Materials Science Forum Vols. 537-538

Abstract: The modified Jominy-test was designed for prediction of hardenability of high-hardenability tool steels and possibility of application of modified Jominy-test in computer simulation of quenching of high-hardenability tool steels has been investigated. Because of high hardenability there are limits in application of original Jominy-specimen in simulation of quenching of steels. The performance of investigated modified Jominy-test in simulation of quenching of high-hardenability tool steels was estimated by comparison of cooling curves of modified Jominy-specimen (JM®-specimen) and cylindrical specimen. The influence of dimension of JM®-specimen on cooling curves has been investigated. The time of cooling, t8/5 relevant for results of quenching was predicted. Modified Jominy-test can be applied in simulation of quenching of steel with higher hardenability rather than original Jominy-test.

Abstract: A study has been made of asperity interaction of unlubricated steel/rubber sliding pair. The aim is to study the effect of the internal friction (hysteresis) of rubber on the friction force. In the two-dimensional finite element analysis, asperities are modeled by cylinders and both the interfacial adhesion and the friction at steel-rubber interface are neglected. Rate-dependent material behavior of rubber is described, as a first approximation, by a three-parameter Zener-model. It is found that the viscoelastic properties of rubber have a strong influence on the hysteresis component of friction. Distribution of energy loss generated over a cycle of contact in the rubber asperity is also studied. It is concluded that the energy dissipation is most intensive at a certain depth below the rubber surface.

Abstract: In order to alter the physical/chemical characteristics of multi-walled carbon nanotubes (MWNTs) we modified them by different organic reactions (Diels-Alder and Sand-Meyer reaction, oxidation) and their d properties were characterized by thermogravimetry/mass spectrometry, photoelectron spealterectroscopy, and nuclear magnetic resonance spectroscopy, as well as by dispersion. The results proved that, depending on the groups built in the MWNTs, the modified carbon nanotubes are more dispersible either in polar or apolar solvents and the suspensions are stable for long time. The presence of the substituents in the MWNTs was proved by methods listed above, e.g. high concentration of sulfur was detected when SO3H groups were inserted onto the MWNTs. The enhanced thermal stability of the modified carbon nanotubes allows their further application.

Abstract: Residual stresses are an inevitable result of mechanical and heat treatments of steel. The stress field generated at the surface and through the cross section of a product is usually non-uniform, and there can be important stress gradients. In general residual stresses can be defined as self-balanced internal stresses existing in a free body on which no external forces are acting. In the present study the focus was on the residual stresses occurring in a steel deep-drawn product of a complex form. The residual stresses occur mainly because of different degrees of plastic deformation at different locations. The residual stresses occurring in the product as a result of the manufacturing process of deep drawing can be comparatively strong. In slitting or cutting a useful product out of a deepdrawn part by laser cutting, a portion of the residual stresses will get relieved and produce distortion of the deep-drawn product. The studies made showed that the distortion of the deep-drawn product depended on the mode of cutting-out. Also the state of the residual stresses after slitting or cuttingout of the deep-drawn product will depend on the mode of cutting-out, i.e., on the sequence of individual cuts chosen. The product used in the analysis made was a deep-drawn steel mud-guard, which makes a component part of a working machine. The various modes of cutting-out a useful product of 1260x800x370 mm in size were performed with laser cutting using cutting oxygen. The residual stresses in the mud-guard were determined with a semi-destructive hole drilling strain gage method.

Abstract: The paper describes the results obtained in an investigation of laser remelting of specimens made of 12% Ni maraging steel suitable for the manufacture of highly thermomechanically loaded machine parts or tool parts. The aim of the investigation was to establish the state of differently heat treated maraging steel after a laser repair processes including melting of the material. An analysis of the specimens with an optical microscope after laser remelting showed, regardless of the initial condition, a cellular-dendritic microstructure in the remelted track. In the heat-affected zone surrounding the laser-remelted track the microstructure varied considerably also regardless of the initial condition. A microstructure analysis supported by microhardness measurements made it possible to define several microstructural zones or sub-zones in the heataffected zone. The measurement of residual stresses using the relaxation hole-drilling method close to the laser-remelted track was used to study the influence of the initial material state on the residual stresses after laser remelting.

Abstract: Laser Shock Processing (LSP) is a process of laser treating of a surface with a pulsed beam of high power density. The process enables hardening of a thin surface layer; therefore, it is suitable for the improvement of fatigue strength of quality materials. Locally directed mechanical waves produce a considerably increased dislocation density in the thin surface layer, which affects the variations of microhardness and residual stresses. The magnitude and variation of the residual compressive stresses in the surface layer are favourable, which ensures higher fatigue strength. Laser shock processing (LSP) is more exacting than conventional shot peening, but it shows certain advantages such as better control of the surface state, processing of locally limited surfaces and a possibility to produce different transitions between the processed surface and the non-processed one. LSP has so far been tested and efficiently applied to various materials, including maraging steels. Relevant publications often deal with LSP mechanisms and the influence of the process on the dynamic strength of maraging steel, but less frequently the influence of individual characteristics such as the microstructure of matrix and of precipitated phases or residual stresses. The present paper deals with LSP of 12% Ni maraging steel. The material chosen is suitable for the production of complex structural parts and dies for die casting, which require high resistance of the material to thermo-mechanical loads. By means of measurement of the state before and after LSP, the value of the mean roughness Ra, surface defects and the variation of residual stresses in the thin surface layer were determined. After LSP of the surface, the influence of processing parameters such as laser-beam diameter and pulse density per unit of area was established.

Abstract: The paper presents the results of investigations made to clear how the different manufacturing technologies of silicone rubber structures affect the mechanical properties of the products. The problem is not obvious since there are numerous devices produced especially for the health care. We also controlled extreme high purity silicone rubbers and compared the results with data measured on simple industrial silicone rubbers. The aim was to see the difference between expensive medical materials and normal ones to simplify the fatigue tests and to decrease the costs of other mechanical investigations.

Abstract: The metallurgical slag is a normal building material in Europe but not in Hungary. I prove by my own researches that the electric arc furnace slag isn’t harmful to the nature especially the ground water and their physical aspects are similar to the natural rocks. It means that we can use slag to replace them to save our environment and to clear away the old slag dumps.

Abstract: Modern vehicle bodies make intensive use of high strength steel grades to improve the weight and the mechanical performance simultaneously. A broad range of medium and extra high strength steel grades is available. These steel grades have different characteristics concerning strength, formability and weldability. For many steel grades microalloying by niobium is the key to achieve their characteristic property profile. In HSLA steels niobium enhances the strength primarily by grain refinement. In interstitial free high strength steels niobium serves as a stabilizing element and also assists in obtaining the bake hardening effect. Some modern multiphase steels rely on niobium to achieve additional strength via grain refinement and precipitation hardening. Microstructural control provides a way to further optimize properties relevant to automotive processing such as cutting, forming and welding. The relevance of niobium microalloying in that respect will be outlined.