外文翻译---离子渗氮对马氏体时效钢微观结构和性能的影响.docx

附录一 英文翻译原文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, Bangalore-560 037, IndiaDepartment of Materials Engineering, Indian Institute of Science, Bangalore-560 012, 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 i.e., at 435, 450 and 465 for 10h duration in order to achieve good wear resistance along with high strength required for the slat track component 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 X-ray diffraction. Various properties, such as, hardness, case depth, tensile, impact, fatigue properties and corrosion resistance were investigated for both un-nitrided and ion-nitrided 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 450C for 10h are found to be optimum when compared to the other two ion nitriding temperatures.Keywords：Ionnitriding;Maragingsteel;Microstructure;Casedepth;Tensile properties;Fatigue properties1. IntroductionSurface engineering means engineering the surface of a material or component 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 substrates chemical composition; these types of processes include transformation hardening, cold deformation, machining and peening, (b) changing the chemistry of the surface region; these types of processes include carburizing, nitriding, anodizing and ion implanting, and (c) adding a layer of material to the surface; these types of processes include weld overlay, painting, metal spraying, plasma spraying, electroplating, bonding, physical vapor deposition and chemical vapor deposition. Nitriding is a surface-hardening 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 nascent (elemental) nitrogen to the surface of a metal part for subsequent diffusion into the material3,4and5. 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 properties3. 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 complex parts. The hardened surface layer becomes 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 component. Ion nitriding provides better control of case chemistry and uniformity 3,6and7.Maraging steels belong to a new class of high strength steels with the combination of strength and toughness that are among the highest attainable in general engineering alloys8. The term maraging is derived from martensite age hardening and denotes the age hardening of a low carbon, ironnickel 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 compounds at a temperature of about 480 11 and 12. Carbon is treated as an impurity and is kept as low as commercially feasible (0.03wt.%) 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 components 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 5005502, which is above the ageing temperature of maraging steel and would result in the over-aging 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 process13. In the present investigation, ion nitriding of Maraging steel (250 grade) has been carried out at three different temperatures, i.e., 435, 450 and 465 for 10h duration. Microstructural changes are examined with the help of optical microscope, scanning electron microscope (SEM) and X-ray 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 composition 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 composition of the material falls within the composition range specified for Maraging steel (250 grade).Table 1.Chemical composition of the alloy (wt. %)ElementSpecifiedObtainedC0.03 max.0.01Mn0.10 max.0.04Si0.10 max.0.04S0.01 max.0.005P0.01 max.0.006Ni17.019.017.29Mo4.605.204.89Co7.08.507.90Ti0.300.500.41Al0.050.150.14Zr0.02 max.0.005B0.003 max.0.001FeBalanceBalanceFig. 1shows a simplified schematic of the ion nitriding installation used for ion nitriding of the material. The work load is supported on a hearth plate inside a double walled, water cooled vacuum chamber, connected to vacuum pumps and gas supply. The chamber is evacuated to a pressure of about 2.5102mbar, 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 ion-nitrided in the nitriding furnace under vacuum. Plasma is obtained by passing the gas mixture of H2and N2gases 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. 1.Schematic of the ion nitriding set up.View high quality image (177K)Photograph of the specimens before ion nitriding and after ion nitriding at 465C are shown inFig. 2. After ion nitriding the specimens were subjected to buffing (cloth polishing) to remove the thin white layer formed.Fig. 2.Photograph of the specimens (a) before ion nitriding,(b) after ion nitriding at 465C.View high quality image (250K)Microstructural examination was carried out on NIKON optical microscope and Leo SEM. XRD was carried out on Philips Analytical X-ray Diffractometer. Hardness was measured using Vickers Hardness Tester (Buelher Micromet 2100) with a load of 100g. Tensile tests were carried out on cylindrical specimens having 4mm gauge diameter and 20mm gauge length using TIRA Test 2820S Universal Testing Machine.Impact tests were conducted on Charpy U notch specimens having a dimension 10mm10mm55mm using FIE Charpy Impact Testing machine. Constant amplitude LCF tests were carried out on smooth round specimens with 4.5mm gauge diameter and 12mm gauge length using Zwick Roell Universal Testing Machine at an applied stress range of 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 10mm10mm18mm for un-nitrided samples and 10mm1mm20mm for ion-nitrided samples using 5% NaCl solution for 144h in a salt spray test chamber. 