The effect of temperature on plasma nitriding behaviour of DIN 1.6959 low alloy steel

A series of experiments have been conducted on DIN 1.6959 low-alloy steel using a 5 kVA DC plasma nitriding apparatus with the aim of elucidating the role of treatment temperature in plasma nitriding process. Treatments were carried out in 75%N₂-25%H₂ atmosphere of 4 mbar for 5 h at temperatures ranging from 350 to 550 °C. Optical microscopy, scanning electron microscopy, X-ray diffraction, along with surface roughness and microhardness measurements were utilized to characterize the treated samples. The depth, microstructure, hardness profile and phase constituents of the nitrided layers as well as the surface roughness of the samples were assessed as a function of treatment temperature. The results suggested that the compound layers were mostly dual phase consisting of gamma prime and epsilon iron nitride phases. Increasing treatment temperature increases compound layer and diffusion layer thicknesses. However, maximum surface hardness and roughness were found on the samples treated at 500 and 550 °C, respectively.     

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.     

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.     

Ammonia gas nitriding of Fe-18Cr-9Ni alloy at lower than 823 K

An Fe-18Cr-9Ni alloy, which it had not previously been possible to nitride at temperatures below 873 K, was found to form nitrides in an ammonia gas atmosphere at temperatures as low as 823 K after annealing at low hydrogen pressure at 1473 K. Microstructure and hardness were examined on cross-sections of the nitrided specimens. An internal nitriding layer had formed beneath an external nitriding layer on the specimen surface. Vickers hardness was above 1000 throughout the internal nitriding layer. The nitrides formed at the specimen surface and in the internal nitriding layer were identified using grazing incidence X-ray diffraction and ordinary X-ray diffraction methods, respectively. The external nitriding layer, which was about 6 to 10 m thick, formed on the surface, which consisted of -Fe_2–3N, -Fe₄N, and CrN. Two types of chromium nitride were precipitated by ammonia gas nitriding of the present alloy: CrN in the external nitriding layer and Cr₂N in the internal nitriding layer.     

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.     

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.     

Hydrogen-free nitriding of ZrTiAlV by double glow plasma discharge improving the wear resistance

A hydrogen-free nitriding method through double glow plasma metallurgy is exploited and a nitrided layer was formed on ZrTiAlV alloy. The nitrided layer was characterised through X-ray diffraction, optical microscopy, scanning electron microscopy and energy-dispersive spectroscopy techniques, as well as through Vickers hardness and friction and wear tests. Results showed that the nitrided layer is 580?µm thick, homogeneous and dense. It mainly consists of TiN, Ti₂N and ZrN phases. The hardness of the nitrided layer on the surface of the ZrTiAlV alloy is nearly 2.5 times higher than that of the ZrTiAlV substrate. The friction coefficient and wear resistance of the alloy considerably improved after nitriding.     

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.     

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.     

Plasma nitriding of AISI 52100 ball bearing steel and effect of heat treatment on nitrided layer

In this paper an effort has been made to plasma nitride the ball bearing steel AISI 52100. The difficulty with this specific steel is that its tempering temperature (~170–200°C) is much lower than the standard processing temperature (~460–580°C) needed for the plasma nitriding treatment. To understand the mechanism, effect of heat treatment on the nitrided layer steel is investigated. Experiments are performed on three different types of ball bearing races i.e. annealed, quenched and quench-tempered samples. Different gas compositions and process temperatures are maintained while nitriding these samples. In the quenched and quench-tempered samples, the surface hardness has decreased after plasma nitriding process. Plasma nitriding of annealed sample with argon and nitrogen gas mixture gives higher hardness in comparison to the hydrogen–nitrogen gas mixture. It is reported that the later heat treatment of the plasma nitrided annealed sample has shown improvement in the hardness of this steel. X-ray diffraction analysis shows that the dominant phases in the plasma nitrided annealed sample are ε (Fe_2 − 3N) and γ (Fe₄N), whereas in the plasma nitrided annealed sample with later heat treatment only α-Fe peak occurs.     

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.     

Microstructure and mechanical properties of the modified layer obtained by low temperature plasma nitriding of nanocrystallized 18Ni maraging steel

In the present study, low temperature plasma nitriding of nanocrystallized 18Ni maraging steel has been carried out at 360 °C from 1 to 24 h in a mixed gas of 25%N₂ + 75%H₂. The surface phase constitutions and microstructures of the nitrided layer have been investigated by X-ray diffraction analysis, transmission electron microscopy and optical microscopy. Nanoindentation and microhardness tests have been performed to determine the surface hardness and the hardness profile in the nitrided layer. The plasticity of the nitrided surface has been analyzed based on the nanoindentation results. The results show that at the initial stage of nitriding, the surface phase consists of a solid solution of nitrogen in α-Fe, and nanoscale nitrides and aging phase are formed with increasing of treatment time. The surfaces nitrided for 8 and 16 h possess the highest hardness. The plasticity factor calculations suggest that the nitrided surfaces have a good wear resistance and possess excellent plasticity.     

