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Gear Material Selection Guidelines

Many factors influence the selection of materials for gears, and the relative importance of each can vary. These factors include:

• Mechanical Properties
• Grade and Heat Treatment

• Cleanliness
• Dimensional Stablility
• Availability and Cost
• Hardenability and Size Effects
• Machinability and Other Manufacturing Characteristics

Mechanical Properties
It is necessary for the gear designer to know the application and design loads and to calculate the stresses before the material selection can begin.

• Hardness. The strength properties are closely related to material hardness, which is used in AGMA gear rating practice. Surface hardness is an important consideration for gear wear. Core hardness is an important consideration for bending and impact strength.
• Fatigue Strength. Contact and bending fatigue strengths are used to predict, at a given stress level, the number of cycles that gearing can be expected to endure before pitting or fracture occurs. Contact and bending fatigue strengths are influenced by a variety of factors such as hardness, microstructure, material cleanliness, surface conditions and residual stresses.
• Tensile Strength. Tensile strength predicts the stress level above which fracture occurs. It is not recommended for use in gear manufacturing specifications.
• Yield Strength. Yield strength determines the stress level above which permanent deformation occurs.
• Toughness. Toughness is determined by impact strength, tensile ductility and/or fracture toughness testing. Although not directly considered in gear rating, toughness may be important for high impact or low temperature applications or both. Toughness of steel gearing is adversely affected by a variety of factors such as:
  • Low temperature
  • Improper heat treatment or microstructure
  • High sulfur - High phosphorus and embrittling type residual elements
  • Nonmetallic inclusions
  • Large grain size
  • Absence of alloying elements such as nickel.
  • NOTE: Gear toughness is adversely affected by design or manufacturing considerations(such as notches, small fillet radii, tool marks, material defects, etc., which act as stress concentrators).
• Heat Treatment. Most wrought ferrous materials used in gearing are heat treated to meet hardness and/or mechanical property requirements. Round and flat stock can be purchased in numerous combinations of mechanical and thermal processing, such as hot rolled, cold rolled, cold drawn, stress relieved, pickled, annealed, and quenched and tempered. Gear blanks are generally given an annealing or normalizing heat treatment, which homogenizes the microstructure for machinability and mechanical property uniformity. Gear blanks can also be quenched and tempered.
• Stock Removal. All rough ferrous gear castings, forgings and bar stock have a surface layer containing decarburization, nonmetallic inclusions, seams, and other surface imperfections. This layer should be removed from critical gearing surfaces. The minimum surface stock removal varies with stock size and type of mechanical working.

Grade and Heat Treatment
The specific gear design will usually dictate the grade of material required as a function of subsequent heat treatment;

Cleanliness
Alloy steelmanufactured with electric furnace practice for barstock and forged steel gear applications is commonly vacuum degassed, inert atmosphere (argon) shielded and bottom poured to improve cleanliness and reduce objectionable gas content (hydrogen, oxygen and nitrogen). Improved cleanliness (reduced nonmetallic inclusion content) results in improved transverse ductility and impact strength, but machinability may be reduced; for example, with sulfur content less than 0.015 percent. Vacuum degassed steel may be further refined by vacuum arc remelting (VAR) or electroslag remelting (ESR) of the steel. These refining processes further reduce gas and inclusion size and content for improved fatigue strength to produce the highest quality steel for critical gearing applications. Significant increase in cost and reducedmachinability, however, must be fully evaluated with respect to the need for improved properties for other than critical gearing applications.

Dimensional Stability
The process to achieve the blueprint design may require material considerations such as: added stock, die steps, restricted hardenability, etc. to minimize distortion and possible cracking.

Cost and Availability
The specific material selection is often determined by cost and availability factors such as standard industry alloys and procurement time.
When specifying parts with small quantity requirements, standard alloys should be specified or engineering drawings should allow optional materials.

Hardenability
Hardenability of steel is the property that determines the hardness gradient produced by quenching from the austenitizing temperature. The as quenched surface hardness is dependent primarily on the carbon content of the steel part and cooling rate. The depth to which a particular hardness is achievedwith a given quenching condition is a function of the hardenability, which is largely determined by the alloy content of the steel grade.

• Determination. Hardenability is normally determined by the Jominy End Quench Test or can be predicted by the Ideal Diameter (DI) concept.
  • Jominy Test Method. A one inch (25mm) diameter bar, four inches (102 mm) in length is first normalized then uniformily heated to a standard austenitizing temperature. The bar is placed in a fixture, then quenched by spraying room temperature water against one end face.
  • Jominy Analysis. Rockwell C hardness measurements are made along the length of the bar on ground flats in one sixteenth of an inch(1.6 mm) intervals. Jominy hardenability is expressed in HRC obtained at each interval starting at the water quenched end face.
  • H-Band Steel. Jominy hardenability has been applied to standard steels. For a given composition the Jominy hardenability data falls within a predicted range. Steels purchased to predicted hardenability ranges are called H-Band steels.
  • Ideal Critical Diameter. The Ideal Critical Diameter Method (DI) is based on chemical analysis.
• Application. Hardenability is constant for a given steel composition; however, hardness will vary with the cooling rate. Therefore, the hardness obtained at any location on a part will depend on carbon content, hardenability, part size, configuration, quenchmedia, and quenching conditions. Typically a steel composition is selected with a hardenability characteristic that will yield an as quenched hardness above the specified hardness so that toughness and machinability can be attained through appropriate tempering. As the section thickness increases, the steel hardenability must be increased in order to maintain a given hardness in the part section.

Machinability
Several factors influence the machinability of materials and in turn affect the economy and feasibility of manufacturing. These factors must be considered at the design stage, particularly when high strength levels are being specified. Factors influencing machinability are:

• Material being cut, including composition, microstructure, hardness, shape, and size.
• Cutting speeds, feeds and cutting tools.
• Condition of machine tools, including rigidity, precision, power, etc.
• Characteristics of the cutting fluid used. There is abundant material published on machinability. The mechanics of the cutting operation will not be considered here. Only metallurgical factors will be discussed.

Chemical composition and microstructure of steel have major influences on machinability, since they affect properties and structures. Metallic oxides like alumina and silica form hard oxide inclusions and contribute to poormachinability. Elements such as sulfur, lead, selenium, and tellurium form soft inclusions in the steel matrix and can benefit machining. Calcium additions (in steel making) form hard, irregular inclusions and can also benefit machining. However, sulfur, lead and calcium inclusions which improve machinability can decrease mechanical properties, particularly in the transverse direction. Calcium treated steel, when used in high stress gear and shaft applications, may significantly reduce fatigue life compared to conventional steelmaking practices. Carbon content over 0.30 percent decreasesmachinability due to increased hardness.Dependent on carbon and sulfur levels, higher manganese also decreases machinability. In general, alloys which increase hardness and toughness decreasemachinability. With good machinability as a base, a fair rating would add 20 to 30 percent to the cost, and poorwould add 40 to 50 percent.