Nitrided Specimens Research Articles

The effect of gas mixture of post-oxidation on structure and corrosion behavior of plasma nitrided AISI 316 stainless steel

In this research, microstructure and corrosion properties of plasma nitriding and post-oxidation treated AISI 316 austenitic stainless steel have been studied. The plasma nitriding treatment was carried out at 450°C for 5h in a D.C. plasma setup with a gas mixture of 25% N2–75% H2 followed by post-oxidation in gas mixtures of O2/H2: 1/3, 1/5, 1/9 and 1/12 for 30min. The treated samples were characterized by SEM, XRD and roughness testing. Potentiodynamic and cyclic polarization tests were also employed to evaluate the corrosion resistance of the samples. The results showed that plasma nitriding treatment decreases corrosion resistance of the steel substrate. However, post-oxidizing treatment improves both uniform and pitting corrosion resistances of the nitrided specimens. This effect was most pronounced by post-oxidizing at lower O2/H2 ratios due to the development of magnetite as a more dense oxide phase over the modified layer.

The effect of nitriding, severe shot peening and their combination on the fatigue behavior and micro-structure of a low-alloy steel

Fatigue strength of mechanical components can be greatly enhanced by generating compressive residual stress, increasing the hardness and reducing the grain size. It is well known that while the use of mechanical treatments is able to generate an effective field of compressive residual stresses and, if severe parameters are used, to cause grain refinement, thermochemical treatments are able to increase the surface hardness. This justifies the interest in developing combined treatments, able to achieve all the just mentioned factors.In this study, the effect of combination of severe shot peening and nitriding on the fatigue limit of a low-alloy steel is investigated. Severe shot peening was conducted by using particular processing parameters to obtain ultra-fine/nano-structured surface layers. Micro-structural observation, micro-hardness, surface roughness and XRD measurement of residual stress were performed on single and hybrid surface treated specimens including nitrided, severely shot peened, nitrided plus severely shot peened and severely shot peened plus nitrided specimens. The fatigue limit of all series was experimentally determined and compared with the as-received specimens. Severe shot peening and Nitriding improved the fatigue limit by 11.6% and 51.3% respectively. Combination of severe shot peening and nitriding improved hardening, residual stress and nitrogen diffusion with respect to the single treatment. Nevertheless, it could not guarantee further improvement in the fatigue limit as compared with the nitrided smooth specimens. The results are critically assessed by considering the local fatigue limit concept.

The Role of Activated Nitrogen Species on Double-folded Screen Nitriding Process

As clean and energy saving surface hardening technology, plasma nitriding techniques have been evolved with object of higher performance in the last decades. Even though the diffusion of nitrogen inward to steel is occurred at the final step, solid diffusion from surface, energy transition from gas molecule of nitrogen to atomic or an activated state have many different steps depending on the plasma conditions, parameters and the design of each equipment. And this study made comparative on nitrogen sources transfer with conventional DC plasma nitriding and novel nitriding process using plasma diagnosis and metallurgical observation. With different vacuum pressure, gas ratio and new designed electrode (double-folded screen cathode electrode), it showed a different behavior of DC plasma nitriding including the nano-sized nitride on the outer surface of specimen due to nitrogen source of determining plasma species. In this study, plasma species was able to identify with optical emission spectroscopy (OES) studies. From these observations, we could understand better role of ions or neutral nitrogen species, like neutral nitrogen (N), N2+ and NHx radicals in plasma nitriding process with different parameters. And cutting layers of nitride specimen were showed the results due to a different species gas flow ratio or plasma conditions.

