Ultra-Clean steels in transmission components

Purpose and goal

The project was led by Swerim and RISE IVF with the industrial partners Ovako, Scania, GKN ePowertrain, Volvo CE and GTO. The goal of the project is to 1) assess performance of ultra-clean steels in case-carburized components and 2) clarify how steel cleanliness is quantified and implemented in company material standards. However, the project has shown that isolating and benchmarking cleanliness with respect to fatigue performance is quite a challenge. Objectively quantifying the cleanliness has also been proven difficult with the limitations of current standards.

Expected results and effects

It was proven challenging to objectively quantify cleanliness on a meso- and micro inclusion level. More work is needed to establish a lab-based fatigue testing method with the ability to benchmark ultra-clean steels with respect to their cleanliness. With the used methods however, it was indicative that components need to be manufactured with high surface hardness, high compressive residual stresses and fine surface finish to take advantage of the advantageous properties of ultra-clean steels.

Planned approach and implementation

Steel grade 18CrNiMo7-6 was manufactured in four different cleanliness qualities. Case-carburizing was done using both low-pressure- and gas carburizing. Gas carb were made with and without shot peening. Cleanliness was assessed using existing standards. Fatigue performance was assessed by using tooth root- and rotating bending fatigue testing. Since no strong conclusions could be drawn regarding the correlation between performance and cleanliness, guide lines for heat treatment of ultra-clean steels were not made. Workshop for dissemination of results was also cancelled.