No-Bake S-Containing Mold/DI Metal Interactions: Consequences and Potential Applications
The ductile iron liquid metal is very reactive with environmental gases and solid molds after a nodulizing treatment. Mold-metal reactions could alter the spheroidal graphite shape to vermicular or flake. In this research, the metal-mold reactions were enhanced by various sulfur additions to the no-bake mold inserts to study these phenomena and to artificially produce layered dual-graphite structure in the casting. Thermodynamic and solidification modeling together with sets of different experimental techniques were applied to a test casting with internal core passages representing the cylinder head of a diesel engine. The effects of insert surface shape and location in the mold; content and type of sulfur-containing agent; coating; pouring temperature and additional chilling were experimentally studied and discussed. These results could be used to better understand the metal-mold interaction, prevent surface defects and design dual-graphite structure processes.
The Mechanism of Formation of Casting Skin in CG Iron and its Effect on Tensile Properties
Sand-cast compacted graphite (CG) cast iron typically presents a surface layer (the casting skin) whose microstructure is significantly different than that of the bulk material. It is generally believed that the casting skin is the result on interaction between the metal and the mold moisture and atmosphere. This reaction results in a decarburized or graphite-free layer. The thickness of this layer is a function of process variables (e.g. cooling rate, pouring temperature, binder, coating, sand fineness, and Mg and inoculation level). The paper presents a summary of recent findings on the negative effect of casting skin on the static properties of CG iron through the use of the skin quality factor for tensile strength, defined as the ratio between the as-cast and machined tensile strength. A mechanism of casting skin formation is proposed and supported with experimental and computational data.
Using Automated Inclusion Analysis for Casting Process Improvements
Different industrial melting and ladle practices (deoxidation, slag, refractory types, etc.) used in steel foundries were analyzed and compared using an ASPEX automated inclusion analyzer for study of inclusions. The effects of deoxidation and pouring practices on the size, type and number of inclusions were evaluated for steel foundries equipped with induction and arc melting furnaces, with capacities ranging from 1 to 20 tons. Samples were collected from the furnace, ladle, and castings. Specific rules were developed for classification of inclusions by composition, size distribution and shape. Inclusion statistics, including composition, quantity, shape, and size during cast steel processing from the furnace to the final casting were used for treatment optimization in the foundry ladles. INTRODUCTION The toughness of steel is very important in many critical applications where fracture resulting in failure could produce catastrophic effects. The toughness of steel depends on the number, size, shape and composition of non-metallic inclusions in the steel matrix. Hence, exact determination of the characteristics of non-metallic inclusions is essential to the success of research aimed at increasing toughness of steel parts. In a steel part, several types of second phase particles are present including non-metallic inclusions, carbides and nitrides. As illustrated in Figure 1, fracture of steel is due to the growth and coalescence of voids nucleated at some of these second-phase particles. In steel, non-metallic inclusions are the primary particles at which void nucleation occurs. Voids are nucleated at one of these second-phase particles, either by decohesion of the particle-matrix interface or by particle fracture. The voids nucleated at the particles grow until they coalesce by impingement or by the process of void sheet coalescence. Void sheet coalescence requires fracture of the ligament between the voids created at the larger non-metallic inclusions. Recent research has shown that decreasing the volume fraction of inclusions that induce void nucleation and increasing the inclusion spacing result in significant improvement in toughness. The majority of the inclusions in steel are the product of the deoxidation process or reoxidation during pouring and casting solidification. The aim of deoxidation or “killing” is to reduce the dissolved oxygen content of the steel. As the steel solidifies, oxygen dissolved in the liquid cannot be accommodated by the solid crystal structure and