Improvement of corrosion and wear resistances of AISI 420 martensitic stainless steel using plasma nitriding at low temperature

The influence of low temperature plasma nitriding on the wear and corrosion resistance of AISI 420 martensitic stainless steel was investigated. Plasma nitriding experiments were carried out with DC-pulsed plasma in 25% N₂ + 75% H₂ atmosphere at 350 °C, 450 °C and 550 °C for 15 h. The composition, microstructure and hardness of the nitrided samples were examined. The wear resistances of plasma nitrided samples were determined with a ball-on-disc wear tester. The corrosion behaviors of plasma nitrided AISI420 stainless steel were evaluated using anodic polarization tests and salt fog spray tests in the simulated industrial environment.The results show that plasma nitriding produces a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer on the AISI 420 stainless steel surface. Plasma nitriding not only increases the surface hardness but also improves the wear resistance of the martensitic stainless steel. Furthermore, the anti-wear property of the steel nitrided at 350 °C is much more excellent than that at 550 °C. In addition, the corrosion resistance of AISI420 martensitic stainless steel is considerably improved by 350 °C low temperature plasma nitriding. The improved corrosion resistance is considered to be related to the combined effect of the solid solution of Cr and the high chemical stable phases of ?-Fe₃N and α_N formed on the martensitic stainless steel surface during 350 °C low temperature plasma nitriding. However, plasma nitriding carried out at 450 °C or 550 °C reduces the corrosion resistance of samples, because of the formation of CrN and leading to the depletion of Cr in the solid solution phase of the nitrided layer.     

Effect of Plasma Nitriding and Nitrocarburizing on HVOF-Sprayed Stainless Steel Coatings

In this work, the effects of plasma nitriding (PN) and nitrocarburizing on HVOF-sprayed stainless steel nitride layers were investigated. 316 (austenitic), 17-4PH (precipitation hardening), and 410 (martensitic) stainless steels were plasma-nitrided and nitrocarburized using a N₂ + H₂ gas mixture and the gas mixture containing C₂H₂, respectively, at 550 °C. The results showed that the PN and nitrocarburizing produced a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer depending on the crystal structures of the HVOF-sprayed stainless steel coatings. Also, the diffusion depth of nitrogen increased when a small amount of C₂H₂ (plasma nitrocarburizing process) was added. The PN and nitrocarburizing resulted in not only an increase of the surface hardness, but also improvement of the load bearing capacity of the HVOF-sprayed stainless steel coatings because of the formation of CrN, Fe₃N, and Fe₄N phases. Also, the plasma-nitrocarburized HVOF-sprayed 410 stainless steel had a superior surface microhardness and load bearing capacity due to the formation of Cr₂₃C₆ on the surface.     

渗扩时间比对AISI 316不锈钢渗氮层组织与性能的影响

采用真空两段渗氮工艺,在不同的强渗、扩散时间下对AISI 316不锈钢进行渗氮处理,通过X射线衍射(XRD)、扫描电镜(SEM)、光学显微镜(OM)、显微硬度测试和摩擦磨损试验等分析了渗氮层的组织和性能。结果表明,经过12 h的真空渗氮后,AISI 316不锈钢表面形成了一层由γ′-Fe₄N、ε-Fe_2-3N和CrN等相组成的渗氮层,其表面硬度和耐磨性能相较于基体均有明显的提高。其中,渗扩时间比为1∶1(强渗6 h、扩散6 h)时的渗层厚度约为96 μm,表面硬度约为1069 HV0.5,是基体表面硬度的4.5倍,在20 N载荷下的磨损量约为基体的1/3;渗扩时间比为1∶2(强渗4 h、扩散8 h)时的渗层厚度约为120 μm,ε-Fe_2-3N相衍射峰增强,在20 N载荷下的磨损量约为基体的1/30。延长扩散时间能增加渗氮层厚度,改善表面形貌,进一步提高不锈钢的耐磨性。     

High temperature wear and friction properties of duplex surface treated bearing steels

