Piotr Dziarski

Fracture Toughness of Plasma Paste-Borided Layers Produced on Nickel-Based Alloys

The boriding treatment is the suitable process which caused an increase in surface hardness and wear resistance of nickel and its alloys. However, the phase composition of boride layers strongly influences on layer properties—especially hardness and brittleness. The method of plasma paste boriding was used in this study to produce the hard boride layers on nickel-based alloys: Ni201, Inconel 600, and Nimonic 80A. This process was carried out at 800 °C (1073 K) for 3 h. The chemical composition of substrate material was the reason for producing of layers which were characterized by different thickness: 55 μm for Ni201, 42 μm for Inconel 600, 35 μm for Nimonic 80A. The lowest hardness (1000–1400 HV) and the highest fracture toughness (up to 2.6915 MPa m1/2) were measured for layer produced on Ni201. In this specimen, only nickel borides were detected. However, due to high content of chromium, in case of Inconel 600-alloy and Nimonic 80A-alloy, the higher hardness (in the range of 1000–2450 HV) and higher brittleness (average value of K c = 0.77 MPa m1/2 for Inconel 600-alloy and K c = 0.67 MPa m1/2 for Nimonic 80A-alloy) were calculated. This situation was caused by the appearance of hard ceramic phases (chromium borides CrB and Cr2B) in borided layer. Simultaneously, at the cross section of each sample, the strong fluctuation of hardness occurred, due to the variable participation of chromium and nickel borides.

Corrosion resistance of laser-borided Inconel 600 alloy

Inconel 600 alloy is used extensively for a variety of industrial applications involving high temperature and aggressive environments. However, under conditions of appreciable mechanical wear (adhesive or abrasive), this material has to be characterized by suitable wear protection. The diffusion boronizing efficiently improved the tribological properties of this alloy. Nevertheless, the long duration of this process was necessary in order to obtain the layers of the thickness up to about 100 μm. In this study, instead of the diffusion process, the laser alloying with boron was used for producing a boride layer on Inconel 600 alloy. During this process, the external cylindrical surface of base material was coated by paste, including amorphous boron, and remelted by a laser beam. In the remelted zone, the three areas were observed: compact borides zone consisting of nickel and chromium borides (close to the surface), zone of increased percentage of Ni–Cr–Fe matrix (appearing in the greater distance from the surface) and zone of dominant percentage of Ni–Cr–Fe matrix (at the end of the layer). The hardness was comparable to that-obtained in case of diffusion boriding. Simultaneously, the laser-borided layer was significantly thicker. In order to evaluate the corrosion behaviour, the immersion corrosion test in a boiling solution of H2O, H2SO4 and Fe2(SO4)3 was used. As a consequence of selective laser alloying, the difference in electrochemical potentials between the layer and base material caused the accelerated corrosion of the substrate in areas without laser-borided layer. The results showed that laser-borided Inconel 600 alloy could be characterized by the excellent corrosion resistance in such corrosive solution if the whole surface would be covered with laser-alloyed layer.

Laser alloying of 316L steel with boron and Stellite-6

Austenitic 316L steel belongs to one of the most numerous groups of alloys with special properties. It is well-known for its most effective balance of carbon, chromium, nickel and molybdenum concentrations for corrosion resistance. However, under conditions of appreciable mechanical wear (adhesive or abrasive), this steel should be characterized by suitable wear protection. Diffusion boronizing and laser alloying with boron were often used in order to improve tribological properties of 316L steel. In this study, the method of laser alloying was modified in this way that alloying material contained the mixture of amorphous boron and Stellite-6 powders. The coated surface was remelted by the laser beam using TRUMPF TLF 2600 Turbo CO2 laser. After the laser alloying process, the composite surface layer was produced. Only two zones occurred in the laser-alloyed 316L steel: remelted zone and the substrate (base material). Heat-affected zone was invisible because the austenitic steel could not be hardened by typical heat treatment. The remelted zone consisted of hard ceramic phases (iron, chromium and nickel borides) in the soft austenitic matrix with the increased concentration of cobalt. Some properties of this layer were investigated and compared to the laser-alloyed layer with boron only. The produced layer was characterized by a compact microstructure which was free of cracks and gas pores. The layer was also uniform in respect of the thickness because of the high overlapping used during the laser treatment (86%). The obtained thickness was significantly higher than that obtained in case of diffusion boriding. In spite of the lower hardness of remelted zone, the increase in wear resistance of the proposed surface layer was observed in comparison with laser-alloyed 316L austenitic steel with boron only.

Laser-borided composite layer produced on austenitic 316L steel

Abstract Abstract Austenitic 316L steel is well-known for its good resistance to corrosion and oxidation. Therefore, this material is often used wherever corrosive media or high temperatures are to be expected. The main drawback of this material is very low hardness and low resistance to mechanical wear. In this study, the laser boriding was used in order to improve the wear behavior of this material. As a consequence, a composite surface layer was produced. The microstructure of laser-borided steel was characterized by only two zones: re-melted zone and base material. In the re-melted zone, a composite microstructure, consisting of hard ceramic phases (borides) and a soft austenitic matrix, was observed. A significant increase in hardness and wear resistance of such a layer was obtained.