Luiz Carlos Casteletti

Wear and corrosion resistance of pack chromised carbon steel

Abstract Pack chromising treatment is an environmentally friendly alternative to hard chromium to form wear and corrosion resistant surface layers. In this work, samples of AISI 1060 steel were pack chromised for 6 and 9 h at 1000 and 1050°C using different activator concentrations. Wear tests were performed in dry conditions and corrosion tests in natural sea water for the pack chromised samples and hard chromium. Pack chromising yielded the formation of layers with high chromium concentrations, high hardness and wear resistance. Increasing activator concentration causes no significant change on the morphology and thickness of the layers. The layers produced at 1050°C yielded only a (Cr,Fe)2N1−x phase, and those obtained at 1000°C are composed of a carbide mixture with (Cr,Fe)2N1−x. The sample treated at 1050°C for 9 h resulted in an optimum condition by means of better wear resistance and corrosion properties, which were close to that exhibited by the hard chrome, indicating that pack chromising is a promising alternative.

Inter-relation of Microstructural Features and Dry Sliding Wear Behavior of Monotectic Al–Bi and Al–Pb Alloys

Abstract Immiscible Al-based alloys of monotectic composition have a particular feature of minority phases embedded into the Al-rich matrix. The disseminated particles may act as in situ self-lubricating agents due to their lower hardnesses compared with that of the Al-rich matrix, favoring good tribological behavior. There is a lack of systematic fundamental studies on the microstructural evolution of monotectic alloys connected to application properties. In the present investigation, the monotectic Al-1.2wt%Pb and Al-3.2wt%Bi alloys have been chosen to permit the effect of microstructural parameters on the wear behavior to be analyzed. Directional solidification experiments were carried out under transient heat flow conditions allowing a large range of cooling rates to be experienced, permitting a representative variation on the scale of the microstructure to be examined. Samples of the monotectic alloys having different interphase spacing, λ, have been subjected to microadhesive wear tests, and experimental laws correlating the wear volume with the microstructural interphase spacing and test time are proposed. It was found that microstructural features such as the interphase spacing and the morphology of the minority phase play a significant role on the wear process and that for the alloys examined λ exhibits opposite effects on the corresponding wear volume.

Plasma nitriding and nitrocarburising of a supermartensitic stainless steel

Supermartensitic stainless steels (SMSSs) are a new generation of the classic 13%Cr martensitic steels, lower in carbon and with additional alloying of nickel and molybdenum offering better weldabilty and low temperature toughness. Several works have shown that plasma nitriding and nitrocarburising of stainless steels at low temperatures produces a hard surface layer which results in increased wear resistance. In this work, SMSS samples were plasma nitrided and nitrocarburised at 400, 450 and 500uC. The plasma treated SMSS samples were characterised by means of optical microscopy, microhardness, X-ray diffraction and dry wear tests. The thickness of the layers produced increases as temperature is raised, for both plasma nitriding and nitrocarburising. X-ray diffraction demonstrates that the chromium nitride content grows with temperature for nitriding and nitrocarburising, which also showed increasing content of iron and chromium carbides with temperature. After plasma treating, it was found that the wear volume decreases for all temperatures and the wear resistance increased as the treatment temperature was raised. The main wear mechanism observed for both treated and untreated samples was grooving abrasion.

Effect of niobium in the phase transformation and corrosion resistance of one austenitic-ferritic stainless steel

Austenitic-ferritic stainless steels containing 0.2% and 0.5% niobium were evaluated with respect to the microstructures, microhardness, pitting potential resistance, electrochemical impedance and wearing tests. Niobium is an alphagenic element and has a great influence on phase transformation of duplex stainless steels. In the present investigation, the samples were annealed at 1050 oC and aged at 850 oC to promote the formation of sigma phase. The pitting potential resistance and electrochemical impedance were evaluated in 3.5 % sodium chloride solution. The wear coefficients were calculated by micro-wear tests conducted using a fixed ball machine and aluminum oxide as abrasive. The results show that the niobium content lead to increase on Laves phase formation. This phase, in association with the sigma phase, causes a significant increase in the hardness and wear resistance with a decrease on corrosion resistance of the austenitic-ferritic stainless steels.