Influence of Plasma Nitriding on the Microstructure, Wear, and Corrosion Properties of Quenched 30CrMnSiA Steel

The oil-quenched 30CrMnSiA steel specimens have been pulse plasma-nitrided for 4 h using a constant 25% N2-75% H2 gaseous mixture. Different nitriding temperatures varying from 400 to 560 °C have been used to investigate the effects of treatment temperature on the microstructure, microhardness, wear, and corrosion resistances of the surface layers of the nitrided specimens. The results show that significant surface-hardened layer consisting of compound and diffusion layers can be obtained when the oil-quenched steel (α′-Fe) are plasma-nitrided at these experimental conditions, and the compound layer mainly consists of e-Fe2-3N and γ′-Fe4N phases. Lower temperature (400-500 °C) nitriding favors the formation of e-Fe2-3N phase in surface layer, while a monophase γ′-Fe4N layer can be obtained when the nitriding is carried out at a higher temperature (560 °C). With increasing nitriding temperature, the compound layer thickness increases firstly from 2-3 μm (400 °C) to 8 μm (500 °C) and then decreases to 4.5 μm (560 °C). The surface roughness increases remarkably, and both the surface and inner microhardness of the nitrided samples decrease as increasing the temperature. The compact compound layers with more e-Fe2-3N phase can be obtained at lower temperature and have much higher wear and corrosion resistances than those compound layers formed employing 500-560 °C plasma nitriding.

Cracking in a multiple gas-nitrided H13 aluminum extrusion mandrel

A failure analysis was carried out to investigate the root cause of cracking in a mandrel used in a local aluminum extrusion plant. The tool was made of AISI H13 and received multiple gas-nitriding case hardening during its service life. The scraped mandrel showed several microcracks in the filleted bridges. Izod impact tests were conducted over a range of temperatures (23–550°C) for un-nitrided and single nitrided H13 specimens to examine the possible susceptibility of the tool steel to temper embrittlement. The effect of single and multiple nitriding cycles on the surface fracture resistance of H13 specimens was characterized using Vickers indentation test. The impact test results showed a low impact-energy regime at temperatures at or below 350°C and a high impact-energy regime at higher temperatures. Temper embrittlement was ruled out as a likely cause of cracking since the extrusion temperature of 425°C is within the high impact-energy regime. Vickers indentations typically showed sharp cracks initiated from the corners of the indentation for a sample taken from the mandrel. No cracks emanated from indentations applied on un-nitrided and single-nitrided specimens. Crack initiation in the mandrel is primarily attributed to multiple gas-nitriding and the subsequent thickening of the brittle nitrided layer which promoted brittle crack initiation in the mandrel bridges under the applied extrusion pressure.

Effect of nitriding on corrosion behaviour of graphite reinforced aluminium alloy composites

The paper evaluates the effect of nitriding treatment on the corrosion behaviour of graphite reinforced aluminium alloy 6061 composites. The composites were prepared using the liquid metallurgy technique. The graphite content in the composites was varied from 1 to 7% (by weight) in steps of 2%. The nitriding process was carried out at 500°C for 24 h. Energy dispersive X-ray analysis was used to confirm the implantation of nitrogen in the composites. Three categories of specimens, namely, the nitrided composites, non-nitrided composites as well as non-nitrided and unreinforced specimens, were tested. Immersion corrosion test using the weight loss method was employed to study the effect of corrosion over a period ranging from 24 to 120 h in steps of 24 h at ambient temperature. The test solution used was aerated 1 N NaCl acid. It was observed that the nitrided specimens exhibited less weight loss compared to the non-nitrided specimens in general. The composites with greater content of graphite exhibited low corrosion rate.

The Influence of Substrate Treatment on Load Carrying Capacity of Ti6Al4VDuplex Coating

In this work, a duplex coating is performed by conducting microwave plasma nitriding on Ti6Al4V, followed by the deposition of hardcoating (CrN) using an arc coating system. Plasma nitriding of Ti6Al4V alloy using microwave plasma technique, created a thick (∼130μm) modified layer (case depth) at processing temperature of 700°C and 5 hour. The load carrying capacity of the duplex coating assessed using Rockwell-C test shows that the case depth increases with an increase in process temperature and time for nitriding. Consequently, it affects the formation of compound layer on the plasma nitrided specimen. Moreover, the formation of compound layer may lead to uneven distributions of the applied hardcoating (CrN) resulted in poor adhesion of hard coating - nitrided substrate.

