Modification of Silicon Microsegregation in Solid-Solution-Strengthened Ductile Iron by Alloying with Aluminum
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Copyright Ó 2020 The Author(s) https://doi.org/10.1007/s40962-020-00412-7
Abstract In solid-solution-strengthened ferritic ductile iron (SSFDI), a silicon content above 4.3 wt% leads to an abrupt decrease in ultimate tensile strength and elongation at fracture. This phenomenon has recently been proven to be attributed to the formation of iron–silicon long-range orderings that lead to an embrittlement of the material. It is assumed that the local tendency to form silicon superstructures is promoted in particular by the occurrence of silicon microsegregation. During solidification of ductile iron, silicon segregates inversely into the austenite. Thus, the highest silicon concentration is larger than the initial concentration of the melt and is located directly at the graphite nodules. As a straight consequence, the presence of silicon superstructures is expected primarily in these areas. Therefore, the focus is on homogenization of the silicon microsegregation profile in order to avoid the
formation of brittle iron–silicon superstructures. For this purpose, in the present study the alloying concept of SSFDI is adapted. Thermodynamic–kinetic simulations as well as experimental investigations indicate that aluminum concentrations of approx. 1.2 wt% lead to an inversion of the silicon microsegregation. The findings provide a promising tool to shift the silicon embrittlement in SSFDI to higher silicon concentrations. This method could be used to increase the maximum strength, to improve toughness properties or to increase the process integrity against deviations in silicon content.
Introduction
iron materials was taken up again. In the mid 1990s, Bjo¨rkegren et al. conducted experiments on solid solution strengthening of SGI with the aim of reducing hardness scattering in ferritic–pearlitic ductile iron grades. Machinability could significantly be improved by adjusting a purely ferritic matrix due to a more homogeneous hardness distribution with very good tensile strengths and elongations, which led to cost savings of about 10% in mechanical machining.2,3 As a result, grades of solid-solutionstrengthened SGI materials were incorporated into the Swedish standards SS 140720 and SS 140725 in 1998 due to their extremely advantageous static mechanical properties. Further investigations in the following years proved the suitability of increased silicon contents for setting an advantageous ratio of tensile strength and elongation at fracture.4,5 In their studies, particularly, materials with minimum values of ultimate tensile strength of 520 MPa and elongation at fracture of 14% were successfully adjusted. In 2012, the solid-solution-strengthened grades
The possibility of strengthening the matrix in ductile iron by increased silicon contents producing a solid-solutionstrengthened ductile iron was rejected for a long time. However, in the 1950s and 1960s, White et al. were able to demonstrate that increased silicon contents of about 4 to 5 wt% lead to the effect of solid solution strengthening, which made it possible to achie
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