Effects of various nitriding parameters on active screen plasma nitriding behavior of a low-alloy steel

Active screen plasma nitriding (ASPN) is a novel surface modification technique that has many capabilities over the conventional DC plasma nitriding (CPN). In this study, 30CrNiMo8 low-alloy steel was active screen plasma-nitrided under various nitriding parameters such as active screen set-up parameters (different screen hole sizes, mesh sheet and plate top lids) and treatment temperature (520, 550 and 580 °C), in the gas mixture of 75% N₂+25% H₂ and chamber pressure of 500 Pa for 5 h. The properties of the nitrided specimens have been assessed by evaluating composition of phases, surface hardness, compound layer thickness and case depth using X-ray diffraction (XRD), microhardness measurements and scanning electron microscopy (SEM). It was found that the screen hole size and top lid type (mesh or plate) play an important role in transition of active species (nitrogen ions and neutrals) toward the sample surface, which in turn can affect the nitrided layer hardness and thickness. Treatment at higher temperature with bigger screen hole size resulted in a thicker compound layer and higher layer hardness. The compound layers developed on the samples treated under different conditions were dual phase consisting of γ′-Fe₄N and ε-Fe_2-3N phases.     

Duplex surface treatment of AISI 1045 steel via plasma nitriding of chromized layer

In this work AISI 1045 steel were duplex treated via plasma nitriding of chromized layer. Samples were pack chromized by using a powder mixture consisting of ferrochromium, ammonium chloride and alumina at 1273 K for 5 h. The samples were then plasma-nitrided for 5 h at 803 K and 823 K, in a gas mixture of 75%N₂ + 25%H₂. The treated specimens were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and Vickers micro-hardness test. The thickness of chromized layer before nitriding was about 8 μm and it was increased after plasma nitriding. According to XRD analysis, the chromized layer was composed of chromium and iron carbides. Plasma nitriding of chromized layer resulted in the formation of chromium and iron nitrides and carbides. The hardness of the duplex layers was significantly higher than the hardness of the base material or chromized layer. The main cause of the large improvement in surface hardness was due to the formation of Cr_xN and Fe_xN phases in the duplex treated layers. Increasing of nitriding temperature from 803 to 823 K enhanced the formation of CrN in the duplex treated layer and increased the thickness of the nitrided layer.     

Novel plasma nitriding-oxidizing duplex treatment of AISI 316 austenitic stainless steel

In this work, plasma nitriding and plasma nitriding-oxidizing treatment have been performed on AISI 316 austenitic stainless steel. In order to evaluate its response to this treatment, their microstructures and wear resistance have been compared with conventional plasma nitrided. The treatment of plasma nitriding was performed at temperature of 450 °C for 5 h with gas mixture of N₂/H₂:1/3 whereas plasma nitriding-oxidizing was performed with the same parameters of plasma nitriding and temperature of 450 °C with gas mixture of O₂/H₂:1/5 for 15, 30 and 60 min. The structural, mechanical and tribological properties were analyzed using XRD, SEM, microhardness testing and pin-on-disk tribotesting. The results showed that oxidation treatment reduces wear resistance of plasma nitrided sample under high loads. Furthermore the tribological evaluation indicates that by increasing the oxidation time further reduction of wear resistance can be occurred. In addition, it was found that oxidation treatment after plasma nitriding provides an important improvement in the friction coefficient against a AISI 52100 steel pin and reduces surface roughness.     

Comparison of ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel

This paper compares the ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel carried out to improve the surface corrosion resistance. The gas composition for plasma nitriding was 85% N₂–15% H₂ and that for plasma nitrocarburizing was 85% N₂–12% H₂–3% CO₂. Both treatments were performed for 5 h, for different process temperatures of 570 and 620 °C for ferritic and austenitic plasma treatment, respectively. Optical microscopy, X-ray diffraction and potentiodynamic polarization technique in 3.5% NaCl solution, were used to study the treated surfaces. The results of X-ray analysis revealed that with increasing the treatment temperature from 570 to 620 °C for both treatments, the amount of ε phase decreased and γ′ phase increased. Nitrocarburizing treatment resulted in formation of a more amount of ε phase with respect to nitriding treatment. However, the highest amount of ε phase was observed in the ferritic nitrocarburized sample at 570 °C. The sample nitrided at 620 °C exhibited the thickest layer. The potentiodynamic polarization results revealed that after plasma nitriding and nitrocarburizing at 570 °C, corrosion potential increased with respect to the untreated sample due to the noble nitride and carbonitride phases formed on the surface. After increasing the treatment temperature from 570 to 620 °C, corrosion potential decreased due to the less ε phase development in the compound layer and more porous compound layer formed at 620 °C with respect to the treated samples at 570 °C.     

