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Involves hardening only the surface of the gear while keeping the core softer and more ductile

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Hardening is a critical process in gear manufacturing to improve surface strength, wear resistance, and fatigue life while maintaining a tough and ductile core to handle shocks and loads. Here are the common methods of gear hardening.

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Heat Treatment Process • All heat-treating processes are similar because they all involve the heating and cooling of metals. • However, there are differences in the methods used such as the heating temperatures, cooling rates and quenching media necessary to achieve the desired properties. Stages: 1—Heat the metal slowly to ensure a uniform temperature. 2—Soak (hold) the metal at a given temperature for a given time. 3—Cool the metal to room temperature.
Hardening Methods • The purpose of hardening is not only to harden steel as the name implies, but also to increase its strength. • While a hardening, heat treatment does increase the hardness and strength of the steel, it also makes it less ductile, and brittleness increases as hardness increases. • To remove some of the brittleness, you should temper the steel after hardening. • Many nonferrous metals can also be hardened and their strength increased by controlled heating and rapid cooling, but for nonferrous metals, the same process is called heat treatment rather than hardening. • For most steels, hardening consists of employing the typical first two stages of heat treatment, but the third stage is dissimilar. With hardening, you rapidly cool the metal by plunging it into oil, water, or brine. • The cooling rate required to produce hardness decreases when alloys are added to steel; this is advantageous since a slower cooling rate also lessens the danger of cracking and warping.
Surface Treatments Diffusion methods • Carburising • Carbonitriding • Nitriding • Cyaniding Selective hardening methods • Flame hardening • Induction Hardening • Laser hardening • Electron beam hardening
Through hardening • Through hardened gears are heated to a required temperature and cooled in the furnace or quenched in air, gas or liquid. Through hardening may be used before or after the gear teeth are formed. There are generally three methods of heat treating through hardened gearing. In ascending order of hardness for a particular type of steel they are; annealing, normalizing (or normalizing and tempering), and quenching and tempering. Modifications of quench hardening, such as austempering and martempering, occur infrequently for steel gearing and are, therefore, not discussed. Austempering is used, however, for through hardened (approximately300 to 480 HB) ductile cast iron gears • Annealing. • Annealing consists of heating steel or other ferrous alloys to 802-899 oC), and furnace cooling to a prescribed temperature [316 oC]. Annealing may be the final treatment or is typically a pretreatment applied to the cast or wrought gear blank in the rough. It results in low hardness and provides improved machinability and dimensional stability(minimum residual stress). • Normalizing. • Normalizing consists of heating steel or other ferrous alloys to 871-982 oC) and cooling in still or circulated air. Normalizing results in higher hardness than annealing, with hardness being a function of grade of steel and the part section thickness. However, with plain carbon steels containing up to about 0.4 percent carbon, normalizing does not increase hardness significantly more than annealing, regardless of section size. Alloy steels are normally tempered at 538-677 oC) after normalizing for uniform hardness, dimensional stability and improved machinability. • Normalizing and Annealing for Metallurgical Uniformity. • The normalizing and annealing processes are frequently used, either singularly or in combination, as a homogenizing heat treatment for alloy steels. These processes are used in wrought steel to reduce metallurgical non-uniformity such as segregated alloy microstructures and distorted crystalline microstructures from mechanical working. Cycle annealing is a term applied to a special normalize/temper process in which the parts are rapidly cooled to (427-538 oC) after normalizing at 871-954 oC), followed by a 649 oC temper with controlled cooling to 316 oC). • Quench and Temper. • The quench and temper process on ferrous alloys involves heating to form austenite at 802-871 oC), followed by rapid quenching. The rapid cooling causes the gear to become harder and stronger by formation of martensite. The gear is then tempered to a specific temperature, generally below (691 oC), to achieve the desired mechanical properties. Tempering reduces the material hardness and mechanical strength but improves the material ductility and toughness (impact resistance). Selection of the tempering temperature must be based upon the specified hardness range, material composition, and the as quenched hardness. The tempered hardness varies inversely with tempering temperature. Parts are normally air cooled from tempering temperatures. The hardness and mechanical properties achieved from the quench and temper process are higher than those achieved from the normalize or anneal process. The quench and temper process should be specified for the following conditions: When the gear application stress analysis indicates that the hardness and mechanical properties for the specified material grade can best be achieved by the quench and temper process. When the hardness and mechanical properties required for a given gear application can be achieved more economically by quench and temper of a lower alloy steel, than by normalizing or annealing. When it is necessary to develop mechanical properties (core properties) in sections of the part which will not be altered by subsequent heat treatments (for example nitriding, flame hardening,induction hardening, electron beam hardening, and laser hardening).
