Through Hardening Processes
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Through Hardening Processes Through Hardening Processes 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 (approximately 300 to 480 HB) ductile cast iron gears. Annealing. Annealing consists of heating steel or other ferrous alloys to 1475-1650_F (802-899_C), and furnace cooling to a prescribed temperature [generally below 600_F (316_C)]. Annealing may be the final treatment (when low hardness requirements permit) 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 1600-1800_F (871-982 _C) 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 1000-1250_F (538-677_C) 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 (banding) and distorted crystaline microstructures from mechanical working. Cycle annealing is a term applied to a special normalize/temper process in which the parts are rapidly cooled to 800-1000_F (427-538_C) after normalizing at 1600-1750_F (871-954_C), followed by a 1200_F (649_C) temper with controlled cooling to 600_F (316_C). Quench and Temper. The quench and temper process on ferrous alloys involves heating to form austenite at 1475-1600_F (802-871_C), 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 1275_F(691_C), 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 (for example nitriding, flame hardening, induction hardening, electron beam hardening, and laser hardening). 1. Processing Considerations. The major factors of the quench and temper process that influence hardness and material strength are: Material chemistry and hardenability Quench severity Section size Time at temperature The steel carbon content determines the maximum surface hardness which can be achieved, while the 2. Through Hardening Processes http://www.gearandrack.com/gear_information/gear_heat_treatment/thr... 1 of 3 18-06-2014 06:54
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Through Hardening Processes
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Through Hardening Processes
Through Hardening Processes
Through hardened gears are heated to a required temperature and cooled in the furnace or quenched in air, gas or liquid. Throughhardening may be used before or after the gear teeth are formed. There are generally three methods of heat treating throughhardened gearing. In ascending order of hardness for a particular type of steel they are; annealing, normalizing (or normalizing andtempering), and quenching and tempering. Modifications of quench hardening, such as austempering and martempering, occurinfrequently 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 1475-1650_F (802-899_C), and furnace coolingto a prescribed temperature [generally below 600_F (316_C)]. Annealing may be the final treatment (whenlow hardness requirements permit) or is typically a pretreatment applied to the cast or wrought gear blank inthe “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 1600-1800_F (871-982 _C) and cooling in stillor circulated air. Normalizing results in higher hardness than annealing, with hardness being a function ofgrade of steel and the part section thickness. However, with plain carbon steels containing up to about 0.4percent carbon, normalizing does not increase hardness significantly more than annealing, regardless ofsection size. Alloy steels are normally tempered at 1000-1250_F (538-677_C) after normalizing for uniformhardness, 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 ahomogenizing heat treatment for alloy steels. These processes are used in wrought steel to reducemetallurgical non-uniformity such as segregated alloy microstructures (banding) and distorted crystalinemicrostructures from mechanical working.Cycle annealing is a term applied to a special normalize/temper process in which the parts are rapidly cooledto 800-1000_F (427-538_C) after normalizing at 1600-1750_F (871-954_C), followed by a 1200_F (649_C)temper with controlled cooling to 600_F (316_C).Quench and Temper.The quench and temper process on ferrous alloys involves heating to form austenite at 1475-1600_F(802-871_C), followed by rapid quenching. The rapid cooling causes the gear to become harder and strongerby formation of martensite. The gear is then tempered to a specific temperature, generally below1275_F(691_C), to achieve the desired mechanical properties. Tempering reduces the material hardness andmechanical strength but improves the material ductility and toughness (impact resistance). Selection of thetempering temperature must be based upon the specified hardness range, material composition, and the asquenched hardness. The tempered hardness varies inversely with tempering temperature. Parts are normallyair cooled from tempering temperatures.The hardness and mechanical properties achieved from the quench and temper process are higher than thoseachieved 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 propertiesfor 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 beachieved more economically by quench and temper of a lower alloy steel, than by normalizing orannealing. When it is necessary to develop mechanical properties (core properties) in sections of the partwhich will not be altered by subsequent heat treatments (for example nitriding, flame hardening,induction hardening, electron beam hardening, and laser hardening).