鉄鋼の窒化処理への沿面放電プラズマの応用

We have been studying the surface discharge plasma generated by insulating capillary tube as an ablation gun for depositing diamond-like carbon films. It is found in our series of experiments that applying a positive voltage to the gun of the system allows us to fill the entire chamber with plasma. In this study, we propose a new application of the surface discharge technique for plasma nitriding of steel.Nitriding experiments under various temperatures were carried out for SKH51 high speed steel substrate. The hardness of the specimen nitrided at 500°C was the highest in this work. Nitriding depth of the specimen seemed to be more than 40μm. Although X-ray diffraction measurements showed no distinguishable nitride diffraction peaks, the hard X-ray photoelectron spectroscopy (HXPS) measurements suggested the presence of Cr-N and W-N chemical bonds in the nitrided SKH51 steel. Furthermore, so-called “white layer (Fe2-3N)” or damage were not found on the surface of nitrided specimens. It can be concluded that this technique can be one of the most superior techniques for nitriding steel.

Increasing Surface Hardness of AISI 1045 Steel by AIH-FPP/Plasma Nitriding Treatment

In order to increase surface hardness of AISI 1045 steel, a new hybrid surface modification; combination of atmospheric-controlled induction-heating fine particle peening (AIH-FPP) and plasma nitriding, was developed. Surface microstructures of plasma nitrided specimens pre-treated with AIH-FPP using Cr shot particles were characterized by an optical microscope, a scanning electron microscope (SEM), an energy dispersive X-ray spectrometer (EDX) and X-ray diffraction analysis (XRD). As results, the nitrided layer was formed at the surface of the specimen with Cr diffused layer induced by AIH-FPP. This nitrided layer showed higher hardness than that of without AIH-FPP specimen. This was because CrN was formed at the surface of the AIH-FPP/Plasma nitriding treated specimen. And, the surface hardness of the nitrided layer tended to increase as the Cr concentration in the surface layer decreased. It was clarified that the pore formation during the AIH-FPP/Plasma nitriding treatment was inhibited by decreasing Cr concentration. These results suggest that the proposed hybrid surface treatment remarkably increases surface hardness of AISI 1045 steel. [doi:10.2320/matertrans.MBW201205]

Fatigue Properties of Solution-Treated Type 304 Stainless Steel after Nitriding

The solution treatment after nitriding (STAN) was performed to stabilize the γ- phase in a metastable austenitic stainless steel, type 304, and to improve the strength of type 304 by the solid solution of nitrogen. Plasma nitriding was conducted at 500°C for 8.5h, and then solution treatment was performed at 1200°C for 1h. As a result, the static strength and the hardness were improved by the STAN. Rotary bending fatigue tests were performed on the specimens with STAN (solid solution strengthened) together with the untreated and the nitrided ones in laboratory air and in 3%NaCl solution. In laboratory air, the fatigue strength of the solid solution strengthened specimen increased compared to that of the untreated specimens, where fatigue limits were 340MPa and 290MPa for the solid solution strengthened and the untreated, respectively. However, the fatigue limit of the solid solution strengthened specimen was lower than that of the nitrided specimen, that is, 380MPa. On the other hand, in 3%NaCl solution, the fatigue strengths of the nitrided specimens and the solid solution strengthened specimens decreased significantly compared to those in laboratory air. After the fatigue tests at the stress level of fatigue limit in laboratory air, the strain-induced martensitic transformation was examined by XRD. In the solid solution strengthened specimens, the strain-induced martensitic transformation was not detected during fatigue tests until 3x107 cycles, indicating that the γ- phase was stabilized by the solid solution of nitrogen.

Fatigue behavior of nitrided and shot peened steel with artificial small surface defects

In this paper the effect of small defects on fatigue threshold of different series of nitrided and nitride-shot peened low alloy steel specimens is investigated by means of experimental rotating bending tests on sandglass specimens. Micro-holes acting as pre-cracks are introduced by means of different methods (electro-erosion and indentation) in order to evaluate the effect of defect size as well as the influence of the process to generate them on the fatigue threshold. The results, interpreted in terms of ΔKth, show good agreement with estimated ones, calculated by a formula that considers the experimentally measured residual stresses, FWHM parameter, micro-hardness values and SEM observation of broken and run-out specimens. A critical discussion of the obtained results highlights the influence of the defect size and of the method used to generate the micro-holes on the fatigue threshold.

