Control of iron nitride formation by a high magnetic field
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J. Lu Department of Mechanical Engineering, the Hong Kong Polytechnic University, Hong Kong, China (Received 12 February 2010; accepted 12 August 2010)
The influence of high magnetic field on nitriding behavior was investigated in a mixture of NH3 and H2. It was found that high magnetic field could shift the equilibrium of nitriding reaction; this proved that the critical nitrogen potential to form g0 -Fe4N and e-Fe3N phase was evidently enhanced compared with conventional nitriding. This research provides a new approach for a selective nitriding process.
It is well known that nitrides are of great industrial importance.1,2 For more than a century, nitriding has been used as a versatile method to improve surface properties of iron and steel. The case nitride can be subdivided into a compound layer consisting mostly of g0 -Fe4N and/or e-Fe3N phases and a diffusion zone where N is dissolved interstitially in ferritic matrix.3,4 To be certain of obtaining the specified properties, it is necessary to control accurately the nitriding process. On the basis of the wellestablished Lehrer diagram, the exact composition of nitrides formed during nitriding depends on the temperature and the nitrogen potential.5–7 In the past decades, a controlled nitriding has been developed using computer technology.8,9 This nitriding layer only contains a diffusion zone and exhibits a good combination property (fatigue strength, hardness, wear resistance, etc.). However, this is not enough for practical application. It is possible that different nitrides are demanded at different places in a workpiece. Selective nitriding is still a major challenge in modern industry, because it is very difficult to adjust local temperature and nitrogen potential in a nitriding furnace. Recently, with the development of superconducting magnet technology, a new branch of electromagnetic processing of materials has been dramatically highlighted.10–12 The magnetic field, especially high magnetic field, has a considerable influence on solid phase transformation in materials because the Gibbs free energy of a phase can be changed by the amount corresponding to its magnetization. Previous investigations showed that the critical nitrogen potential to form g0 -Fe4N or e-Fe3N phase is decided by the standard a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0279 2082
J. Mater. Res., Vol. 25, No. 11, Nov 2010
Gibbs free energy of nitriding reaction.1,13 Hence, it is reasonable to expect that the formation of nitride may be controlled by a high magnetic field. This article investigates the formation of nitride under a high magnetic field during gas nitriding, using pure iron as an example. Experimental observations indicate that a magnetic field can obviously affect the nitriding behaviors of iron, and different nitrides can be obtained at the same nitriding condition when a high magnetic field is applied. Nitriding is performed in a tube furnace with a continuous flow of a mixture of NH3 and H2. Figure 1(a) shows
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