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Processing Considerations.The major factors of the quench and temper process that influence hardness and material strength are:
Material chemistry and hardenability Quench severity Section size Time at temperature
The steel carbon content determines the maximum surface hardness which can be achieved, while the
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alloy composition determines the hardness gradient which can be achieved through the part.Tempering.Tempering lowers hardness and strength, which improves ductility and toughness or impact resistance.The tempering temperature must be carefully selected based upon the specified hardness range, thequenched hardness of the part, and the material. The optimum tempering temperature is the highesttemperature possible while maintaining the specified hardness range. Hardness after tempering variesinversely with the tempering temperature used. Parts are normally air cooled from the temperingtemperature.CAUTION: Some steels can become brittle and unsuitable for service if tempered in the temperaturerange of 800-1200_F (425-650_C). This phenomenon is called “temper brittleness” and is generallyconsidered to be caused by segregation of alloying elements or precipitation of compounds at ferriteand prior austenite grain boundaries.If the part under consideration must be tempered in this range, investigate the specific material’ssusceptibility to temper brittleness and proceed accordingly. Molybdenum content of 0.25-0.50 percenthas been shown to eliminate temper brittleness in most steels. Temper brittleness should not beconfused with the tempering embrittlement phenomenon from tempering in a lower range (500-600_F)often referred to as “500_F or A-Embrittlement.”
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Designer Specification.The designer should specify the following on the drawing.
Grade of steelQuench and temper to a hardness range. The hardness range should be a 4 HRC or 40 HB pointrange. The designer should not specify a tempering temperature range on the drawing. It is bestto specify a hardness range and allow the heat treater to select the tempering temperature toobtain the specified hardness. Specifying both tempering temperatures and hardness ranges ona drawing causesan impractical situation for the heat treater. Tempering below 900_F(482_C)should be approved by the purchaser.Any testing required.For example, hardness tests, or any non-destructive tests such as magnetic particle inspectionor dye penetrant inspection, including the frequency of testing.
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Specified Hardness.The specified hardness of through hardened gearing is generally measured on the gear tooth end faceand rim section. Historically, this has been interpreted to mean that the specified hardness must bemet at this location. Designers often interpret this to mean that minimum hardness is to be obtained atthe roots of teeth for gear rating purposes. Since depth of hardening depends upon grade of steel(hardenability), controlling section size and heat treat practice, achieving specified hardness on thesesurfaces may not necessarily insure hardness at the roots of teeth. If gear root hardness is critical to aspecific design criteria, the gear tooth root hardness should be specified. However, care should betaken to avoid needlessly increasing material costs by changing to a higher hardenability steel whereservice life has been successful.
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Maximum Controlling Section Size.The maximum controlling section size is based upon the hardenability of alloy steel for throughhardened gear blanks.
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Additional Information.For more information, consult the following:The ASM Handbook, Volume 4, Heat Treating, 8th or 9th edition.Military specification MIL-H-6875 and Mil-STD-1684.
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Stress Relief.Stress relief is a thermal cycle used to relieve residual stresses created by prior heat treatments, machining,cold working, welding, or other fabricating techniques. The ideal temperature range for full stress relieving is1100-1275_F (593-691_C). Lower temperatures are sometimes used when 1100_F (593_C) temperatureswould reduce hardness below the specified minimum. Lower temperatures with longer holding times aresometimes used.NOTE: Stress relief below1100_F(593_C) reduces the effectiveness. Stress relief below 900_F(482_C) is notrecommended.Heavy Draft, Cold Drawn, Stress Relieved Steel Bars.Heavy draft, cold drawn, stress relieved bars may be used as an alternative to quench and tempered steel.However, fatigue properties of this steel may not be equivalent to quench and tempered steel with the sametensile properties.
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