Nitrided Specimens Research Articles

Steam oxidation and surface hardness of power plant valve materials under the ultra super critical steam condition

Current supereritical steam power plants operate at 3,600 psi and 1,000°F. If the steam temperature is raised from 1,000 °F (538 °C) to 1,150 °F (621°C), the efficiency increases by 2%. Therefore, study on the high temperature corrosion of power plant materials under ultra-superciritical conditions (USC) is necessary to protect the plant from corrosion. In this study, valve materials of 17% Cr martensitic steels (17Cr steel), Incoloy 901 (1901) and their surface nitrided specimens were exposed to USC of 621 °C and 3600 psi (255 kg/cm2) steam for 200 °C, 400 °C, and 800 h. The oxidation of both 17Cr steel and 1901 under the USC for 800 h is very small due to the formation of a protective thin oxide layer formation on the surface. The USC oxidation of both nitrided specimens were increased due to the decomposition and formation of active nitrogen from the non protective nitrides such as Fe4N, Fe2–3N, and CrN. The oxidation of nitrided 17Cr steel (n17Cr steel) is about two times higher compared to nitrided 1901 (n1901). The surface hardness is improved by more than two times near the surface by nitriding, and the degradation of hardness by USC oxidation is rapid for n17Cr steel, but slow for n1901.

Manufacture of fibrous β-Si 3N 4-reinforced biomorphic SiC matrix composites for bioceramic scaffold applications

The manufacturing of the Si 3N 4 reinforced biomorphic microcellular SiC composites for potential medical implants for bone substitutions with good biocompatibility and physicochemical properties was performed in a two step process. First, wood-derived porous Si/SiC ceramics with various porosities were produced by liquid silicon infiltration (LSI) at 1550 °C with static nitrogen atmosphere protection (0.1 MPa), followed by subsequent partial removing of the Si in vacuo at 1700 °C for different periods of time. Secondly, the final porous Si 3N 4 fiber/SiC composite was obtained by further chemical reaction of nitrogen with the infiltrated residual silicon at 1400 °C for 4 h under high concentration flowing nitrogen atmospheres (0.5 MPa). The bending strengths of the porous Si 3N 4 fiber/SiC composite at axial and radial direction were measured as 180.03 MPa and 90 MPa respectively. The improvement in bending strength was primarily attributed to grain pull-out and bridging enhanced by the elongated β-Si 3N 4 grains cross-linked in the depth of the pore channels. The TG analysis showed an obvious improvement in oxidation resistance of the nitride specimens.

Influence of Deep-Nitriding and Shot Peening on Rolling Contact Fatigue Performance of 32Cr3MoVA Steel

The effects of deep-nitriding and following shot peening on microhardness, rolling contact fatigue property of 32Cr3MoVA steel were investigated and Scanning Electron Microscope (SEM) was employed to evaluate the failure surface of rolling contact fatigue specimens. The fatigue results indicate that the depth of deep-nitrided layer is about 0.75 mm and following shot peening can improve the rolling contact fatigue life. The SEM test shows that the area of failure for nitrided and following shot-peened specimens is smaller and the depth of pit is lower compared with nitrided specimens.

Surface Erosion of GaN Bombarded by Highly Charged 208Pbq+-Ions

Surface change of gallium nitride specimens after bombardment by highly charged Pbq+-ions (q = 25, 35) at room temperature is studied by means of atomic force microscopy. The experimental results reveal that the surface of GaN specimens is significantly etched and erased. An unambiguous step-up is observed. The erosion depth not only strongly depends on the charge state of ions, but also is related to the incident angle of Pbq+-ions and the ion dose. The erosion depth of the specimens in 60° incidence (tilted incidence) is significantly deeper than that of the normal incidence. The erosion behaviour of specimens has little dependence on the kinetic energy of ion (Ek = 360,700keV). On the other hand, surface roughness of the irradiated area is obviously decreased due to erosion compared with the un-irradiated area. A flat terrace is formed.