Duplex treatment of 304 AISI stainless steel using rf plasma nitriding and carbonitriding

Surface of 304 AISI austenitic stainless steel has been modified using duplex treatment technique of nitriding and carbonitriding. A thick modified nitrided layer, of approximately 20 µm, has been achieved when rf inductively coupled plasma was adjusted at 450 W for processing time of only 10 min. After performing the nitrided layer, the nitrided samples were carbonitrided using the same technique at different acetylene partial pressure ratios ranges from 10% to 70%, the balance was pure nitrogen. Different amount of nitrogen and carbon species are diffused underneath the surface through the nitrided layer during carbonitriding process and are found to be gas composition dependent. The treated samples were characterized by glow discharge optical spectroscopy, X-ray diffractometry, scanning electron microscopy and Vickers microhardness tester. The microstructure of the duplex treated layer indicates the formation of γ?-Fe₄N, Fe₃C, CrN and nitrogen-expanded austenite (γ_N). The thickness of the duplex treated layer increases with increasing the acetylene partial pressure ratio. The surface microhardness of duplex treated samples has been found to be gas composition dependent and increased by 1.29 fold in comparison to the nitrided sample.     

Effect of plasma on nitriding of Fe-18Cr-9Ni alloy

The plasma nitriding behaviour of Fe-18Cr-9Ni alloy was compared with gas nitriding. The alloy was nitrided under the following conditions: specimen temperature: 823 K, nitriding time: mainly 108 ks, total pressure: 0.4–0.7 kPa, mixture ratio of N₂ and H₂∶ 0.25, discharge voltage: 350–450 V, current: 0.8–1.1 A. Formation of a surface layer of iron nitrides was not observed. Formation of a homogeneous internal nitriding layer, consisting of small precipitates of CrN and the γ-phase matrix, was, however, noted. The lattice constant at the specimen surface was smaller than that at greater depth. This may have been because the sputtering effect decreased the dissolved nitrogen content at the specimen surface. The sputtering of iron nitrides at the specimen surface by the plasma was experimentally confirmed through γ′-Fe₄N formation on Si beside an alloy specimen. The characteristics of the plasma nitriding mentioned above are discussed in relation to the sputtering.     

Compositional depth profile investigation of plasma nitriding by multiple analyses techniques

Compositional depth profile in plasma nitriding is investigated by several experimental techniques including EDS, GDOES and SIMS as well as a calculated method. Plasma nitriding was carried out on high purity iron substrate at a temperature of 550 °C in an atmosphere of 75 vol.% H₂-25 vol.% N₂ for time periods of 1, 2, 5 and 10 h. SEM and XRD methods were used for microstructural evaluation and phase identification. According to EDS, GDOES and calculated data, composition of the compound layer reached nearly to Fe-8 wt% N and Fe-6 wt% N indicating ε-Fe_2-3N and γ′-Fe₄N nitrides were formed, respectively. Although nitrogen concentration was decreased to nearly zero close to the nitrided surface, calculated data and SIMS profiles show very smooth gradient in diffusion zone down to several hundreds of micrometers. The results of compositional depth profiling by EDS, GDOES and SIMS indicated good agreement between experimental findings and, thus, the techniques completed one another. It was found that EDS and GDOES are appropriate for analysis of Fe and N in the compound layer, but both have limitations for profiling of nitrogen in the diffusion zone. SIMS, on the other hand, was distinguished as a professional technique for accurate measurement of nitrogen within the diffusion zone. The experimental depth profiles indicated good consistency with calculated diffusion profiles for all treatment cycles.     

Corrosion behavior of nitride layer obtained on AISI 316L stainless steel via simple direct nitridation route at low temperature

