The formation of a nitrided layer on steel change its electrical (conductivity) and magnetic properties, among which the relative permeability and magnetic coercive field strength are the most sensitive to phase and structural changes in nitrided steel. The diffusion layer at ambient temperature and up to a temperature of approx. 590°C is ferromagnetic. In contrast, the iron nitride surface layer built of the ε-Fe2N1 - x and γʹ-Fe4N1 - x phases is paramagnetic at the process temperature. The coercion and iron nitride saturation depend on the nitrogen concentration in the nitrides — with increasing nitrogen content, the coercion decreases, while the saturation increases .
Changes in the magnetic properties of unalloyed steel AISI 1085 and alloyed steel AISI 420-C before and after the controlled gas nitriding (CGN) process have been investigated in the manuscript.
2. TESTING METHODOLOGY
AISI 1085 (C85) and AISI 420 (X20Cr13) steels in the form of balls, with chemical composition and diameters as shown in Table 1, were subjected to nitriding processes.
The controlled gas nitriding (CGN) processes were realized in an Nx609 type industrial soaking furnace with working space dimensions of Ø600×900 mm. Test steel samples were nitrided in two-stage processes which differed in temperature, time and nitriding atmosphere nitrogen potential [2, 10]. Table 2 summarizes the basic parameters of the nitriding processes.
After the nitriding processes, the steel balls were subjected to standard physical metallurgy and X-ray examinations. The nitrided layer structural examinations were conducted using a Zeiss Neophot 2 light microscope on Nital-etched microsections, and included also the thickness measurement of the nitride layers formed in the nitriding process.
The use of the ball-shaped test samples allowed the accuracy of nitride layer thickness measurement to be increased. A schematic diagram of the iron nitride layer więcej »
Gas nitriding is a thermochemical processing implemented at the temperature range 400÷580°C. At a constant temperature, depending on the value of the nitriding potential, the subsurface iron nitride layer formed may consist of only the γʹ-Fe4N phase or a mixture of phases γʹ-Fe4N and ε-Fe2-3N. A diffusion zone is formed under the iron nitride layer, in which nitrogen is dissolved interstitially in a ferritic matrix and carbonitrides of iron and alloying elements occur. The thickness and phase composition of the layers of iron nitrides are decisive on the resistance to corrosion and the abrasive wear of steel after nitriding. The diffusion zone, in the case of alloy steels, increases the fatigue strength of steel. .
Generally, there are three types of nitrided layers :
class 0 — nitrided layers without a subsurface iron nitride layer, which is a solution of nitrogen in iron α-Feα(N),
class 1 — nitrided layers with a subsurface layer of iron nitrides with a thickness of up to 13 μm, composed of a solution layer α-Feα(N) and a nitride layer Fe4N (phase γʹ) or (ε + γʹ),
class 2 — nitrided layers with a subsurface layer of iron nitrides with a thickness of 25 μm, composed of a solution layer α-Feα(N) and a nitride layer F4N (phase γʹ) and a nitride layer Fe2-3N (phase ε).
Obtaining these types of layers requires the application of appropriate process parameters, i.e. selecting the appropriate values of the nitriding potential (Np) for the process temperature set. Mutual relations of the nitriding potential and temperature result from the Lehrer system  and for carbon steel and low-alloy steel are as follows:
Increase of the solution layer α, without nitrides γʹ i ε, takes place with nitriding potential Np < Npα/γʹ where Npα/γʹ means the potential corresponding to t więcej »
Modification of austenitic stainless steel surface aiming at improvement of mechanical and functional properties, such as hardness or wear resistance, has been one of the main tasks of surface engineering for several decades. One of the methods to improve these properties is obtaining a nitrogen and/or carbon supersaturated solid solution in austenite which is called S-phase (also γN or expanded austenite). The most frequently used method is low-temperature nitriding. S-phase layers have higher hardness and comparable to or sometimes even better corrosion resistance than austenitic stainless steel [1÷7]. Low-temperature treatment is a process carried out at temperature below 500°C. It prevents the formation of nitrides (mostly chromium nitrides), the presence of which significantly decreases corrosion resistance of austenitic stainless steel.
It is also possible to obtain S-phase as a coating with magnetron sputtering. Austenitic stainless steel is then sputtered in reactive atmosphere containing nitrogen [1, 2, 8÷22]. This method allows the temperature of the preparation of S-phase to be reduced even below 200°C. Moreover, an easy control of nitrogen concentration in the coating is also possible by regulation of the nitrogen content in the reaction chamber. Thanks to the combination of these two basic parameters the deposition of coatings with varying thickness, morphology and mechanical properties is possible.
2. MATERIAL AND METHODS
Coatings were deposited by means of reactive magnetron sputtering method (RMS). Discs of 50 mm in diameter made of austenitic stainless steel (wt %: 18.5 Cr, 9 Ni, 2 Mn, 0.5 Si, 0.4 Cu, Fe in balance) were used as targets. Substrates were made of the same steel grade. Prior to the deposition they were ground using abrasive papers down to 1200 and then with diamond pastes down to 1 μm. After grinding the substrates were vibro-polished with Al2O3 in order to remove the layer of ferrite resulting f więcej »
Tramway are a common city transportation of special performance and functionality. To increase tram transport effectiveness and safety, better understanding of wheel-rail contact wear is required. The most severe wear of wheel is observed on the running gear, especially on the rolling surface.
Problems regarding the durability of the of the wheel-rail contact are associated with many different forms of wear caused by the surface contact friction and fatigue. In literature, models for calculating the of wheel-rail load could be found . It should be underline that, not only rolling friction in wheel-rail contact is present, but mixed: rolling-slip (sometimes, in the case of rail curves, wheel flanges are exposed to almost pure sliding friction [2, 3]).