Effects of Nitriding Processes on the Mechanical Properties of SACM 645 Steel

In this project, the alternating torsion fatigue test was carried out to investigate the effect of the nitrided case produced by gas and plasma nitriding processes on the fatigue endurance of the JIS SACM 645 steel. The surface layers of the nitrided specimens exhibited hardness profiles in the range between 1000–1100HV0.1, and the white layer of the nitrided specimen was consisted of Fe3N and Fe4N. Wear test result indicated that the mass loss of JIS SACM 645 steel was greatly improved by nitriding processes. From the mass loss data, the wear resistance of the steel was significantly influenced by nitriding time. The fatigue strength of the 35C-GN48 nitrided specimen rose 51.9% to 632 MPa, which was the maximum fatigue strength in this study.

The nitrogen-absorption isotherm for Fe–21.5 at. % Cr alloy: dependence of excess nitrogen uptake on precipitation morphology

Nitriding of Fe–21.5 at. % Cr alloy leads to a “discontinuously coarsened”, chromium-nitride/ferrite lamellar precipitation morphology in the nitrided zone. The nitrogen-absorption isotherm for this alloy with this precipitation morphology was determined at 560°C. To assure a constant precipitation morphology the Fe–21.5 at. % Cr specimen was first homogeneously pre-nitrided (at 580°C in an ammonia/hydrogen gas mixture of nitriding potential 0.103 atm−1/2) and then de-nitrided (at 470°C in hydrogen gas atmosphere). The amount of nitrogen remaining in the de-nitrided specimen indicated that the composition of the nitride precipitates is CrN and not (Fe, Cr)N. The measured nitrogen-absorption isotherm revealed the presence of excess nitrogen in the nitrided specimen, which is a surprise in view of the coarse, lamellar precipitation morphology. The occurrence of this excess nitrogen could be ascribed to an unexpected, minor fraction of the total chromium content in the alloy present as coherent, tiny nitride platelets within the ferrite lamellae of the “discontinuously coarsened” lamellar precipitation morphology, as evidenced by transmission electron microscopy. A possible kinetic background for this unusual phenomenon was discussed.

Nitride precipitation and coarsening in Fe–2.23 at.% V alloys: XRD and (HR)TEM study of coherent and incoherent diffraction effects caused by misfitting nitride precipitates in a ferrite matrix

Specimens of Fe–2.23 at.% V alloy were nitrided in a NH 3/H 2 gas mixture at 580 °C. The nitrided microstructure was investigated by X-ray diffraction (XRD), and by conventional and high-resolution transmission electron microscopy ((HR)TEM). For specimens homogeneously nitrided for relatively short times no separate VN reflections developed but instead sidebands associated with ferrite reflections, most pronouncedly for the α-Fe-2 0 0 reflection, appeared. The diffractograms measured for the different specimens were interpreted as the result of coherent diffraction of the nitride platelets with the surrounding ferrite matrix, which is tetragonally distorted: the distorted ferrite matrix and the nitride platelets are represented by a single body-centered tetragonal lattice, whereas the remaining part of the ferrite is described by a body-centered cubic lattice. Analysis of the microstructure of the nitrided specimens using (HR)TEM confirmed the existence of very tiny VN platelets, coherent with the surrounding matrix. Annealing at elevated temperatures (up to 750 °C) after nitriding led to (moderate) coarsening of the nitride precipitates. The coarsening is associated with the occurrence of local disruptions/bending of lattice planes in the VN platelet. This effect causes the VN platelets to appear segmented in the diffraction–contrast images. The specific changes in the X-ray diffractograms, as function of the stage of aging, could be consistently described as consequences of the transition from coherent to incoherent diffraction of the nitride platelets with reference to the surrounding ferrite matrix.