Through hardening • Normalizing and Annealing for Metallurgical Uniformity. • The normalizing and annealing processes are frequently used, either singularly or in combination, as a homogenizing heat treatment for alloy steels. These processes are used in wrought steel to reduce metallurgical non-uniformity such as segregated alloy microstructures (banding) and distorted crystalline microstructures from mechanical working. Cycle annealing is a term applied to a special normalize/temper process in which the parts are rapidly cooled to 427-538 oC after normalizing at 871-954 oC, followed by a 649 oC temper with controlled cooling to 316 oC. • Quench and Temper. • The quench and temper process on ferrous alloys involves heating to form austenite at 802-871 oC), followed by rapid quenching. The rapid cooling causes the gear to become harder and stronger by formation of martensite. The gear is then tempered to a specific temperature, generally below 691 oC), to achieve the desired mechanical properties. Tempering reduces the material hardness and mechanical strength but improves the material ductility and toughness (impact resistance). Selection of the tempering temperature must be based upon the specified hardness range, material composition, and the as quenched hardness. The tempered hardness varies inversely with tempering temperature. Parts are normally air cooled from tempering temperatures. The hardness and mechanical properties achieved from the quench and temper process are higher than those achieved from the normalize or anneal process. Applications. The quench and temper process should be specified for the following conditions: When the gear application stress analysis indicates that the hardness and mechanical properties for the specified material grade can best be achieved by the quench and temper process. When the hardness and mechanical properties required for a given gear application can be achieved more economically by quench and temper of a lower alloy steel, than by normalizing or annealing. When it is necessary to develop mechanical properties (core properties) in sections of the part which will not be altered by subsequent heat treatments.
Case hardening • Case hardening is an ideal heat treatment for parts which require a wear-resistant surface and a tough core, such as gears, cams, cylinder sleeves, and so forth. The most common case-hardening processes are carburizing and nitriding. During the case-hardening process, a low-carbon steel (either straight carbon steel or low-carbon alloy steel) is heated to a specific temperature in the presence of a material (solid, liquid, or gas) which decomposes and deposits more carbon into the surface of a steel. Then, when the part is cooled rapidly, the outer surface or case becomes hard, leaving the, inside of the piece soft but very tough.
Induction hardening • Induction heating is an extremely versatile heating method that can perform uniform surface hardening, localized surface hardening, through hardening, and tempering of hardened pieces. Heating is accomplished by placing a steel ferrous part in the magnetic field generated by high frequency alternating current passing through an inductor, usually a water-cooled copper coil. The depth of heating produced by induction is related to the frequency of the alternating current, power input, time, part coupling and quench delay. The higher the frequency, the thinner or more shallow the heating. • Therefore, deeper case depths and even through hardening are produced by using lower frequencies. The electrical considerations involve the phenomena of hysteresis and eddy currents. Because secondary and radiant heat are eliminated, the process is suited for in-line production. Some of the benefits of induction hardening are faster process, energy efficiency, less distortion, and small footprints. Care must be exercised when holes, slots, or other special geometric features must be induction hardened, which can concentrate eddy currents and result in overheating and cracking without special coil and part designs.
Flame hardening • Flame hardening consists of austenitizing the surface of a steel by heating with an oxyacetylene or oxyhydrogen torch and immediately quenching with water or water-based polymer. The result is a hard surface layer of martensite over a softer interior core with a ferrite-pearlite structure. There is no change in composition, and therefore, the flame-hardened steel must have adequate carbon content for the desired surface hardness. The rate of heating and the conduction of heat into the interior appear to be more important in establishing case depth than the use of a steel of high hardenability. • Flame-heating equipment may be a single torch with a specially designed head or an elaborate apparatus that automatically indexes, heats, and quenches parts. Large parts such as gears and machine toolways, with sizes or shapes that would make furnace heat treatment impractical, are easily flame hardened. With improvements in gas-mixing equipment, infrared temperature measurement and control, and burner design, flame hardening has been accepted as a reliable heat treating process that is adaptable to general or localized surface hardening for small and medium-to-high production requirements.
