New Vacuum Processes Achieve Mechanical Property Improvement in Gearbox Components
A research program was launched by ECM using the flexibility of low-pressure vacuum carburizing in combination with high-pressure gas quenching (LPC+HPGQ) to investigate several new vacuum processes:
- LPC+HPGQ+StopGQ® Quenching
- LPCN (low-pressure carbonitriding) +HPGQ
- LPCN+HPGQ+StopGQ® Quenching
StopGQ® Quenching ConceptConventional high-pressure gas quenching looks to bring the entire workload to room temperature as rapidly as possible. Once a workload has been hardened or case hardened, tempering is necessary. Interrupted quenching involves halting the cooling process in the temperature range of 350-400°F (180-200°C) and introducing an isothermal hold (Fig. 1) in order to perform an “auto-tempering” step in the gas quenching cell, thus avoiding the need for subsequent tempering. DataPaq®-instrumented, full-load trials of gears (Fig. 2) helped determine the correct time delay before initiation of quench interruption as a function of quench pressure.
LPCN StudiesAtmosphere carbonitriding is typically performed in the temperature range of 1475-1650°F (800-900°C). Typical case depths are 0.010–0.020 inches (0.25–0.50 mm), although deeper and shallower cases can be achieved. Surface carbon is normally in the range of 0.6–0.8%, and surface nitrogen content is in the range of 0.15–0.3%.
The principle of low-pressure vacuum carbonitriding is to alternate the boost/diffuse gas mixtures between hydrocarbon gas (propane or acetylene) and nitrogen or hydrocarbon gas and ammonia (Fig. 3). Processing pressure was equivalent to LPC – in the range of 4–15 torr (5–20 mbar). Ammonia is added during the later boost/diffuse steps and during the final soak. The amount and duration of the carbonitriding steps depend on the depth and nitrogen concentration desired.
Fatigue and Impact StudiesSpecification targets were selected for gears of SAE 5130 (29MnCr5) material. Hardness and effective case depth results (Table 3) achieved targeted values. This confirmed that there was no metallurgical difference to influence the fatigue strength results.
The result of impact testing when compared to LPC+HPGQ revealed, as one might expect, a strong benefit of tempering on impact properties. Noteworthy is the improvement over LPC+HPGQ achieved by interrupted quenching or by LPCN+HPGQ. These results indicate that additional testing is required to optimize results. The use of LPCN+HPGQ with StopGQ® quenching (Fig. 8) resulted in impact values exceeding those of LPC+HPGQ+Tempering.
Realized ObjectivesAn improvement in fatigue strength was realized using the auto-tempering effect achieved by StopGQ® quenching. The affect of nitrogen present in the surface layer of LPCN parts was revealed in higher impact and fatigue-strength values.
Dilatometry studies (Fig. 12) showed less contraction with an interrupted quench compared to direct high-pressure gas quenching. During the StopGQ® quench, tetragonal martensite is transformed into cubic martensite+e-carbides, resulting in an auto-tempering effect.
These trials allowed the following conclusions to be reached:
- Conditions were established to predict and control carbon and nitrogen concentration profiles.
- The metallurgical parameters, which act on resistance in fatigue inflection of gear teeth for a fixed hardened depth, were better understood.
- Results indicate improvements in fatigue resistance of gear teeth compared to low-pressure vacuum-carburizing treatments applied to gearboxes.
Future StudiesFuture research and development efforts will target further understanding of the role of nitrogen on mechanical properties, including microstructural analysis of grain boundaries, and will determine if the grain size has been refined. In addition, further optimization of the influence of StopGQ® quenching temperature and hold time will be investigated.
AcknowledgmentThe author would like to thank ASCOMETAL for their assistance in the testing and evaluation of certain technical results presented in this article.
For more information: Aymeric Goldsteinas is prospective and innovation manager for ECM, 46, rue Jean Vaujany – TECHNISUD – 38029 Grenobe Cedex 2 – France; e-mail: firstname.lastname@example.org
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: low-pressure carburizing, gas quench, auto-temper, fatigue strength, rotating bending fatigue