Fatigue Properties of Nitrided Ultrafine Ferrite-Cementite Steels under Rotating Bending Fatigue Testing

Fatigue tests under rotating bending were conducted on nitrided ultrafine ferrite-cementite steels. The ultrafine ferrite-cementite steels included carbon-increased and phosphorus-added versions to investigate the effects of grain growth suppression during nitriding. In the carbon-increased versions, grain growth was successfully suppressed both in the nitrided layer and in the core region. On the other hand, the low-carbon versions showed grain growth in the core region, even in the phosphorus-added types, although grain growth was suppressed in the nitrided layer. In the fatigue tests, many of the nitrided specimens revealed fish-eye fractures originating from inclusions located in or beneath the nitrided layer. In spite of the occurrence of fish-eye fractures, the fatigue strength of the carbon-increased versions was markedly improved due to nitriding, whereas it was a little improved in the low-carbon versions. The fatigue strength of the nitrided specimens was closely related to hardness at the fracture origin, even when fish-eye fractures occurred. This was why nitriding markedly improved the fatigue strength of the carbon-increased versions in which grain growth was successfully suppressed and high hardness was maintained beneath the nitrided layer.

Improvement of mechanical properties of martensitic stainless steel by plasma nitriding at low temperature

A series of experiments were carried out to study the influence of low temperature plasma nitriding on the mechanical properties of AISI 420 martensitic stainless steel. Plasma nitriding experiments were carried out for 15 h at 350°C by means of DC-pulsed plasma in 25%N2+75%H2 atmosphere. The microstructure, phase composition, and residual stresses profiles of the nitrided layers were determined by optical microscopy and X-ray diffraction. The microhardness profiles of the nitridied surfaces were also studied. The fatigue life, sliding wear, and erosion wear loss of the untreated specimens and plasma nitriding specimens were determined on the basis of a rotating bending fatigue tester, a ball-on-disc wear tester, and a solid particle erosion tester. The results show that the 350°C nitrided surface is dominated by ɛ-Fe3N and αN, which is supersaturated nitrogen solid solution. They have high hardness and chemical stabilities. So the low temperature plasma nitriding not only increases the surface hardness values but also improves the wear and erosion resistance. In addition, the fatigue limit of AISI 420 steel can also be improved by plasma nitriding at 350° C because plasma nitriding produces residual compressive stress inside the modified layer.

Influence of the microstructure on the residual stresses of nitrided iron–chromium alloys

Different iron–chromium alloys (4, 8, 13 and 20 wt.%Cr) were nitrided in a NH 3/H 2 gas mixture at 580 °C for various times. The nitrided microstructure was characterized by X-ray diffraction, light microscopy and hardness measurements. Composition depth profiles of the nitrided zone were determined by electron-probe microanalysis. Residual stress–depth profiles of the nitrided specimens were measured using the (X-ray) diffraction sin 2 ψ method in combination with cumulative sublayer removals and correction for corresponding stress relaxations. Unusual, nonmonotonous changes of stress with depth could be related to the microstructure of the nitrided zone. A model description of the evolution of the residual stress as function of depth and nitriding time was given.

Comparison of fatigue criteria for combined bending-torsion loading of nitrided and virgin specimens

This study deals with fatigue life of plasma-nitrided and virgin specimens made of a low-alloy high-strength steel. Specimens were subjected to an in-phase combined bending-torsion loading. The plasma-nitrided specimens exhibited a significantly improved fatigue resistance. The criterion proposed by McDiarmid was found to be the most precise in the fatigue life prediction for virgin specimens. On the other hand, the Matake criterion was the most successful for nitrided specimens.

Effect of Low Temperature Gas Nitriding and Low Temperature Gas Carburizing on High Cycle Fatigue Property in SUS316L

It is known that nitrogen and carbon S phases are formed in the diffusion layer on the surface of austenitic stainless steels if nitriding or carburizing is performed at the temperature of 500°C or less. In order to investigate the effect of the nitrogen and carbon S phases on high cycle fatigue properties of type316L austenitic stainless steel, rotating bending fatigue tests were carried out for four specimens with different treatments : One was gas carburized at 470°C. The other three were gas nitrided at 420°C, 460°C and 570°C, respectively. The former three specimens had the carbon or the nitrogen S phase and the last one had no S phase in the diffusion layer, depending on the temperature. As the fatigue tests result, the S phase is effective to enhance the fatigue properties. The effect of fatigue properties improvement of the nitrogen S phase is greater than that of the carbon S phase. The fatigue strength increases with an increase in the thickness of the diffusion layer in the nitrided specimens. External observation suggests that the fatigue crack initiated from the chipped part on the surface due to fatigue loading. Although the chipping behavior depended on the diffusion species, the propagation behavior of fatigue cracks did not depend on them.