X-ray diffraction characterisation of expanded austenite and ferrite in plasma nitrided stainless steels

Abstract The S phase, known as expanded austenite, is formed on the surfaces of austenitic stainless steels that are nitrided under low temperature plasma. A similar phase was observed for nitrided ferritic stainless steels and was designed as expanded ferrite or ferritic S phase. The authors treated samples of austenitic AISI 304L and AISI 316L and ferritic AISI 409 stainless steels by plasma nitriding at different temperatures and then studied the structural, morphological, chemical and corrosion characteristics of the modified layers by X-ray diffraction, scanning electron microscopy/energy dispersive spectroscopy and electrochemical tests. For both austenitic AISI 304L and AISI 316L stainless steels, the results showed that a hard S phase layer was formed on the surfaces, promoting an anodic polarisation curve displacement to higher current density values that depend on the plasma nitriding temperature. A layer having a high amount of nitrogen was formed on the ferritic AISI 409 stainless steel. X-ray diffraction measurements indicated high strain states for the modified layers formed on the three stainless steels, being more pronounced for the ferritic S phase.

Nitriding of Stainless Steels

Because of their corrosion resistance, stainless steels are essential for the modern industrial civilization, especially in the chemical, petrochemical, and food industries. The stainless steel family has the following members: austenitic, ferritic, duplex, martensitic, and precipitationhardening alloys. The last two show higher levels of hardness. The others possess low hardness and consequently low wear resistance, mainly of the tribochemical type—especially the austenitic alloys, which are the most widely used. Among the possible solutions to this problem was the development of appropriate coatings. Thus, a research effort was undertaken with the goal of producing layers of high hardness on these steels to improve their tribological performance without degrading their corrosion resistance. The nitriding and carburizing treatments proved to be the most adequate to attain these properties.

Microstructural Characterization of Layers Produced by Plasma Nitriding on Austenitic and Superaustenitic Stainless Steel Grades

Dept. of Mechanical and Materials Engineering Portland State Univ., P.O. Box 751, Portland, OR, 97207-0751

Decomposition of expanded austenite in AISI 316L stainless steel nitrided at 723K

Expanded austenite (cN), which can be produced during plasma nitriding of austenitic stainless steels, provides high levels of strength, toughness and corrosion resistance by comparison with traditional nitride layers. However, expanded austenite properties can be lost due to decomposition caused its thermodynamic metastability. In the present work, austenitic stainless AISI 316L steel was plasma nitrided at 723 K for 5 h at 500 Pa and microstructurally characterised by X-ray diffraction (XRD), and optical and transmission electron microscopy (TEM) which confirmed the presence of fcc expanded austenite with a lattice parameter up to 9?5% larger than untreated austenite. TEM analyses of thin foils showed that fine nitrides were formed in the cN layer and some areas were observed with a singular lamellar morphology very similar to the pearlite colonies found in carbon steels. Selected area electron diffraction (SAED) analysis suggests that these areas are composed of bcc ferrite and cubic chromium nitrides produced after a localised decomposition of the expanded austenite layer. Amorphous expanded austenite was observed in some areas of the investigated samples. The occurrence of cN decomposition was associated with microsegregation of ferrite stabilisers (Cr, Mo) and depletion of an austenite stabiliser (Ni) in localised regions of the expanded austenite layer.

Effect of binders and surface finish on wear resistance of HVOF coatings

Abstract The high velocity oxygen fuel (HVOF) thermal spray process produces highly wear and/or corrosion resistant coatings. Tungsten carbide with a metallic binder is often used for this purpose. In this work, tungsten carbide coatings containing cobalt or nickel binder were produced by HVOF and characterised by optical and electron microscopy, hardness and a dry sand/rubber wheel abrasion test. The HVOF process produced dense coatings with low porosity levels and high hardness. The wear resistance of the specimens, which were surface treated, increased as the roughness percentage decreased. Tungsten carbide nickel based coating yielded the best wear resistance in the as sprayed condition. However, the wear rate and wear of the two coatings converged to the same values as the number of revolutions increased. Wear behaviour in the ground condition was similar, although the tungsten carbide cobalt based coating yielded better performance with increasing distance travelled during the wear test.

Avaliação da resistência à corrosão do aço AISI 420 depositado por processos variados de aspersão térmica

Among the techniques used to improve materials performance, deposition on the surface of components is a proper way of recovering worn elements. Thermal spraying processes were developed during the last few years and they are a very suitable method to obtain layers with high hardness for protecting or repairing the base component. Employing these processes, it is possible to overlay metallic substrates with polymers, metals and ceramics. Among these processes are: HVOF, Arc-Spray and Flame-Spray. The selection of a particular type of stainless steel for an application involves some considerations, as the corrosion resistance of the alloy, mechanical properties, manufacture feasibility and cost. In this work, used were samples of AISI 1045 steel, coated with stainless steel AISI 420, using the techniques of Arc-Spray, HVOF and Flame-Spray for the comparative study of their corrosion resistance in sea water, aimed at producing low-cost alternative pieces, compared with massive pieces of steel. The best performances in terms of hardness, porosity levels and corrosion resistance of the layers occurred in the following sequence growing: Flame-Spray, Arc-Spray, and HVOF.