Simulteneous Dual Frequency Induction Hardening of Gears

Abstract

Gear-performance characteristics (including load condition and operating environment) dictate the re-quired surface hardness, core hardness, hardness profile, residual stress distribution, grade of steel, and the prior microstructure of the steel. In recent years, gear manufacturers have gained additional knowledge about how technology can be used to produce higher quality induction hardened gears and gear-like components with less distortion. The application of this knowledge has resulted in gears that are quieter, lighter, and lower cost with an in-creased load-carrying capacity to handle higher speeds and torques, while generating a minimum amount of heat and requiring minimum or no post grinding. Steels containing 0.40 to 0.60% carbon content are commonly specified for induction hardening of gears [1-3] ; for example, AISI 1045, 1552, 4140, 4150, 4340, and 5150. In some cases, steels with high carbon content are used (e.g., 52100). De-pending on the application, tooth hardness after tem-pering typically is in the 48 to 60 HRC range. Gear Spin Hardening (Encircling Inductors) Spin hardening is the most popular approach for hardening gears having fine-and medium-size teeth. Gears are rotated during heating to ensure an even distribution of energy. Single-turn or multi-turn induc-tors that encircle the whole gear can be used [1] . When applying encircling coils, it is possible to obtain substantially different hardness patterns by varying process parameters. Figure 1 illustrates a di-versity of induction hardening patterns that were ob-tained with variations in frequency, heat time, and coil power [1] . As a rule, when it is necessary to harden only the tooth tips, a higher frequency and high power den-sity should be applied; to harden the tooth roots, use a lower frequency (Fig. 2). A high power density in combination with the relatively short heat time gen-erally results in a shallow pattern, while a low power density and extended heat time produces a deep pat-tern with wide transition zones. Quite often, to prevent problems such as pitting, spalling, tooth fatigue, and endurance, it is required to harden the contour of the gear, or to have gear-contour hardening (Fig.3). This often maximizes ben-eficial compressive stresses within the case depth and minimizes distortion of as-hardened gears. In some cases, obtaining a true contour hardened pattern can be a challenging task due to the differ-ence in current density (heat source) distribution and heat transfer conditions within a gear tooth. Two main factors must be optimized to obtain the contour hard-ness profile.