Accurate measurement and evaluation of the nitrogen depth profile in plasma nitrided iron

Nitrogen depth profile of plasma nitrided pure iron was measured and evaluated by accurate experimental techniques. Plasma nitriding cycles were carried out on high purity iron substrate in an atmosphere of 75% H₂-25% N₂. Nitrogen concentration depth profiles in the compound layer and the diffusion zone were characterized by glow discharge optical emission spectroscopy (GDOES) and secondary ion mass spectroscopy (SIMS), respectively. Nitrogen diffusion depths were measured accurately by optical and scanning electron microscopy as well as SIMS technique at different nitriding times. Experimental results indicated good agreement between SIMS data and microscopic evaluations for various nitriding cycles. The results of SIMS showed the nitrogen diffusion depth of about 2000 μm in the diffusion zone for 10 h plasma nitriding at 550 °C. Such high depth had not been detected in previous investigations in which the conventional methods such as EDS, GDS, XPS, EPMA or ion probe techniques were used.     

Structural characterization of fluidized bed nitrided steels

The nitriding behaviour of 34CrAlNi7, 42CrMo4 and 40CrMnMoS86 steels was investigated nitrided in the fluidized bed processes. The nitriding processes were carried out at a temperature of 575 °C for treatment times of 6, 12 and 18 h. The nitrided samples were fully characterized using metallographic, microhardness and XRD techniques. Test results indicated that thickness of the compound layer on the steel surface changed in the range from 10 to 18 μm depending on steel type and treatment time, and γ′-Fe₄N and ε-Fe₂₋₃N formed in the compound layer. The hardness of the diffusion layer was over 1000 HV. Depending on the chemical composition of steels, the case depth ranged from 155 to 525 μm. Kinetics studies showed that the effective diffusion coefficients are 298×10⁻¹⁴, 525×10⁻¹⁴ and 68.8×10⁻¹⁴ m² s⁻¹, for 34CrAlNi7, 42CrMo4 and 40CrMnMoS86 steels, respectively. The fluidized bed process realizes the highest hardness of the case layer, 1095 HV, with fairly high growth rates, 27 μm/h.     

Phase formation in selected surface-roughened plasma-nitrided 304 austenite stainless steel

Direct current (DC) glow discharge plasma nitriding was carried out on three selected surface-roughened AISI 304 stainless steel samples at 833 K under 4 mbar pressures for 24 h in the presence of N₂:H₂ gas mixtures of 50 : 50 ratios. After plasma nitriding, the phase formation, case depth, surface roughness, and microhardness of a plasma-nitrided layer were evaluated by glancing angle x-ray diffractogram, optical microscope, stylus profilometer, and Vickers microhardness tester techniques. The case depth, surface hardness, and phase formation variations were observed with a variation in initial surface roughness. The diffraction patterns of the plasma-nitrided samples showed the modified intensities of the α and γ phases along with those of the CrN, Fe₄N, and Fe₃N phases. Hardness and case depth variations were observed with a variation in surface roughness. A maximum hardness of 1058 Hv and a case depth of 95 μm were achieved in least surface-roughened samples.     

Phase formation in selected surface-roughened plasma-nitrided 304 austenite stainless steel

Abstract

Direct current (DC) glow discharge plasma nitriding was carried out on three selected surface-roughened AISI 304 stainless steel samples at 833 K under 4 mbar pressures for 24 h in the presence of N₂:H₂ gas mixtures of 50 : 50 ratios. After plasma nitriding, the phase formation, case depth, surface roughness, and microhardness of a plasma-nitrided layer were evaluated by glancing angle x-ray diffractogram, optical microscope, stylus profilometer, and Vickers microhardness tester techniques. The case depth, surface hardness, and phase formation variations were observed with a variation in initial surface roughness. The diffraction patterns of the plasma-nitrided samples showed the modified intensities of the α and γ phases along with those of the CrN, Fe₄N, and Fe₃N phases. Hardness and case depth variations were observed with a variation in surface roughness. A maximum hardness of 1058 Hv and a case depth of 95 μm were achieved in least surface-roughened samples.     

