Systematic laboratory and field exposure tests allow to compare nine different stainless steels (three ferritic, three austenitic, and three duplex grades) for civil engineering applications. The influence of surface finish was also taken into account by choosing five different industry‐specific features. The three duplex stainless steels revealed excellent corrosion resistance under most of the test conditions. Especially for applications in civil engineering the lean duplex steels offer distinct advantages. The manganese‐alloyed 1.4376 and the ferritic steel 1.4509 cannot be recommended as alternative materials as they did not perform satisfactorily.
The corrosion resistance of stainless steels is massively influenced by the condition of their surface. The surface quality includes the topography of the surface, the structure and composition of the passive layer, and the surface near structure of the base material. These factors are influenced by final physical/chemical surface treatments. The presented work shows significantly lower corrosion resistance for mechanical machined specimens than for etched specimens. It also turns out that the rougher the surface, the lower the corrosion resistance gets. However, there is no general finding which shows if blasted or grinded surfaces are more appropriate, but a dependency on process parameters and the characteristics on corrosive exposure in terms of corrosion behavior. The results show that not only the surface roughness Ra has an influence on corrosion behavior but also the shape of peaks and valleys which are evolved by surface treatments. Imperfections in the base material, like sulfidic inclusions lead to a weaker passive layer, respectively, to a decrease of the corrosion resistance. By using special passivating techniques the corrosion resistance of stainless steels can be increased to a higher level in comparison to common passivation.
AnlaB der Untersuchungen war das Versagen von Abhangern aus austenitischem Niro-Stahl einer Betonunterdecke in einem Hallenbad in Uster (Schweiz). Untersucht wurden hochlegierte nichtrostende Stahle bei definierten mechanischen und umweltrelevanten Belastungen. Dazu wurden in einer Versuchsreihe Salzspots an BUgelproben aufgebracht und bei 40 "C und 35 bzw. 70 % rel. Feuchte ausgelagert. In einer Spannungsriss-Priifanlage wurden zugbelastete Rundproben bei den selben Temperatur-und Feuchtigkeitsbedingungen untersucht. Die beste Korrosionsbestandigkeit zeigen die hochstickstoffhaltigen Stahle 1.4529 und 1.4565. Den grijBten Korrosionsangriff erlitten die Stahle 1.4401, 1.4462 und 1.4539. Spannungsrisskorrosion (SpRK) konnte metallographisch nur an den Stahlen 1.4401 und 1.4462 bei 35 % rel. Feuchte nachgewiesen werden. Ein Zusammenhang zwischen Wirksumme der Stahlzusammensetzung und Korrosionserscheinung ist deutlich zu erkennen.The work was started on the occasion of failures of stainless steel components at the indoor swimming pool atmosphere in Uster (Switzerland). Highly-alloyed stainless steels were tested at defined mechanical and environmental conditions. Therefore U-bend specimens with salt spots were examined at 40°C and 35 and 70% rel. humidity respectively. A further test series was realised with round specimens under constant load in a stress cracking testing system at the same temperature and humidity conditions.The highly-nitrided steels 1.4529 and 1.4565 showed the best corrosion resistance. The steels 1.4401, 1.4462 and 1.4539 incured the highest corrosion attacks. Stress corrosion cracking (SCC) was determined at the steels 1.4401 and 1.4462 at 35 % rel. humidity by metallographic structure micrography only. A distinctive relationship was observed between the pitting resistance equivalent and the kind of corrosion attack.
Shape Memory Alloys (SMAs) are materials that have the ability to return to a predetermined shape when heated. Being cold or below their transformation temperatures or in their martensitic forms (Fig. 1b), they have a very low yield strength and can be quite easily deformed into any new shape (Fig. 1c). However, when one alloy is heated above its transformation temperature, it undergoes a change in crystal structure, reverts to austenite and recovers its previous shape (Fig. 1a). If the SMA faces any resistance during this transformation, it can generate extremely large forces. This process is known as one way shape memory effect (OWME). [1] In trained SMAs two way shape memory effect (TWME) can be observed in the sense that no mechanical deformation is needed to complete the strain cycle, but only thermal cycling. The phase transformation does not occur at a single temperature but over a range of temperatures which varies with each alloy system. A method of determining and naming transformation temperatures is to thermally cycle a specimen under load, producing a T-e-curve, as shown in Figure 2. Most of the transformation occurs over a relatively narrow temperature range, although the beginning and end of the transformation during heating or cooling actually extends over a much larger temperature range. The transformation also exhibits hysteresis in that the transformations on heating and on cooling do not overlap. This transformation hysteresis (shown as T 1 in Figure 2) varies with the alloy system. Differ-ential Scanning Calorimetry (DSC) and Electrical Resistance Testing are also used to determine transformation temperatures.Crystallography of SMAs: Thermoelastic martensites are characterised by their low energy and glissade interfaces, which can be driven by small temperature or stress changes. The needle structure of martensite essentially consists of ADVANCED Among other special characteristics Shape Memory Alloys (SMAs) have the ability to return to a predetermined shape when heated. In fact, the phase change of an existing element can strongly be influenced by thermal and thermo-mechanical treatments. Up to now, SMEs have been discovered in various materials, which can generally be classified into noble-metal based, Cu-based, Fe-based, Ni-Tibased alloy systems and non-metallic SMAs. In this paper a general overview of the Ni-Ti system and a detailed review over the Ni-Ti-Cu system will be given with special regard to the influence of heat treatments upon the phase change behavior. Fig. 1. (a) Beta phase crystal. (b) Self-accommodating twin-related variants, A, B, C, and D, after cooling and transformation to martensite. (c) Variant A becomes dominant when stress is applied. Upon heating, the material reverts to the beta phase and recovers its original shape [2].
In automotive a lot of electromagnetically, pyrotechnically or mechanically driven actuators are integrated to run comfort systems and to control safety systems in modern passenger cars. Using shape memory alloys (SMA) the existing systems could be simplified, performing the same function through new mechanisms with reduced size, weight, and costs. A drawback for the use of SMA in safety systems is the lack of materials knowledge concerning the durability of the switching function (long-time stability of the shape memory effect). Pedestrian safety systems play a significant role to reduce injuries and fatal casualties caused by accidents. One automotive safety system for pedestrian protection is the bonnet lifting system. Based on such an application, this article gives an introduction to existing bonnet lifting systems for pedestrian protection, describes the use of quick changing shape memory actuators and the results of the study concerning the long-time stability of the tested NiTi-wires. These wires were trained, exposed up to 4 years at elevated temperatures (up to 140°C) and tested regarding their phase change temperatures, times, and strokes. For example, it was found that A P -temperature is shifted toward higher temperatures with longer exposing periods and higher temperatures. However, in the functional testing plant a delay in the switching time could not be detected. This article gives some answers concerning the long-time stability of NiTi-wires that were missing till now. With this knowledge, the number of future automotive applications using SMA can be increased. It can be concluded, that the use of quick changing shape memory actuators in safety systems could simplify the mechanism, reduce maintenance and manufacturing costs and should be insertable also for other automotive applications.
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