High chromium martensitic steels are designed to provide high corrosion resistance in combination with high strength. Some of these steel grades contain primary carbides for improving the wear resistance, e.g. the steel 440C. The present paper mainly deals with the effect of chemical composition and microstructure on the corrosion properties. Different experimental alloys were produced in the shape of small ingots. The influence of the alloying elements chromium, molybdenum, cobalt, and carbon on the corrosion properties was studied. The results can be summarized as follows: Chromium and molybdenum improve the corrosion resistance, however, only the content of these elements in solid solution in the steel matrix is effective. In case of cobalt the corrosion resistance decreases.The reason is the interaction between cobalt and carbon and its effect on the chromium content in the steel matrix. The calculated pitting resistant equivalent number of high chromium martensitic steels is only limited valid, because there is a major effect of carbide precipitation on the corrosion behaviour. Further investigations were focused on the heat treatment. Especially the effect of the tempering temperature of these steels was studied. The tempering temperature is most relevant for secondary hardening carbide precipitation, which lowers the chromium content of the matrix with detrimental influence on the corrosion properties. The carbide precipitation and chromium distribution was characterized by means of energy filtered transmission electron microscopy (EFTEM).
The material requirements on aircraft engine mainshaft bearings increase due to an elevated speed index (bearing bore diameter multiplied by rotational shaft speed) and slip ratios [1]. The formation of reaction layers on surfaces in mechanical contact is strongly affected by the tribological loading conditions, the materials used, the lubricant, and the service temperature. An appropriate reactivity between material and lubricant in tribological systems decreases wear and friction and increases the durability [2,3]. Goal of the paper is to compare wear and friction properties of standard aerospace bearing steel AMS 6491 (M50) with that of the high strength stainless steel grade AMS 5898. The nominal chemical compositions are 0.82C-4.1Cr-1V-4.2Mo (wt%) and 0.3C-0.4N-15.2Cr-1Mo (wt%), respectively. In order to characterize the material behavior under pure sliding conditions, ball on disc (BOD) experiments were performed with a contact pressure of 1GPa and a sliding speed of 10 cm/s at room temperature and at 150°C. As lubricant the jet engine oil Mobil Jet II was used. It is assumed that the reaction layer formation depends on the material composition and is also effected by the counterpart and the lubricant additives. Thus, the experiments were performed with two different ball materials. The first ball material was the same standard aerospace bearing steel (M50) as mentioned above and the second was a ceramic (Si3N4). The homogeneity and the distribution of the reaction layers as well as the wear rate were determined in the contact zone by means of optical profilometry, scanning electron microscopy (SEM) and Fourier transformation infrared spectroscopy (FTIR). The study showed that wear is significantly higher on the stainless steel grade compared to M50 (Fig. 1a and Fig. 1b). The dark areas in Fig. 2 are phosphorus rich regions on the wear track of M50. This reaction layer is mainly built up of phosphates, which result from the TCP lubricant additive. In the FT-IR spectra (Fig. 3) absorption bands at 1130 cm−1 (room temperature-BOD test) and at 1160 cm−1 (150°C-BOD test) are visible, which result from the P=O stretching [4]. The shift of the absorption band to lower wave numbers with decreasing test temperature is probably due to hydrogen bonding [5]. Contrary to AMS 6491 no measurable reaction layer was found on AMS 5898 after testing at room temperature (Fig. 1b). The friction coefficients of the two steels against Si3N4 balls determined in the BOD tests are compared in Fig. 4. AMS 5898 shows an abrupt increase of the friction coefficient after a sliding distance of 3.5 m. The reason for that is a material transfer of disc material to the ceramic ball as can be seen in Fig. 5. This transfer material causes a plowing of the disc and thus, an increased wear can be observed. The different Cr-contents and consequently oxide layers of AMS 5898 and AMS 6491, which react differently with TCP, might explain this behavior. AMS 5898 does not sufficiently react with TCP at room temperature to form a protective layer. Consequently, material transfer and increased wear occurs. In case of AMS 6491 an increase of the operating temperature cause a change of the reaction layer (see Fig. 3) and to an increase in the wear rate.
Abrasive and erosive properties of coal can lead to serious operational problems in mining industry. Here the abrasive medium is varied, containing often fractions of quartz, pyrite, and contents of coal which can be also mixed with water resulting in an abrasive slurry mixture. A better understanding of the dependence between the environmental and systematic conditions of the tribological processes that happen in ore mining and the wear behaviour of steels can be achieved only by analysing each parameter that forms these tribological processes. The Steel Wheel Abrasion Test (SWAT) represents a testing method which is capable of experimentally modelling the high stress wear environment typical for mining industry. The aim of this work exposes a comparison between the dry and slurry tribocontact in terms of wear, as well as the influence of coal simulating real field conditions.
The formation of reaction layers on surfaces in mechanical contact is strongly affected by the tribological loading conditions, the materials used, the lubricant and the service temperature. An appropriate balance between reactivity of material and lubricant in tribological systems decreases wear and friction and increases the durability. The goal of this paper is to compare the reaction layer formation on a standard aerospace bearing steel AMS 6491 (M50) with that on a high strength stainless steel grade AMS 5898 exhibiting a nominal chemical compositions of 0.82C-4.1Cr-1V-4.2Mo (wt. %) and 0.3C-0.4N-15.2Cr-1Mo (wt. %), respectively. As lubricant jet engine oil Mobil Jet II has been used. Rolling contact fatigue (contact pressure: 6 GPa) and ball-on-disk tests (contact pressure: 1.6 GPa, sliding speed 10 cm/s) at room temperature and at 150°C were employed to study the effect of extreme loading conditions and temperature dependence of the reaction layer formation. The contact areas were inspected by means of optical profilometry, scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS) in order to determine type, thickness, homogeneity, and distribution of the reaction layers in the contact zone. The main result of the study is that the reaction layer formation is significantly less on the stainless steel grade compared with M50. SIMS depth profiles were determined in order to explain the differences in the wear characteristics of the two materials. The reaction layers are mainly built up of PXOY molecules, which might be phosphates resulting from the tricresyl phosphate (TCP) lubricant additive. The role of the chemical reaction of the steel and TCP regarding the layer formation will be discussed.
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