G. S. Takeya

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.

Characterization of Coatings Obtained by Boriding Niobizing Treatment of an AISI H13 Steel

Wear is responsible for numerous industrial problems leading to increased maintenance costs due to the necessity of replacing worn components or due to equipment failure and manufacturing process downtime. Surface treatments can improve performance because the component maintains its ductile interior but with significantly improved surface wear resistance while using a minimal amount of material. Niobizing the surface of tool steel leads to the formation of very hard coatings, in the range of 2300 HV, composed of niobium carbide but with limited thickness. Boriding also produces very hard coatings, on order of 2000 HV, but with a much thicker layer than attainable by the niobizing treatment. If both were applied to the same material they could potentially complement each other by forming a duplex coating with a thin, but very hard, surface coating supported by an inner layer with slightly lower hardness but substantially thicker. The objective of this work was to evaluate the wear resistance of a coating formed by the niobizing and boriding diffusion treatments. Boron and niobium coatings were prepared on AISI H13 tool steel by thermo-reactive diffusion treatment in molten borax with 10 wt% aluminum followed by the niobium carbide pack process. The boriding treatment was performed at 900°C for 2 h, followed by a pack process at 1000°C for 2, 4, and 6 h. Optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, micro-adhesive wear test, and Vickers micro-hardness were used to analyze the samples. The boriding and niobium carbide pack process produced coatings with thicknesses above 45 and 4 μm, respectively.

Aging of a Fe-Mn-Al Steel Using Plasma Nitrocarburizing

The aeronautics and automotive industries face increasing demands to lower fuel consumption and consequent CO2 emissions. One method of accomplishing this is to use materials with high strength/weight ratio. Alloys of steels using high manganese and aluminum shows promising results, with densities between 10% and 13% lower than those of conventional steels and high strength because of precipitation of κ-carbides. The performance of these alloys can be broadened with use of surface hardening techniques, attached to suitable heat treatment. In this work, the mechanical characteristics of conventional aging were compared with plasma nitrocarburizing in the Fe-31.2Mn-7.5Al-1.3Si-0.9C (wt. %) steel. The layers produced were characterized using optical micrograph and hardness and wear tests. The treatment produced layers with wear resistance superior to that the substrate, which also had its wear resistance increased because of aging. The increase in hardness was about 2× in the surface and 1.2× in the substrate, which resulted in wear resistances 9× higher than a substrate without any treatment.

Heat Treatment of Precipitation-Hardening Stainless Steels Alloyed With Niobium

Precipitation-hardening stainless steels are iron-nickel-chromium alloys containing precipitation hardening elements such as aluminum, titanium, niobium, and copper. In this work, heat treatment of a novel precipitation hardening stainless steel using niobium as a forming element for the hardening precipitates was carried out in order to increase its hardness. The steel composition was 0.03C - 0.22Si - 17.86Cr - 3.91Ni - 2.19Mo - 1.96Nb (in wt.%). The samples were solution annealed at 1100°C for 2 h. Cooling was done in oil and the samples were subsequently aged at 500, 550, and 600°C. The solution annealed samples exhibited an average hardness of 30 Hardness Rockwell–Scale C and after the aging treatments, the hardness increased to 46 HRC. The hardness increases during the aging treatments were very fast. A 5 min treatment achieved hardness levels that were close to the maximum obtained for this alloy. Niobium was an efficient precipitation hardeners forming a Laves phase of the type Fe2Nb.

Production of Aluminide Layers on Copper and Copper Alloy

Intermetallic coatings were produced on pure copper and aluminum-bronze substrates using the powder aluminizing process. The treatments were carried out at 550°C and 600°C during 30, 60, 120, and 180 min. The layers were characterized by optical and scanning electron microscopy (SEM) associated with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and wear and hardness testing. The treatments performed at 600°C resulted in larger layer thicknesses compared to those produced at 550°C for both copper and bronze substrates. XRD analysis indicated the presence of intermetallic phases FeAl, FeAl3, and CuAl on aluminum-bronze. The layers produced on the Cu and aluminum-bronze substrates exhibited maximum hardness of approximately 750 HV and 870 HV, respectively. The adhesive wear resistance of all formed layers was higher than those of low-hardness substrates (100 HV in the case of copper and 300 HV for the aluminum-bronze). There were no significant differences in the wear resistances of the layers produced on the same substrate. The abrasion resistance of the aluminum-bronze layers were higher than those of the copper layers, because of the greater hardness of the layers formed on the aluminum-bronze.

Influence of Boronizing Treatment on Fe–Mn–Al–Si–C Steel

Austenitic Fe–Mn–Al–Si–C steel provides excellent cold formability and lower density compared to conventional stainless steels. These steels owe their corrosion and oxidation resistance to their aluminum and silicon contents, and the stability of their gamma phase is due to manganese and carbon alloying elements. The production of a boride layer with high hardness can significantly increase its surface hardness and consequently its wear resistance. In this work, Fe–31Mn–7.5Al–1.3Si–0.9C steel samples were subjected to a boriding treatment for 4 h at 900°C. The composition of the bath was 90 % borax and 10 % aluminum. Boride layers with high hardness levels were obtained (1900 HV). There was also a marked increase in wear resistance of the material.