Duplex treatments by thermo reactive diffusion (TRD) chromizing and puls plasma nitriding were carried out on AISI 52100 and 8620 bearing steels. Tribological behaviors of TRD chromized and duplex treated bearing steels were investigated against Al₂O₃ ball in ball-on-disc system at room temperature and 500 °C. The samples were pack chromized in a furnace at temperature of 1000 °C for 5 h. After chromizing, the samples were puls plasma nitrided for 5 h at 500 °C. The coated steels were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), scratch and microhardness testing. Plasma nitriding of chromized steels increased the total thickness of the compound layer. The subsequent plasma nitriding increased the surface hardness to 2135 HK_0.025 due to the formation of CrN and Cr₂N. The surface hardness and scratch resistance of coating can be increased with duplex treatment of chromizing followed by plasma nitriding, resulting in high wear resistance. Tribological tests indicated that puls plasma nitriding process decreased the coefficient of friction values and wear rate of the chromized steels at room temperature and 500 °C. Also, examination of the worn surface of the samples showed that particularly at high temperature, the oxidized compact layer occurs and tribo-oxidation played an important role in oxidation behaviour of the steels after the duplex treatment.     

Effect of the initial microstructure on the plasma nitriding behavior of AISI M2 high speed steel

The nitriding behavior of AISI M2 steel was studied on samples previously submitted to two different heat treatments in order to investigate the effects of the initial microstructure on the thickness and hardness of nitrided layer. Prior to nitriding, one group of samples was fully annealed while the other group was quenched and tempered, thus acquiring the lowest and highest hardness respectively. Plasma nitriding was performed at 450 °C for 8 h with a mixture of N₂ and H₂ in a plasma reactor working under floating potential. Structural and mechanical properties of nitrided layers were characterized using X-ray diffraction (XRD), optical microscopy and microhardness testing. Variations in surface roughness were obtained by 3D surface profilometry analysis. The thicker nitrided layer was obtained for the fully annealed samples, in which the nitrided layer is composed of γ′-Fe₄N and ε-Fe_2-3N phases plus a diffusion zone. For the hardened-tempered samples, the nitrided region mainly consisted of a diffusion zone. Plasma nitriding increased the surface hardness of the fully annealed samples by 330% and that of the quenched-tempered samples by 50%. The nitrided depth was also estimated using cross-sectional microhardness profiles; giving about 140 µm and ∼ 70 µm for the fully annealed and quenched-tempered samples, respectively. Due to the grain to grain nitrogen diffusion, plasma nitriding also increased the surface roughness. The largest roughness was obtained for the fully annealed samples, in accordance with the largest nitrided depth. The difference in the nitriding behavior was explained on the basis of the microstructural aspects of the substrates such as the concentration of the freely dispersed alloying elements and the level of compressive residual stresses.     

Microstructure and tribological characteristics of nitrided layer on 2Cr13 steel in air and vacuum

A 2Cr13 steel was gas nitrided in pure NH₃ gas atmosphere at 793 K for 20 h. The microstructure, composition and microhardness of the nitrided samples were examined. The tribological behaviour of the nitrided 2Cr13 steel in air and vacuum was investigated in order to analyse effects of the nitriding on wear resistance of the 2Cr13 steel. The results show that the nitrided layer consists of a compound layer and diffusion zone. The nitriding increases both the surface hardness and wear resistance of 2Cr13 steel in air and vacuum, and the anti-wear characteristic of the nitrided 2Cr13 steel in vacuum is much higher than that in air. The nitrided layer exhibits a mild wear in air, and avoids the severe wear that happens on the unnitrided steel. While the adhesion dominates the wear process in vacuum. The material transfer between the wear couples helps to improve the tribological characteristics of the nitrided layer in vacuum.     

