Babette Tonn

Theory of Graphite Nucleation in Lamellar Graphite Cast Iron

The precise knowledge of the fundamentals of nucleation is the key to a reliable production process for the fabrication of high quality cast iron products. In most foundries the production process is optimized based on practical experience and not on precise theoretical knowledge. Our work focuses on the research of graphite nucleation in lamellar cast iron. The results of the microscopical investigations are showing for the first time centres of graphite lamella. They are present in the middle of the MnS-particles. The nucleation of graphite on MnS particles was confirmed by microstructure simulations. Studies on the influence of Mn and S on the grain microstructure show that the contents of both elements play an important role. By treating the melt with small contents of Mg, MnSMg particles could also be observed in the matrix as well as in the graphite centres.

Nucleation of graphite in cast iron melts depending on manganese, sulphur and oxygen

Abstract The form and distribution of graphite in grey iron influences the mechanical properties and depends on numerous factors, such as nucleation and cooling speed conditions. The main focus of the present work is the influence of manganese, sulphur and oxygen on the nucleation of graphite in lamellar cast iron melts. Previous studies showed that the nucleation in GJL melts is initiated by a MnS particle. For investigations in the field of nucleation in grey iron the authors examined several EN-GJL-200 specimens. The specimens were cast with and without inoculant. The studies of the specimens were realised using light microscopy, SEM-EDS (energy dispersive X-ray analysis), WDS (wavelength dispersive X-ray analysis). The experimental results were compared with the software Thermo-Calc calculations. From the experimental results and the Thermo-Calc calculations it can be concluded, that Mn and S contents and the Mn/S ratio respectively plays a very important role for the nucleation. A direct influence of oxygen on the nucleation of graphite could not be observed.

Experimental Investigations on the Influence of the Thermal Conditions During Composite Casting on the Microstructure of Cu–Al Bilayer Compounds

Metallic material compounds are of technical interest as they allow the combining of specific properties of different materials into one component. In particular, compounds of copper (Cu) and aluminum (Al) are a current object of research. There are different fabrication processes, such as roll cladding or overlay welding. In recent years, a variety of alternative bonding methods were investigated, among other composite casting processes. A notable problem of Cu–Al composites is the formation of hard and brittle interface layers which reduce the mechanical properties of the compounds. Therefore, the reduction of these layers is one of the objectives of the development of new fabrication processes to realize suitable industrial applications. In this study, gravity casting experiments are carried out to produce Cu–Al bilayer composites and to determine the influence of the thermal process parameters on their properties. Two experimental setups using sand molds are developed, and casting experiments with varying thermal parameters are carried out. Specimens of the cast composites are extracted, metallographic examined and the hardness profile is determined. Hence, it is possible to identify the influence of thermal parameters on microstructure and hardness of cast Cu–Al bilayer composites.

New Wear Resistant Hypereutectic AlSi4Cu4FeCrMn Alloys for High Pressure Die Casting

In order to adapt the AlSi14Cu4FeCrMn alloy to the production of monolithic engine blocks in HPDC as well as to improve their tribological properties, the following study was performed. The silicon content in the hypereutectic Al-Si alloy was reduced to allow for a lower casting temperature. The addition of iron to the alloy compensated for the reduced volume fraction of hard primary silicon particles and improved the tribological properties.

Ductile Cast Iron with High Toughness at Low Temperatures

High life expectancy of cast components and good material performance at dynamic load are a prerequisite to cater for future trends in wind energy generators. To remain competitive in this ever evolving sector challenges reside in alloy development. In this work fractional factorial design has been applied to ferritic ductile iron with varying contents of silicon (1.6‑2 wt%), nickel (0‑1 wt%), cobalt (0‑3 wt%) and copper (0‑0.2 wt%). The minimum criteria the new alloy should meet were a minimum yield strength of 240 MPa and an impact work of minimal 8 J at a temperature of -20 °C for wall thicknesses of 60‑200 mm. To obtain these mechanical properties thick-walled castings with additional insulation were produced to achieve a higher thermic module. They provided the material for test specimens to perform static tensile tests, Charpy impact tests at varying temperatures and a microstructure analysis. With these results, a sweet spot plot has been created. That way, an optimum alloy composition could be found and has been proven by validation experiment.The optimum alloy for thick-walled castings is composed of Si = 1.6 wt%, Cu = 0.2 wt%, Ni = 0 wt% and Co = 0 wt%. It offers an enhancement in yield strength and acceptable impact work at low temperatures for massive castings in as cast state. The heat treated, full ferritic material could even improve these results.

Impact of Molybdenum on Heat-Treatment and Microstructure of ADI

Austempered Ductile Iron (ADI) is characterized by high tensile strength with acceptable ductility. Steel, as a large competitor to ADI, also meets the tensile and yield strength. Nevertheless, the main advantages of ADI compared to steel are the lower density (7.2 g/cm3 to 7.85 7.87 g/cm3) for weight reduction and lower manufacturing costs because of less energy consumption during the production. One of the main problems of producing ADI is the quenching process during heat treatment of thick-walled castings. The inner part of a massive casting – in contrast to the outer part – cools down more slowly, resulting in a heterogeneous microstructure with parts of pearlite and ferrite embedded in austenite before reaching the isothermal transformation temperature. Molybdenum is, besides nickel, copper and manganese, one of the possible alloying elements that postpone the transition point of ferrite and/or pearlite. To investigate the influence of molybdenum in thick-walled castings experiments with different molybdenum contents were performed. In dependence on the molybdenum content, different austenisation and ausferritisation temperatures and times are examined in order to investigate the transformation points, fraction and morphology of different phases. The mechanism of molybdenum in ADI has been investigated by means of dilatometer tests, microstructure analysis and mechanical tests.

