M. Ortíz-Domínguez

Diffusion model for growth of Fe2B layer in pure iron

Abstract A simple diffusion model is proposed to estimate the growth kinetics of Fe2B layers created at the surface of pure iron. This model employs the mass balance equation at the Fe2B/substrate interface to evaluate the boron diffusion coefficient (DFe2B) in the boride layer. The Fe2B layers were formed using the paste boriding process, at four temperatures with different exposure times. Analysing the results, the evolution of the parabolic growth constant (k) of the Fe2B layer is presented as a function of boron concentration and boride incubation time [t0(T)]. Furthermore, the instantaneous velocity of the Fe2B/substrate interface and the weight gain of borided pure iron were estimated for different boriding temperatures. Finally, to validate the diffusion model, the boride layer thicknesses were predicted and experimentally verified for two boriding temperatures and for different treatment times.

Growth Kinetics of Boride Layers: A Modified Approach

This study evaluates the boron diffusion in the Fe2B phase formed at the surface of AISI 1018 steels during the paste boriding process. The treatment was carried out at temperatures of 1123, 1173, 1223 and 1273 K with 2, 4, 5, 6 and 8 h exposure times for each temperature using a 4 mm layer thickness of boron carbide paste over the material surface. The boron diffusion coefficient Fe2B D was determined by the mass balance equation and the boride incubation time assuming that the boride layers obey the parabolic growth law, while the boron concentration profile along the interphase Fe2B/substrate was unknown. The boron diffusion coefficient was interpreted as a function of the treatment temperature, obtaining the activation energy value for diffusion controlled growth of Fe2B boride phase.

Determination of Boron Diffusion Coefficients in Borided Tool Steels

The boron diffusion in the Fe2B and FeB borided phases formed at the surface of AISI H13 tool steels during the paste boriding process was estimated. The treatment was carried out at temperatures of 1173, 1223 and 1273 K with 2, 4, 6 and 8 h exposure times for each temperature using a 4 mm layer thickness of boron carbide paste over the material surface. The boride layers were characterized by the GDOES technique to determine in quantitative form the presence of the alloying elements on the borided phases. The boron diffusion coefficients and were determined by the mass balance equation and the boride incubation time assuming that the boride layers obey the parabolic growth law. Also, the mass gain produced by both boride layers at the surface of the tool steels was determined. Finally, the boron diffusion coefficients were interpreted as a function of the treatment temperature, obtaining the activation energy values for the diffusion controlled growth of Fe2B and FeB hard coatings.

Properties and Characterization of Hard Coatings Obtained by Boriding: An Overview

Some physicochemical and mechanical properties of surface hard coatings obtained by the paste-boriding process are summarized in this work. Different grades of borided ferrous alloys were used to develop the formation of surface layers type Fe2B or FeB/Fe2B. Furthermore, in order to characterize the nature of boride layers, some classical techniques are presented and discussed such as Glow Discharge Optical Emission Spectrometry (GDOES), Atomic Force Microscopy (AFM) and estimation of residual stresses by X-Ray Diffraction method. Also, the morphology of borided interfaces was evaluated by concepts of fractal theory.

Dependence between the Boron Surface Concentration and the Growth Kinetics of Boride Layers in AISI 4140 Steels

The present work estimated the growth kinetics of Fe2B layers formed at the surface of AISI 4140 steels. The thermochemical treatment was applied in order to produce the Fe2B phase, considering temperatures of 1123, 1173, 1223 and 1273 K with five exposure times (2, 4, 5, 6, and 8 h), using a 4 mm thick layer of boron carbide paste over the material surface. The growth of boride layers was described by the mass balance equation between phases in thermodynamic equilibrium, assuming that the growth of boride layers obeys the parabolic growth equation and the boron concentration at the interfaces remains constant. Also, the boron diffusion coefficient at the Fe2B ( ) was established as a function of boriding temperature. Likewise, the parabolic growth constant (k), the instantaneous velocity (v) of the Fe2B/substrate interface and the weight-gain of borided steels were established as a function of the parameters and , which are related to the boride incubation time ( ) and boron surface concentration ( ), respectively.

Anisotropy of Boride Layers: Effect on the Mechanical Properties of AISI 4140 Borided Steels

The growth of iron borides over the surface of different steels is of high anisotropy. It was determined that the anisotropy of borided phases reveals a significant instability of properties in service. One of the techniques to determine the effect of anisotropy on the mechanical properties of boride layers is the induced-fracture by Vickers microindentation. During the present work, the fracture toughness (KC) of the Fe2B hard coatings has been estimated at the surface of AISI 4140 borided steels. The force criterion of fracture toughness was determined from the extent of brittle cracks originating at the tips of an indenter impression. The indentation loads were established between 1.9 to 9.8 N at three different distances from the borided surface. The KC values were expressed as a function of temperature, treatment time and the indentation distances from the surface. Likewise, the adherence of the coated system was evaluated by Rockwell-C indentation, where the borided steel showed sufficient adhesion.

Fracture Indentation on AISI 1018 Borided Steels

Fracture indentation was applied to estimate the fracture toughness of AISI 1018 borided steels. The Fe2B hard layers were formed using the powder-pack boriding process for two temperatures with 4 and 8 h of exposure times. The fracture toughness of the iron boride layer of the AISI 1018 borided steels was estimated using a Vickers microindentation induced-fracture testing at distances of 15 and 30 m from the surface, applying four loads (0.49, 0.98, 1.96 and 2.9 N). The microcracks generated at the corners of the Vickers microindentation were considered as experimental parameters, which are introduced in a Palmqvist crack model to determine their corresponding fracture toughness KC. As a result, the experimental parameters, such as exposure time and boriding temperature are compared with the resulting fracture toughness of the borided phase.

Measurement of Fracture Toughness in AISI 1018 Borided Steels by Vickers Indentation

This study evaluates the fracture toughness of Fe2B boride layers formed by the paste boriding thermochemical process on an AISI 1018 steel surface. The samples were placed in acrylic molds for the impregnation of boron carbide paste with thickness of 4mm over the sample surfaces to produce the diffusion into the steel. The aforementioned treatment considered one temperature, T= 1273 K, and three exposure times t=5, 6 and 8 h. Later, the borided samples were prepared metallographically to determine the mean values of the layer thicknesses and to produce Vickers microindentations at 45 m from the surface, applying four loads (1.9, 2.9, 4.9 and 9.8 N). The microcracks generated at the corners of the Vickers microindentation were considered as experimental parameters, which are introduced into two Palmqvist cracks models to determine their corresponding fracture toughness KC. As a result, the experimental parameters, such as exposure time and applied load are compared with the resulting fracture toughness of the borided phase.