【正文】 618 177。 2 620 177。 7200 618 177。 2 626 177。 2 636 177。 5150 626 177。 8 642 177。 5 714 177。 5100 689 177。 4 734 177。 8 822 177。C50 749 177。C Ion nitrided at450176。4 values obtained at different depth from the surface after ionnitridingDistance from Hardness (VHN)the edge(181。2At 465176。3At 450176。3After ion nitriding At 435176。1172MPa having stress ratio of ?1. Hardness values were measured at five places for each sample, for tensile and impact three samples were tested at each condition and for LCF five samples were tested at each condition and the average value along with standard deviation is reported. Corrosion tests were carried out on specimens having a dimension 10mm?10mm?18mm for unnitrided samples and 10mm?1mm?20mm for ionnitrided samples using 5% NaCl solution for 144h in a salt spray test chamber. 3. Results and discussions. Microstuctural examinationOptical and scanning electron micrographs of unnitrided and ionnitrided specimens are shown in Fig. 3 and Fig. 4. These micrographs show a case hardened nitrided layer and martensitic core structure. The thickness of the nitrided layer increases with the increase in ion nitriding temperature due to greater diffusivity at higher temperatures. Fig. micrographs of (a) unnitrided specimen, (b) ion nitrided at 435℃, (c) ion nitrided at 450℃ and (d) ion nitrided at 465℃.View high quality image (2381K) Fig. electron micrographs of (a) unnitrided specimen, (b) ion nitrided at 435℃, (c) ion nitrided at 450℃ and (d) ion nitrided at 465℃.View high quality image (2252K)XRD patterns obtained from the surface of unnitrided and ionnitrided specimens are shown in Fig. 5. The unnitrided specimen exhibits diffraction peaks only due to Fe, whereas the ionnitrided specimens exhibit additional peaks due to Fe4N as well. The intensity of the Fe peaks progressively decreases and the intensity of the Fe4N peaks progressively increases with the increase in ion nitriding temperature. Thus, the iron nitride formed in the surface hardened layer on ion nitriding is Fe4N. The ratio of H2 and N2 in the gas mixture used for ion nitriding in our investigation is 3:1, which is termed as γ′ gas and forms mono phase (called as γ′ phase) crystal structure of Fe4N in the pound layer. Nitrogen atoms go in to the interstitial sites of iron lattice inanordered manner and form Fe4N [14].Fig. diffraction pattern of (a) unnitrided specimen, (b) ion nitrided at 43℃, (c) ion nitrided at 450℃ and (d) ion nitrided at 465℃. View high quality image (534K). Hardness and case depthThe hardness values obtained on unnitrided and ion nitrided samples are given in Table 2. The average surface hardness of the unnitrided samples is 616VHN. The average surface hardness of the samples ion nitrided at three different temperatures is more or less the same and is about 50% higher than the unnitrided sample. Hardness values of the ionnitrided samples obtained at various depths from surface for the three ion nitriding temperatures are listed in Table 3 and are plotted inFig. 6. The continuous decrease of hardness from surface to the core of the sample suggests the presence of a diffusion zone in which precipitates of nitrides of iron and other metals are formed at the grain boundaries as well as within the grains. These precipitates distort the lattice and pin crystal dislocations and thereby increase the hardness of the surface layer of the ionnitrided samples[15]. Case depth is taken as the distance from the surface where hardness value is 100 units more than the core hardness. Accordingly, case depth is measured by drawing a line at a hardness value 716VHN and is obtained as 81, 87 and 99181。 high quality image (250K)Microstructural examination was carried out on NIKON optical microscope and Leo SEM. XRD was carried out on Philips Analytical Xray Diffractometer. Hardness was measured using Vickers Hardness Tester (Buelher Micromet 2100) with a load of 100gases in the ratio 3:1 into the chamber and maintaining the pressure of 5mbar. Ion nitriding is carried out at 435℃., 450℃ and 465℃ for10h..Fig. of the ion nitriding set high quality image (177K)Photograph of the specimens before ion nitriding and after ion nitriding at 465176。2mbar, a pressure low enough for the background level of oxygen to be within the acceptable limits (less than 50ppm) and is then filled with a low pressure mixture of hydrogen and nitrogen. The use of auxiliary AC heaters to heat the cathode to 250℃ is desirable to minimize cycle time. It can also help to provide better temperature uniformity of the part in ion nitriding treatment. The discharge is ignited using a DC power supply, and pressure and temperature are raised to the desired operating values by controlling gas flow and pressure, applied voltage and current. The discharge can be monitored by control panel and viewed through inspection windows. The work is cathode and the vessel is anode. The furnace is electrically grounded, cool to touch, and quiet in operation. Maraging steel (250 grade) specimens are prepared from the material solutionized and aged at 480℃ for 3h. These aged specimens are ionnitrided in the nitriding furnace under vacuum. Plasma is obtained by passing the gas mixture of H2 these types of processes include weld overlay, painting, metal spraying, plasma spraying, electroplating, bonding, physical vapor deposition and chemical vapor deposition. Nitriding is a surfacehardening process by the introduction of nitrogen into the surface of steel [2]. Process methods for nitriding include gas nitriding, liquid or salt bath nitriding and plasma or ion nitriding. In gas nitriding this is done using a mixture of ammonia gas and dissociated ammonia in suitable proportions. In plasma or ion nitriding a glow discharge technology is used to introduce nasc