Diffusion kinetics of explosively treated and plasma nitrided Ti-6Al-4V alloy

Ti-6Al-4V alloys, which were exposed to an explosive shock process, were nitrided in nitrogen plasma in the temperature range of 700-900°C for 3-12 h. During the plasma nitriding, the surface layer consisted of TiN (δ), Ti₂N (ε) and nitrogen solid solution layers (α-Ti). The growth rate of nitride and solid solution layers were found to be controlled by the diffusion of nitrogen. An effective nitriding was achieved due to high dislocation density and vacancy concentration. Based on the present layer growth data, an analytical model for multiphase diffusion was used to estimate the effective nitrogen atom diffusion coefficient in the nitride layers. The interface velocity equations were derived from Fick's law and a numerical method has been used to compute the diffusion coefficients of nitrogen in a binary multiphase Ti-TiN system. Depending on temperature and layer thickness, the activation energies of nitrogen in TiN and Ti₂N phases were found to be 18,950 (±2116) and 27,925 (±1105) cal/mole, respectively.     

The study of tribological and corrosion behavior of plasma nitrided 34CrNiMo6 steel under hot and cold wall conditions

This paper reports on a comparative study of tribological and corrosion behavior of plasma nitrided 34CrNiMo6 low alloy steel under modern hot wall condition and conventional cold wall condition. Plasma nitriding was carried out at 500 °C and 550 °C with a 25% N₂ + 75% H₂ gas mixture for 8 h. The wall temperature of the chamber in hot wall condition was set to 400 °C. The treated specimens were characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness and surface roughness techniques. The wear test was performed by pin-on-disc method. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were also used to evaluate the corrosion resistance of the samples. The results demonstrated that in both nitriding conditions, wear and corrosion resistance of the treated samples decrease with increasing temperature from 500 °C to 550 °C. Moreover, nitriding under hot wall condition at the same temperature provided slightly better tribological and corrosion behavior in comparison with cold wall condition. In consequence, the lowest friction coefficient, and highest wear and corrosion resistance were found on the sample treated under hot wall condition at 500 °C, which had the maximum surface hardness and ε-Fe_2–3N phase.     

Comparison of conventional and active screen plasma nitriding of hard chromium electroplated steel

This paper considers the nitriding behavior of hard chromium electroplated steel by conventional plasma nitriding (CPN) and active screen plasma nitriding (ASPN) methods. Indentation test along the cross-section of the treated samples reveals that duplex treatment performed by two methods exhibits almost the same hardnesses. Furthermore, an increase in the time of plasma nitriding from 5 h to 10 h restores 30% of the hardness decline. Morphological studies show that surface particles formed on active screen plasma nitrided samples have orderly formed geometrical shapes while in conventional plasma nitriding they are in cauliflower shape. The reason for reaction between chromium and nitrogen seems to be the difference between thermal expansion coefficient of chromium oxide, chromium and steel substrate which results in partial breakdown of the oxide film. Moreover, the reducing of chromium oxide by hydrogen promotes the process. It looks as if nitriding treatment changes the corrosion behavior of the chromium coating from severe localized to uniform corrosion. Also active screen plasma nitriding treatment lowers the anodic dissolution 50-100 orders of magnitude which would be the result of full closure of surface microcracks.     

Effect of pulsed plasma nitriding on mechanical and tribological performance of Ck45 steel

We studied the mechanical properties and wear performance of AISI 1045 (Ck45) carbon steel under the influence of pulsed plasma nitriding. The treatments were performed at temperatures of 500 and 550 degrees C in N2:H2 gas ratios of 1:3 and 3:1 and the working pressure of 10 mbar for 1 to 4 hours. Samples were examined by X-ray diffraction, optical, electron and atomic force microscopy, microhardness tests, roughness measurements and wear tests. Nitride layers were mainly composed of epsilon-(Fe2-3N) or gamma'-(Fe4N) depending on the gas ratio and/or temperature and time. When the nitriding time is increased, the composition of the compound layer varies from monophase gamma'-(Fe4N) to the two phase of epsilon-(Fe2-3N) and gamma'-(Fe4N). The highest thickness and hardness of the layers were obtained at 550 degrees C in the N2:H2 gas ratios of 3:1 for 4 h. The topographical evolution and surface roughness of the samples showed that all the roughness parameters increase with increasing the temperature. The friction coefficient of all samples was higher than that of untreated material. Wear performance of all nitrided samples was significantly better than that of untreated material.     