A methodology for modelling the effects of nitriding on fatigue life

A methodology is presented for modelling the effects of nitriding for fatigue life prediction. The method is illustrated by application to tooth bending fatigue failure in notch conditions representative of the fillet regions in spline teeth. The methodology first involves the prediction of nitrided layer stress distributions which take account of the graduation in material stress—strain curve through the nitrided layer and the effects of residual stresses. Second, the life prediction aspect requires that the effects of the graded material properties and residual stresses on mean stress levels and thus on fatigue constants be accounted for. Comparisons between predicted and experimentally-determined lives show that the methodology significantly improves the predictions for nitrided specimens.

Effect of Fine Particle Peening Treatment prior to Nitriding on Fatigue Properties of AISI 4135 Steel

In this study, a new hybrid surface modification process, fine particle peening (FPP) treatment prior to nitriding, was proposed. In order to clarify the effects of FPP treatment prior to nitriding on the fatigue strength of notched AISI 4135 steel with a stress concentration factor Kt of 2.36, fatigue tests were conducted at room temperature using a rotational bending fatigue testing machine. Hardness and residual stress distributions were measured in order to characterize the surface-modified layer. As a result, the surface hardness of the FPP-treated specimen before nitriding was higher than that of the nitrided specimen. The surface microstructures of treated specimens were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The crystal structure of the compound layer of the FPP-treated specimen before nitriding was different to that of the nitrided specimen. The compound layer of the FPP-treated specimen was dense. This suggests that FPP treatment prior to nitriding is very effective for improvement of the fatigue strength of steel.

Influence of zirconia addition on reaction bonded silicon nitride produced from various silicon particle sizes

Reaction bonded silicon nitride ceramics have been produced from Si powders of different particle size and both with and without zirconia as a nitride catalyzing element. The effect of the ZrO 2 on the nitridation rate of the specimens was studied as a function of temperature, and the mechanism of enhanced nitridation was discussed. At all temperatures, the degree of nitridation of specimens sintered with ZrO 2 was higher than those without additions for all particle sizes. In the case of nitridation of specimens produced without ZrO 2 , the degree of nitridation was dependent on particle size, with larger sizes resulting in less nitridation. For specimens with ZrO 2 the finest powder also showed highest transformation, but there was no difference in the degree of nitridation of medium and coarse powders. It was found that during heating of the specimens the ZrO 2 was transformed to ZrN at low temperature and enhanced nitridation is considered to be due to this phase re-converting to ZrO 2 at higher temperature and thereby supplying nitrogen to the interior of the specimens. At lower temperatures, the strength of the specimens with ZrO 2 were higher than those without, attributed to the enhanced nitridation. However, comparing strengths of fully nitrided specimens, those with ZrO 2 were lower than those without, thought to be due to the presence of ZrO 2 particles disrupting the connectivity of adjacent silicon nitride grains.

Thermal Crack Propagation Behavior on Nitrided H13 Hot Work Die Steel

The influence of nitriding type and condition on the thermal crack propagation behavior of hot work die steel was investigated. Thermal fatigue tests were carried out using a special apparatus, which is consisted of induction heating and water spray cooling unit. The sum of crack length per unit specimen length, Lm is proposed as an index representing the susceptibility to crack propagation. The Lm values of the gas and ion nitrided specimens were lower than that of as-heat treated specimen. But in the case of maximum and average crack length of nitrided specimen was higher than those of as-heat treated specimen. The nitrogen diffusion layer still remained although the oxide scale was formed and fell off during thermal fatigue test. After 1000 cycles of the thermal fatigue test, the interior region of nitrided specimen showed lower hardness by softening. In particular, ion nitrided specimens of relatively less softening condition showed reduction both the depth and number of cracks compared with the other surface conditions.

