Grinding away the mythology of recycled steel

Thomas Björk, PhD, Group Technical Specialist at Ovako, outlines a recent research program that shows both recycled steel and ore-based steel are equally good as raw materials for automotive crankshafts.

Historically, a number of automotive industry OEMs have been reluctant to use recycled steel for their crankshafts. This is despite its considerable potential to reduce the carbon footprint of their products. The reason is an historic belief that engine crankshafts should be manufactured from ore-based steel, mainly because it performs better during the grinding operations necessary to create high-quality bearing surfaces. Recycled steel is considered by some to be more susceptible to “grinding burns”. This is a type of thermal damage that decreases surface hardness, introduces tensile residual stress and shortens fatigue life.

A number of major players in the industry decided it was time to challenge this by carrying out a test program to benchmark the performance of recycled and ore-based steels. With financial support from the Swedish government, a three-year project was devised - Grindability of recycled steel: automotive crankshafts (CRANK-STEEL). It was coordinated by Chalmers University of Technology, with project members including Volvo Group, Volvo Cars, Scania, Bharat Forge, Ovako and RISE IVF.

How to assess the grindability of a material?

The first challenge for the project was in how to assess the grindability of steel. This is because there is no grinding-specific definition. It was therefore decided to use the general concept of “machinability”. This describes the ease with which a certain material can be machined under given process conditions and it may be assessed by one or more of the criteria:

• Tool life & tool wear
• Material removal rate (MRR)
• Cutting force
• Surface fininsh
• Chip shape

Grinding tests

To carry out the tests, tubular, hardened workpieces (see Figure 1) were fabricated from bars in a steel grade used regularly for forging. These were divided into two groups from different steel makers:

• Steel production based on recycled material
• Virgin ore-based steel

• Specific energy - Is there a difference in energy required to grind the materials?
• Wheel wear - Is there a difference in how the grinding wheel wears down?
• Surface integrity - Are there differences in the surface integrity of the ground surfaces?