【正文】 ent (elemental) nitrogen to the surface of a metal part for subsequent diffusion into the material[3],[4]and[5]. The plasma assisted surface modification techniques offer a great flexibility and are capable of tailoring desirable chemical and structural surface properties independent of the bulk properties[3]. It has other advantages, such as, no or very thin white layer forms after nitriding and there is no machining or grinding involved after the process, which is particularly beneficial for plex parts. The hardened surface layer bees an integral part of the base material and there is no significant reduction in properties of the base material. It is also known to provide the modified surface without dimensional change and distortion of the ponent. Ion nitriding provides better control of case chemistry and uniformity [3],[6]and[7].Maraging steels belong to a new class of high strength steels with the bination of strength and toughness that are among the highest attainable in general engineering alloys[8]. The term maraging is derived from martensite age hardening and denotes the age hardening of a low carbon, iron–nickel lath martensite matrix [9] and [10]. These steels typically have very high nickel, cobalt and molybdenum and very low carbon content. They are strengthened by the precipitation of intermetallic pounds at a temperature of about 480℃ [11] and [12]. Carbon is treated as an impurity and is kept as low as mercially feasible (.%) in order to minimize the formation of titanium carbide (TiC), which can adversely affect strength, ductility and toughness [9] and [11]. Different maraging steels are designed to provide specific levels of yield strength from 1030 to 2420MPa (150 to 350ksi). Maraging steel (250 grade) possesses a yield strength of 250ksi.For aeronautical ponents requiring high strength and good wear resistance, such as, slat track, Maraging steel possesses high strength, and good wear resistance can be achieved by ion nitriding process of case nitriding. In conventional gas nitriding process the nitriding temperature is 500℃–550℃[2], which is above the ageing temperature of maraging steel and would result in the overaging of the core. However, ion nitriding can be carried out at a temperature lower than the aging temperature. Ozbaysal and Inal have demonstrated that the surface hardening of maraging steels without a reduction in core hardness is possible using the ion nitriding process[13]. In the present investigation, ion nitriding of Maraging steel (250 grade) has been carried out at three different temperatures, ., 435℃, 450℃ and 465℃ for 10h duration. Microstructural changes are examined with the help of optical microscope, scanning electron microscope (SEM) and Xray diffraction (XRD). Various properties, such as, hardness, case depth, tensile properties, impact strength, low cycle fatigue (LCF) and corrosion resistance are evaluated. These properties are explained with the help of microstructural observations.2. Experimental procedureThe specified chemical position for Maraging steel (250 grade) and the actual chemical analysis of the material obtained from optical emission spectroscopy is shown in Table 1. As can be seen, the chemical position of the material falls within the position range specified for Maraging steel (250 grade).Table 1. these types of processes include transformation hardening, cold deformation, machining and peening, (b) changing the chemistry of the surface region。Fatigue properties1. IntroductionSurface engineering means ‘engineering the surface’ of a material or ponent to confer surface properties, which are different from the bulk properties of the base material [1]. The purpose may be to reduce wear, minimize corrosion, increase fatigue resistance, reduce frictional energy losses, act as a diffusion barrier, provide thermal insulation, exclude certain wave lengths of radiation, promote radiation, electronic interactions, electrically insulate or simply improve the aesthetic appearance of the surface. Surface engineering processes may broadly be grouped in three categories as (a) modifying surfaces without altering the substrate39。Casedepth。Maragingsteel。C for 10h duration in order to achieve good wear resistance along with high strength required for the slat track ponent of aircraft. The microstructure of the base material and the nitrided layer was examined by optical and scanning electron microscope, and various phases present were determined by Xray diffraction. Various properties, such as, hardness, case depth, tensile, impact, fatigue properties and corrosion resistance were investigated for both unnitrided and ionnitrided materials. It is observed that the microstructure of the core material remains unaltered and Fe4N is formed in the hardened surface layer after ion nitriding at all the three temperatures employed. Surface hardness increases substantially after ion nitriding. Surface hardness remains almost the same but case depth increases with the increase in ion nitriding temperature due to greater diffusivity at higher temperatures. Tensile strength, fatigue strength and corrosion resistance are improved but ductility and energy absorbed in impact test decrease on ion nitriding. These results are explained on the basis of microstructural observations. The properties obtained after ion nitriding at 450附錄一 英文翻譯原文EFFECT of ION NITRIDING ON the MICROSTRUCTURE AND PROPERTIES of MARAGING STEEL (250 GRADE)Kishora ShettyS. Kumarb,P. Raghothama Rao。Regional Centre for Military Airworthiness (Foundry and Forge), CEMILAC, DRDO, Bangalore560 037, IndiaDepartment of Materials Engineering, Indian Institute of Science, Bangalore560 012, IndiaReceived 25 June 2008. Accepted 28 November 2008. Available online 10 December 2008AbstractIn the present investigation, ion nitriding of Maraging steel (250 grade) has been carried out at three different temperatures ., at 435℃, 450℃ and 465℃ for 10176。h are found to be optimum when pared to the other two ion nit