Kinetics and wear behaviour of M50NiL steel plasma nitrided at low temperature

The quenched M50NiL steel was plasma nitrided at 460°C for different time to investigate the effects of the duration time on the microstructure, microhardness and wear resistance of the nitrided layers. The results show that the plasma nitrided layer depth increases with increasing nitriding time. The plasma nitrided layer includes only the diffusion layer without compound layer. The main phases in the nitrided surface layer are nitrogen expended α′-Fe and γ′-Fe₄N. The microstructure of the nitrided layer is refined. The wear resistance of the nitrided samples can be improved significantly by plasma nitriding. The sample nitrided for 4?h possesses the highest wear resistance, due to its relatively smooth surface and ultra-fine grains in the nitrided layer.     

Effect of treating atmosphere in plasma post-oxidation of nitrocarburized AISI 5115 steel

In this study quenched and tempered AISI 5115 steel was plasma-nitrided and nitrocarburized at 550 °C for 5 h in atmospheres of 80% N₂ balanced with various amounts of CO₂ and H₂ gases. The amount of CO₂ varied from 0 to 10 vol%. The highest amount of ε phases was formed in the compound layer after treating in atmosphere containing 7 vol% CO₂. Optimized compound layer was post-oxidized for 1 h at 450 °C under O₂/H₂ volume ratios of 1/1 and 3/1 as well as 100% oxygen. The treated samples were characterized using metallographic techniques, XRD, SEM, roughness measurement and potentiodynamic methods. The results showed that the growth rate of the oxide layer increased with increasing O₂ in the oxidizing gas mixture. X-ray diffraction analysis of oxidized layers confirmed the formation of highest amount of magnetite at post-oxidation in an atmosphere with the O₂/H₂ volume ratio of 1/1. Electrochemical polarization tests proved the enhancement of corrosion resistance by plasma post-oxidation and the highest corrosion resistance obtained after oxidizing under an O₂/H₂ volume ratio of 1/1.     

Microstructural changes in the upper surface of austenitic steel induced by ion sputtering and gas nitriding

The results of investigations of the influence of ion sputtering parameters applied as a pre-treatment on the formation of a nitrided layer during gas nitriding are presented. Previous work had shown that a low reproducibility of layer composition occurred after gas nitriding, thought to be the result of uncontrolled variation of sputtering parameters during the plasma initiation. In the present studies a two-stage sputtering process in nitrogen at a pressure of 3-5 Pa was used: firstly for a few seconds to 1 min with the plasma parameters 1.8 kV and 3 mA/cm², and secondly followed by sputtering with the parameters 1.35 kV, 0.5 mA/cm² for 15 min. The samples were gas nitrided immediately following these processes in 100% ammonia. It was found that even a few seconds of the first plasma treatment enabled magnetic “expanded austenite” to be formed in the layer. Surface layers were analysed using grazing angle X-ray diffraction (GXRD), light microscopy (LM), scanning electron microscopy (SEM), electron back scattered diffraction (EBSD), magnetic and atomic force microscopy (AFM and MFM) and electron probe micro analysis (EPMA) that revealed a complex microstructure. The results have emphasised the need for good control over plasma sputtering parameters in the initial process stage.     

Nitriding of AISI 304 stainless steel in a 85% H2/15% N2 mixture with an inductively coupled plasma source

We present some results obtained from nitriding of AISI 304 stainless steel substrates. The process take place in an inductively coupled plasma discharge supplied by a ∼300 W source at 13.56 MHz in a 85% hydrogen and 15% nitrogen gas mixture. The samples, placed into a pyrex glass discharge chamber (3.5 cm diameter and 60 cm long) were biased by DC power from 0 to −300 V with respect to ground. The resulting hardness is a function of the increasing nitrogen concentration in the substrate and this, in turn, of the temperature which varied from 200 to 470 °C. After nitriding at −300 V, hardness values of 1790 HV under 10 g loads and of 632 HV for 300 g were measured. X-ray diffraction indicates that the expanded phase amplitude (γ_N) depends on the substrate temperature.     