Plasma nitriding of plastic mold steel to increase wear- and corrosion properties

In this study, the wear- and corrosion resistance of the layers formed on the surface of a precipitation hardenable plastic mold steel (NAK55) by plasma nitriding were investigated. Plasma nitriding experiments were carried out at an industrial nitriding facility in an atmosphere of 25% N₂ + 75% H₂ at 475 °C, 500 °C, and 525 °C for 10 h. The microstructures of the nitrided layers were examined, and various phases present were determined by X-ray diffraction. Wear tests were carried out on a block-on-ring wear tester under unlubricated conditions. The corrosion behaviors were evaluated using anodic polarization tests in 3.5% NaCl solution.The findings had shown that plasma nitriding does not cause the core to soften by overaging. Nitriding and aging could be achieved simultaneously in the same treatment cycle. Plasma nitriding of NAK55 mold steel produced a nitrided layer consisted of a compound layer rich in ε-nitride and an adjacent nitrogen diffusion layer on the steel surface. Increasing the nitriding temperature could bring about increase in the thickness of the nitrided layer and the nitride volume fraction. Plasma nitriding improved not only surface hardness but also wear resistance. The anti-wear property of the steel was found to relate to the increase in the thickness of the diffusion layer. Corrosion study revealed that plasma nitriding significantly improved corrosion resistance in terms of corrosion potential and corrosion rate. Improvement in corrosion resistance was found to be directly related to the increase in the nitride volume fraction at the steel surface.     

Increasing surface hardness of austenitic stainless steels by pack nitriding process

This paper investigated the possibility of increasing the surface hardness of austenitic stainless steels under very low nitrogen dissociation pressures of metal nitride powders using pack nitriding process. Thin sheet of 304 type of stainless steel of approximately 1 mm in thickness was used as a substrate for the study. Based on the results of thermochemical calculations, Cr₂N powder was selected as a nitrogen source from a series of metal nitride powders considered for the pack nitriding process, which included Si₃N₄, Mn₄N, BN, AlN and TiN. The pack nitriding was carried out in a sealed alumina retort at temperatures of 860 °C and 910 °C for up to 48 h. The surface was then characterised using techniques of SEM, XRD and microhardness testing. It was observed that the process used increased the surface hardness of the steel, but it also induced precipitation of chromium nitrides in the matrix even under the nitrogen dissociation pressures below 50 Pa. It was also observed that, in the nitrided layer, the γ phase of the steel was partially transformed to the α phase under the pack nitriding process conditions studied.     

Use of cathodic cage in plasma nitriding

Cylindrical samples of 1020 steel and 316 stainless steel were nitrided under the conditions by conventional dc plasma nitriding (DCPN) and by a new technique denominate cathodic cage plasma nitriding (CCPN). The 1020 and 316 stainless steel samples were treated during 3 h and 5 h, respectively, in 773 K and 360 Pa. The samples were characterized by optical microscopy, X-ray diffraction and microhardness testing. All the samples nitrided by DCPN process presented erosion rings on the surface exposed to the plasma. In comparison, in samples nitrided by CCPN, the erosion rings were completely eliminated, without loss of the mechanical properties in the different phases of existence in the nitrided layer.     

Effects of Temperature on Microstructure and Wear of Salt Bath Nitrided 17-4PH Stainless Steel

Salt bath nitriding of 17-4 PH martensitic precipitation hardening stainless steels was conducted at 610, 630, and 650?°C for 2?h using a complex salt bath heat-treatment, and the properties of the nitrided surface were systematically evaluated. Experimental results revealed that the microstructure and phase constituents of the nitrided surface alloy are highly process condition dependent. When 17-4PH stainless steel was subjected to complex salt bathing nitriding, the main phase of the nitrided layer was expanded martensite (????), expanded austenite (??_N), CrN, Fe₄N, and (Fe,Cr) _x O _y . In the sample nitrided above 610?°C, the expanded martensite transformed into expanded austenite. But in the sample nitrided at 650?°C, the expanded austenite decomposed into ??_N and CrN. The decomposed ??_N then disassembled into CrN and alpha again. The nitrided layer depth thickened intensively with the increasing nitriding temperature. The activation energy of nitriding in this salt bath was 125?±?5?kJ/mol.     

