
Electromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...
in the second winding by switching the electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
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
Faraday's law of induction
Faraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...
was born. Once discovered, these principles were employed over the next century or so in the design of dynamo
Dynamo
- Engineering :* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field- Software :...
s (electrical generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
s and electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s, which are variants of the same thing) and in forms of electrical transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
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
Induction furnace
An induction furnace is an electrical furnace in which the heat is applied by induction heating of metal. The advantage of the induction furnace is a clean, energy-efficient and well-controllable melting process compared to most other means of metal melting...
. After general acceptance of the methodology for melting steel, engineers began to explore other possibilities for the utilisation 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
Midvale Steel
Midvale Steel was a succession of steel-making corporations whose flagship plant was the Midvale Steel Works at Nicetown, Philadelphia, Pennsylvania, which operated from 1867 until 1976...
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 utilisation of the induction hardening process and by the late 1950s many systems utilising motor generators and thermionic emission
Thermionic emission
Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the binding potential, also known as work function of the metal. The charge carriers can be electrons or ions, and...
triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
oscillators were in regular use in a vast array of industries. Modern day induction heating units utilise the latest in semiconductor technology and digital control systems to develop a range of powers from 1 kW to many megawatts.
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
Ferrite (magnet)
Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...
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 coilInduction coil
An induction coil or "spark coil" is a type of disruptive discharge coil. It is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current supply...
progressively and a following quench spray or ring is utilised. 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 utilise 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
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
encoder
Encoder
An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size.-Media:...
and programmable logic controller
Programmable logic controller
A programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...
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.
Power required
Power supplies for induction hardening vary in power from a few kilowatts to hundreds of kilowatts dependent of 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
The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....
calculations and experience. Modern techniques typically utilise finite element analysis and Computer-aided manufacturing
Computer-aided manufacturing
Computer-aided manufacturing is the use of computer software to control machine tools and related machinery in the manufacturing of workpieces. This is not the only definition for CAM, but it is the most common; CAM may also refer to the use of a computer to assist in all operations of a...
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.| Case depth [mm] | Bar diameter [mm] | Frequency [kHz] |
|---|---|---|
| 0.8 to 1.5 | 5 to 25 | 200 to 400 |
| 1.5 to 3.0 | 10 to 50 | 10 to 100 |
| >50 | 3 to 10 | |
| 3.0 to 10.0 | 20 to 50 | 3 to 10 |
| 50 to 100 | 1 to 3 | |
| >100 | 1 |
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.
See also
- Case hardeningCase hardeningCase hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy...
- Induction forgingInduction forgingInduction forging refers to the use of an induction heater to pre-heat metals prior to deformation using a press or hammer. Typically metals are heated to between and to increase their malleability and aid flow in the forging die.-Process:...
- Induction heaterInduction heaterAn induction heater is a key piece of equipment used in all forms of induction heating. Typically an induction heater operates at either medium frequency or radio frequency ranges. Three main components form the basis of a modern induction heater including the power unit , the work head and the...
- Induction shrink fittingInduction shrink fittingInduction shrink fitting refers to the use of induction heater technology to pre-heat metal components between and thereby causing them to expand and allow for the insertion or removal of another component. Typically the lower temperature range is used on metals such as aluminium and higher...
External links
- Frequently Asked Questions About The Induction Hardening Process with examples of Induction Heating Applications
- The National Metals Centre offering Design, Modeling & Simulation (DMS) technologies relating to Induction Hardening processes - NAMTEC
- Induction Hardening Centre within Research & Development of New Technologies - TWI