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In spite of the growing commercial maturity of a wide range of surface engineering technologies, there are no comprehensive design systems for surface engineering. Accordingly, the overall objective of the present research proposal is, in the anticipation of an increased industrial awareness and application of surface engineering, to develop a design system for surface engineering through mathematical modelling and computer simulation of various plasma enhanced surface engineering processes as well as t properties of the resultant surface engineered components.

An extensive and comprehensive data base has been built up on the plasma nitriding process and on the relevant properties of plasma nitrided steels. This includes both available published literature and additional experimental data. This data base has been used in the mathematical models developed in the present work to simulate the plasma nitriding process and the resultant service behaviour of low alloy steels.

Various experimental techniques have been used to measure the thermodynamic data of the nitriding process and mechanical properties of nitrided steels, including the diffusion coefficient of nitrogen, reaction rate of the nitriding process, elastic moduli of gamma'-Fe4N layers and TiN coatings, hardness profiles, friction and wear behaviour and residual stress profiles. The elastic modulus of nitrided gamma'-Fe4N layer has been measured for the first time, using the nano-indentation, electrodynamic balance and sound velocity methods. These methods produced similar results, i.e E = 130-140 GPa for gamma'-Fe4N layers.

The X-ray diffraction technique has been used to measure the residual stress profiles developed in various nitrided specimens. Detailed profiling investigations have revealed that the maximum compressive residual stress level is developed in bright nitrided specimens, in particular, those produced at low temperatures. With increasing temperature and time, the compressive stress level is reduced to such an extent that a tensile stress may develop some distance below the surface. This phenomenon, discovered for the first time for plasma nitriding, indicates the detrimental effect of high temperature and long time nitriding on residual stress distribution and the resultant fatigue properties of nitrided components. A mathematical model has been developed to simulate the development of residual stresses in nitrided low alloy steels.

Mathematical models have been developed to simulate the plasma nitriding process of low alloy steels, using both finite difference and finite element methods. A software package PLASMA, has been developed based on the models, which can run on any IBM PC. PLASMA has various functions, including self-supporting windows, graphic facilities, and the ability to simulate one-stage and two-stage processes, as well as to predict required nitriding times.

A mathematical model has been developed using the numerical integral transform technique, to simulate the elastic contact problems of layered surfaces, including nitrided single layer and duplex treated double layer surfaces. The model is the most advanced available so far, considers (1) different properties in various layers; (2) real rough surfaces; (3) friction effects and (4) residual stresses. The model has been successfully applied to predict the load bearing capacity of various materials under rolling-sliding testing and gear testing of nitrided and duplex treated steels.

A design system has therefore been developed in the present work for plasma nitriding of low alloy steels. The package comprises three models, i.e. the nitriding model, the residual stress model and the contact model, and various input data established by experiments.
The detailed scientific tasks of the programme are :

- to produce a design database for the Processing and Properties of Plasma Nitrided Low Alloy Steels

- to produce a Finite Element Model software package capable of predicting the plasma nitriding processing performance of low alloy steels

- to produce a Finite Element Model software package capable of predicting the load bearing properties of plasma nitrided and duplex treated low alloy steels.

This will mean that not only will design engineers be able to use the models in engineering design, but also materials processing engineers can use the models in the selection of optimum process parameters.


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