Experimental evaluation and modelling of the boronizing kinetics of AISI H13 hot work tool steel

Abstract

In the study for this contribution, the AISI H13 hot work steel was pack-boronized between 2 and 6 h of exposure time within the temperature range of 800 – 1000 °C. The boriding agent was composed of a powder mixture containing (in weight percent): 90 % of boron carbide (B4C) and 10 % of sodium tetrafluoroborate (NaBF4). The SEM observations showed a less pronounced jagged interface between the boronized layer and the transient zone. A double phase boride layer (FeB and Fe2B) was identified over the surface of AISI H13 steel with the presence of metallic borides inside this compound layer. The mean diffusion coefficient (MDC) method was applied to analyze the growth of iron borides (FeB and Fe2B) as compact layers over the surfaces of AISI H13 steel. The boron activation energies in the two iron borides were also assessed from the present kinetic approach by assuming the Arrhenius relationships. Afterwards, the kinetic model was checked experimentally by considering two extra boriding conditions (925 °C for 1 and 3 h). Finally, the predicted layer thicknesses are in accordance with experimental measurements.

Prediction models for the kinetics of iron boride layers on AISI 316L steel

“…In this kinetic model [11], a linear boron concentration profile through each boride layer (FeB or Fe 2 B) has been considered. Based on the MDC method, the expressions of boron diffusion coefficients in FeB and Fe 2 B are given by Equations ( 22) and ( 23):…”

“…By using the alternative diffusion model (ADM), the layers' thicknesses can be determined by solving numerically the following set of differential Equations (10) and (11) with the Runge-Kutta method [16]: (10) and…”

“…4) and ( 5) ) in iron borides deduced from the Arrhenius relationships at 1123, 1173 and 1223 K, it is seen the calculation results vary with the diffusion models used. For instance, the results provided for the boron diffusion coefficients in iron borides by the alternative diffusion model [9] differ by a factor of 1.1 to 1.4 for FeB and between 2.8 and 3.2 for Fe 2 B in comparison to the two models [10,11] and the bilayer diffusion model. With the assumption that the k FeB and k Fe 2 B parameters of the Dybkov model are the measures of boron mobilities in FeB and Fe 2 B, the comparison between the models is feasible.…”

“…In terms of kinetic modelling of boriding process, a certain number of mathematical models was implemented and dedicated to the growth of FeB and Fe 2 B layers on some steels [7][8][9][10][11][12][13]. In the following mathematical models [7][8][9][10]12], the boride incubation periods were considered during the growth of both phases.…”

“…Keddam and Jurci [10] proposed two different approaches (the integral diffusion model and a modified version of the Dybkov model [13] with the inclusion of boride incubation periods) for the bilayer growth (FeB/Fe 2 B) onto the AISI M2 steel. Finally, Nait Abdelah et al [11] suggested a numerical treatment regarding the boriding kinetics of AISI H13 steel by employing the mean diffusion coefficient (MDC) method and ignoring the incubation periods for the formation of iron boride layers.…”