1. Mechanical properties improvement

"Low-pressure carbonitriding®

 improves mechanical properties"

> Better performing finished products

ECM has developed a carbonitriding® process at low-pressure and thus, revealed the potential of this technology in terms of fatigue strength improvements. Compared to traditional carbonitriding® and oil quenching, tests on gears with PSA showed a 34% improvement on tooth root fatigue limit with low-pressure carbonitriding® followed by gas quenching. With greater fatigue strengths limits, transmission gears support increased torque in transmissions. To improve resistance of its valves, Sun Hydraulics has deployed ECM low-pressure carbonitriding® on an industrial scale.

>Measured improvements

Low-pressure carbonitriding® consists of alternative boost of acetylene C2H2 for enrichment and nitrogen N2 and ammonia NH3 in diffusion phases at a 5 to 20 mbar pressure. Duration of each phase depends on targeted case depth and nitrogen concentration. On samples, low pressure carbonitriding® followed by gas quenching improves fatigue strength by 15% and impact resistance by 60%, compared to low pressure carburizing and gas quenching.

"Stop gas quenching for mechanical properties improvement (StopGQ)"

>Improve fatigue strength

More and more parts are subject to high sensible stress, such as transmission gears. Stop Gas Quenching is an adapted quenching solution to improve fatigue strength resistance of those parts.
ECM has developed StopGQ – a quenching process with an anticipated stop around 200°C– that causes an auto-tempering effect. It has been validated by tests on real parts, run by GM Powertrain Sweden and Scania. Parts treated this way are 30% more resistant to fatigue, as Wöhler curves show.

>Opening prospects

Mechanical improvement with ECM StopGQ gas quenching process opens up interesting prospects. Combining StopGQ with other operations like ECM low-pressure carbonitriding®  or shot peening will bring higher level of fatigue strength. With a whole transmission in mind, StopGQ might help engineering increase torque or design thinner and lighter gears.

>Auto-tempering effect

StepGQ is a 3-phase gas quenching principle, achieved by adjusting the two parameters of ECM gas quenching: gas pressure and mixing rate. ECM innovation is to literally stop – for about a couple of second ¬– the cooling process, at a temperature around 180-200°C, before it resumes. This causes an effect comparable to auto tempering.

2. Distortion control with step quenching (StepGQ)

>Minimize deformation

For components subject to distortion, ECM has developed StepGQ. StepGQ is a 3-step gas quenching principle, achieved by adjusting the two parameters of ECM gas quenching: gas pressure and turbine speed. The key innovation is to mark a step - few second pause - during the cooling process, before austenite starts transforming in martensite (Ms point). Temperature is then homogenized within the part: this prevents excessive temperature difference between surface and core, a residual stress factor as steel structure changes.

>Closer to continuous cooling transformation

Different series of tests on deformation-prone real components, like General Motrors transmission rings, have confirmed the resulting reduction distortion. Out-of-roundness defects can be reduced by a factor of four, an improvement that is very uniform within a batch of components. This leads to savings in straightening operations.