Influence of nitriding on corrosion resistance of martensitic X17CrNi16‐2 stainless steel

Nitriding increases surface hardness and improves wear resistance of stainless steels. However, nitriding can sometimes reduce their corrosion resistance. In this paper, the influence of nitriding on the corrosion resistance of martensitic stainless steel was investigated. Plasma nitriding at 440 °C and 525 °C and salt bath nitrocarburizing were carried out on X17CrNi16‐2 stainless steel. Microhardness profiles of the obtained nitrided layers were examined. Phase composition analysis and quantitative depth profile analysis of the nitrided layers were preformed by X‐ray diffraction (XRD) and glow‐discharge optical emission spectrometry (GD‐OES), respectively. Corrosion behaviour was evaluated by immersion test in 1% HCl, salt spray test in 5% NaCl and electrochemical corrosion tests in 3.5% NaCl aqueous solution. Results show that salt bath nitrocarburizing, as well as plasma nitriding at low temperature, increased microhardness without significantly reducing corrosion resistance. Plasma nitriding at a higher temperature increased the corrosion tendency of the X17CrNi16‐2 steel.     

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.     

Structure and wear resistance of the composite layers produced by glow discharge nitriding and PLD method on AISI 316L austenitic stainless steel

The rapid technical development enhances the demands on constructional materials in terms of their resistance to frictional wear, resistance to corrosion and erosion, high hardness, high tensile and fatigue strength. These demands can be satisfied by e.g. applying various surface engineering techniques that permit to modify the microstructure, phase and chemical composition of the surface layers of the treated parts. A prospective line of the development of surface engineering is the production of composite layers by combining various surface engineering methods. The paper presents the results of examinations of the phase composition and frictional wear resistance of the layers produced by hybrid processes, i.e. such that combined glow discharge assisted nitriding performed at 450 °C and 550 °C with a pulsed laser deposition of boron nitride coatings (PLD method). It has been shown that the boron nitride coatings formed on nitrided AISI 316L steel increase its frictional wear resistance.     

Characteristics of the nitrided layer formed on AISI 304 austenitic stainless steel by high temperature nitriding assisted hollow cathode discharge

A series of experiments have been conducted on AISI 304 stainless steel using a hollow cathode discharge assisted plasma nitriding apparatus. Specimens were nitrided at high temperatures (520–560 °C) in order to produce nitrogen expanded austenite phase within a short time. The nitrided specimen was characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, potentiodynamic polarization and microhardness tester. The corrosion properties of nitrided samples were evaluated using anodic polarization tests in 3.5% NaCl solution. The nitrided layer was shown to consist of nitrogen expanded austenite and possibly a small amount of CrN precipitates and iron nitrides. The results indicated that rapid nitriding assisted hollow cathode discharge not only increased the surface hardness but also improved the corrosion resistance of the untreated substrate.     

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.     

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.     

Wear assessment of plasma nitrided AISI H11 steel

Plasma nitriding is one of the effective methods for improvement of the hardness, wear and corrosion resistance of steels. In this research AISI H11 hot working tool steel was plasma nitrided in various gas mixtures for different times and temperatures. The morphology, size and composition of nitride nanoparticles formed on the surface of the specimens were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD). The wear behavior of plasma nitrided samples was studied by means of unlubricated pin-on-disc method under constant load of 80 N, sliding speed of 1 m/s, sliding distance of 2000 m at room temperature. The results showed plasma nitriding process improved the wear behavior of H11 steel. The increase in time and temperature of plasma nitriding decreased the hardness and increased the wear weigh loss of the specimens.     

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.     

纯铁及结构钢快速气体渗氮工艺及机理研究

研究了纯铁及38CrMoAlA钢分别在500℃、0~0.4MPa压力和510℃、0~0.5MPa压力条件下的氨气渗氮行为。提高渗氮压力可显著加速气体渗氮动力学过程,纯铁在500℃和0.4 MPa下气体渗氮处理5h后渗氮层厚度(1160μm)可同比达到常规渗氮层厚度(205μm)的5倍以上,而38CrMoAlA钢经510℃和0.5 MPa压力下渗氮5h后的渗氮层厚度(400μm)几乎与常规渗氮50h所得硬化层厚度(440μm)相当。同时,纯铁及38CrMoAlA钢渗氮层中ε-Fe2-3N与γ′-Fe4N的相比例、氮势及表层硬度均随压力的提高呈现先增加后降低的变化趋势。提出了一种合金结构钢表面高强高韧渗氮层快速复合制备工艺(增压渗氮+冷轧)。与一段式常规渗氮及增压渗氮工艺相比,复合工艺处理表层硬度及韧性均较优良,尤其高剪切应力磨损条件下复合处理表层的耐磨性能最优,在20~600℃热循环处理10~300次条件下复合处理表层的耐热疲劳性能最佳。研究了42CrMo钢在既定的渗氮周期内(6h)以NH3为介质,530℃及不同压力循环次数条件下的气体渗氮行为。在渗氮温度和总时间相同条件下,循环压力气体渗氮样品化合物层随压力循环次数的增加逐渐减薄,渗氮层整体厚度随压力循环次数的增加逐渐增加,同时渗氮表层韧性随压力循环次数的增加逐渐增强。     

