GM Powertrain Sweden - Scania tests

Better fatigue strength with a stop during gas quenching

Thanks to two sets of tests on transmission gears, GM Powertrain Sweden and Scania have just confirmed gains in fatigue strength linked to a stop during gas quenching. Invaluable results considering that tomorrow's gearboxes will have to be more powerful without any increase in size or weight.

The flexibility of gas quenching holds a potential for improving mechanical properties. This is the conclusion of several series of tests conducted during 2006 by GM Powertrain Sweden, Scania and ECM on transmission gears. By testing a gas quenching procedure known as "stopGQ" - for stop Gas Quenching - characterized by a long stoppage of quenching in the martensite phase, the three partners obtained a 30% improvement in fatigue strength compared to direct, continuous gas quenching. This will no doubt have a major impact on processes and products.

Industrial validation

GM Powertrain Sweden and Scania have just validated the process at industrial level. Repeated after an interval of several weeks, their "stopGQ" gas quenching tests carried out on real components - planetary gears made from 16MnCr5 steel grade - gave consistent results. The values shown by Wöhler curves are even more convincing: for an equivalent metallurgical quality, the limit fatigue strength of the gear after two million cycles was found to be σ_a=685 MPa (with R=0.1) for the "stopGQ" gas quenching method compared to σ_a=525 MPa for direct gas quenching.

"We wanted to use our own environment to check the benefits of stopping the gas quenching process, as recommended by ECM", explains Sune Larsson, heat treatment manager at GM Powertrain Sweden. As early as 2005, ECM had in fact already shown, during research work on 29MnCr5 steel samples, the benefits of stopping the gas quenching process for about one minute at around 180-200°C, at the moment when martensite transformation at the surface of the component was initiating. The gain in fatigue strength measured at that time, compared to a direct gas quench, was 28%.

The Swedish manufacturers took inspiration from the work of ECM to establish their own gas quenching method that included three stages. The quench started with full-scale cooling (maximum pressure and gas mixing), before marking a pause - the famous gas quench stop - for one minute at 180°C (reduced pressure, no mixing). Finally, the component is simply brought to a temperature where it can be handled (reduced pressure, maximum mixing).

Future products

What applications are Scania and GM Powertrain Sweden considering for "stopGQ" gas quenching? "The results obtained, equivalent to or better than those for oil quenching, have enhanced the credibility of gas quenching, a process that is now already being used by some of our suppliers", mentioned Krister Johansson, responsible for heat treatment processes in Scania's material laboratory, currently equipped for traditional and oil quench case-hardening.

"With better fatigue strength, the torque of existing transmissions gears can be increased", comments Sune Larsson for GM Powertrain Sweden, that already treats 680,000 components every year by low-pressure, gas quench carburizing, "but, above all, these results will have a significant impact on the design of our future products". One of the possibilities being considered is thinner, lighter gears or gears made from a lower steel grade. Stop gas quenching would then contribute to the production of less expensive, more fuel-efficient vehicles.

Self-tempering and transgranularity

Several factors can explain the improvements provided by stop gas quenching. Dilatometric analysis on the components treated by this method indicates stress relaxation, thus confirming a "self-tempering" effect. In addition, an examination of cracked zones during fatigue tests reveals transgranular fractures whereas these fractures are intergranular in components cooled by direct quenching. A more detailed analysis of these structures is currently in progress.