Influence of Plasma Radical Nitriding on Fatigue Properties of SCM435 Steel

The effect of plasma radical nitriding treatment on fatigue properties of SCM435 steel in super long life region was investigated. Fatigue tests were carried out using a dual-spindle rotating bending fatigue-testing machine at room temperature in air for the specimens nitrided at 773 K and 823 K for 3 hrs. The fatigue strength of nitrided specimen was greater than that of un-nitrided specimen and the crack initiation mode changed from the surface cracking of un-nitrided specimen to the subsurface cracking of nitrided specimen. Hardening layer and compressive residual stress were formed by nitriding, which resulted in the improvement of the fatigue strength. The stress intensity factor was calculated using facet area in Fish-eye fracture mode. As a result, the stress intensity factor indicated almost constant value, ~ 3-4 MPa·m1/2, regardless of the number of cycles to failure.

A comparative study: The effect of surface treatments on the tribological properties of Ti–6Al–4V alloy

The effect of different surface treatments on the wear resistance of Ti–6Al–4V alloy has been investigated. For this purpose, plasma nitriding treatment was performed in a gas mixture 75% N 2–25% Ar, for 1 h treatment time and at 750 °C. The thin films were deposited using CFUMBS technique. The results showed that more surface roughness was obtained for nitrided specimens compared with thin film deposited specimens. It was also observed that both surface treatments improved the wear resistance of Ti–6Al–4V alloy. It was determined that plasma nitrided specimens exhibited excellent wear resistance compared with thin film deposited ones when applied load increased. Similar results were obtained from surface hardness measurements, and it was observed that load bearing capacity increased after plasma nitriding. The corrosion resistance of both surface treatments showed similar properties.

Wear characteristics of microscopic bushings for MEMS applications investigated by an AFM

The wear characteristics of silicon-based materials that are widely utilized in MEMS applications were investigated. Silicon (1 0 0), silicon oxide, silicon nitride and polysilicon specimens with microscopic bushings were fabricated for the wear tests performed under 100 µN normal force. The wear tests were conducted for a relatively long sliding distance of 5.6–187 km. Following the sliding experiment, the wear characteristics of the bushings were assessed by using an atomic force microscope. It was found that the wear rate was highest for the silicon oxide specimens and decreased in the order of polysilicon, silicon (1 0 0) and silicon nitride. Observation of the wear region showed that the scratches due to wear particles were often formed on the silicon (1 0 0), silicon oxide and polysilicon specimens. However, silicon nitride specimens showed signs of burnishing wear. The wear coefficients of the specimens were in the order of 10−8 to 10−7. Based on the wear characteristics and the wear debris behavior, the application of microgrooves for wear debris removal at the contacting interface was proposed.

Plasma nitriding behavior of Ti6Al4V orthopedic alloy

The influence of plasma nitriding on mechanical, corrosion and tribological properties of Ti6Al4V has been investigated using X-ray diffraction, microhardness tester, scanning electron microscopy, pin-on-disc tribotester, electrochemical polarization and impedance spectroscopy. Plasma nitriding treatment of Ti6Al4V has been performed in 25%Ar–75%N 2 gas mixture, for treatment times of 1–4 h at the temperatures of 650–750 °C. The corrosion tests were carried out in Ringer solution at 37 °C, and the wear tests were performed in dry sliding conditions. XRD analyses confirm the formation of δ-TiN and tetragonal ɛ-Ti 2N phases in the modified layer. It was observed that the surface hardness and wear resistance increase as the treatment time and temperature increase. The electrochemical impedance measurements indicate a decrease in double layer capacitance value and increase in charge transfer resistance for the nitrided specimens compared to the untreated ones.