Nitriding • Nitriding is a surface-hardening heat treatment that introduces nitrogen into the surface of steel at a temperature range (500 to 550 C, or 930 to 1020 F), while it is in the ferritic condition. Because nitriding does not involve heating into the austenite phase with quenching to form martensite, nitride components exhibit minimum distortion and excellent dimensional control. Nitriding has the additional advantage of improving corrosion resistance in salt spray tests. The mechanism of nitriding is generally known, but the specific reactions that occur in different steels and with different nitriding media are not always known. Nitrogen has partial solubility in iron. It can form a solid solution with ferrite at nitrogen contents up to approximately 6%. At approximately 6% N, a Fig. 4 Equilibrium diagram for reaction 2CO at pressure of one atmosphere. Source: Ref 6 !C + CO2 compound called gamma prime (g0), with a Fig. 5 Equilibrium percentages of carbon monoxide and carbon dioxide required to maintain various carbon concentrations at 975 C (1790 F) in plain carbon and certain low-alloy steels. K = 89.67. Introduction to Surface Hardening of Steels / 393 composition of Fe4N, is formed. At nitrogen contents greater than 8%, the equilibrium reaction product is E compound, Fe3N. Nitrided cases are stratified. The outermost surface can be all g0, and, if this is the case, it is referred to as the white layer (it etches white in metallographic preparation). Such a surface layer is undesirable; it is very hard but is so brittle that it may spall in use. Usually it is removed; special nitriding processes are used to reduce this layer or make it less brittle. The E zone of the case is hardened by the formation of the Fe3N compound, and below this layer there is some solid-solution strengthening from the nitrogen in solid solution (Fig. 6). The Fe3N (E) formed on the outer layer is harder than Fe4N, which is more ductile. Controlling the formation of each of these compound layers is vital to application and degree of distortion.
Tuftriding • This method is similar to nitriding except that nitrogen and carbon are introduced into the surface of the work piece. This is achieved by heating the part in a molten salt hath for 2—4 hours. The result is a very hard but thinner layer, which is more resistant to shock loading conditions. Its chief advantages are: • A very hard but less brittle surface. • Resistance to shock loading conditions. • Short process times Cost-effectiveness. • The work piece is heated to 500—550 °C in a nitrogen- and carbon-rich powder bath. The part is allowed to cool slowly to avoid the formation of undesirable stresses.
Hardening defects • Defects of heat treatment which may encounter during the Heat treatment process are; • Soft Spots • Lower hardness and strength • Oxidation and Decarburization • Formation of Cracks • Overheating and burning • Distortion and wrapping • Temper Embrittlement
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Involves hardening only the surface of the gear while keeping the core softer and more ductile

  • 1. Heat Treatment Process • All heat-treating processes are similar because they all involve the heating and cooling of metals. • However, there are differences in the methods used such as the heating temperatures, cooling rates and quenching media necessary to achieve the desired properties. Stages: 1—Heat the metal slowly to ensure a uniform temperature. 2—Soak (hold) the metal at a given temperature for a given time. 3—Cool the metal to room temperature.
  • 2. Hardening Methods • The purpose of hardening is not only to harden steel as the name implies, but also to increase its strength. • While a hardening, heat treatment does increase the hardness and strength of the steel, it also makes it less ductile, and brittleness increases as hardness increases. • To remove some of the brittleness, you should temper the steel after hardening. • Many nonferrous metals can also be hardened and their strength increased by controlled heating and rapid cooling, but for nonferrous metals, the same process is called heat treatment rather than hardening. • For most steels, hardening consists of employing the typical first two stages of heat treatment, but the third stage is dissimilar. With hardening, you rapidly cool the metal by plunging it into oil, water, or brine. • The cooling rate required to produce hardness decreases when alloys are added to steel; this is advantageous since a slower cooling rate also lessens the danger of cracking and warping.