Saddle field fast atom beam source: A new low pressure plasma nitriding method for a alloy Ti-6Al-4V

Ti and its alloys (Ti-6Al-4V) have been used in different engineering applications due to their several outstanding properties. Nevertheless, their use in practical applications is limited in many cases due to their poor tribological property. Researches are ongoing on surface modification of Ti based materials by different plasma and ion based techniques to overcome this problem. However, the conventional plasma nitriding techniques have several problems such as formation of an arc, increased possibility of surface contamination due to a comparatively higher operating pressure, production of a very thin nitrided layer after a long processing time, etc. In this present work, the possibility of a new low-pressure plasma nitriding process using a Plasma Enhanced Chemical Vapor Deposition (PECVD) based saddle field fast atom beam source on a Ti-6Al-4V alloy sample is investigated. Plasma nitriding was carried out at 900 °C and at a pressure 0.1 Pa for 8 h by using a beam current 0.5 A. Optical Microscopy investigation of the cross-section of the nitrided sample revealed a compound nitrided layer (thickness approximately 16 μm) followed by a diffusion layer. X-Ray Diffraction (XRD) analysis confirmed the presence of a TiN phase in the nitrided layers. A roughly three fold higher hardness value (1578 HV_0.015) in the top nitriding layer was observed by Vickers microhardness testing compared to hardness value of untreated sample (568 HV_0.015),with a gradually decreasing hardness in the core material. The results show that this is a promising method for low pressure plasma nitriding of Ti alloy within a short processing time compared to the conventional nitriding process.     

Corrosion behavior of ion nitrided AISI 316L stainless steel

In the present work the corrosion susceptibility of ion nitrided AISI 316L stainless steel was investigated for two different nitriding times and compared with the corrosion susceptibility of the untreated material. Plasma nitriding for short times (30 min) produced the “S” phase or expanded austenite (γ_N), with a thickness of ∼ 5 μm and a micro-hardness of 1300-1400 HV_0.025 (6.5 times higher than the untreated material). Plasma nitriding for long times (6 h) resulted in the precipitation of iron and chromium nitrides.To evaluate the corrosion resistance of both untreated and nitrided samples, anodic potentiodynamic polarization curves and immersion tests were performed in 1 M NaCl at room temperature. It was found that the corrosion resistance depends on the nitriding time. Samples nitrided for half an hour developed a much better corrosion resistance - close to that observed in the untreated samples - than those nitrided for 6 h. Samples nitrided for half an hour showed high roughness probably due to the presence of sliding bands developed in the expanded austenite phase. These sliding bands provide appropriate sites for the developing of the corrosion process. This would explain the results obtained in the corrosion tests. Samples ion nitrided for 6 h showed a severe and massive surface damage due to corrosion.Ion nitriding of AISI 316L stainless steel for short periods of time (30 min in the present case) may be an interesting surface treatment process that efficiently improves the surface hardness of the steel with some reduction in its corrosion resistance.     

Research on new rapid and deep plasma nitriding techniques of AISI 420 martensitic stainless steel

Cyclic plasma oxynitriding and cyclic plasma nitriding catalyzed by rare earth La of AISI 420 martensitic stainless steel were performed and compared with conventional plasma nitriding. The nitrided layers were investigated by means of an optical microscope, microhardness tester, Auger electron spectroscopy (AES), X-ray diffraction (XRD), wear machine, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The results show that the wear resistance of AISI 420 martensitic stainless steel is improved significantly by the two new rapid and deep plasma nitriding techniques. The new techniques increase the surface hardness of the nitrided layers and make the microhardness profiles gentler, which are consistent with the nitrogen concentration depth profiles. Meanwhile, the nitrided effect improves with increasing cycles. It was also found that the optimum phase compositions of nitrided layers with more γ′ phases and less ? phases for long-term service conditions can be obtained by the two new techniques, which is in agreement with the microstructure. In addition, traces of Fe₃O₄ were found in the cyclic plasma oxynitrided sample. Combining the SEM and EDS analysis indicated the existence of La in the nitrided layer of the sample under cyclic plasma nitriding catalyzed by rare earth La.     

Modifying the properties of AISI 316L steel by glow discharge assisted low-temperature nitriding and oxynitriding

Apart from titanium, its alloys and CoCrMo alloys, austenitic steels are widely used in medical applications. In order to improve the frictional wear resistance of these steels, they are subjected to various surface treatments such that the good corrosion resistance of the steels is preserved.The paper analyzes the structure and phase composition of AISI 316L steel after subjecting it to low-temperature nitriding and oxynitriding under glow discharge conditions. The treatments produced diffusion-type surface layers composed of nitrogen-expanded austenite (known as the phase S, i.e. supersaturated solution of nitrogen in austenite) with a thin surface layer of chromium nitride (CrN) zone (after nitriding) or chromium oxide (Cr₂O₃) zone (after oxynitriding). It has been shown that the treatments substantially increase the hardness and frictional wear resistance of the steel without degrading its good corrosion resistance (examined in the Ringer physiological solution at a temperature of 37 °C).