High strength solution-strengthened ferritic ductile cast iron

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master_Okunnu_Rilwan_2015.pdf (7.93 MB)

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Insinööritieteiden korkeakoulu | Master's thesis
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Date

2015-08-24

Department

Major/Subject

Mechanical Engineering

Mcode

IA3027

Degree programme

Master’s Programme in Mechanical Engineering

Language

en

Pages

81 + 9

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Abstract

High Silicon ductile iron (SSFs) represents a state-of-art in standardized ductile Iron grades, because it uniquely combines uniform hardness, intermediate strength and ductility rivalling that of some steels. Also backed by economic and technical potentials, it is imperative to take advantage of these unique properties to explore avenues to reach even higher strength levels in this grade. Within the SSF grade, strengthening limit with Silicon is reached at 4.3wt% with mechanical properties markedly plunging at this point, necessitating the need to investigate alternative strengthening measures aside expensive heat treatments, hence thesis objective to research methods to develop high strength SSF ductile irons as-cast. Casting process and production methods to mitigate against challenges of current SSF irons also form part of this objective. Researching and developing high strength SSF offers to the engineering materials pool, aside from obvious financial benefits, additional options in material selection for different applica-tions, energy savings due to lighter castings, improved tool life from uniform hardness and boost knowledge on suitability of cast irons as viable replacements for steel in some applications. The solution approach in the thesis started with extensive literature reviews and background researches on DI standardized grades with emphasis on the SSFs and strengthening mechanisms to understand present challenges of these grades. The idea was then to aim for higher strength using the EN-GJS-600-10, the SSSF grade with the highest Silicon content and strength as both reference and base iron for designing the experimental high strength SSF. Using a suc-cession of elimination methods in the product development process based on identified pa-rameters for high strength in DIs, such as improving graphite properties, reducing carbidic pres-ence etc., controlled chemical composition with Cobalt (Co) as alloying element was proposed. The result of the casting trials proved that Co indeed fortified the experimental SSF showing increase in strength and elongation. Graphite properties were improved, with refined nodules and higher nodule counts. In addition, no apparent graphite degeneracy was noticed in the microstructure with a homogenous ferritic matrix. The overall qualities of the Co-alloyed irons were better compared to the reference EN-GJS-600-10. The significance of the results is that silicon limit in current SSF grade does not affect the possi-bility of reaching higher strength via controlled alloying with other element. Co alloying provid-ed better graphite attributes in the Iron which influenced elongation and cast quality. Although from the result data, it is statistically insufficient to ascertain if further increase in Co will be proportional to strength or elongation, Co however, similar in effect to Si, stabilizes austenite contributing positively to strength, increased nucleation and nodularisation efficiency.

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Supervisor

Orkas , Juhani

Thesis advisor

Jalava, Kalle

Keywords

cast irons, high strength ferritic ductile irons, solid solution strengthening, alloying and composition control, casting and foundry, high silicon irons

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https://urn.fi/URN:NBN:fi:aalto-201509184417

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[dipl] Insinööritieteiden korkeakoulu / ENG