Abstract
High life expectancy of cast components and good material performance at dynamic load are a prerequisite to cater for future trends in wind energy generators. To remain competitive in this ever evolving sector challenges reside in alloy development. In this work fractional factorial design has been applied to ferritic ductile iron with varying contents of silicon (1.6‑2 wt%), nickel (0‑1 wt%), cobalt (0‑3 wt%) and copper (0‑0.2 wt%). The minimum criteria the new alloy should meet were a minimum yield strength of 240 MPa and an impact work of minimal 8 J at a temperature of -20 °C for wall thicknesses of 60‑200 mm. To obtain these mechanical properties thick-walled castings with additional insulation were produced to achieve a higher thermic module. They provided the material for test specimens to perform static tensile tests, Charpy impact tests at varying temperatures and a microstructure analysis. With these results, a sweet spot plot has been created. That way, an optimum alloy composition could be found and has been proven by validation experiment.The optimum alloy for thick-walled castings is composed of Si = 1.6 wt%, Cu = 0.2 wt%, Ni = 0 wt% and Co = 0 wt%. It offers an enhancement in yield strength and acceptable impact work at low temperatures for massive castings in as cast state. The heat treated, full ferritic material could even improve these results.
Highlights
Technological progress in renewable energies in the field of ultra large wind generators in regions of unpredictable and/or very low temperatures demand ductile iron (DI) with advanced qualities
The chemical analysis of all alloys is in good agreement with those the design of experiments (DoE) demanded
With this model prognosis alloy properties of new compositions could become predictable but the linear approach does not cover the physical background of the connection between elements, microstructure and mechanical properties
Summary
Technological progress in renewable energies in the field of ultra large wind generators in regions of unpredictable and/or very low temperatures demand ductile iron (DI) with advanced qualities. The basis of our decision-making can be found in previous research: In thick-walled castings with restrained cooling rates the eutectoid reaction predominantly follows the stable system which yields a primarily ferritic structure In this regard the graphite distribution has a great influence as it defines the diffusion length of carbon which has a direct impact on the ferrite/pearlite ratio [1, 2]. The improvement of yield strength and impact toughness with cobalt is induced by hardening of ferrite It increases the transition temperature and lowers the low temperature toughness [17]. From [20] it can be derived that the maximum alloying contents of elements in the present work (0.2 wt% Cu, 3 wt% Ni, 1 wt% Co) at 2-2.2 wt% Si provoke an increase of yield strength with nickel (followed by copper, no effect caused by cobalt). A selection of samples is heat treated at 750 °C for 13 hours with subsequent oven cooldown to obtain full ferritic specimens for tensile and impact tests as well as microscopic analysis
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