therefore reacts with dissolved carbon forming CO gas most of which is trapped in the casting as porosity or pinholes. To avoid porosity, additions are made to the liquid steel to form solid deoxidation products that can be Figure 1: Nucleation of voids on primary inclusions (a, b) and fracture of steel (c) 62 SFSA T&O Automated Inclusion Analysis page 2 floated to the surface and removed. However, some of the inclusions (especially smaller and slow floating particles) are trapped in the steel casting as inclusions. Many of the inclusions in castings are also formed by reoxidation in which liquid steel picks up oxygen from contact with the air during pouring and transport through the gating system. In addition, inclusions can be formed by reaction of the liquid steel with water vaporizing from the molding sands and debris in the gating system. Elements used for deoxidation are those that have a greater affinity for oxygen than carbon and form thermodynamically more stable oxides than iron oxide. These elements include aluminum, silicon, manganese, calcium, zirconium, titanium, magnesium, boron and rare earth metals (REM). Figure 2 summarizes the deoxidation effects of the common deoxidizers: Figure 2: Deoxidation results of the common deoxidizers The most common deoxidation practice utilized for steel castings is the use of aluminum, which produces solid particles of Al2O3. The coalescence of these particles forms aggregates of irregular shape called ‘alumina clusters’. They significantly affect the mechanical and fatigue properties of steel, and may also result in the generation of surface defects. One technique used to diminish the harmful effects of Al2O3 inclusions is calcium treatment. The addition of calcium promotes partial reduction of the Al2O3 inclusions, giving rise to the formation of liquid calcium aluminates with low melting point and spherical morphology, which can easily float out. Most of these liquid inclusions separate easily from the melt, and those not removed are less harmful to the mechanical properties of the final steel product. The reaction sequence followed is: Al2O3 -> CA6 -> CA2 -> CA -> C12A7 where C and A denote CaO and Al2O3, respectively. The CaOAl2O3 phase diagram in Figure 3 shows the presence of CA2, CA, C12A7 in liquid state at steelmaking temperatures (~ 1650 C). 62 SFSA T&O Automated Inclusion Analysis page 3 Figure 3: CaOAl2O3 binary phase diagram 5 As these reactions progress, the activity of Al2O3 decreases gradually in the calcium aluminates, allowing more of the Al to react with oxygen, providing even better deoxidation. These inclusions can also react with the sulfur present to form oxy-sulfide inclusions. Sulfide morphology has been classified under Type I, Type II and Type III inclusions, while modified oxy-sulfides have been classified as Type IV inclusions as shown schematically in Table I. Table I. Different types of sulfides and oxy-sulfides CLASSIFICATION OXIDATION STATE SHAPE OF SULFIDE Type I Under Deoxidized ,e.g., Very low soluble Al ,High residual oxygen Random Large Globules Type II Almost Complete Deoxidation, e.g., Low soluble Al, Low residual oxygen Intergranular Fine Precipitate Type III Deoxidation with Excess Deoxidant, e.g., High soluble Al, Very low residual oxygen Random Large Angular Precipitate Type IV As Type III + Ca , RE , etc Modified Globular 62 SFSA T&O Automated Inclusion Analysis page 4 Type II sulfides are to be avoided as they cause a reduction in toughness. Sulfide morphology is not as critical if sulfur is adequately low (<0.010%), but above 0.025% sulfide morphology becomes critical. An aluminum level greater than 0.03 % has been considered a ‘safe’ chemistry to avoid type II sulfides. However, complex deoxidation reactions, chemistry, local reoxidation, and other unknowns can affect the formation of type II sulfides. Figures 4 and 5 show the effect of chemistry on sulfide morphology, which varies with aluminum content. Figure 4: Effect of chemistry on sulfide morphology Figure 5: Effect of Carbon and Aluminum on Sulfide Shape As shown in Figure 6, the actual sulfide shape depends on the type and amount of the deoxidizers added. Such morphology has a significant effect on the resultant mechanical properties of the cast steel product. Therefore, deoxidized steel castings may still suffer from various detrimental mechanical property values due to poor removal of the deoxidation products. Hence the final deoxidation and pouring practices should be such that they control the number, composition and morphology of inclusions present in the steel. 62 SFSA T&O Automated Inclusion Analysis page 5