Mass Transfer Processes

Chapter 16

Technique

Abstract

Martín Ortiz Domínguez

Comparison and Analysis of

Steel by Using Powder-Pack

microscopy and energy dispersive X-ray spectroscopy.

diffusion coefficient, growth kinetics

1. Introduction

319

Keywords: diffusion model, activation energy, parabolic growth law,

Surface hardening of steel can be achieved, mainly through two procedures: modifying the chemical composition of the surface by diffusion of some chemical element (carbon, nitrogen, boron, sulfur, etc.) in which case it is known as thermochemical treatment (Table 1) or modifying only the microstructure of the sur-

face by thermal treatment, then known as surface treatment. The current technological demands highlight the need to have metallic materials with high performance under critical service conditions, consequently, the increase in the

wear resistance, preserving its ductility and the toughness of the core. According to Table 1, there are three methods of surface hardening:

Diffusion Models for the Fe2B

Layers Formed on the AISI 12L14

Boriding is a thermochemical surface treatment, a diffusion process similar to carburizing and nitriding in that boron is diffused into a metal base. An indispensable tool to choose the suitable process parameters for obtaining boride layer of an adequate thickness is the modeling of the boriding kinetics. Moreover, the simulation of the growth kinetics of boride layers has gained great interest in the recent years. In this chapter, the AISI 12L14 steel was pack-borided in the temperature range of 1123–1273 K for treatment times between 2 and 8 h. A parabolic law for the kinetics of growth of Fe2B layers formed on the surface of AISI 12L14 steel was deducted. Two diffusion models were proposed for estimating the boron diffusion coefficients through the Fe2B layers. The measurements of the thickness (Fe2B), for different temperature of boriding, were used for calculations. As a result, the boron activation energy for the AISI 12L14 steel was estimated as 165.0 kJ/mol. In addition, to extend the validity of the present models, two additional boriding conditions were done. The Fe2B layers grown on AISI 12L14 steel were characterized by use of the following experimental techniques: X-ray diffraction, scanning electron