3. Results and discussions3.1. Microstuctural examinationOptical and scanning electron micrographs of un-nitrided and ion-nitrided 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. 3.Optical micrographs of (a) un-nitrided specimen, (b) ion nitrided at 435, (c) ion nitrided at 450 and (d) ion nitrided at 465.View high quality image (2381K) Fig. 4.Scanning electron micrographs of (a) un-nitrided 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 un-nitrided and ion-nitrided specimens are shown in Fig. 5. The un-nitrided specimen exhibits diffraction peaks only due to Fe, whereas the ion-nitrided 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 gas and forms mono phase (called as phase) crystal structure of Fe4N in the compound layer. Nitrogen atoms go in to the interstitial sites of iron lattice inanordered manner and form Fe4N 14.Fig. 5.X-ray diffraction pattern of (a) un-nitrided specimen, (b) ion nitrided at 43, (c) ion nitrided at 450 and (d) ion nitrided at 465. View high quality image (534K)3.2. Hardness and case depthThe hardness values obtained on un-nitrided and ion nitrided samples are given in Table 2. The average surface hardness of the un-nitrided 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 un-nitrided sample. Hardness values of the ion-nitrided 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 ion-nitrided samples15. 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 99m at 435, 450 and 465 ion-nitriding temperatures respectively from Fig. 6. Case depth increases with the increase in ion nitriding temperature due to greater diffusivity at higher temperatures. It should be noted that the core hardness at all the three ion nitriding temperatures remains the same as the hardness of the un-nitrided sample. This confirms that the core does not soften by over-aging at all the three ion nitriding temperatures.Table2.Surface hardness of un-nitrided and ion-nitrided specimensBefore ion nitriding 616VHN3After ion nitriding At 435C 906 VHN3At 450C 901 VHN2At 465C 907 VHN4Table3.Hardness values obtained at different depth from the surface after ion-nitridingDistance from Hardness (VHN)the edge(m) Ion nitrided at435C Ion nitrided at450C Ion nitrided at465C50 749 7 810 8 822 1075 725 4 734 4 737 5100 689 12 690 5 714 5125 633 8 642 5 659 5150 626 6 631 2 636 3175 618 2 626 4 628 7200 618 3 618 2 620 2225 618 2 618 2 619 2Core 614 3 618 2 618 2Fig. 6.Variation of hardness with depth from the surface for samples ion nitrided at 435, 450and 465.View high quality image (145K)3.3. Tensile propertiesTensile properties for the un-nitrided and ion-nitrided specimens at three different ion-nitriding temperatures are given inTable 4. It is observed that, as compared to the un-nitrided specimen, the nitrided specimens exhibit an increase in ultimate tensile strength (UTS) and 0.2% proof stress (0.2% PS) and a decrease in percentage elongation to fracture (%El) and percentage reduction in area (%RA). The higher UTS and 0.2% PS values and lower %El and %RA values exhibited by nitrided samples can be attributed to the hardened surface layer on ion nitriding.Table 4 Tensile, Impact and LCF properties of un-nitrided and ion-nitrided specimensProperty Un nitrided Ion-nitridedat435 C Ion-nitrided at450 C Ion-nitrided at465 CUTS (MPa) 1697 4 1740 105 1863 15 1699 140.2% PS (MPa) 1616 9 1711 120 1793 32 1628 16% El 9.1 0.3 5.8 1.1 6.2 0.5 7 0% RA 57.0 2.6 34.5 0.7 27.7 2.5 35 0Energy absorbed inimpact tes t 21.7 0.6 18.7 0.6 18.3 1.2 14 1(Joules) No. of cycles to4700 1113 11017 5748 9366 3557 9675 6811The nitrided specimen can be treated as a composite material consisting of martensitic core of about 3.7mm diameter surrounded by a thin nitrided layer of about 0.15mm thickness (as shown inFig.6), and a rough estimate of 0.2% PS can be obtained by applying the rule of mixture, as follows:(1)where,C 0.2% PS of the nitrided specimenf 0.2% PS of the nitrided layer (calculated below)m 0.2% PS of the un-nitrided core (assumed to be the same as that of the un-nitrided specimen, i.e., 1616MPa)AC cross section area of the nitrided specimen (12.57mm2)Af cross section area of the nitrided layer (1.82mm2)Am cross section area of the un-nitrided core (10.75mm2).It is observed fromFig. 6that the hardness of the nitrided specimens approaches the hardness of the core material at a depth of about 150m from the surface for all the three ion nitriding temperatures. The average hardness of the nitrided layer is obtained by calculating the area under the polynomial curve fitted up to a depth of 150m and dividing it by 150m in Fig. 6for the samples ion nitrided at 435, 450 and 465. These average hardness values are converted into 0.2% PS using Tabors relation16and17(2)f=VHN/3.Thus,fvalues at 435, 450 and 465ion nitriding temperatures are obtained as 2390, 2449 and 2487MPa respectively. TheCvalues using Eq. (1) at 435, 450and 465 ion nitriding temperatures are obtained as 1728, 1737 and 1742MPa respectively. Thus calculated 0.2% PS value matches closely with the experimentally obtained value for the specimens nitrided at 435, as shown inTable 4. The estimated value of 0.2% PS should further increase with the increase in ion nitriding temperature. However, the experimental results inTable 4show an increase in 0.2% PS value from 435 to 450but a decrease from 450 to 465. Perhaps, more number of tests is required t