János Ginsztler

Activation processes of stress relaxation during hold time in 1Cr–Mo–V steel

Abstract A quantitative analysis of activation process during hold time under creep–fatigue interaction conditions has been interpreted for 1Cr–Mo–V steel. The apparent activation energy for stress relaxation at the saturated stage was the same for the lattice diffusion activation energy of iron i.e. 251 kJ mol −1 independent of the total strain range. Analyzing the value of the activation volume for the initial transient relaxation behavior, it is suggested that the rate controlling dislocation mechanism is either cross slip, or overcoming Peierls–Nabarro stress. The trend of increasing activation energy with the relaxed stress was found to be due to the effective stress which was decreased with time and approached to zero.

Stress dependence on stress relaxation creep rate during tensile holding under creep-fatigue interaction in 1Cr-Mo-V steel

A quantitative analysis of the stress dependence on stress relaxation creep rate during hold time under creep-fatigue interaction conditions has been conducted for 1Cr-Mo-V steel. It was shown that the transient behavior of the Norton power law relation is observed in the early stage of stress relaxation in which the instantaneous stress is relaxed drastically, which occurs due to the initial loading condition. But after the initial transient response in a 5 hour tensile hold time, the relations between strain rate and instantaneous stress represented the same creep behavior, which is independent of the initial strain level. The value of stress exponent after transition was 17 which is the same as that of the typical monotonic creep suggested from several studies for 1Cr-Mo-V steel. Considering the value of the activation energy for the saturated relaxation stage, it is suggested that the creep rate is related to instantaneous stress and temperature by the Arrhenius type power law.

Microstructural Stability of Duplex Stainless Steel Weldments

Welding experiences are demonstrated, which were developed on 2205 type duplex and 2509 type superduplex stainless steels (DSS and SDSS). The welded joints were produced by activated TIG welding and laser beam welding. For the ATIG process, which is a special kind of the TIG-welding, silica powder was used as activating flux. Microstructural characterizations were applied for the measurement of ferrite/austenite phase ratio in the weldments. The welded joints were thermally aged at temperature of 400 °C. The phase transformations produced during thermal ageing were investigated by optical and scanning electron microscopy and thermoelectric power measurement. Investigations show that the thermal ageing induced phase transformations are similar at ATIG-welded weldment and DSS base material, but the incubation time is significantly shorter in the weldment than in the base material when laser welding is applied.

Infiltration Characteristics and Compressive Behaviour of Metal Matrix Syntactic Foams

The most promising process for metal matrix syntactic foam (MMSF) production is pressure infiltration. In case if it can be advanced to die casting the cost of the MMSFs will drop significantly. The first step on this road is to characterize the kinetics of the pressure infiltration with respect to infiltration pressure and time. Experimental infiltration equipment was built and many preliminary tests were performed on the AlSi12 + SLG system. The load bearing capacity is also important, therefore the compressive behaviour of MMSFs were investigated. According to the results engineering factors (matrix material, size of the microballoons, applied heat treatment, temperature of the tests) have significant effects on the compressive properties.

Life extension by cavity annihilation heat treatment in AISI 316 stainless steel under creep-fatigue interaction conditions

The creep-fatigue life extension of AISI 316 austenitic stainless steel by heat treatment for cavity annihilation was investigated. Different heat treatments were conducted to find better conditions which could enhance the effect of treatments for the life extension. Cavities formed during creep-fatigue tests were found to be annihilated during solution heat treatment, and the cavity annihilation led the materials to have longer creep-fatigue lives. Having different heat treatments, it can also be suggested that the extension of the creep-fatigue life can be maximized by changing the ageing condition, after solution treatment, to have the lower density of grain boundary carbides that serve as nucleation sites for the cavities. The specimen aged at a higher temperature which has the lower value of cavity nucleation factor, P′, has a longer extended creep-fatigue life. In this specimen, cavity re-generation during the loading after annihilation of previous cavities is retarded owing to the lower density of the grain boundary carbide.