Corrosion resistance of the layers formed on the surface of plasma-nitrided AISI 304L steel

Austenitic stainless steels have good corrosion resistance, but their low hardness and low wear resistance limit their use whenever surface hardness is required. Nitriding treatments have been successfully applied to stainless steels to improve their mechanical and tribological properties; however, at temperatures above 723 K, gas or salt bath nitriding processes decrease the corrosion resistance due to the formation of CrN and other phases within the modified layer. Chromium compounds draw chromium and nitrogen from the adjacent regions, degrading the corrosion resistance. The plasma nitriding technique permits the use of treatment temperatures as low as 623 K without promoting degradation in the corrosion resistance of stainless steel. In this work, the pulsed glow discharge (PGD) technique was used for nitriding steel (AISI304L) in order to investigate the effect of the temperature of this treatment in the morphology and, as a consequence, in the anodic behavior of the formed layers, in solution with and without chloride ions. Four different temperatures were employed (623, 673, 723, and 773 K). The samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), microhardness measurements, and electrochemical tests with potentiodynamic anodic polarization curves. The nitriding temperature alters the anodic behavior due to a displacement of the polarization curve towards higher currents, in a solution free of chloride ions. In a chloride solution, the nitriding temperature increases the pitting potential up to the oxygen evolution region.     

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.     

Determination of the s-phase formation coefficient of plasma nitrided austenitic steel

Plasma nitriding is an effective surface hardening treatment for austenitic stainless steels. During plasma nitriding, s-phase formation takes place which is not only responsible for high hardness and wear resistance but also for good corrosion resistance. In order to estimate the thickness of the s-phase for austenitic stainless steel in a plasma nitriding process, an empirical model is devised. A number of plasma nitriding processes of austenitic stainless steel (304 L) were carried out with varying treatment temperature from 360 °C to 450 °C and process duration ranging from 10 hours to 24 hours with constant pressure, voltage, pulse-to-pause-ratio and gas mixture. A time-temperature dependent s-phase formation coefficient is determined by measuring the thickness of the s-phase using a scanning electron microscope (SEM) and glow discharge optical emission spectroscopy (GDOES). The developed model is verified by three controlled experiments. This model fits the thickness of the s-phase with an error of less than 6 %.     

Structure and wear resistance of the composite layers produced by glow discharge nitriding and PLD method on AISI 316L austenitic stainless steel

The rapid technical development enhances the demands on constructional materials in terms of their resistance to frictional wear, resistance to corrosion and erosion, high hardness, high tensile and fatigue strength. These demands can be satisfied by e.g. applying various surface engineering techniques that permit to modify the microstructure, phase and chemical composition of the surface layers of the treated parts. A prospective line of the development of surface engineering is the production of composite layers by combining various surface engineering methods. The paper presents the results of examinations of the phase composition and frictional wear resistance of the layers produced by hybrid processes, i.e. such that combined glow discharge assisted nitriding performed at 450 °C and 550 °C with a pulsed laser deposition of boron nitride coatings (PLD method). It has been shown that the boron nitride coatings formed on nitrided AISI 316L steel increase its frictional wear resistance.     

FACTORS CONTROLLING THE FATIGUE STRENGTH OF NITRIDED TITANIUM

Abstract— The factors affecting the fatigue strength of nitrided titanium were clarified. The fatigue strength depended strongly on the fracture strength of the compound layer formed on the surface by nitriding. We found a Hall-Petch relationship between the fatigue strength of nitrided titanium and the grain size. The findings indicated that the reduction in the fatigue strength by nitriding results from both the formation of the compound layer possessing low fracture strength and grain growth occurring from ordinary nitriding. Furthermore, low-temperature nitriding (620°C, 24 h) was proposed to suppress grain growth. This treatment method improved not only the wear resistance and the corrosion resistance but also the fatigue strength of titanium.     

Influence of layer microstructure on the corrosion behavior of plasma nitrided cold work tool steel

The influence of layer microstructure on the corrosion behavior of plasma nitrided cold work tool steel, of commercial name “DC53”, in 3.5% NaCl solution is reported. The specimens were nitrided at 520 °C for different treatment times using a constant [N₂ + H₂] gaseous mixture by a DC-pulsed plasma system. The microstructure of the nitrided layers was investigated by optical microscopy and X-ray diffraction. The corrosion behavior was evaluated by potentiodynamic polarization experiments. The plasma nitriding process considerably improves the corrosion resistance of material in NaCl environment as compared to the unnitrided DC53 steel. The modified surface layer consisting mainly of ε-nitride (Fe_2–3N) and a small amount of γ′-nitride (Fe₄N) confers this outstanding behavior. The corrosion resistance dependence on specific nitriding processes is reported and the role of the ε-nitride is discussed. In particular, the correlation of pitting current density, density of pits, and volume fraction of ε-nitride with nitriding time is analyzed. The results denote that the most important parameter for controlling the corrosion resistance of the material is the volume fraction of ε-nitride and the nitrided layer thickness. It is expected that a nitrided layer would be thicker and rich in ε-nitride phase to achieve a high corrosion resistance.     

