Influence of Carbide-Promoting Elements on the Microstructure of High-Silicon Ductile Iron

Abstract

Because of its low cost, steel scrap is one of the most important raw materials for the production of ductile iron (DI). The amount of carbide-promoting elements in steel scrap, such as chromium, manganese, molybdenum, niobium and vanadium, is expected to increase in the future. Most of these elements have a negative impact on the microstructure and mechanical properties of DI. The solubility of carbide-promoting elements in solid solutionstrengthened DI materials, standardized in DIN EN 1563:2011, is modified by the high silicon content. For these new materials, the tolerance limits for carbide-promoting elements and their mutual influence must be known to ensure a sustainable production process. To investigate the individual and combined impact of carbide-promoting elements on the carbide content in high-silicon ductile iron EN-GJS-500-14, experimental investigations and thermodynamic-kinetic microstructure simulations were carried out. Microstructure was characterized using metallographic analysis, and quantitative relations between chemical composition and microstructure were developed by means of regression analysis. Besides this quantitative analysis, it was found that the formation of grain boundary carbides can be detected via thermal analysis. Furthermore, experiments and simulations showed that vanadium promotes the formation of chunky graphite in high-silicon DI castings.

Determination of Corrosion Resistance of High-Silicon Ductile Iron Alloyed with Nb

“…For all HSDI samples with different Nb content, a change in the phase fraction in the metallic matrix was observed, as well as the presence of small irregular particles randomly distributed and corresponding to polygonal primary NbC carbides. These particles form before the casting due to the high affinity between Nb and C atoms, so they are called primary carbides [18,21,22,[24][25][26][27][28][29][30][31]. Unlike other carbide-forming elements (Mo, Cr, and V), precipitation of NbC carbides occurs randomly throughout the material rather than by positive segregation.…”

“…A new DI classification called High-Silicon Ductile Iron (HSDI) has recently been introduced for DI with Si content ranging from 3.2 to 4.3 wt.%. It has been reported that with increasing Si content, hardness, tensile strength, and yield strength properties are promoted, while elongation to failure decreases [8,[12][13][14] and machinability and corrosion resistance improve [15][16][17][18]. These results suggest a potential advantage in the production process and performance over DI with lower Si content [10,11].…”

“…With the purpose of modifying the structure and properties of HSDI, the addition of different carbide-forming elements, such as V, Cr, Mo, and Mn has been investigated by several authors [18][19][20]. Riebisch et al [18] have studied the effect of carbide-forming elements on HSDI with 3.8 wt.% Si.…”

“…With the purpose of modifying the structure and properties of HSDI, the addition of different carbide-forming elements, such as V, Cr, Mo, and Mn has been investigated by several authors [18][19][20]. Riebisch et al [18] have studied the effect of carbide-forming elements on HSDI with 3.8 wt.% Si. They have found that V has a significant negative influence on graphite morphology, as it promotes the formation of chunky graphite due to a long solidification time and its effect of slowing down the diffusion of C into austenite; on the other hand, Cr and Mn have the greatest influence on pearlite area fraction, due to their segregation.…”

Mechanical Properties and Impact Toughness of Molybdenum Alloyed Ductile Iron

“…Additionally, elevated amounts of Si promote a stable eutectic solidification of carbon, resulting in higher tolerances against carbide formation, particularly in thin wall castings. 7,8 This advantageous combination of static mechanical properties and good machinability has led to a large number of casting components in a wide range of technical applications such as rotor hubs and other wind power components as well as various automotive applications like wheel suspensions and suspension arms made of high silicon ductile iron in the last years. 9 -11 However, dynamic mechanical properties of high silicon ductile iron such as the Charpy impact energy can be described as significantly disadvantageous.…”