  • 3. Surface Treatments Diffusion methods • Carburising • Carbonitriding • Nitriding • Cyaniding Selective hardening methods • Flame hardening • Induction Hardening • Laser hardening • Electron beam hardening
  • 4. Through hardening • Through hardened gears are heated to a required temperature and cooled in the furnace or quenched in air, gas or liquid. Through hardening may be used before or after the gear teeth are formed. There are generally three methods of heat treating through hardened gearing. In ascending order of hardness for a particular type of steel they are; annealing, normalizing (or normalizing and tempering), and quenching and tempering. Modifications of quench hardening, such as austempering and martempering, occur infrequently for steel gearing and are, therefore, not discussed. Austempering is used, however, for through hardened (approximately300 to 480 HB) ductile cast iron gears • Annealing. • Annealing consists of heating steel or other ferrous alloys to 802-899 oC), and furnace cooling to a prescribed temperature [316 oC]. Annealing may be the final treatment or is typically a pretreatment applied to the cast or wrought gear blank in the rough. It results in low hardness and provides improved machinability and dimensional stability(minimum residual stress). • Normalizing. • Normalizing consists of heating steel or other ferrous alloys to 871-982 oC) and cooling in still or circulated air. Normalizing results in higher hardness than annealing, with hardness being a function of grade of steel and the part section thickness. However, with plain carbon steels containing up to about 0.4 percent carbon, normalizing does not increase hardness significantly more than annealing, regardless of section size. Alloy steels are normally tempered at 538-677 oC) after normalizing for uniform hardness, dimensional stability and improved machinability. • Normalizing and Annealing for Metallurgical Uniformity. • The normalizing and annealing processes are frequently used, either singularly or in combination, as a homogenizing heat treatment for alloy steels. These processes are used in wrought steel to reduce metallurgical non-uniformity such as segregated alloy microstructures and distorted crystalline microstructures from mechanical working. Cycle annealing is a term applied to a special normalize/temper process in which the parts are rapidly cooled to (427-538 oC) after normalizing at 871-954 oC), followed by a 649 oC temper with controlled cooling to 316 oC). • Quench and Temper. • The quench and temper process on ferrous alloys involves heating to form austenite at 802-871 oC), followed by rapid quenching. The rapid cooling causes the gear to become harder and stronger by formation of martensite. The gear is then tempered to a specific temperature, generally below (691 oC), to achieve the desired mechanical properties. Tempering reduces the material hardness and mechanical strength but improves the material ductility and toughness (impact resistance). Selection of the tempering temperature must be based upon the specified hardness range, material composition, and the as quenched hardness. The tempered hardness varies inversely with tempering temperature. Parts are normally air cooled from tempering temperatures. The hardness and mechanical properties achieved from the quench and temper process are higher than those achieved from the normalize or anneal process. The quench and temper process should be specified for the following conditions: When the gear application stress analysis indicates that the hardness and mechanical properties for the specified material grade can best be achieved by the quench and temper process. When the hardness and mechanical properties required for a given gear application can be achieved more economically by quench and temper of a lower alloy steel, than by normalizing or annealing. When it is necessary to develop mechanical properties (core properties) in sections of the part which will not be altered by subsequent heat treatments (for example nitriding, flame hardening,induction hardening, electron beam hardening, and laser hardening).
  • 5. Through hardening • Normalizing and Annealing for Metallurgical Uniformity. • The normalizing and annealing processes are frequently used, either singularly or in combination, as a homogenizing heat treatment for alloy steels. These processes are used in wrought steel to reduce metallurgical non-uniformity such as segregated alloy microstructures (banding) and distorted crystalline microstructures from mechanical working. Cycle annealing is a term applied to a special normalize/temper process in which the parts are rapidly cooled to 427-538 oC after normalizing at 871-954 oC, followed by a 649 oC temper with controlled cooling to 316 oC. • Quench and Temper. • The quench and temper process on ferrous alloys involves heating to form austenite at 802-871 oC), followed by rapid quenching. The rapid cooling causes the gear to become harder and stronger by formation of martensite. The gear is then tempered to a specific temperature, generally below 691 oC), to achieve the desired mechanical properties. Tempering reduces the material hardness and mechanical strength but improves the material ductility and toughness (impact resistance). Selection of the tempering temperature must be based upon the specified hardness range, material composition, and the as quenched hardness. The tempered hardness varies inversely with tempering temperature. Parts are normally air cooled from tempering temperatures. The hardness and mechanical properties achieved from the quench and temper process are higher than those achieved from the normalize or anneal process. Applications. The quench and temper process should be specified for the following conditions: When the gear application stress analysis indicates that the hardness and mechanical properties for the specified material grade can best be achieved by the quench and temper process. When the hardness and mechanical properties required for a given gear application can be achieved more economically by quench and temper of a lower alloy steel, than by normalizing or annealing. When it is necessary to develop mechanical properties (core properties) in sections of the part which will not be altered by subsequent heat treatments.
  • 6. Case hardening • Case hardening is an ideal heat treatment for parts which require a wear-resistant surface and a tough core, such as gears, cams, cylinder sleeves, and so forth. The most common case-hardening processes are carburizing and nitriding. During the case-hardening process, a low-carbon steel (either straight carbon steel or low-carbon alloy steel) is heated to a specific temperature in the presence of a material (solid, liquid, or gas) which decomposes and deposits more carbon into the surface of a steel. Then, when the part is cooled rapidly, the outer surface or case becomes hard, leaving the, inside of the piece soft but very tough.
