Metallurgical Characterisation Comparing Recycled and Ore-based Steels for Automotive Crankshafts
Typ
Examensarbete för masterexamen
Program
Production engineering (MPPEN), MSc
Publicerad
2020
Författare
Pautzke, Joshua Michael
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Steel crankshafts are a substantial component in internal combustion engines. A
critical feature of crankshafts is their bearing surfaces which have to meet tight
requirements regarding hardness and compressive residual stresses in order for
the component to function properly. The properties of the bearing surfaces are to
a large extent determined by a grinding operation which is used as the final step
in the manufacturing chain. In practice, it is commonly experienced that there
are batch-to-batch material variations which can cause grindability issues leading
to unwanted high scrap rates when the surface requirements are not met. These
issues are especially pronounced when both steels made from recycled resources
and from iron-ore are fed into the production line. The underlying reasons for
these grindability variations are largely unknown. The present master thesis
project addresses metallurgical differences between a recycled and an iron-ore
based steel batch and investigates potential effects on the grindability. Both steel
bars (as supplied by steel mills) and the final ground crankshafts are examined.
Samples taken from different positions of the crankshaft were investigated using
optical and scanning electron microscopy, Vickers hardness testing, Barkhausen
noise analysis and XRD residual stress measurements. The results show that there
is a significant difference in grain size between the microstructure of the recycled
and ore-based initial, as received steel bars. However, metallurgical examination
of the unhardened material of the corresponding crankshaft material indicates that
these grain size differences have largely been eliminated during the processing
steps. When considering the crankshafts, the recycled steel batch is clearly more
inhomogeneous but also has a clearly higher hardenability, while the hardness
close to the bearing surface only shows insignificant differences. The estimated
prior austenite grain sizes and residual stresses seem to differ more within different
spots in a material than between recycled and ore-based material.
Beskrivning
Ämne/nyckelord
Crankshaft , steel , recycled , one-based , grindability , Microstructure