ECHT QDE 2021 with a focus on distortion

The European conference ECHT2021 on heat treatment was recently held online and was entirely dedicated to distortion engineering. Dimensional change and distortion are inevitably associated with the heat treatment of metal, and attention is still required for this in industrial practice. A look back at the online event.

This year's two-day conference was organised by the German AWT and consisted of nine thematic sessions. This is a short overview of the main presentations:

In the keynote session, RISE’s Eva Troell gave an overview of the possibilities and advantages of gas quenching. Furnace manufacturers ALD from Germany and ECM from France further demonstrated this with their commercial, modular systems and illustrated this with typical applications such as a gear with internal teeth. Both the type of quenching gas and the pressure and rotation speed of the fan in the furnace affect the quenching speed and therefore the end result.

The subject of quenching was also particularly considered, for example, in relation to induction hardening of heavy bearing rings (Rostock University). The study of water jets for quenching was modelled to achieve the most optimal cooling of a flat plate.

In a subsequent keynote, Scott MacKenzie, an expert in quenching solutions, presented an interesting overview of the phenomena that take place during quenching in a liquid medium. The physical approach to the complex convection process using dimensionless parameters such as the Nusselt and Reynolds numbers and their interrelationships were also shown.

The performance of different types of quenching oils was thoroughly discussed by Burgdorf GmbH. Apart from mineral oils, synthetic oils are the most widely used, but new evolutions, such as vegetarian oils and renewable oils based on biomass, are making an appearance. However, these oils usually perform less well than the conventional oils due to lower stability.

The Cologne University of Applied Sciences demonstrated the influence of an electrical potential on the quenching behaviour of an aluminium rod. The boiling stage during the quenching process could thus be accelerated. 

UK company Phoenix presented a new type of data logger that can cover the entire heat treatment process in a specially insulated box, allowing in-situ temperature recording.   

In the section on processes, much attention was paid to quenching & partitioning (QP), a variant of conventional quenching & tempering (QT). The intention of QP, in contrast to QT, is to interrupt the martensite transformation, thereby intentionally forming retained austenite. A well-known application for this is TRIP steel, which is already in use for some parts in car bodywork. The research at the University of Applied Sciences Upper Austria focused on both martensitic stainless steel and experimental medium Mn steel.

The influence of tempering on the properties of ball-bearing steel was investigated by the Foundation Institute of Materials Engineering IWT Bremen, while the company HEESS studied the austempering of a shaft with controlled spray cooling. The resulting bainite structure offers high toughness in addition to a relatively high hardness.

In the section on the study of distortion, IWT Bremen proposed a new method for mapping distortion using complex numbers (vectors). They successfully applied this method to a massive steel ring.

IWT Bremen also proposed a method of compensating for the distortion during the heat treatment of a steel pinion through dimensional adjustments. It compared different quenching methods (oil, salt bath and gas) and, as expected, found a very small distortion when quenching in gas.

In a keynote, IWT professor Fechte-Heinen gave an overview of all factors that contribute to distortion: geometrical asymmetries, segregation and residual stresses as well as the gas flow contribute to the final distortion. The importance of 4D gas cooling was illustrated compared to oil cooling.

A possible solution for distortion in carbonated gears is EMA's inductive fixture hardening machine. In this process, the pieces are induction heated after conventional carburizing and then cooled with a polymer solution in a clamped mould. Afterwards, the pieces are inductively tempered. The clamping ensures high dimensional stability and low oxidation when working with a shielding gas.

The distortion of asymmetric steering columns for cars after induction hardening was investigated by the Bosch company. By adjusting the parameters, the curvature could be reduced without the need for mechanical straightening.

The American firm Dante proposed a software that can control the cooling of a component and keep the distortion within narrow limits.

The TRIP effect mentioned above was later also seen in PM samples carbonitrided d by KIT from Germany. It was shown that carbon, nitrogen and density influence plasticity.

Conclusion

During this conference, various aspects of quenching and distortion after hardening were discussed. The increasing use of high-pressure quenching in vacuum furnaces, the improvement of quenching media and the monitoring of parameters such as temperature, bath circulation... are important factors.

The numerical simulation of the distortion was discussed but had a smaller share than expected. This year, more attention appears to have been paid to the setting parameters for the hardening process and their impact on the hardening result.

The written presentations can be obtained from the author upon request.

This article was written with the financial support of VLAIO within the DCT4Cut Cornet project.