11 th International Research/Expert Conference ”Trends in the Development of Machinery and Associated Technology” TMT 2007, Hammamet, Tunisia, 05-09 September, 2007. TEMPERATURE UNIFORMISATION AT PLASMA NITRIDING BY USING SPECIAL PROTECTION SCREENS Marius Bibu Valeriu Deac Cristian Deac Claudiu Isarie "Lucian Blaga" University of Sibiu, Romania Emil Cioran str., 4, 550025 Sibiu Romania ABSTRACT Despite some very well established advantages, during the plasma nitriding process important problems related to temperature uniformisation may occur, especially during the thermochemical treatment of metallic parts that contain areas with complex geometry. After an examination of the theoretical aspects pertaining to furnaces without auxiliary heating, the authors present an original method based on the usage of adjustable protection screens, for correcting the temperature on certain surfaces of a large-size part subjected to plasma nitriding. Keywords: plasma nitriding, protection screens, temperature control 1. INTRODUCTION Among the thermochemical processes for the improvement of surface proprieties of components, plasma nitriding holds an important position in industry today. Unlike processes employing conventional methods of heating, in plasma nitriding the energy is impartes directly to the work piece by ion and fast-neutron bombardment. The energy efficiency of this mechanism is quite high (70- 90%) and depends on pressure and gas composition. On the other hand, under certain conditions, this form of heating may give rise to substantial and unacceptable temperature nonuniformity between different components of the load or between different areas of the same component. The main factors which can lead to this temperature nonuniformity were analysed in a previous paper. It was found that the jigging system could be modified in order to ensure good temperature uniformity of the work piece in the treatment chamber. In the present paper, a method of redistribution of the input energy on the surface of the load is described. Using this technique, a significant correction of the temperature in ceratin areas of large components has been achieved. As an example, some aspects of the plasma nitriding of a balancing chamber for a 700MW turbine are also presented. 2. THEORETICAL ASPECTS During the heating process, the energy imparted to the work piece by ion bombardment and by other sources is partly used to heat up the components, the rest being dissipated by conduction, convection and radiation. This physical equilibrium may be expressed by the following equation: r cv c a S disch P P P t T c m P dS j U + + + Δ Δ ⋅ ⋅ = + ⋅ ⋅ ⋅ ∫ η (1) where: η = energy efficiency of the heating mechanism U disch = discharge voltage applied to the load j = current density, which is often non-uniform on the surface of the components S = surface area of the component or a part of it 279