Frederico Augusto Pires Fernandes

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.

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.

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.

Influence of alloying elements Cu, Ni and Mo on mechanical properties and austemperability of austempered ductile iron

Austempered ductile iron (ADI) is the most recent development in the nodular iron family. The austempering treatment produces a unique microstructure, ausferrite, which provides high mechanical strength combined with ductility, toughness, and good fatigue and wear resistance. The effect of alloying elements Cu, Ni and Mo on the mechanical properties and austemperability of ADI is reported. The mechanical strength and toughness decreased with the addition of Mo, but both wear resistance and austemperability increased with Mo content.

Wear Evaluation of Pack Boronized AISI 1060 Steel

Boronizing is used to obtain very high hardness and wear resistance on ferrous and non-ferrous materials and some super alloys. The process can be carried out in solid, liquid, or gaseous medium. In this work, samples of AISI 1060 steel were treated by the pack boriding method during 2 and 4 h at 900 and 1000°C. Optical microscopy, x-ray diffraction, and micro-hardness analyses were conducted and wear tests were performed using a micro-wear machine with a fixed-ball configuration. The pack boriding resulted in the formation of layers with high hardness and wear resistance. All the borided layers presented top hardness around 2000 HV (Vickers pyramid number). Raising time and temperature of treatment increased the layer thickness. X-ray diffraction patterns showed the presence of only Fe2B for samples treated at 900°C for 2 and 4 h. The samples borided at 1000°C for 2 and 4 h presented layers composed by a mixture of Fe2B and FeB. The wear resistance of the samples borided at 1000°C was higher than that borided at 900°C, probably because of the difference in chemical composition.

Effect of the strain-hardening on the wear and corrosion behaviour of a Fe-Mn-Al-C alloy

Iron alloys with appreciable amounts of aluminium yield a corrosion resistant alumina surface film. The addition of Mn to these alloys makes them austenitic, significantly increasing ductility. Due to these factors, it is interesting to develop austenitic Fe–Mn–Al alloys that can substitute, in some applications, traditional stainless steels. This study aims to evaluate the wear and corrosion resistance of a Fe–32Mn–8Al–1C alloy after cold–rolling. Rolling was carried out with thickness reductions of 20%, 40% and 80%. Wear tests were performed using a micro–wear machine and corrosion tests performed in natural sea water. The results showed that Fe–Mn–Al–C alloy exhibits good cold–forming ability with a progressive increase of hardness with thickness reduction. The wear resistance increased as the reduction level was increased and corrosion resistance decreased with the increase of the strain–hardening, due to deformation. Such behaviour may be associated with stress corrosion, which is amplified by the degree of reduction.

Estudo do mecanismo de corrosão por pites em água do mar de aços inoxidáveis supermartensíticos microligados com Nb e Ti

Supermartensitic stainless steel (SMSS) is increasingly used in harsh environments due to its good weldability and mechanical properties at higher temperatures and high resistance to corrosion under stress. Applications in oil exploration demand a superior combination of properties and duplex and superduplex stainless steels have been widely applied in this area, despite their high costs. SMSS provides a technical and economical alternative for these steels. In this research, additions of Nb and Ti were made in order to minimize the sensitization effect and to promote grain refinement, studying the microstructural aspects and the pitting corrosion resistance in seawater. Pitting formation and evolution were accompanied by corrosion testing, optical and electronic microscopy. The Ti alloyed steel showed the best corrosion performance, with the greatest corrosion potential and the lowest pitting potential. The steel with Nb addition presented a higher corrosion potential than that of the steel without additions but had a lower pitting potential.