Improvements in Dimensional Control of Heat Treated Gears

The following study involves applying low pressure vacuum carburizing technology utilizing either oil or high pressure gas quenching in order to eliminate distortion.

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The goals and objectives of this investigation focused on highly distortion-prone gearing (Figure 1) to determine if dimensional improvements could be realized by applying low pressure vacuum carburizing technology utilizing either oil or high pressure gas quenching. The gears in question are traditionally atmosphere carburized and plug quenched.

Test Plan

Full production loads (Figure 2) were run using two different carburizing methods (atmosphere, low pressure vacuum) in combination with free quenching in either oil at 165°F (75°C) or high pressure gas (nitrogen) at 11 bar.

Sampling Method

Gears were taken from multiple locations throughout each load for analysis (Table 1). Parts for metallurgical evaluation were selected from the center of each load. Multiple areas on each part were then analyzed for microstructure, case depth, and hardness (surface, profile, core). Dimensional checks (out of round, gear tooth profiles) were conducted on the gears before and after heat treatment. Although only a portion of the complete test program could be presented here, the results are representative of the entire study. Figure 3

Process Parameters
Carburizing was performed at 1760°F (960°C) for 3.34 hours followed by either oil quenching (variable agitator speed) or high pressure gas quenching. Targeted surface carbon content was 0.72 percent C (vacuum) and 0.80-0.90 percent C (atmosphere). Gas quenching utilized four changes in speed and pressure made through the critical transformation range of the material while the oil quench utilized two changes in speed (70 and 40 percent). Tempering was performed at 300°F (150°C) for two hours at temperature.

Test Results
Surface hardness of all low pressure vacuum carburized gears was in the 64-65 HRC range. Atmosphere carburized gears exhibited a slightly lower surface hardness, in the 62-63 HRC range. Figure 4, %%0708Figure5%%

The depth of high hardness (> 58 HRC) was 0.002”-0.005” (0.05-0.13 mm) deeper for the low pressure vacuum carburized gears than for the atmosphere carburized gears (Table 2). The root-to-pitch line case depth ratio was 92-94 percent (vacuum carburizing) versus 63 percent (atmosphere carburizing). See selected data found in Tables Table 2, Table 3, Table 4, Table 5, Table 6.

Distortion
Dimensional variation was determined by measuring both out of round (Table 7) and by coordinate measuring machine (CMM) measurement of the gear tooth profiles (Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13). With respect to the gear charts shown, the lead was measured across the tooth or spline from side to side at the pitch diameter. This method checked for excessive taper. The involute measurement was taken on the tooth form (active profile), starting from the root diameter to the tip of the tooth. Indexing (index error) measured the tooth spacing from tooth to tooth around the gear. Gear or spline run-out measured variation of concentricity of the centerline (datum) of the gear.

Microstructure

Analysis of part microstructures from low pressure vacuum carburized gears (Photomicrograph 1, Photomicrograph-No2, Photomicrograph 3,) taken from all areas (tip, mid-radius, root) revealed a tempered martensite structure with small amounts of retained austenite. Atmosphere carburized gears (Photomicrograph 4, Photomicrograph 5) revealed the presence of large amounts of retained austenite (tip, mid-radius).

Conclusions

The following are the principle results of this investigation:

• Low pressure vacuum carburizing in combination with high pressure gas quenching produced consistent repeatability. This degree of predictable movement is capable of being compensated for in the manufacturing process.
• Low pressure vacuum carburizing in combination with high pressure gas quenching allowed for the replacement of atmosphere carburizing and plug quenching on the gears investigated in this study.
• The depth of high hardness (> 58 HRC) was greatest in the low pressure vacuum carburized samples.
• The root-to-pitch line case depth ratio was superior in vacuum carburizing (approximately 93 percent, versus 63 percent).
• Atmosphere carburizing resulted in unacceptable levels of retained austenite.
• Gear charts indicated an average movement of 0.003” (0.08 mm). The involute form remained intact after low pressure vacuum carburizing and gas quenching, as did the lead on the gear teeth and splines.

Acknowledgments:
The authors would like to thank Andrews Products, in particular Mark O’Neill, gear engineer, for his insights into the test results, and Dan Guy and Eddy Cespedes of the gear department for their assistance with measuring the test gears.