Gabriela Sikora

Effect of Compaction Pressure Applied to TiC Reactants on the Microstructure and Properties of Composite Zones Produced In Situ in Steel Castings

Compacts for the synthesis of composite zones in castings were obtained by cold pressing powders of the TiC reactants under a pressure of 250, 300, 500 and 600 MPa. The all compacts made under different pressures were placed in a mould cavity and poured with liquid unalloyed low-carbon steel. From the resulting casting, four composite zone A, B, C, D, produced in this casting by in situ method. In all composite zones, TiC and ferrite (αFe) were obtained. Additionally, in zones A, B and C the presence of graphite (Cgr) was also stated. The surface friction (Sf) of Cgr decreases in composite zones A ÷ D, while both the Sf of TiC and hardness VH30 increase in these zones with increasing compaction pressure of the reactants. Too low compaction pressure applied to the TiC reactants impedes the effective propagation of the reaction of synthesis.

Identification of Mg 2 Cu particles in Cu-alloyed austempered ductile iron

In the present work, the Mg2Cu precipitates in copper-alloyed austempered ductile iron (ADI) were identified by analyzing techniques such as TEM and SEM with EDS. It was revealed that, in castings made of ADI-containing copper, highly dispersed particles of Mg2Cu are formed, whose size does not exceed <1 μm. The research work was carried out on ductile iron that was austenitized at 900 °C, followed by austempering at 380 °C. The microstructure was investigated using various techniques, including optical microscopy, XRD, SEM, and TEM. In addition to this, the exhibited impact properties of castings with Cu, Ni, and Cu+Ni were also determined. This study casts a new light on the formation of the structure of Cu-alloyed ADI. The highly-dispersive and brittle Mg2Cu particles that are located in the vicinity of the graphite nodules have a negative effect on the impact properties of ADI. It has also been shown that impact strength decreases from levels of 160-180 J (for copper-free ADI) to 90-120 J (for copper-and copper-nickel-alloyed ADI).

Transformation Kinetics and Mechanical Properties of Copper-Alloyed and Copper-Nickel Alloyed ADI

In this study the effect of copper and nickel in shaping the structure and properties of ADI (Austempered Ductile Iron) was investigated. The austenitization and austempering transformations were studied in order to follow the changes exhibited in transformation kinetics. The dilatometric results indicated that the addition of Cu and the addition of both Cu and Ni resulted in reducing relative expansion during austenitization, due to a larger pearlite fraction in the microstructure. In the initial stage of the austempering process, the addition of Cu, and to a greater extent, additions of both Cu and Ni led to a reduction in the transformation rate, shifting the maximum transformation rate values toward longer times. X-ray diffraction, dilatometric, metallographic and magnetic examinations allowed us to determine the phases fraction in the structure of ADI with the presence of Cu and Ni. From SEM-EDS analysis, it follows that in the copper alloyed ADI, highly dispersed particles are formed containing Mg and Cu, whose size does not exceed <1 µm. The exhibited mechanical properties were determined as a function of Cu and Ni additions and also variable austempering period of time. It was found that the addition of Cu resulted in increased tensile strength and hardness but simultaneously decreased the impact strength of ADI. The outcome of this work indicates that in order to obtain a satisfactory combination of static and dynamic mechanical properties of ADI, an optimal combination - aside from proper heat treatment - Cu and Ni should be selected.

Effect of Different Molding Materials on the Thin-Walled Compacted Graphite Iron Castings

This article addresses the effects of six mold materials used for obtaining thin-walled compacted graphite iron castings with a wall thickness of 3 mm. During this research, the following materials were analyzed: fine silica sand, coarse silica sand, cerabeads, molohite and also insulated materials in the shape of microspheres, including low-density alumina/silica ceramic sand. Granulometric and SEM observations indicate that the sand matrix used in these studies differs in terms of size, homogeneity and shape. This study shows that molds made with insulating sands (microspheres) possess both: thermal conductivity and material mold ability to absorb heat, on average to be more than five times lower compared to those of silica sand. In addition to that, the resultant peak of heat transfer coefficient at the mold/metal interface for microspheres is more than four times lower in comparison with fine silica sand. This is accompanied by a significant decrease in the cooling rate of metal in the mold cavity which promotes the development of compacted graphite in thin-walled castings as well as ferrite fractions in their microstructure.

Effect of cooling rate and of titanium additions on the microstructure of thin-walled compacted iron castings

This article addresses the effect of cooling rate and of titanium additions on the exhibited microstructure of thin-walled compacted graphite iron (TWCI) castings as determined by changing moulding media, section size and ferrotitanium. Various moulding materials were employed (silica sand and insulating sand ‘‘LDASC’’) to achieve different cooling rates. This study shows that the cooling rates exhibited in the TWCI castings varies widely (70–14 °C/s) when the wall thickness is changed from 2 to 5 mm. In turn, this is accompanied by a significant variation in the compacted graphite fraction. The resultant cooling rates were effectively reduced by applying an insulating sand in order to obtain the desired graphite compactness. Ti additions in combination with LDASC sand moulds were highly effective in promoting the development of over 80% compacted graphite in castings with wall thicknesses of 2–3 mm as evidenced by quantitative metallographic analyses.