Fig. 1. StopGQ® quench concept

Increased performance requirements in automotive and other industries demand higher strength characteristics from components such as gears and pinions, which translate into a need for improved impact properties and fatigue strength. The technical solution to address the need for expanding the performance envelope required the development and optimization of new processes.

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
Marked improvements in mechanical properties were found as a result of the optimization of both surface enrichment techniques and quench parameters. The conventional process – LPC+HPGQ+Tempering – was used as a benchmark for all tests.

Fig. 2. StopGQ® quench time vs. pressure relationship

StopGQ® Quenching Concept

Conventional 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.

Fig. 3. LPCN boost/diffuse steps

LPCN Studies

Atmosphere 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.

Fig. 4. LPCN carbon and nitrogen profiles

Trials were conducted on SAE 5130 (29MnCr5) pinions at 1615°F (880°C) followed by direct quenching using 8-bar nitrogen (Fig. 4). Samples were placed in the top, center and bottom baskets of the load. Analysis of carbon and nitrogen content involved two different technologies: GDOS (Glow Discharge Optical Emission Spectrometry) to measure the nitrogen concentration of the surface to a depth of 0.0015 inch (40 µ) and WDS (Wavelength Dispersive X-Ray Spectroscopy) accurate to a depth of up to 0.060 inch (1.5 mm). These techniques confirmed that the profiles using LPCN were equivalent to what was produced by atmosphere carbonitriding.

Fig. 5. LPCN elevated temperature test results

Additional LPCN tests (Fig. 5) were conducted at higher temperatures to investigate cycle time savings. Typical carburizing temperatures of 1700°F (930°C) and 1750°F (960°C) were selected for study. Process parameters were similar to the trials at 1615°F (880°C). GDOS profiles were conducted to determine the nitrogen content at the near surface up to 0.0012 inch (30 µ). Results indicated a nitrogen content of 0.5% falling rapidly to 0.15% below the near surface at 1700°F (930°C), and the same behavior was observed at 1750°F (960°C). The nitrogen profile was achieved at 1700°F (930°C) up to 0.015 inch (0.4 mm) and up to 0.040 inch (1.0 mm) at 1750°F (960°C). These tests indicate that the process can work at high temperatures as well.

Fatigue and Impact Studies

Specification 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.

Fig. 8. Improvement of impact properties over LPC+HPGQ by process

One of the mechanical property tests employed was to determine impact properties. Impact samples were tested in a pendulum-type impact tester at 50J.

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.

Fig. 10. Improvement of rotating beam fatigue properties over LPC+HPGQ by process

Another mechanical test employed was that of rotating bending fatigue involving a notched sample geometry. The test was run for 1x107 cycles. The results of rotating bending fatigue testing (Fig. 10) indicate that all of the new processes improve strength values over those of either LPC+HPGQ or LPC+HPGQ+Tempering.

Fig. 12. Dilatometry studies of the StopGQ® quenching method

Realized Objectives

An 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.

Fig. 13. Rupture analysis comparison by process

This was demonstrated by rupture analysis (Fig. 13), which revealed higher ductility in the core of the material processed with LPCN+StopGQ® Quenching.

Fig. 14. Comparison of LPC and LPCN microstructures

Scanning Electron Microscopy (SEM) analysis at 3700X comparing LPC and LPCN microstructures found fine precipitates of carbonitrides in the latter which is believed to have a strong influence on fatigue strength (Fig. 14).

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 Studies

Future 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.

Acknowledgment

The 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: a.goldsteinas@ecm-ip.com

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