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Distortion of gearing during heat treatment is inevitable and varies with the hardening process. The part design and manufacturing process must consider movement during heat treatment. Tolerancing must consider these changes. Section size modification may be required along with added stock for grinding or machining after heat treatment.
  • Causes.
    Dimensional changes of gearing resulting from heat treatment occur principally when steel is quenched. These changes occur in both quenched and tempered and surface hardened gears. Distortion is due to mechanical and thermal stresses
    and phase transformation. Process variables and design considerations have a significant effect upon the amount of distortion. High induced stress can result in quench cracking. Thermal processes such as annealing, normalizing, and diffusion controlled surface hardening processes such as nitriding, which do not require liquid quenching, result in less distortions than processes that require liquid quenching.
  • Quenching and Tempering.
    Quenched and tempered gearing changes size and distorts due to mechanical and thermal stresses and microstructural transformations. Quenching the structure to martensite prior to tempering results in steel growing in size. Tempering of the hardened structure reduces the volume, but the combined effects of quenching and tempering still result in a volume and size increase.
    Distortion of quenched and tempered gearing occurs generally as follows:
    1. Gears
      • Outside and bore diameters grow larger and go out of round.
      • Side faces become warped, and exhibit runout.
    2. Pinions. Pinions become bowed, with the amount of bowing increasing with higher length/diameter ratios and smaller journal diameters; amount of bowing or radial runout is often confined to journal diameters and shaft extensions for integral shaft pinions.
      Normally, rough gear blanks (forging, bars tock, or casting) have sufficient stock provided so distortion can be accommodated by machining. High L/D ratio pinions may require straightening and a thermal stress relief prior to finish machining. In some exceptional instances, straightening, thermal stress relief, rough machining, and a second stress relief prior to finish machining may all be necessary to keep the pinion dimensionally stable during finish machining.
      Sequence of manufacture is dependent upon design considerations and the temperature used for stress relief. Stress relief temperature is dependent upon specified hardness and temper resistance of the steel.
      Modified methods of quench hardening, such as austempering of ductile iron, reduces distortion and forms a modified hardened structure at higher quenchant temperatures than those conventionally used.
  • Surface Hardened Gearing.
    Distortion must be minimized, controlled and made predictable to minimize costly stock removal (lapping, skiving, or grinding), when tooth accuracy requirements dictate. Selective surface hardening of gear teeth by flame and induction hardening results essentially in only distortion of the teeth because only the teeth are heated and quenched. Amount of distortion increases with case pattern depth and increases as more of the tooth cross section is hardened, compared to profile hardened tooth patterns. Distortion is not limited to gear teeth, however, when the entire gear is heated and quenched as with carburizing.
    1. Carburized Gearing.
      Distortion of carburized gearing makes it one of the least repeatable of surface hardened processes. Lack of repeatability is due to the greater number of variables which affect distortion. Close control is, therefore, required. Distortion results from microstructural transformation, and residual stress (from thermal shock, uneven cooling, etc.) considerations. Transformation in the case results in growth which sets up residual surface compressive stress. This stress is balanced by corresponding residual tensile stress beneath the case. Principal variables affecting the amount of growth, distortion, and residual stress include:
      • Geometry.
      • Hardenability (carbon and alloy content) of the base material. Higher hardenability increases growth and distortion.
      • Fixturing techniques in the furnace and during quenching.
      • Carbon potential of the carburizing atmosphere.
      • Carburizing temperature and temperature prior to quenching.
      • Time between quench and temper for richer alloys.
      • Quenchant type, temperature and amount of agitation.
      • Resultant metallurgical characteristics of the case, such as carbon content, case depth, amount of retained austenite, carbides, etc.
      NOTE: Direct quenching generally results in less distortion than slow cooled, reheated and quenched gears, providing gears are properly cooled from the carburizing temperature to the quench temperature before hardening.
      Once a component is designed to minimize distortion, processing techniques should be optimized to make distortion consistent. At times, redesign of components may be required to reduce distortion.
      Stock removal by grinding after carburize hardening should be limited to approximately 0.007 inch (0.18mm) per tooth surface or 20 percent of the case depth, which ever is less. Exception may be made for coarser pitch gearing with cases 0.080 inches (2 mm) or greater. Surfaces other than the tooth flanks and roots may tolerate greater stock removal.
      General design considerations of carburized gearing related to distortion include the following:
      • Larger teeth (lower DP) distort more.
      • Rim thickness should be the same at both end faces.
      • Radial web support section under the rim should be centrally located. Web support section thickness under the rim is recommended to be not less than 40-50 percent of the face width for precision gears. Near solid “pancake” gear blanks, designed with moderate recess on both sides of the web section, distort less. The recess is provided to enable clean-up grinding of the rim and hub end faces after hardening.
