Metallurgical Quality for Heat-Treated Gears
Gears perform the critical functions of transmitting power or carrying motion in mechanical assemblies. To perform these functions effectively, gears need to have certain properties that enable them to meet specified quality requirements.
Gear manufacturers and design engineers often designate “gear quality” based on the so-called AGMA gear-quality number. While this number provides an indication of the geometric accuracy of the gear teeth, it does not provide information on the metallurgical soundness of the gear. Therefore, the ANSI/AGMA 200 A88 Gear Classification and Inspection Handbook also provide specifications for the type of material and the heat-treatment process to be used in gear manufacture. The heat-treated gear in this study was evaluated based on its microstructure and its hardness values from the hardened surface to the softer core.
Material and Heat-Treatment Process Selection
A good starting point in gear manufacture is the selection of an appropriate gear material: nonferrous materials or plastics for motion-carrying gears and steels or cast irons for power-transmitting gears. This is followed by the careful selection of a suitable heat treatment and process conditions that will impart the right microstructure that will, in turn, enable the gear to achieve certain properties that assure its satisfactory performance.
Heat treatment is a critical component of gear manufacturing, accounting for about 30% of the manufacturing costs. Different heat-treatment processes are used in gear manufacturing depending on intended end use. For example, while annealing and normalizing are used to soften the gear, others such as carburizing, nitriding, carbonitriding, etc. are used to harden the gear.
Any of these processes can be tailored to achieve desired end results by modifying process parameters such as furnace temperatures and atmospheres, cycle times, quench media, tempering cycles, etc. In general, heat treatment of power-transmission gears seeks to impart hard, wear-resistant surfaces while maintaining a relatively tough and ductile interior.
Thus, metallurgical properties such as martensitic structure and physical properties such as surface hardness, case depth and core hardness can vary greatly depending on the type of heat-treatment process used and how well the process parameters are controlled. The gear examined in this study was a helical gear that had been surface-hardened by carburizing.
Evaluation of Metallurgical Quality
To verify that the gears have been appropriately heat treated, many companies perform metallography. Metallography includes sample preparation and evaluation of the true structure. The true structure gives an accurate understanding of the material’s microstructure and thus mechanical properties and suitability for the intended application.
In almost every type of examination, a small sample must be sectioned from the larger part. Due to the gear’s complex geometry, multiple clamping tools were utilized to make the series of cuts (Fig. 1). The gear was isolated from the assembly and then cut into pie-shaped sections. The 30-degree wedges were sectioned further to fit inside a 2-inch-diameter mount (Fig. 2).
The mounting resin used was chosen for its wear resistance. Ideally, the removal rate for the sample material and mounting resin should be equivalent to ensure a perfectly flat sample with maximum edge retention. After mounting, the hardened gears underwent plane grinding, fine grinding and polishing. Between each preparation step, the samples were thoroughly cleaned using an ultrasonic cleaner to remove abrasives, lubricant and particles of sample material that could cause cross-contamination of the preparation cloths.
Repeatable sample preparation is necessary for reliable hardness test data. By performing a coarse grinding step only, the sample may show artificially high hardness readings. Lower hardness loads exaggerate this problem, so sample preparation is critical for microhardness testing.
The prepared mounts were etched with 5% nital to expose the microstructure. A microscope with bright-field illumination was used to capture images of the etched surface. The hardened case of the sample showed a martensitic microstructure, while the core showed a ferrite/pearlite microstructure (Fig. 3).
During a heat treater’s quenching cycle, steel is heated sufficiently to form an austenitic microstructure and quenched to prevent a normalized microstructure from forming. The austenite transforms into martensite due to the rapid cooling process. This quenched microstructure is a non-equilibrium structure and, as such, yields the highest hardness possible for the grade of steel being used.
A side effect of this very hard microstructure is a very low ductility, meaning that there is a high potential for fracture if used in the wrong way. Ideally, the steel being used can be processed in such a way that only the surface of the material cools fast enough to form this martensitic structure and the core cools slowly enough to allow a more normalized structure to form. This optimal situation will achieve a hard surface with its associated appealing wear properties while keeping the ductility and toughness of the core to add strength to the component.
In an atmosphere carburizing process, the sample is heated in a carbon-rich environment to a point at which the austenitic structure will allow additional carbon to diffuse into the surface due to the higher carbon solubility of the austenite. A quench at this point will prevent the carbon from diffusing back out and lock it in place, forming a non-equilibrium structure.
The high-carbon case and its subsequent high hardness value improves wear resistance. The lower-carbon microstructure and corresponding low hardness values in the core allow the material to retain its ductility and toughness. These are ideal features for a gear tooth.
Hardness testing was chosen to evaluate the material properties and verify that the heat treatment yielded the correct case depth. Because the gear-teeth samples were mounted, 500 gram-force Vickers testing was utilized. The mount was fixtured in a specimen holder to ensure perpendicularity between the indenter and surface and to avoid flexing under the indenter’s load application.
An automated Vickers hardness tester was used to set up multiple traverses perpendicular to the surface (Fig. 4). ASTM E92 gives recommendations for dwell time, indent spacing and test forces. Multiple rows of indents were run on the tip, midpitch and root of each gear tooth with a 10-second dwell time per indent. The tip row showed the deepest effective case depth, while the midpitch and root traverses had equivalent effective case-depth values (Fig. 5).
The case-depth results are critical to understanding the gear’s pitting fatigue life. The case depth must be appropriately deep to handle the load applied to the gear tooth. Too shallow a case will lead to pitting, while too deep a case may cause the tip to break away from the tooth. The optimal case depth is related to the size of the tooth. Larger gear teeth require deeper cases to handle the load application.
In addition to the case-depth traverses, hardness maps were created from the gear-teeth samples. An array of indents was dropped across the gear tooth, and the resulting hardness values were displayed as a color map (Fig. 6).
The hardness map may include over 500 indents and take too much time for routine use by high-volume heat treaters. However, hardness maps have proven useful when developing new heat-treatment processes or when performing other R&D tasks. High hardness values were shown as red and orange colors, and low hardness values were shown as blue and green colors. The hardness map is a useful way to visualize the changes in hardness across the sample from the martensitic case to the ferrite/pearlite core.
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