  • 7. Induction hardening • Induction heating is an extremely versatile heating method that can perform uniform surface hardening, localized surface hardening, through hardening, and tempering of hardened pieces. Heating is accomplished by placing a steel ferrous part in the magnetic field generated by high frequency alternating current passing through an inductor, usually a water-cooled copper coil. The depth of heating produced by induction is related to the frequency of the alternating current, power input, time, part coupling and quench delay. The higher the frequency, the thinner or more shallow the heating. • Therefore, deeper case depths and even through hardening are produced by using lower frequencies. The electrical considerations involve the phenomena of hysteresis and eddy currents. Because secondary and radiant heat are eliminated, the process is suited for in-line production. Some of the benefits of induction hardening are faster process, energy efficiency, less distortion, and small footprints. Care must be exercised when holes, slots, or other special geometric features must be induction hardened, which can concentrate eddy currents and result in overheating and cracking without special coil and part designs.
  • 8. Flame hardening • Flame hardening consists of austenitizing the surface of a steel by heating with an oxyacetylene or oxyhydrogen torch and immediately quenching with water or water-based polymer. The result is a hard surface layer of martensite over a softer interior core with a ferrite-pearlite structure. There is no change in composition, and therefore, the flame-hardened steel must have adequate carbon content for the desired surface hardness. The rate of heating and the conduction of heat into the interior appear to be more important in establishing case depth than the use of a steel of high hardenability. • Flame-heating equipment may be a single torch with a specially designed head or an elaborate apparatus that automatically indexes, heats, and quenches parts. Large parts such as gears and machine toolways, with sizes or shapes that would make furnace heat treatment impractical, are easily flame hardened. With improvements in gas-mixing equipment, infrared temperature measurement and control, and burner design, flame hardening has been accepted as a reliable heat treating process that is adaptable to general or localized surface hardening for small and medium-to-high production requirements.
  • 9. Nitriding • Nitriding is a surface-hardening heat treatment that introduces nitrogen into the surface of steel at a temperature range (500 to 550 C, or 930 to 1020 F), while it is in the ferritic condition. Because nitriding does not involve heating into the austenite phase with quenching to form martensite, nitride components exhibit minimum distortion and excellent dimensional control. Nitriding has the additional advantage of improving corrosion resistance in salt spray tests. The mechanism of nitriding is generally known, but the specific reactions that occur in different steels and with different nitriding media are not always known. Nitrogen has partial solubility in iron. It can form a solid solution with ferrite at nitrogen contents up to approximately 6%. At approximately 6% N, a Fig. 4 Equilibrium diagram for reaction 2CO at pressure of one atmosphere. Source: Ref 6 !C + CO2 compound called gamma prime (g0), with a Fig. 5 Equilibrium percentages of carbon monoxide and carbon dioxide required to maintain various carbon concentrations at 975 C (1790 F) in plain carbon and certain low-alloy steels. K = 89.67. Introduction to Surface Hardening of Steels / 393 composition of Fe4N, is formed. At nitrogen contents greater than 8%, the equilibrium reaction product is E compound, Fe3N. Nitrided cases are stratified. The outermost surface can be all g0, and, if this is the case, it is referred to as the white layer (it etches white in metallographic preparation). Such a surface layer is undesirable; it is very hard but is so brittle that it may spall in use. Usually it is removed; special nitriding processes are used to reduce this layer or make it less brittle. The E zone of the case is hardened by the formation of the Fe3N compound, and below this layer there is some solid-solution strengthening from the nitrogen in solid solution (Fig. 6). The Fe3N (E) formed on the outer layer is harder than Fe4N, which is more ductile. Controlling the formation of each of these compound layers is vital to application and degree of distortion.
  • 10. Tuftriding • This method is similar to nitriding except that nitrogen and carbon are introduced into the surface of the work piece. This is achieved by heating the part in a molten salt hath for 2—4 hours. The result is a very hard but thinner layer, which is more resistant to shock loading conditions. Its chief advantages are: • A very hard but less brittle surface. • Resistance to shock loading conditions. • Short process times Cost-effectiveness. • The work piece is heated to 500—550 °C in a nitrogen- and carbon-rich powder bath. The part is allowed to cool slowly to avoid the formation of undesirable stresses.
  • 11. Hardening defects • Defects of heat treatment which may encounter during the Heat treatment process are; • Soft Spots • Lower hardness and strength • Oxidation and Decarburization • Formation of Cracks • Overheating and burning • Distortion and wrapping • Temper Embrittlement