      • Holes in the web section close to the rim, to reduce the weight or provide holes for lifting, may cause collapsing of the rim section over the holes.
      • High length/diameter ratio pinions distort more. Journals may be required to be masked in order to prevent carburizing and then be finish machined after hardening with sufficient stock for clean-up. Masking can also be used for ease of straightening.
      • Cantilever pinions, with teeth on the end of the shaft, and “blind ended” teeth on pinions, where the adjacent diameter is larger than the root diameter, present problems from both distortion and finishing standpoints.
      Distortion of carburized gearing also exhibits the following typical characteristics :
      • Reduction in tooth helix angle (“helix unwind”), which often requires an increased helix angle to be machined into the element prior to carburizing (more prevalent in pinions). Teeth on larger diameter, smaller face width gears may exhibit “helix wind-up” after hardening.
      • End growth on gear teeth at both ends of the face due to increased case depth (carburizing from two directions, 90 degrees apart, followed by improved quench action for the same reason) may appear as reverse tooth crowning on narrow face gearing.
        Teeth are often crown cut prior to hardening to compensate for reverse crown or are chamfered at the ends of teeth. Teeth may also be both crown cut and chamfered.
      • Eccentricity (radial run-out) of gears and their bores is dependent upon how they are fixtured in the furnace.
      • Taper across the face (tapered teeth), bore taper and “hour-glassing” of the gear bore can occur due to non-uniform growth of teeth across the face and non-uniform shrinking of the bores.
      • Bowing of the integral shaft pinions. Integral shaft pinions should, whenever possible, be hung or fixtured in the vertical position (axes vertical) to minimize bowing.
        Gears may be fixtured vertically through the bores or web holes on a support rod (axes horizontal), or fixtured horizontally (individually or stacked) to minimize distortion, depending on size and face width. Larger ring gears are positioned horizontally with sufficient stock for clean-up of the teeth. Bores and web sections can be masked to prevent carburizing, and enable subsequent machining. Thin section gears, such as bevel ring gears, may be press quenched to minimize distortion.
    2. Flame and Induction Hardened Gearing.
      Flame and induction hardened gearing generally distort less than carburized gearing because only the teeth are heated and subsequently quenched. Contour induction hardening of tooth profiles produce less distortion and growth than spin hardening methods.
      During both spin flame and spin induction hardening, the entire tooth cross section is often hardened to the specified depth below the roots of teeth.
      For high bending strength applications, it is not desirable to have the hardening pattern terminate in the roots of the teeth because of residual tensile stress considerations. Distortion increases as a greater cross-section of a tooth is hardened. Spin flame and spin induction hardening generally produce the following distortion characteristics:
      • Helical unwinding of the gear teeth, as with carburized pinions.
      • Increased growth of the teeth (greater than for carburized gearing) because the entire tooth cross section may be hardened in finer pitch gearing.
      • Crowning or reverse crowning of the teeth across the face dependent upon the heat pattern.
        Crowning is more desirable from a tooth loading standpoint.
      • Taper of teeth due to varied heat pattern and case depth across the face.
        Distortion of the teeth from spin induction hardening is often considered more repeatable than with spin flame hardening, because of fewer human error factors involved during machine and inductor set-ups with induction hardening. Spin flame hardening involves more manual set-up factors, which include positioning of the flame, gas flows, etc. However, spin flame hardening can be engineered with special flame heads and fixtures for required control.
        CAUTION: Deep spin hardening of gear teeth may cause excessive tooth growth and may affect bore size.
    3. Nitrided Gearing.
      Nitriding of gearing results in less distortion, compared to carburize, flame, and induction hardening. Prior quench and temper heat treatment, which results in distortion, is done before machining and nitriding. Parts are also not heated above the transformation temperature or previous tempering temperature of the steel during nitriding, and are not quenched, as occurs during carburizing, flame or induction hardening. Therefore, nitrided gear teeth are not generally required to be ground or lapped after hardening to meet dimensional tolerance requirements. Bearing diameters of shaft extensions are often ground after nitriding with only minimum stock provided. Surfaces can also be masked for subsequent machining.
      When close tolerances are required, gearing can be rough machined and stress relieved at 50_F(28_C) below the prior tempering temperature to relieve rough machining residual stress prior to finish machining and nitriding.
      During nitriding, outer surfaces grow approximately 0.0005-0.001 inch (0.013-0.025 mm). Bores size may shrink up to 0.0015 inch (0.04 mm) depending upon size.

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