Mean Diffusion Coefficient Method in Studying Armco Iron Boriding Kinetics
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MEAN DIFFUSION COEFFICIENT METHOD IN STUDYING ARMCO IRON BORIDING KINETICS M. Keddam1 and M. Kulka2 Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 30 – 34, May, 2020.
Activation energy of boron diffusion during pack boriding of armco iron with a constant incubation time for boride formation is estimated using a mean diffusion coefficient model. Experimental verification is accomplished for the model by boriding armco iron at 1253 K for 5 h. Good correspondence is obtained between experimental and calculated Fe2B layer thicknesses.
Key words: boriding, iron borides, incubation time, mean diffusion coefficient (MDC) method, activation energy.
through the thickness. In this model in to estimate the boron diffusion coefficient through the Fe2B layer formed at a steel AISI 1045 steel surface with a constant prolonged incubation period a dimensionless parameter was introduced. In [8] in order to estimate the boron diffusion coefficient in an Fe2B layer fore at the surface of AISI P20 steel at 1123 – 1223 K we propose an approach based on an integral method. In the present work in order to estimate the boron diffusion coefficient in an Fe2B layer during the incubation period of boride formation, independent of boriding temperature, a new (kinetic) approach has been used based on the average diffusion coefficient. This approach has been used previously during armco iron boriding from powder mixtures in the temperature range 1123 – 1273 K. Comparative analysis of the predicted and experimental thicknesses of the Fe2B layer prepared at 1253 K for 5 h showed good conformity. In a previous work [14] a new approach has been used in studying a double-boride layer (FeB and Fe2B) formed during boriding of steel AISI D2 from powder mixtures. The aim of the present work is to develop a procedure for calculating the boron diffusion coefficient in an Fe2B layer formed at the surface of armco iron during boriding from a powder mixture using the average diffusion coefficient method.
INTRODUCTION Boriding is used in order to strengthen the surfaces of metal material that proceeds due to formation of hard borides [1]. This form of chemical heat treatment facilitates improvement of tribological properties and corrosion resistance for ferrous and nonferrous alloys in some acid and alkaline media [2]. Boriding is conducted in sold, liquid, and gaseous media at 800 – 1050°C for 0.5 – 10 h [3]. Due to the simplicity of the technology and economic advantage of solid-phase boriding from powder mixtures (powder-pack boriding) it is a preferred process [4]. During steel and armco iron boriding within a surface layer there may be formation of two forms of borides. From a practical point of view it is preferable to prepare a single-phase layer of Fe2B since within a double boride layer FeB + Fe2B at the FeB/Fe2B interface there is crack formation. There has been development of several processes in order to model boriding kinetics for steels [1, 5 – 8], cast iron [9, 10], and armco iron [
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