Unusual nucleation and growth of γ′ iron nitride upon nitriding Fe–4.75 at.% Al alloy

The influence of substitutionally dissolved Al in ferritic Fe–4.75 at.% Al alloy on the nucleation and growth of γ′ iron nitride (Fe4N1− x ) was investigated upon nitriding in NH3/H2 gas mixtures. The nitrided specimens were characterised employing optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis and X-ray diffraction. As compared to the nitriding of pure ferrite (α-Fe), where a layer of γ′ develops at the surface, upon nitriding ferritic Fe–4.75 at.% Al an unusual morphology of γ′ plates develops at the surface, which plates deeply penetrate the substrate. In the diffusion zone, nano-sized precipitates of γ′ and of metastable, cubic (NaCl-type) AlN occur, having, with the ferrite matrix, a Nishiyama–Wassermann orientation relationship and a Bain orientation relationship, respectively. The γ′ plates contain a high density of stacking faults and fine ε iron nitride (Fe2N1− z ) precipitates, although the formation of ε iron nitride is not expected for the employed nitriding parameters. On the basis of dedicated nitriding experiments it is shown that the unusual microstructural development is a consequence of the negligible solubility of Al in γ′ and the obstructed precipitation of the thermodynamically stable, hexagonal (wurtzite-type) AlN in ferrite.

Influence of strain on thermal conductivity of silicon nitride thin films

We present a micro-electro-mechanical system-based experimental technique to measure thermal conductivity of freestanding ultra-thin films of amorphous silicon nitride (Si3N4) as a function of mechanical strain. Using a combination of infrared thermal micrography and multi-physics simulation, we measured thermal conductivity of 50 nm thick silicon nitride films to observe it decrease from 2.7 W (m K)−1 at zero strain to 0.34 W (m K)−1 at about 2.4% tensile strain. We propose that such strong strain–thermal conductivity coupling is due to strain effects on fraction–phonon interaction that decreases the dominant hopping mode conduction in the amorphous silicon nitride specimens.

Development of eco-friendly surface modification process for 316L austenitic stainless steel weld cladding

Austenitic stainless steels are used for industrial applications due to their improved corrosion resistance properties. Their poor tribological behaviour is a barrier for their wider application under corrosion–wear conditions taking place between rolling and sliding contacts. The present work has been conducted to develop a low temperature liquid nitriding process for austenitic stainless steel claddings to impart better resistance to corrosion–wear. This work presents the results of the corrosion and wear tests performed on liquid nitrided AISI 316L stainless steel claddings. The wear tests were conducted with a Ducom pin on disc machine under rolling–sliding conditions. Surface and subsurface were studied by an optical microscope. The hardness and depth of the modified layer were measured in the sectioned samples using a microhardness tester. The optical metallographic microstructures of the liquid nitrided specimens were revealed using Marbles reagent. The electrochemical behaviour of the liquid nitrided specimens was investigated with a dc anodic potentiodynamic polarisation technique.

Nanoindentation Test and Stress Fields Simulation of Hot-Pressed Silicon Nitride Ceramic

In order to investigate material properties of silicon nitride ceramic, nanoindentation tests are carried out on Hot-press silicon nitride specimen. The micro-hardness and elastic modulus are obtained. simultaneously, load-displacement curve is acquired. The whole process is simulated in ansys. Through comparing, there are satisfactory consistency between experimental curve and simulation curve. In addition, simulation can also indicate the distribution of stress fields during loading and unloading process. Base on these informations, the mode of cracks development and mechanism of material removal can be known.

