Chunky graphite formation in ductile cast irons: effect of silicon, carbon and rare earths

Abstract

Use of rare earths, high silicon and carbon contents, and low cooling rates are reported as possible reasons for formation of chunky graphite in ductile iron castings. The understanding of this graphite degeneration is however limited, and the above conclusions are still controversial. To get further insight into this topic, ductile cast iron melts have been prepared with various carbon and silicon contents and using nodularizing alloys with various rare earth levels. These melts have been cast in blocks with different sizes to include also the effect of cooling rate on graphite degeneration. Metallographic investigation revealed that the chunky-affected areas enlarge when decreasing the cooling rate, when increasing the silicon content and without low-level addition of rare earth. In the discussion, a schematic based on change of liquid/graphite interfacial energy with alloying elements is proposed which describes the conditions for the formation of chunky graphite instead of nodular graphite.

Effects of high silicon contents on graphite morphology and room temperature mechanical properties of as-cast ferritic ductile cast irons. Part I – Microstructure

“…Although addition of inoculant has been reported as promoting chunky graphite formation [6,12,13], this effect still remains con troversial. Thus, alloys #29, #30 and #31 were prepared using dif ferent inoculation additions, namely no addition, 0.15 wto/o and 0.30 wt %, so as to check the effect of inoculation on chunky graphite forma tion.…”

“…Such alloys have much higher strength than standard ferritic cast irons with comparatively low silicon contents though at the expense of lower ductility. On the other hand, the appearance of some graphite degen eration such as chunky graphite has also been related to high silicon contents in ductile irons [4][5][6][7]. As for the mechanical properties, an optimum silicon content of 4.3 wto/o has been reported on the basis of a limited number of alloys [8] which called for a more extensive study.…”

Detection of chunky graphite in iron casting using ultrasonic scattering

“…Here, often a too large amount of inoculants of silicon or rare earths can be detected, which are added to the melt to form spheroidal graphite instead of lamellar graphite. 2 Usually, the appearance of chunky graphite is not visible at first. Since it affects the mechanical properties of the involved work piece, especially locally but also overall, its detection is all the more important.…”

“…If chunky graphite is present in large quantities, this can often be assumed visually at the interfaces by a darker shading of the affected areas. 1,2 Such areas are to be examined in the following with ultrasonic scattering.…”

Analysis of Local Conditions on Graphite Growth and Shape During Solidification of Ductile Cast Iron

“…Several researchers have contributed to the understanding of factors influencing growth of graphite work that in 2017 was comprehensively compiled by Stefanescu in ASM Handbook [1]. The use of certain alloying elements to control the shape of graphite during solidification is extensively reported, we now know what causes graphite to grow in the form of spheres or flakes or intermediate forms [3][4][5][6][7][8][9][10]. However, in ductile cast irons, it is common to find a variation in graphite shape, size and distribution, which results in significant variations in mechanical properties [11].…”