As soon as the grey cast iron hardened in its mould at Volvo’s Skövde foundry, the brick of still-hot metal was lifted with pliers, placed in a fireproof steel box filled with sand, and loaded into a car. Within hours, it would be sliced, and studied at MAX IV’s ForMAX beamline.
It was a four-hour drive to Lund, where the metal finally cooled to room temperature. Then, Juan Manuel Bello Bermejo, a PhD candidate in Production and Materials Engineering at Lund University, cut the ingot into 50-micron thick slices – about the width of a strand of hair. Thirty hours after the metal had been cast, the sample was scanned at ForMAX.
Using WAXS, a technique which studies how X-ray beams scatter at wide angles after being directed at a material, researchers studied how the structure of the cast iron changed over time. The resulting patterns reveal how atoms are arranged in the material. The sample was measured 30 hours after the metal was cast, and again after two months. In between measurements, the sample was left to age naturally at room temperature and under low humidity.
Those in the metals industry already knew from hands-on experience – “folk knowledge,” in Bello Bermejo’s words – that grey cast iron is nearly impossible to machine straight out of the mould and is best handled after being aged for at least a few days. They didn’t know what drove this change.
Through comparing the sets of WAXS data, researchers found iron nitride particles distributed in tiny clusters throughout the material. These clusters are concentrated around eutectic cells, regions that form as the cast iron cools and solidifies and spread outwards until they collide into other cells. These meeting points are called eutectic cell boundaries, and it is along these boundaries that nitrides occur. The findings suggest that nitrides play a key role in the aging process that increases the tensile strength of cast iron and makes it easier to machine.
The role of nitrogen – in the iron nitride – as a player in aging came as a surprise to Lena Magnusson Åberg, Senior Material Specialist at Volvo Powertrain. Foundries have long been adding nitrogen to grey cast iron to strengthen the material, but now there was scientific evidence backing their work.
“Since foundries use nitrogen to enhance the strength, I was sceptical at first,” Magnusson Åberg said. She was convinced after Bello Bermejo redid the experiment and found the same nitrides.
Since these measurements, researchers have already returned to MAX IV, using the NanoMAX beamline to get a more detailed view of the changes the cast iron undergoes as it ages.
The current findings from ForMAX have offered important insights – and savings – to Volvo, turning years of foundry know-how into scientific understanding.