Effect of Mn Contents on Fatigue Properties of Plasma-Nitrided Fe-C-Mn Steels

Rotating bending fatigue tests were conducted for a series of plasma-nitrided Fe-C-Mn steels in order to investigate the effect of manganese contents. 5 types of steels were prepared, i.e., 0.0Mn, 0.5Mn, 1.0Mn, 2.0Mn and 4.0Mn. Microstructures of the steels were ferrite-pearlite except 4.0Mn whose microstructure was martensite. 2.0Mn and 4.0Mn of the nitrided specimens had a hardened layer of 0.5 to 1 mm in depth, while the no hardening was observed in 0.0Mn, 0.5Mn and 1.0Mn of the nitrided specimens. In case of 0.0Mn, 0.5Mn and 1.0Mn, the nitrided specimen showed surface fracture and increase of fatigue strength was very small. The 2.0Mn nitrided specimen showed surface fracture and fatigue strength were largely improved by plasma-nitriding. On the other hand, in case of 4.0Mn, the nitrided specimen showed internal fracture beneath the hardened layer and the fatigue strength were lower than that of the 2.0Mn nitrided specimen.

Studies on distortions in nitrided steel cylinders using computer simulation

Gas nitriding is well established as a suitable process for realising low distortion in precision parts under optimal conditions. However, efforts for optimising the distortions by trial and error, which are not avoidable, have been repeated over many years. A software tool based on the finite element method is developed by use of the latest models of nitriding for resolving the problems, which was applied to the nitrided austenitic stainless steels specimens. In this study, distortions in nitrided steel cylinders, which were measured systematically in 1930s, were investigated using the tool for understanding their tendencies derived from different conditions. The fruits will be used for enhancing the tool by additional verifications, and applying finally to distortion problems in practical parts.

AN INVESTIGATION OF FATIGUE PROPERTIES OF PRE-QUENCHED-TEMPERED S45C STEEL WITH PLASMA NITRIDING TREATMENT

Quenched and tempered (QT) S45C steel was processed by plasma nitriding (a mixture of 40% N 2-60% H 2, 400 Pa , 773 K ) with two different durations: 8 h and 48 h. The microhardness, surface residual stress and nitriding layer compound were separately detected and the effect was discussed with the fatigue test result. The rotating bending fatigue properties was test with QT, QT nitiding 8 h, QT nitriding 48 h specimens. Fish-eye type crack formation occurred and the two crack initiation modes, fish-eye crack initiation and surface crack initiation, were observed by the scanning electron microscope and discussed. The result showed that the fatigue strength of QT specimens were improved by 54% at least compared with the untreated specimens. But the QT and un-QT have the mainly same fatigue strength in the same plasma nitriding condition, especially no-improvement with the 48 h plasma nitriding specimens.

비정질합금 박판 제조용 노즐 재료의 내구성평가

Erosion and thermal shock resistance of several refractory materials have been investigated, which are expected to be used as nozzles in a planar flow casting equipment for amorphous alloys. The test was conducted on five materials; graphite, boron nitride, fused silica, alumina and zirconia. Test specimens were preheated and dipped into the melt of carbon steel and amorphous alloys. Some test specimens were rotated to develop high erosion and to shorten the test periods. Fused silica and boron nitride specimens showed the excellent erosion and thermal shock resistance irrespective of the kind of melt and melting atmosphere.

Effects of Plasma Nitriding on the Tensile Properties of Al-Mg-Si

Effects of plasma nitriding on the tensile behavior of Al-Mg-Si alloy were investigated in this study. The specimens were nitrided at 70 W input pulsed DC power at 1 mbar pressure of nitrogen for 4 h using glow discharge plasma. The formation of aluminum nitride layer on the specimen's surface was confirmed by the XRD analysis. Stress-strain curves of both un-nitrided and nitrided specimens were obtained using Universal Testing Machine. The comparison of these curves reveals that yield stress, ultimate tensile stress, percentage elongation, and stress relaxation rate decrease after plasma nitriding. The changes in the tensile properties after nitriding have been correlated with the changes in the microstructure of the specimens as observed using scanning electron microscope.