Newly developed low-temperature nitride synthesis route was used to introduce interstitial nitrogen into the passive layer of as-received and as-polished 316L stainless steel. The new thermochemical route is based on treating the stainless steel samples in potassium nitrate melt in an ultra pure nitrogen atmosphere at 450 °C. Electrochemical impedance spectroscopy (EIS) and dc polarization measurements have been used to evaluate the nitride layer performance in 3.5% NaCl solution. Results showed a marked increase in the corrosion resistance of nitrided stainless steel even after maintaining two weeks in NaCl solution. The effect of the treatment temperature was also studied. Data showed that the as-polished samples nitrided at 450 °C have the highest corrosion resistance. The polarization resistance (R_p) for the as-polished and as-received blank stainless steel samples was estimated by EIS were approximately 4.0 × 10⁴ Ω cm² and 2.0 × 10⁴ Ω cm², respectively. The R_p increased by a factor of 2.5–5 for the nitrided samples. Increasing the nitriding temperature from 450 to 600 °C affects negatively the corrosion resistance of stainless steel in NaCl solution. The R_p of the samples nitrided at 600 °C decreased sharply being almost 1/30 of the R_p of the samples nitrided at 450 °C. Linear polarization measurements showed that the lowest corrosion rates and highest polarization resistances obtained from the as-polished nitrided samples at 450 °C. It has been found from the potentiodynamic measurements that the E_corr of the as-polished nitrided samples at 450 °C is nobler than that measured from the other groups. The surface morphology was analysed by optical microscope and SEM-EDS under different nitriding conditions.     

Evaluation of the effects of plasma nitriding temperature and time on the characterisation of Ti 6Al 4V alloy

The effects of plasma nitriding (PN) temperature and time on the structural and tribological characterisation of Ti 6Al 4V alloy were investigated. PN processes under gas mixture of N₂/H₂ = 4 were performed at temperatures of 700, 750, 800 and 850 °C for duration of 2, 5 and 10 h. Cross section and surface characterisation were evaluated by means of SEM, AFM, XRD and microhardness test techniques. Dry wear tests were performed using a pin on disc machine. Mass loss and coefficient of friction were measured during the wear tests. Three distinguished structures including of a compound layer (constituted of δ-TiN and ɛ-Ti₂N), an aluminium-rich region and a diffusion zone (interstitial solid solution of nitrogen in titanium) were detected at the surface of plasma nitrided Ti 6Al 4V alloy. These structures increased surface hardness of Ti 6Al 4V alloy significantly and gradually distributed the hardness from the surface to the substrate. The "surface hardness", "surface roughness", "wear resistance" and "coefficient of friction" of the alloy were increased due to plasma nitriding process. Moreover, rising both process temperature and time led to increasing of "layers thicknesses", "surface hardness", "surface roughness", "dynamic load-ability" and "wear resistance" of Ti 6Al 4V alloy.     

Investigation of the formation of Al,Fe, N intermetallic phases during Al pack cementation followed by plasma nitriding on plain carbon steel

Plain carbon steels are not suitable for nitriding as they form an extremely brittle case that spalls off readily, and the hardness increment of the diffusion zone is small. In this research, the effect of plasma nitriding time and temperature variation on the microstructure of the pack cemented aluminized plain carbon steel is investigated. All samples were aluminized at 900 °C for 2 h; the aluminized samples were subsequently plasma nitrided at 500 °C, 550 °C and 600 °C for 2.5, 5, 7.5 and 10 h. The phases formed on the sample surface were detected by X-ray diffraction (XRD). The cross section and samples surface were investigated by optical and scanning electron microscopy (SEM). Microhardness test was conducted to determine hardness change from the surface to the sample core. Results showed that by aluminizing the steel, Fe₃Al phases as well as Fe–Al solid solution were formed on the surface and some aluminum rich precipitates were formed in solid solution grain boundaries. Plasma nitriding of the aluminized layer caused the formation of aluminum and iron nitride (AlN, Fe₄N) on the sample surface. Consequently, surface hardness was improved up to about eight times. By increasing the nitriding temperature and time, aluminum-rich precipitates dissociated. Moreover, due to the diffusion of nitrogen through aluminized region during ion nitriding, iron and aluminum nitrides were formed in aluminized grain boundaries. Increasing nitriding time and temperature lead to the growth of these nitrides in the grain boundaries of the substrate. This phenomenon results in the increment of sample hardness depth. Plasma nitriding of aluminized sample in low pressure chamber with nitrogen and hydrogen gas mixture reduced surface aluminum oxides which were formed in aluminizing stage.     

Effects of DC plasma nitriding parameters on microstructure and properties of 304L stainless steel

A wear-resistant nitrided layer was formed on a 304L austenitic stainless steel substrate by DC plasma nitriding. Effects of DC plasma nitriding parameters on the structural phases, micro-hardness and dry-sliding wear behavior of the nitrided layer were investigated by optical microscopy, X-ray diffraction, scanning electron microscopy, micro-hardness testing and ring-on-block wear testing. The results show that the highest surface hardness over a case depth of about 10 µm is obtained after nitriding at 460 °C. XRD indicated a single expanded austenite phase and a single CrN nitride phase were formed at 350 °C and 480 °C, respectively. In addition, the S-phase layers formed on the samples provided the best dry-sliding wear resistance under the ring-on-block contact configuration test.     

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.