F. E. Mariani

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.

Austempering and Boro-Austempering Treatments in Gray Cast Iron

Austempering heat treatment of gray cast iron can significantly improve its mechanical properties. In addition, increasing its surface hardness by means of thermochemical treatments such as boriding can further extend its range of use. In this work, gray cast iron samples with the composition 3.6 % C - 2.1 % Si - 0.43 % Mn - 0.26 % Cr - bal Fe were subjected to austempering treatment using austenitizing at 900°C, with subsequent cooling using austempering salt baths at temperatures of 240°C, 300°C, and 360°C for 1, 2, 3, and 4 hours with subsequent cooling in air. Another set of samples was subjected to boro-austempering treatments, which consisted of boriding at 950°C for 2 to 4 hours in molten borax and followed by cooling in salt baths at temperatures of 240°C, 300°C, and 360°C for 1, 2, 3, and 4 hours, with subsequent air cooling. This approach avoids the need for further heating the work piece to perform the austempering treatment. Subsequently, the samples were characterized for micro-hardness and adhesive wear behavior. The austempering treatment significantly increased material performance, and the boro-austempering treatment further improved its properties.

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.

Wear Behavior of a Borided Nickel-Based Self-Fluxing Thermal Spray Coating

Increasing the hardness of materials used in machines can lead to a longer useful life while decreasing maintenance downtime. However, high hardness usually results in an increase in brittleness of the material, which can be avoided by surface treatments, such as thermal spray coatings that are largely used to produce coatings with high resistance to wear and corrosion. This method has advantages, such as easy application, relatively inexpensive in comparison with other methods, and does not increase the brittleness of the base material. However, in some applications, the wear resistance may not be sufficient. To further improve the wear resistance of these coatings, a thermochemical boriding treatment can be subsequently applied to produce a duplex coating. In this work, an SAE 1020 steel was used as the substrate for the application of a thermal sprayed surface coating. The alloy applied is nickel based and self-fluxing with composition NiCrBSiC. The boriding treatment was subsequently performed at 850°C for 2 h. Optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and Vickers micro-hardness and micro-abrasive wear tests were used to characterize the samples. The flame spray produced coatings of approximately 900 HV, which was increased to 1600 HV after the boriding treatment.

Characterization of Layers Produced by Boriding and Boriding-PVD on AISI D2 Tool Steel

Boride layers with high hardness and wear resistance were produced on AISI D2 tool steel by thermo-reactive treatment using borax with the addition of 10 wt. % aluminum. Subsequently, PVD treatments were performed on the borided layer. The samples were characterized using optical microscopy, scanning electron microscopy, X-ray diffraction, micro-hardness testing, and free-ball micro-abrasive wear test. The boriding treatment time was 4 h, followed by cooling in oil or air. The average thickness of the borided layers was 120 μm with a hardness ranging from 1400 to 1700 HV. The micro-abrasive wear tests were performed using an abrasive solution of silicon carbide. All layers produced exhibited a wear resistance much higher than that of the substrate. Samples with duplex PVD plus boriding treatments yielded the best wear performance, followed by the borided steel samples, and finally, the untreated quenched and tempered samples.