Influence of time on the microstructure of AISI 321 austenitic stainless steel in salt bath nitriding

Influence of nitriding time on the microstructure and microhardness of AISI 321 austenite stainless steel was investigated, using a complex salt bath heat-treatment at low temperature, 430 °C. Experimental results revealed that after salt bath nitriding, a modified layer was formed on the surface of substrate with the thickness ranging from 2 μm to 30 μm with changing treating time. The nitrided layer depth thickened extensively with increasing nitriding time. The growth of the nitrided layer takes place mainly by nitrogen diffusion according to the expected parabolic rate law. Scanning electron microscopy and X-ray diffraction showed that in 321 stainless steel subjected to complex salt bathing nitrided at such temperature for less than 8 hours, the main phase of the nitrided layer was expanded austenite (S phase) by large. When the treatment time is prolonged up to 8 hours and more, S phase is formed and subsequently transforms partially into CrN, and then the secondary CrN phase precipitated. With treating time prolonged, more CrN precipitates formed along the grain boundaries in the outer part. In the inside part between the some CrN and the substrate, there is still a broad single S phase layer. All treatments can effectively improve the surface hardness.     

Advance Complex Liquid Nitriding of Stainless Steel AISI 321 Surface at 430 °C

Liquid nitriding of type 321 austenite stainless steel was conducted at low temperature at 430 °C, using a type of a complex chemical heat-treatment; and the properties of the nitrided surface were evaluated. Experimental results revealed that a modified layer was formed on the surface with the thickness ranging from 2 to 30 μm varying with changing treatment time. When the stainless steel subjected to the advanced liquid nitriding less than 8 h at 430 °C, the main phase of the nitrided coating layer was the S phase generally. When the treatment time prolonged up to 16 h, S phase formed and partially transformed to CrN subsequently; and then the fine secondary CrN phase precipitated. All treatments performed in the current study can effectively improve the surface hardness. The nitrided layer thickness changed intensively with the increasing nitrided time. The growth of the nitride layer took place mainly by nitrogen diffusion according to the expected parabolic rate law. The highest hardness value obtained in this experiment was about 1400 Hv_0.25. Low-temperature nitriding can improve the corrosion resistance of the 321 stainless steel against diluted vitriolic acid. The immerse test results revealed that the sample nitrided for 16 h had the best corrosion resistance than the others. SEM examinations indicated that after nitriding, the corrosion mechanisms of the steel had changed from serious general corrosion of untreated sample to selectivity corrosion of nitrided samples in the diluted vitriolic acid.     

Corrosion and mechanical properties of duplex-treated 301 stainless steel

Plasma nitriding is a widely used technique for increasing the surface hardness of stainless steels, and consequently, for improving their tribological properties. It is also used to create an interface between soft stainless steel substrates and hard coatings to improve adhesion. This paper reports on the mechanical and corrosion properties of AISI301 stainless steel (SS) after a duplex treatment consisting of plasma nitriding followed by deposition of Cr bond coat and CrSiN top layer by magnetron sputtering. Mechanical properties of the deposited films, such as hardness (H) and reduced Young's modulus (E_r), were measured using depth-sensing indentation. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were carried out to evaluate resistance to localized and to general corrosion, respectively. The corrosion behavior has been correlated with the microstructure and composition of the surface layers, determined by complementary characterization techniques, including XRD, SEM, and EDS. The CrSiN layers exhibited an H value of 24 GPa, whereas the nitrided layer was shown to present a gradual increase of H from 5 GPa (in the nitrogen-free SS matrix) to almost 14 GPa at the surface. The electrochemical measurements showed that the nitriding temperature is a critical parameter for defining the corrosion properties of the duplex-treated SS. At a relatively high temperature (723 K), the nitrided layer exhibited poor corrosion resistance due to the precipitation of chromium nitride compounds and the depletion of Cr in the iron matrix. This, in turn, leads to poor corrosion performance of the duplex-treated SS since pores and defects in the CrSiN film were potential sites for pitting. At relatively low nitriding temperature (573 K), the nitrided interface exhibited excellent corrosion resistance due to the formation of a compound-free diffusion layer. This is found to favor passivation of the material at the electrode/electrolyte interface of the duplex-treated SS.     

