The choice of material must be made only after careful consideration of the performance demanded by the application. For example, gears under load (Fig. 1) are subject to gradient stresses both on the active flank and at the root fillet. Proper material selection as well as heat treatment will produce strength gradients that are adequate to withstand these stresses and provide an acceptable margin of safety.
Material choice must be a balance between overall cost and required service life. The Model of Gear Engineering (Fig. 2) tells us that cost is the most important consideration among purchasers of raw material followed by material properties. From an engineering perspective, however, key design considerations require an analysis of the type of applied load, whether gradual or instantaneous, and the desired mechanical properties, such as bending fatigue strength or wear resistance. The required mechanical properties are the critical consideration and will define core strength and heat-treating requirements. Manufacturing economics plays an important role as well, and once again, cost manifested in reducing the number of manufacturing operations is paramount.
Proper material hardenability selection is critical to the success of any product since this affects how the material can be quenched (Fig. 3). If the material hardenability is too high, the material will be costly, susceptible to quench cracking and produce high core hardness. For a given gear-tooth pitch size, for example, the result will be more distortion and higher mid-tooth core hardness than is necessary or desirable. If the material hardenability is too low, the material will exhibit low hardness with non-martensitic transformation products (NMTP) present in the microstructure, often exhibit uneven hardness and be prone to distortion and unpredictable size changes. For gears, the core hardness needs to be in a specific range to support the case but not too high to cause cracking at the case/core interface. Many manufacturers find that the use of tighter hardenability (RH-band) steels ensures that the proper mid-tooth core hardness is consistently achieved.
H-band and RH-band steels incorporate slightly different carbon and possibly other chemistry ranges (Table 1) and hardenability (Table 2); generally have a broader chemistry range than standard alloy steels (to allow steel producers to design the optimum alloy combination); and have a more restricted hardenability range at each Jominy distance.
In general, H-band steels offer a wide range of mechanical properties that depend on the development of tempered martensite after quenching and tempering. RH-band steels will exhibit a hardness range not greater than 5 HRC at the initial position (on the end-quench hardenability bar) and not greater than 65% of the hardness range for standard H-band steels in the “inflection” region. Generally, the RH-band follows the middle of the range corresponding to standard H-band.
Finally, the center of a 3.375-inch (85-mm) round of 4340H material can be austenitized and oil quenched to 80% martensite (46HRC). By contrast, when quenching to 95% martensite (55HRC), the bar size drops to a 2-inch (50-mm) round (Fig. 6).
Final ThoughtsWith the vast array of domestic and foreign steels available to choose from, with standard and non-standard chemistry, the challenge of selecting the right steel for a given application seems daunting. The use of H-band steels simplifies this task because direct comparisons can be made between the various steels to quantify their response to heat treatment. Different steels are often grouped by their hardenability requirements, allowing greater selectivity within a given design and making the job of the heat treater easier.
RH-band steels provide even more confidence that, for example, core hardness within a specific range on a gear tooth can be achieved repeatedly, whereas an H-band steel might harden to the high end in one load and to the low end of the range in another load. IH