The Role of Primary Austenite Morphology in Hypoeutectic Compacted Graphite Iron Alloys

Juan Carlos Hernando,Ehsan Ghassemali,Attila Diószegi,Arne K Dahle,Jessica Elfsberg

doi:10.1007/s40962-020-00410-9

Juan Carlos Hernando, Ehsan Ghassemali + Show 3 more

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https://doi.org/10.1007/s40962-020-00410-9

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Abstract

This work investigates the role of primary austenite morphology on the eutectic and eutectoid microstructures and the ultimate tensile strength (UTS) in a hypoeutectic compacted graphite iron (CGI) alloy. The morphology of primary austenite is modified by isothermal coarsening experiments in which holding times up to 60 min are applied to the solid–liquid region after coherency. The cooling conditions for the subsequent eutectic and eutectoid reactions are similar. Miniaturized tensile tests are performed to evaluate the UTS. The morphological characteristics related to the surface area of primary austenite, the modulus of primary austenite, {M}_{upgamma} , and the hydraulic diameter of the interdendritic region, {D}_{text{ID}}^{text{Hyd}} , increase with the cube root of coarsening time. The eutectic and eutectoid microstructures are not significantly affected by the morphology of primary austenite, thus indicating that the morphology of the interdendritic regions does not control the nucleation frequency and growth of eutectic cells or graphite. UTS decreases linearly with the increasing coarseness of primary austenite for similar eutectic and eutectoid microstructures, demonstrating the strong influence of primary austenite morphology on the UTS in hypoeutectic CGI alloys.

Highlights

Global environmental targets demand improved performance and fuel economy
The surface area of the dendrites is reduced, increasing the length scale, and an overall more globular microstructure is observed. This suggests that dendrite fragmentation occurs simultaneously with Ostwald ripening as a result of coarsening
As the number of eutectic cells is similar to the number of nucleation events occurring during the eutectic reaction, these results show that the morphology of primary austenite does not significantly influence the eutectic nucleation frequency in compacted graphite iron (CGI)

Summary

Introduction

Global environmental targets demand improved performance and fuel economy. Urged by these goals, automotive manufacturers strive to design improved fuel-efficient engines, which entail higher peak pressures during operation and subject the material to higher thermal and mechanical loads. Automotive manufacturers strive to design improved fuel-efficient engines, which entail higher peak pressures during operation and subject the material to higher thermal and mechanical loads In this context, the exceptional balance between thermal and mechanical properties achieved with compacted graphite cast iron (CGI) alloys[1] makes them a preferred material for structural elements in heavy-duty engines, such as cylinder heads or cylinder blocks. Large dendrite arms thicken following an Ostwald ripening process[8] in which their length scale, e.g., the secondary dendrite arm spacing (SDAS), is proportional to the cube root of total solidification time, t1/3.4 Further coarsening leads to a globular

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