High frequency and low voltage plasma immersion ion implantation of nitrogen on industrial pure iron at different Rf power

Traditional plasma ion immersion implantation (PIII) can effectively improve material mechanical property and corrosion resistance. But the modified layer by PIII is too thin for many industrial applications. High frequency and low voltage plasma immersion ion implantation (HLPIII) has advantages of PIII and nitriding. Comparing with traditional ion nitriding, HLPIII can obtain higher implantation energy and create a thick modified surface layer. In the present paper nitriding layers were synthesized on industrial pure iron using high frequency and low voltage plasma immersion ion implantation with different RF power (400 W, 600 W, and 800 W). The microstructure of the nitriding layers was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical properties such as microhardness and wear resistance were analyzed using HXD1000 microhardness and CSEM pin-on-disk wear testing machine. The anodic polarization characteristics were measured in a 0.9% NaCl solution at room temperature to examine the corrosion resistance of the nitriding layer. The results reveal that Fe₂N, Fe₃N and Fe₄N coexist in the nitriding layer. The nitriding layer is a corrosion protective coating on industrial pure iron in 0.9% NaCl solution. The hardness, wear resistance and corrosion resistance of the nitrided layers on industrial pure iron increase with RF power.     

Improving the surface properties of A286 precipitation-hardening stainless steel by low-temperature plasma nitriding

Plasma nitriding over a wide range of treatment temperatures between 350 and 500 °C and time from 5 to 30 h on A286 austenitic precipitation-hardening stainless steels has been investigated. Systematic materials characterisation of the plasma surface alloyed A286 alloy was carried out in terms of microstructure observations, phase identification, chemical composition depth profiling, surface and cross-section microhardness measurements, electrochemical corrosion tests, dry sliding wear tests and corrosion-wear tests. Experimental results have shown that plasma nitriding can significantly improve the hardness and wear resistance of A286 stainless steels owing to the formation of nitrogen supersaturated S-phase; the surface layer characteristics (e.g. microstructure, case depth and hardness) of the plasma surface alloyed cases are highly process condition dependent and there are possibilities to provide considerable improvement in wear, corrosion and corrosion-wear resistance of A286 steel.     

A novel high-strength,high-ductility and high-corrosion-resistance FeAlMnC low-density alloy

The as-quenched Fe–8.68 wt.% Al–30.5 wt.% Mn–1.85 wt.% C alloy is plasma-nitrided at 500 °C for 8 h. The nitrided layer obtained is 40 μm thick and composed predominantly of AlN, with a small amount of Fe₄N. The resultant surface hardness (1860 Hv), substrate hardness (550 Hv), ductility (33.6%) and corrosion resistance in 3.5% NaCl solution in the present nitrided alloy are far superior to those obtained previously in optimally nitrided high-strength alloy steels, as well as martensitic and precipitation-hardening stainless steels.     

Surface modification of austenitic stainless steel with plasma nitriding for biomedical applications

In the present study, plasma nitriding of AISI type 303 austenitic stainless steel (SS) specimens was performed using a microwave system. The nitrided layers were characterized by performing scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and a Vickers microhardness test. The antibacterial activities of the nitrided layers were evaluated. XRD and TEM showed that a single γ_N phase was formed by plasma nitriding at the plasma power of 700 W and 450 °C. The analytical results demonstrated that the hardness of type 303 specimens could be enhanced by plasma nitriding because of the formation of the γ_N phase. A bacterial test also demonstrated that the nitrided layer exhibited excellent antibacterial properties.     

