Effect of Si and Ni Addition on Graphite Morphology in Heavy-Section Spheroidal Graphite Iron Parts

Abstract

Abstract. Metallographic analysis is applied to the study of the chunky graphite morphology in heavy-section castings of spheroidal graphite cast irons. Three castings with different Si and Ni contents were prepared. Three positions in casting from the edge to the centre, with different cooling rates, were chosen for microstructure observation. The effect of the Si and Ni content on the graphite morphology and mechanical properties of heavy-section spheroidal graphite cast iron parts was investigated. Cerium containing commercial inoculant was used for in-stream inoculation. Chunky graphite area was estimated in micro-and macrostructure. Mechanical properties were determined on tensile test bars taken from the centre of the casting. Macro-and microstructure examination showed that the castings with high Si-content and Ni addition had chunky graphite present, while the castings produced by use of low Si and Ni containing charge had no chunky graphite. High Si-content is strong chunky graphite promoter, especially in castings with slow cooling rate. Ni addition also promotes chunky graphite formation, but only in thermal centre of the casting (where the cooling rate is the lowest). The elongation is severely lowered when chunky graphite appears in the microstructure.

Soundness Discrimination in Ferrite Ductile Irons Through Tensile Data Analysis

“…No detailed micrographs of chunky graphite are reported in Reference 18, even if the presence of chunky graphite has been widely reported for heavy section products. 9,10,19,20 In Table 3, also in 4.5 wt% HSSDI some graphite degeneracy was present with considerable nodularity decrease, and some chunky graphite is also reported in Figure 4. So, even if no correlation as reported in References 19 and 20 between the unusual plastic behavior and chunky graphite was possible, there was an evident connection between unusual plastic behavior and graphite degeneracy that is known to be detrimental for tensile mechanical properties.…”

“…In DIs produced through heavy sections where the solidification rates are very slow, the nodule count decreases and nodule size increases, while graphite degenerates causing the decrease in nodularity and even the possible appearance of chunky graphite. 9,10 All of this leads to decreasing and scattering mechanical properties. 11,12 So in heavy sections, the sizes and the density of defects are expected to affect significantly the strain hardening behavior and the quality assessment procedure might need some corrections.…”

Chunky Graphite in Ferritic Spheroidal Graphite Cast Iron: Formation, Prevention, Characterization, Impact on Properties: An Overview

“…Published studies reveal different quantitative results on a maximum tolerable Si content (1.8-2.4%) depending on the solidification time (wall thickness) and the contents of trace elements (also called tramp or accompanying elements). 6,9,13,25,33,37,41,42,49 With the recently standardized grades of solid solutionstrengthened ferritic SGI, the Si content is a matter of special interest. [50][51][52] With these SGI grades, Si is limited to approximately 4.2% 50 since above this limit Fe-Si superstructures gain detrimental influence on the mechanical properties [53][54][55] and chunky graphite appears.…”

“…74 chunky graphite. 1,3,8,9,14,20,47,49,51,66,73,82,[91][92][93][101][102][103] Consequently, premature failure of the whole chunky graphite affected region in a component may arise, 1,8,47,82 and cleavage fracture in areas otherwise consisting exclusively of ferrite and graphite nodules is promoted. 82,91,92,102,103 The fundamental correlations between chunky graphite and strength as well as ductility demonstrated for low Si ferritic SGI have been confirmed in Reference 52 for silicon contents up to 4.0%.…”

Study of the Influence of Alloying Elements on the Mechanical Characteristics and Wear Behavior of a Ductile Cast Iron

“…They decrease the quantity of carbon soluble in the austenite and thus increase the quantity of free carbide in the iron at a given carbon content. Recent research works [13,15,[19][20][21][22][23][24] have shown that, Si, Mn, Ni, Mo, V, Cu, Ti, Nb, W, Sn… are the typical alloying elements used to control ferrite and pearlite contents and increases mechanical properties in ductile iron as-cast grades. Mn and Cu are used to promote pearlite, Si is used to promote ferrite and to strengthen it.…”