The wear and corrosion resistance of shot peened–nitrided 316L austenitic stainless steel

316L austenitic stainless steel was gas nitrided at 570 °C with pre-shot peening. Shot peening and nitriding are surface treatments that enhance the mechanical properties of surface layers by inducing compressive residual stresses and formation of hard phases, respectively. The structural phases, micro-hardness, wear behavior and corrosion resistance of specimens were investigated by X-ray diffraction, Vickers micro-hardness, wear testing, scanning electron microscopy and cyclic polarization tests. The effects of shot peening on the nitride layer formation and corrosion resistance of specimens were studied. The results showed that shot peening enhanced the nitride layer formation. The shot peened–nitrided specimens had higher wear resistance and hardness than other specimens. On the other hand, although nitriding deteriorated the corrosion resistance of the specimens, cyclic polarization tests showed that shot peening before the nitriding treatment could alleviate this adverse effect.     

Effect of annealing on corrosion behaviour of nitrogen S phase in austenitic stainless steel

Abstract

The corrosion behaviour of both as nitrided and nitrided then annealed 316 austenitic stainless steel has been investigated. Results show that low temperature plasma nitriding can, to some extent, enhance the corrosion resistance of 316 steel, and this can be attributed to the high nitrogen content in the nitrogen S phase layer. However, when annealed at certain conditions, the corrosion resistance of the S phase layer would be impaired. This is mainly because thermodynamically the S phase is a metastable phase, which will decompose into stable chromium nitride and the alpha phase if the kinetic conditions (temperature and time) are favourable. Clearly, the improved corrosion resistance conferred by low temperature plasma nitriding can be retained provided the S phase layer remains untransformed under service conditions.     

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.     

Axial fatigue of a gas-nitrided quenched and tempered AISI 4140 steel: effect of nitriding depth

Fatigue testing under fully reversed axial loading (R=?1) and zero‐to‐tension axial loading (R= 0) was carried out on AISI 4140 gas‐nitrided smooth specimens. Three different treatment durations were investigated in order to assess the effect of nitriding depth on fatigue strength in high cycle fatigue. Complete specimens characterization, i.e., hardness and residual stresses profiles (including measurement of stabilized residual stresses) as well as metallographic and fractographic observations, was achieved to analyse fatigue behaviour. Fatigue of the nitrided steel is a competition between a surface crack growing in a compressive residual stress field and an internal crack or ‘fish‐eye’ crack growing in vacuum. Fatigue life increases with nitriding depth until surface cracking is slow enough for failure to occur from an internal crack. Unlike bending, in axial fatigue ‘fish‐eye’ cracks can initiate anywhere in the core volume under uniform stress. In these conditions, axial fatigue performance is lower than that obtained under bending and nitriding depth may have no more influence. In order to interpret the results, special attention was given to the effects of compressive residual stresses on the surface short crack growth (closure effect) as well as the effects of internal defect size on internal fatigue lives. A superimposed tensile mean stress reduces the internal fatigue strength of nitrided steel more than the surface fatigue strength of the base metal. Both cracking mechanisms are not equally sensitive to mean stress.     

Mechanical and structural properties of AISI 1015 carbon steel nitrided after warm rolling

Nitriding is usually applied to alloyed steels with the scope of increasing their surface hardness and wear resistance. Warm working has been found to produce a fine-grained microstructure, which makes possible further treatment of low carbon steels. In combination with a low temperature thermochemical treatment, such as nitriding, warm working can be used to produce machine parts with a though core and with a hard, wear resistant surface layer. This paper presents a study of mechanical and structural properties of AISI 1015 carbon steel nitrided after warm rolling. The rolling was performed in the following conditions: temperature 670–550°C, rolling speed 1.39 s^-1 and deformation ratio 36.4%. After rolling, the samples were reheated to 550°C for a duration varying from a few minutes to 10 hours. The microstructural changes were assessed by light microscopy and quantitative microscopy analysis. Warm rolled samples were ion nitrided at 510–520°C in dissociated ammonia. The microstructure was analyzed by scanning electron microscopy and the mechanical properties were evaluated by tensile testing, surface hardness and friction coefficient measurements. Prior application of warm rolling makes possible (in the sense that is a viable solution) the ion nitriding of low carbon steels in order to produce machine parts with improved mechanical properties in the core (due to warm rolling) and longer service life (due to ion nitriding).