Plasma nitriding of 90CrMoV8 tool steel for the enhancement of hardness and corrosion resistance

The aim of the study is to apply a plasma nitriding process to the 90CrMoV8 steel commonly employed in wood machining, and to determine its efficiency to improve both mechanical and electrochemical properties of the surface. Treatments were performed at a constant N₂:H₂ gas mixture and by varying the temperature and process duration. The structural and morphological properties of nitrided layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with EDS microanalyses. Surface hardening and hardness profiles were evaluated by micro hardness measurements. To simulate the wood machining conditions, electrochemical tests were carried out with an oak wood electrolyte with the purpose of understanding the effects of the nitriding treatment on the corrosion resistance of the tool in operation.X-ray diffraction analyses revealed the presence of both γ′ (Fe₄N) and ε (Fe_2-3N) nitrides with a predominance of the ε phase. Moreover, α-Fe (110), γ′ and ε diffraction peaks were shifted to lower angles suggesting the development of compressive stresses in the post nitrided steel. As a result, it was shown that nitriding allowed a significant hardening of steel with hardness values higher than 1200 HV. The diffusion layers were always composed of an outer compound layer and a hardened bulk layer which thickness was half of the total diffusion layer one. No white layer was observed. Similarly, no traces of chromium nitrides were detected. The temperature seemed to be a parameter more influent than the process duration on the morphological properties of the nitrided layer, while it had no real influence on their crystallinity. Finally, the optimal nitriding conditions to obtain a thick and hard diffusion layer are 500 °C for 10 h.On the other hand, to verify the effect of these parameters on the corrosion resistance, potentiodynamic polarization tests were carried out in an original “wood juice” electrolyte. After corrosion, surface was then observed at the SEM scale. Electrochemical study indicated that the untreated steel behaved as a passive material. Although the very noble character of steel was somewhat mitigated and the corrosion propensity increased for nitrided steels, the passive-like nature of the modified surface was preserved. For the same optimized parameters as those deduced from the mechanical characterization (500 °C, 10 h), surface presented, in addition to a huge surface hardening, a high corrosion resistance.     

Plasma nitriding behavior of Ti6Al4V orthopedic alloy

The influence of plasma nitriding on mechanical, corrosion and tribological properties of Ti6Al4V has been investigated using X-ray diffraction, microhardness tester, scanning electron microscopy, pin-on-disc tribotester, electrochemical polarization and impedance spectroscopy. Plasma nitriding treatment of Ti6Al4V has been performed in 25%Ar-75%N₂ gas mixture, for treatment times of 1-4 h at the temperatures of 650-750 °C. The corrosion tests were carried out in Ringer solution at 37 °C, and the wear tests were performed in dry sliding conditions. XRD analyses confirm the formation of δ-TiN and tetragonal ?-Ti₂N phases in the modified layer. It was observed that the surface hardness and wear resistance increase as the treatment time and temperature increase. The electrochemical impedance measurements indicate a decrease in double layer capacitance value and increase in charge transfer resistance for the nitrided specimens compared to the untreated ones.     

钒对Cr12Mo模具钢渗氮层及其摩擦学行为的影响

在Cr12Mo和Cr12MoV模具钢表面进行气体渗氮处理,对比分析了V对Cr12Mo模具钢渗氮层显微结构及其摩擦学行为的影响。结果表明,气体渗氮后两种模具钢的表面均制备出深度约120 μm的渗氮层,由表及里依次为渗氮层、扩散层和基体;V提高了模具钢的耐磨性,Cr12MoV表现出较好的耐磨效果;相比V对模具钢基体和扩散层硬度的提升而言,V对渗氮层最大硬度值附近区域的硬度提升幅度更为明显;V对两种模具钢渗氮层耐磨性的影响并不明显,但V的主要贡献在于促进了渗氮时N的有效渗入,大大提高了渗氮层与扩散层间的界面结合力,避免了渗氮层与扩散层间的开裂,促使磨损机制由疲劳磨损转变为黏着磨损,进一步提高了渗氮层的服役寿命。