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.
1. 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).
2. 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 alloy composition determines the hardness gradient which can be achieved through the part.
3. 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, the quenched hardness of the part, and the material. The optimum tempering temperature is the highest temperature possible while maintaining the specified hardness range. Hardness after tempering varies inversely with the tempering temperature used. Parts are normally air cooled from the tempering temperature.
  CAUTION: Some steels can become brittle and unsuitable for service if tempered in the temperature range of 800-1200_F (425-650_C). This phenomenon is called “temper brittleness” and is generally considered to be caused by segregation of alloying elements or precipitation of compounds at ferrite and prior austenite grain boundaries.
  If the part under consideration must be tempered in this range, investigate the specific material’s susceptibility to temper brittleness and proceed accordingly. Molybdenum content of 0.25-0.50 percent has been shown to eliminate temper brittleness in most steels. Temper brittleness should not be confused with the tempering embrittlement phenomenon from tempering in a lower range (500-600_F) often referred to as “500_F or A-Embrittlement.”
4. Designer Specification.
  The designer should specify the following on the drawing.
  - Grade of steel
  - Quench and temper to a hardness range. The hardness range should be a 4 HRC or 40 HB point range. The designer should not specify a tempering temperature range on the drawing. It is best to specify a hardness range and allow the heat treater to select the tempering temperature to obtain the specified hardness. Specifying both tempering temperatures and hardness ranges on a 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 inspection or dye penetrant inspection, including the frequency of testing.
5. Specified Hardness.
  The specified hardness of through hardened gearing is generally measured on the gear tooth end face and rim section. Historically, this has been interpreted to mean that the specified hardness must be met at this location. Designers often interpret this to mean that minimum hardness is to be obtained at the 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 these surfaces may not necessarily insure hardness at the roots of teeth. If gear root hardness is critical to a specific design criteria, the gear tooth root hardness should be specified. However, care should be taken to avoid needlessly increasing material costs by changing to a higher hardenability steel where service life has been successful.
6. Maximum Controlling Section Size.
  The maximum controlling section size is based upon the hardenability of alloy steel for through hardened gear blanks.
7. 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.
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 is 1100-1275_F (593-691_C). Lower temperatures are sometimes used when 1100_F (593_C) temperatures would reduce hardness below the specified minimum. Lower temperatures with longer holding times are sometimes used.
NOTE: Stress relief below1100_F(593_C) reduces the effectiveness. Stress relief below 900_F(482_C) is not recommended.
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 same tensile properties.