In case of wheel-rail interaction rolling contact fatigue (RCF) wear can occur . Fatigue wear in this friction pair can proceed as spalling or shelling.
Slides in curves, speeding up, braking etc. can cause heating of the surface layer above austenitizing temperature in a very short time (e.g. a few seconds) which leads to the formation of martensite. Then, further movement of the tram and cyclic loading of the wheel cause spalling. For instance, those slips at the wheel-rail contact are considered to be the common reason of faster wheel wear of diesel multiple units in district of Wielkopolska . Heating (due to friction) above austenitizing temperature is a reason of so-called ‘white layers’ formation. Mostly, the phenomenon of the white layers creation takes place in the cutting hard steels. White layers could arise as a consequence of dynamic processes affecting intensive deformation and the related with them thermal effects [6, 7]. Because of their high hardness and brittleness they can favour spalling wear. Such layers were observed in the case of rail [6, 8] and also in case of tram wheels of Solaris Tramino after 146 000 km of approximate total millage . The p więcej »
Collecting electrodes along with emitting ones are the most important components of electrostatic precipitators. On Polish market and abroad there are available a few topologies of collecting electrodes (Fig. 1) .
Combustion and industrial gases are loaded with mineral matter like ash. Collecting electrodes are designed in order to collect this matter deposited by electrostatic charge. Afterward this deposit are removed by forced vibration caused by hitting the assembly beam with a hammer (Fig. 2) . A collecting electrode should effectively collect ash during changing conditions of ESP (ESP — electrostatic precipitators) operation. Leading producers of ash removing devices use formed profiles of Sigma type. This topology of electrodes combines the following basic features: high elasticity and rigidity resulting in efficient ash removal, fast dynamic reaction to hitting; limited release, erosion of the ash into the gas because of profile of the electrodes [3, 4]. Sigma type electrodes hang on the upper beam where their twist is limited. The bottom assembly of electrodes is rigid due to screws and a beam. Such structure allows accelerations in the range of 100÷140 g during ash removal by forced vibration.
According to  efficient ash removal from collecting electrodes requires around 100 g acceleration. Efficiency of ash collection with collecting electrodes and how ease they can be cleaned depends on their topology and material used for manufacturing. The more elastic this material is and faster vibrations are transferred, the higher efficiency can be expected . Taking into account this relation producers state their demands concerning mechanical features of steel strips for production of collecting electrodes by roll forming. It is often observed that cheap steel grades are used because the market situation leads to growing competition between ESP producers, and price is the decisive criterion. To date progress in techno więcej »
Progress in aerospace is linked with the development and application of the new structural materials. Those materials are characterized by higher strength, lower density and good resistance to environmental factors also at high temperature, while compared with materials which are in use. With the development of materials new technologies are also developed for their processing. They allow the use of the material in real components and lead to improved properties of the elements design, materials savings and reduced manufacturing cost. This applies to both: new and conventional materials used in the construction of aircraft engines [1÷3].
The near-β titanium alloys are characterized by good ductility and low susceptibility to cracking, which classify them as alloys suitable for deformation . One of them, Ti-10V-2Fe-3Al alloy, can be used at elevated temperature and under variable loads, is resistant to atmospheric and sea water corrosion as well . All these aspects along with weight savings facilitated its use in forged components of aircraft structures.
According to the literature data, the morphology of microstructure constituents and thus operational properties of the near-β titanium alloys may be changed suitably controlling the heat treatment parameters (temperature and time). Solution treatment from the temperature, where both phases: α and β exist allows to create globular precipitates of phase α. Heating the alloy to a temperature above the phase transformation temperature and the subsequent cooling below the temperature of β phase stability makes possible the precipitation of α phase in the form of needles in an amount dependent on the heat treatment parameters .
It is worth to notice that during cooling of near-β titanium alloys the martensitic transformation is possible. This transformation is induced by previous plastic deformation, when the α phase volume fraction materia więcej »
Development of high efficiency turbine engines leads to evolution of heat-resistant superalloys of different types. The nickel-based superalloys are still being the most frequently used in high temperature applications, whereas new types of γ-γʹ cobalt-based analogues are becoming gradually more effective and popular [1÷3]. These alloys exhibit better oxidation, corrosion and wear resistance than Ni-based alloys, although have inferior strength. Solidus temperature of these alloys is 100÷150°C higher than those of commercial nickel alloys, such as CMSX-4 [4, 5]. The microstructure of γ-γʹ Co-based superalloys contains face-centered cubic matrix γ, strengthened by γ′ phase, which is a ternary compound with the L12 structure and usually Co3(Al, W) formula . Furthermore, in the microstructure occurs variety of carbides (M23C6, M7C3 and MC). In fact, formation of γ′ phase is difficult due to required value of lattice mismatch less than 1% . The γ′ Co3(Al, W) exists owing to Al and W content. The alloying elements such as Ti, Ta, Nb, Mo and V promote the γ′ formation and increase solvus temperature. It is also confirmed that the phase stability of the γʹ phase greatly increases by Ni substitution for Co because the Ni3Al with the L12 structure is very stable and the γʹ phase exists in a wide composition range in the Ni-Co-Al ternary phase diagram . One of the most popular alloy from this group is tungsten containing Co-9Al-9W and Co-20Ni-7Al-7W (at. %) alloys [9, 10].
Co-Al-W alloys after high temperature oxidation are characterized by multilayered oxide structure [11, 12]. Surface layer has been characterized as cobalt monoxide, which is harmful to humans and dangerous for the environment. Middle layer consists of mixed oxides of Al, W and other elements present in alloy . Furthermore, several authors reported an external Al2O3 scale in cobalt-based więcej »