ENDURECIMIENTO SUPERFICIAL DE UNA ALEACION BASE Fe-Cr-Ni: CINETICA DE CRECIMIENTO Y CARACTERIZACION MECANICA DE CAPAS BORURADAS

Abstract

The growth kinetics and fracture toughness of the FeB and Fe2B iron boride layer has been evaluated in this study on Fe-Cr-Ni steel surfaces. The boron diffusion coefficient of the borided phase was calculated with the second Fick’s law and with the mass balance equation at the growth interphases, considering that borided layers obey the parabolic growth law. Also, four models considering the Palmqvist type cracking regime were employed for the evaluation of the fracture toughness on boride layers. The paste boriding process was carried out at temperatures of 850, 900, 950 and 1000 ºC, with exposure times of 2, 4, 5, 6, 8 and 10 h, using 4 mm of boron carbide paste thickness at the surface of the samples. FeB and Fe2B monolayer with a saw-toothed morphology at the growth interphase was obtained due to the absence of alloying elements, such as Cr, Ni, Mo, V, W, in the substrate and a low boron potential at material surface. The mechanical characterization of the borided layers was carried out in two stages: the first, consisted of determining the influence of the experimental parameters (time and temperature treatment) in the FeB and Fe2B phase hardness; Knoop micro-hardness tests were carried out at a 1.9 N constant load modifying the micro-indentation distances from the surface as a function of the thickness of the borided layers and the diagonal size impression. On the other hand, in the second stage, the fracture toughness was experimentally determined by means of induced-fracture indentation using the Palmqvist type cracking, with samples boronized at a constant time of 8 h for each of the treatment temperatures, and varying micro-indentation distances and applied loads in 1.9, 2.9, 4.9, and 9.8 N.