Degenerated Graphite Growth in Ductile Iron

As part of a study devoted to the effect of trace elements on graphite degeneracy, near-eutectic ductile iron melts were prepared to which minute amounts of lead and of both lead and cerium were added. The melts were cast into an insulated Y4 mould, giving a solidification time of about 1 hour and a cooling time to room temperature of about 15 hours. In the thermal centre of the Pb containing sample graphite spheroids as well as intergranular lamellar graphite have been found. At the same location of the casting containing both Pb and Ce, exploded as well as chunky graphite could be observed, while the formation of intergranular lamellar graphite has been suppressed. Deep etching of the samples allowed reaching the following conclusions: i) intergranular graphite in the SG-Pb sample often, if not always, originates on graphite nodules and extends towards the last to freeze areas; ii) in one location of the SG-PbCe sample, chunky graphite strings were observed to originate on an exploded nodule, thus confirming the close relationship between these two forms of graphite. Because of the over-treatment in cerium of the SG-PbCe sample, other unusual degenerate graphite was observed which appears as coarse aggregates of porous graphite after deep etching.

Occurrence and behaviour of Mo containing precipitates in nodular cast iron at high temperatures

In nodular cast iron, molybdenum is alloyed in the range of 0.5 to 1.5u200awt.-% to increase the mechanical properties for high temperature application, especially for thick walled castings like housings of gas or steam turbines. The behaviour of Mo precipitates in the as cast state as well as under long exposure to heat was the object of this investigation. In the as cast state, Mo forms metastable carbidic grain boundary precipitates with iron, silicon and carbon. These transform into stable MoC precipitates during sustained temperature influence. Furthermore, there are finely dispersed Mo containing precipitates found in the matrix. These increase the high temperature strength of Mo alloyed cast iron due to the Orowan mechanism and the occurrence of additional grain boundaries. However, it has been shown that these Mo containing precipitates underlie Ostwald ripening under continuous heat exposure, and a precipitate free area occurs along the ferritic grain boundary.

Application of Internal Fusible Chills in Thick‐Walled Castings Made of EN‐GJS with an Optimized Microstructure

The degeneration of graphite in thick‐walled components made of ductile niron due to slower solidification affects the mechanical properties and is unacceptable for all safety‐relevant components. The inoculation of the melt no longer leads to a fine microstructure. After exceeding the critical solidification time, degenerated shapes of graphite are to be expected. The external cooling with a chill‐mould does not eliminate graphite degeneration in the thermal centres. The positive effect of these chills is also limited by the wall thickness. The aim of this study was to increase the heat dissipation of the melt by positioning the internal fusible chills in the thermal centre of the mould cavity. This should lead to accelerated solidification. The plate‐shaped chills were placed in the middle of rectangular samples. The solidification processes were first simulated with Magmasoft in order to optimize the size and shape of the fusible chills and to thus guarantee a complete dissolving of the chills. A reduction in the solidification time of approximately 15% was achieved. In the experiments thick‐walled samples were cast with and without internal fusible chills and compared. Areas with degenerated graphite, including chunky graphite, were found in the centres of the cast samples without internal cooling. Placing fusible chills in castings increased the number of graphite spheroids in the microstructure and exhibited no graphite degeneration. A homogenous microstructure was developed—no residues of the chills were found. Differences in microstructure and mechanical properties between the edges and centres of the casting could be nullified. The optimized graphite morphology of the casting with internal cooling led to an increase in tensile strength in the thermal centre of about 30 MPa (8%). This process was successfully implemented in an industrial environment. Blocks out of EN‐GJS‐400 for use in hydraulic engineering with a total weight of eight tonnes were cast in permanent moulds using fusible cooling plates at Gontermann‐Peipers GmbH. Homogenous microstructures as well as higher mechanical properties were achieved. This new method shows good perspectives for further applications in composite casting technologies.

Substitution of Nickel by Combined Addition of Cobalt and Zirconium in Alloy A 332

Due to the increasing international competition and the resulting pricing pressure it is imperative to avoid the use of expensive alloying elements during the production of aluminium castings. The piston alloy A 332 shows an optimum combination of mechanical and casting properties and an attractive cost‐performance ratio whereas nickel is the most expensive alloying element. A substitution of nickel by a combined addition of low contents of cobalt and zirconium has the potential capacitiy to increase the mechanical properties and reduce the costs of the alloy. At Clausthal University of Technology Thermo‐Calc simulations and casting experiments were carried out to investigate the effect of the nickel subtitution. Thermo‐Calc‐simulations were made to analyze the intermetallic phases in these alloys. These simulations were evaluated by observations under optical microscope and SEM of specimens poured into permanent moulds. The size and morphology of the intermetallic phases and the primary silicon was analy...