DRUŠTVO LIVARJEV SLOVENIJESLOVENIAN FOUNDRYMEN SOCIETY
Član The World Foundry OrganizationMember of World Foundry Organization

Meanwhile the calculation of cooling and solidification is a well introduced tool in the foundry industry for calculating gating and riser systems. However, knowing the areas of last solidification is often no answer to the question whether there are any defects in the casting or not. Of course one requirement is an exact description of the heat flow and the heat transfer between melt and mould. But furthermore the liquid/liquid and liquid/solid shrinkages influence the temperature fields due to a convective heat flux. Also auxiliary materials influence the liquid/solid behaviour. Auxiliary materials can on one hand be the inoculation material and on the other chills with and without coating. This is the reason why defects near the gating often can not be detected by a conventional solidification calculation which does not take into account the impact of the auxiliary material on the convective flow. With new algorithms within the FEM program SIMTEC/WinCast ® it is possible to incorporate this behaviour within the simulation of solidification. Special attention was directed to the expansion of graphite in ductile irons. The graphite first nucleates in the liquid and then continues to grow in the solid phase depending on the alloy composition and the solidification parameters. With the knowledge of the physical properties of the alloy the location and the size of defects can be predicted precisely.
Key words: liquid/solid shrinkage, influence of auxiliary material, graphite growth

 

The aim of this work is to study the influence of mechanical and metallurgical properties of hot-work tool steels determined by heat treatment and thermal-chemical surface treatment on in-service life of pressure die-casting tooling. The study is performed on Uddeholm hot-work tool steels; Dievar, Orvar Supreme, QRO 90 Supreme, and Vidar Supreme. A number of heat treatment parameters, such as austenitizing temperature, quenching speed, heat treatment system, and different tempering approaches are studied. Thermal-chemical surface treatments such as nitriding and oxidation are also studied. To characterize the material, hardness and toughness are measured and metallographic examination is performed. A test apparatus is developed for the assessment of thermal fatigue resistance of the materials, which is estimated by the surface area of cracks developed on the surface of test specimens subjected to cyclic thermal loading. Material characterization is performed at different stages of the thermal fatigue test to study the evolution of mechanical and metallurgical properties throughout the service life. Temperature transients at different locations of test specimens are measured and used in computation of transient stresses performed by finite elements. A relationship between the thermal fatigue resistance and surface stresses is developed. It is also expressed as a function of the initial material properties and the material type.
Key words: High-pressure die casting, hot-work tool steel, heat treatment, thermal fatigue, toughness, hardness, microstructure, finite element modelling.

Relations between chemical composition, microstructure and mechanical properties of low-alloyed Cr-Mo steel, of the WC 9 quality, and high-alloyed Cr-Mo-V steel, of the C12A quality (according to ASTM SA-217/SA-217M), for the production of valve housings for elevated temperature service had been analyzed in this paper. Applied heat treatment process that consisted of normalization and tempering at high temperatures (above 700 °C) resulted the formation of ferrite regions and regions of proeutectoid ferrite with fine dispersed carbides inside the ferrite grains and on the boundaries of original austenite grains in the case of low-alloyed WC9 steel. Microstructure of high-alloyed C12A steel consisted after the heat treatment process (normalization and tempering at high temperatures) of ferrite and bainite with finely dispersed carbides. This microstructure resulted in achieving values of mechanical properties and of retention of the attained mechanical properties at elevated temperatures, i.e. high creep resistance. The obtained results of examinations revealed that performed production process with the heat treatment of steel that followed gave satisfactory microstructures and mechanical properties as demanded by standards.
Key words: low-alloyed Cr-Mo cast steel, high-alloyed Cr-Mo-V cast steel, normalization, high-temperature tempering, elevated temperature service