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Residual Stress Effects

Residual stresses play an important role in the manufacture and performance of gears. Residual stresses created by machining and heat treating operations are responsible for much of the distortion that occurs during manufacture.
The residual stress distribution in finished gears can determine whether or not the gears will survive in service. Residual stresses (either favorable or unfavorable) are induced mechanically, thermally, by phase transformation, or by modification of surface chemistry (such as by nitriding). Each of these, singularly and in combination (such as by carburizing), can affect the degree of in-process distortion and the residual stress state present in the finished
parts. The following sections briefly discuss the causes of each type of induced residual stress.
  • Mechanically Induced Residual Stresses.
    There are two types of mechanically induced residual stresses, machining stresses and finishing operation stresses. Machining stresses are created by the cutting of the gear shape and can be either beneficial or detrimental. Parts given a final heat treatment after finish machining may have the gross residual stresses from milling, turning, and hobbing minimized by intermediate stress relief heat treatments in order to prevent significant distortion during the final heat treatment. Machining cuts taken just prior to final heat treatment must be light enough so as not to create significant residual stresses. Grinding after final heat treatment must be performed very carefully since it can create residual tensile stresses in the surface of the gear which can adversely affect performance. Lapping, honing or careful grinding of gears after final heat treatment maintains beneficial compressive residual stresses. Finishing operations such as shot peening  and roller burnishing also impart beneficial compressive residual stresses when properly controlled. These operations are typically performed on finished gears to improve the pitting and surface bending fatigue resistance.
    Use of cubic boron nitride (CBN) grinding may have a favorable effect on the residual stresses in the finished gear. Under extreme grinding conditions, however, CBN grinding may also induce surface tempering residual tensile stresses. Other hard gear finishing methods (e.g. skiving)will need to be individually evaluated as to effect on residual stress levels.
  • Metallurgically Induced Residual Stress.
    The other types of residual stress, although quite different, can all be categorized as being metallurgically induced. Thermal, phase transformation and modification of surface chemistry stresses result from heat treatment of steel.
    1. Thermal and Phase Transformation Stresses.
      Thermal stresses result from the heating and cooling of materials. Quenching, one type of thermal stress, can also be considered a phase transformation stress. Quenching, particularly fast quenching to form martensite, generates both thermal and phase transformation stresses. For example, two types of residual stress patterns can form on quenching of a round bar. The most common type of residual stress pattern in small diameter bars is a tensile stress at the surface and a compressive stress at the center. This stress pattern results from the surface of a bar cooling faster than the center. The phase transformation to martensite creates volume expansion producing tensile stress at the surface. This in turn creates a compressive stress at the center.
      The second and opposite type of residual stress pattern occurs during quenching of large diameter bars. In this situation, the surface hardens but the center remains at an elevated temperature for some extended period of time. The thermal contraction exceeds the expansion of the transformation to martensite, setting up residual tensile stress at the center and residual compressive stress at the surface.
      These two types of stress patterns are determined by two variables, size of the bar and speed of the quench. When the sum of these two variables is large, for example large diameter bar with a fast quench, the stress pattern will be of the second type with residual tensile stress at the center and residual compressive stress at the surface. When the cooling rates of the surface and center are similar, the thermal contraction can not overcome the expansion from the martensitic formation and residual tensile stress will form at the surface, while the center will consist of residual compressive stress.
    2. Residual Stresses by Modification of Surface Chemistry.
      This type of residual stress must also be considered in conjunction with thermal residual stress because modification of surface chemistry requires heating, and heating can introduce thermal stresses, which must be taken into account. Carburizing, the most common type of surface chemistry modification, will serve as a good example of these types of residual stresses. In quenched carburized steels, the transformation temperature of austenite to martensite in the core occurs at a much higher temperature than the case, and as discussed in the previous section, the austenite to martensite transformation creates a volume expansion. Therefore, as the part is cooling, transformation begins in the core and moves outward toward the case setting up tensile stresses in the core. The expansion of the case is opposed by the previously transformed core imparting beneficial compressive stresses in the case. Compressive stresses in the case help reduce surface pitting caused by tooth contact stress above and below the pitch line. They help counteract tensile stresses caused by bending in the root.

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