Problems, and even failures, occurring from heat treatment are generally created by the stresses that are induced when phase transformations are taking place. During the transformation either thermal expansion or contraction can take place, which induces stress. Internal stresses can occur with through-hardened, carburized, or nitrided gears. Residual stresses are compressive on the surface and tensile in the core. The most serious—and also the most spectacular—defects that can occur in heat treatment are those due to the buildup of residual stresses: breakage, and distortion.
Residual stresses are controllable by material specification, the kinetic energy in relation to size and additional operations. The steels with a higher carbon content, up to eutectoid (0.83 percent) or with a coarse grain, will be more susceptible to cracking. Unpredictable distortion takes place as the result of the combination of the physical shape of the gear and the induced stresses. The stress level is influenced by the minimum flaw size and can be reduced by geometrical affects. Highly stressed gears may be affected by the internal stresses after many cycles and their contact pattern changes. Such gears may require regrinding, carburizing limits, or even a new design with different materials. The initial carburizing process does not permit a high tempering temperature, and the stresses in some instances remain in the gear. With higher hardenability there is an increased chance of quench cracking. Quench cracks are more probable in gears that are hardened after cutting. It is not unusual for them to develop with time, even in storage, or they appear when the gear reaches room temperature.
Cracks from heat treatment grow along grain boundary lines, as opposed to material defects. Hardened gears that are highly stressed are treated with a weak nitric acid solution, and a microscopic examination will reveal white layers in the area of high shear stresses. Intergranular oxidation (IGO) is the surface oxidation that occurs along the grain boundaries of carburized steel. The depth is usually proportional to the carburizing time, seldom exceeding 0.001”. Nitrided gears suffer the least distortion. Flame or induction hardened teeth can result in a transition zone at the location of the highest tooth bending stress. Induction hardened gears also leave a discontinuity at the tooth tips. The distortions are usually corrected during the finish grinding or hard cutting process, with material having been left on the gear for this purpose. When the distortion is not uniform, an excessive amount might be removed from one flank. The case is then excessively thin on that flank.
The problems that may then occur should be blamed on the heat treatment process, and not the finishing process. Poor heat treatment can also result in the physical separation of the case and core during the quenching process. Dr. Valery I. Rudnev studied the cracking of induction parts and concluded that there are six major groups of factors that lead to their unexpected cracking. The cracking is related to material, quenching, geometry, inductor design, and power cycle, among other factors such as overheating, burns, decarburization, and excessive grain growth, etc. When we quench large gears the process cools down the periphery faster than the core. Quench cracks are easily recognizable as they run in a relatively straight line, and the crack has a tendency to open. The crack will run from the surface toward the core. Sharp changes in sections, particularly keyways, promote the formation of cracks unless they have been plugged. When the quench medium is water, the possibility of cracking is
increased. Carburized gears of coarse pitch combined with excessively thin tips experience cracking at the tip.
Such a crack does not extend all the way to the ends of the teeth. Many of these type of cracks make their appearance after shot peening, and before the gear has been placed in service. Form and structure defects can also occur in any casting, molding, or sintering process.