Development and Manufacturing of Coronary Stents in Hungary

Revascularisation by endovascular implant (stent) is of great importance in the treatment of coronary artery diseases. Hungary has a stent production since 1995, which is the only one in the new EU member countries. The paper presents the antecedents, achievements and main future objectives of the project, that have been started within the frameworks of the National Research and Development Program. The aim of the project is the development of a new stent family based on the results of the material science researches have started for 10 years and the clinical and production experiences of the experts of the consortium, which could result an equivalent Hungarian product to the market leader products in case of several product lines.

Polyurethane Coating on Coronary Stents

Stents are special metallic or polymer endoprostheses of meshed structure and tube shape. Their function is to prevent restenosis in the arteries. Stents can be coated or uncoated. In the expanded part of the artery the chance of restenosis is bigger even without a stent so it is practical to coat the stents. The aim of this work is to present the results of the coating experiments made on the coronary stents. Three types of commercially available polyurethanes were used for these experiments. The coatings were produced by a dipping method. Electro-polished and non-electro-polished metallic sheets and stents were used for these experiments. Contact angle measurements were done to examine the wetting properties of the three different polyurethane coatings. The quality and the changing of the coatings were examined by different methods (stereomicroscope, scanning electron microscope and energy dispersive spectrometry).

Mechanical Behaviours of Coronary Stents

The role of the stents is to prevent restenosis. The rapid growth of stents’ application in the treatment of cardiovascular diseases resulted in the unique development of these implants. This is mainly due to the effective clinical trials, the success of which determined the use of these endoprostheses. In this study the surface properties of the coronary stents were described by using different methods (stereomicroscopy, scanning electron microscopy and energy dispersive analysis) before and after balloon expansion. Furthermore, the most frequent failures caused by the expansion were introduced. For investigating fatigue properties two high cycle fatigue test equipments were used: the first one simulates the bending stress, and the second one simulates the effect of the pulsating mechanical strain. Surface features of the stents were examined after the fatigue tests as well: macroscopic damages were not originated on the stents, and the implants were not broken down. Only small traces of fatigue occurred on the surface, which became rough; and slip lines and grain boundaries were outlined.

Investigation of Notch Sensitivity and Blade Breakage of Bandsaw Blade Steels

Bandsaw blades are one of the most commonly used tools of the wood cutting industry. Their base materials are mostly unalloyed or poorly Cr-alloyed tool steels. After the review of the bandsaw blade’s cyclic load characteristics, we present the typical failures, which can be, originate in it’s cracking. With the use of these information an overall systematization has been set up by the bandsaw blade cracking types and its root causes. The main place of the failure is the weld and the heat-affected zone’s area where the welding can failure or the lack of toughness can produce cracking. An other zone, where cracking may appear is the bandsaw blade’s tooth gullet because it raises the stress intensity factor. Special material testing methods have been made to investigate the tooth gullet’s notch sensitivity from the test results that a comparison of several bandsaw blade base material, are reviewed. With the use of the results a notch sensitivity rating system has been worked out, which can be used to rate different bandsaw blade base material’s notch sensitivity.

Measuring the Heparin Binding Capability of Polyurethane-Coated Stainless Steel Stents with a Label-Free Biosensor

In the development of modern drug eluting implants it is crucial to be able to measure the long term desorption processes which determine the drug elusion. In this article we set up a simple model for these measurements, which consists of stainless steel stent-model samples with polyurethane coatings incubated in heparin, and a label-free OWLS (Optical Waveguide Lightmode Spectroscopy) biosensor to measure the elusion of heparin from these samples. We found that porous coatings bind heparin better than smooth coatings, and that the PUR materials tested by us all have different binding properties. We also constructed a dual-flow set-up for the instrument, which enabled us to do real-time elusion measurements instead of sample injection, which would make the monitoring of desorption processes much clearer, although this measurement method is yet to be fully developed.