Influence of processing parameters on the characteristics of surface layers of low temperature plasma nitrocarburized AISI 630 martensitic stainless steel

Plasma nitrocarburizing was performed on solution-treated AISI 630 martensitic precipitation hardening stainless steel samples with a gas mixture of H₂, N₂, and CH₄ with changing temperature, discharge voltage and amount of CH₄. When nitrocarburized with increasing temperature from 380 °C to 430 °C at fixed 25% N₂ and 6% CH₄, the thickness of expanded martensite (α'_N) layer and surface hardness increased up to 10 μm and 1323 HV_0.05, respectively but the corrosion resistance decreased. Though the increase of discharge voltage from 400 V to 600 V increased α'_N layer thickness and surface hardness (up to 13 μm and 1491 HV_0.05, respectively), the treated samples still showed very poor corrosion behavior. Thus, to further improve the corrosion resistance, the influence of variation of the amount of CH₄ in the nitrocarburizing process was investigated. Increasing the CH₄ percentage aided higher corrosion resistance, although it decreased the α'_N layer thickness. The most appropriate conditions for moderate α'_N layer thickness, high surface hardness and better corrosion resistance than the solution-treated bare sample were established, which is plasma nitrocarburizing at 400 °C with 400 V discharge voltage and containing 25% N₂ and 4% CH₄.     

Development of plasma nitrocarburizing and post-oxidation technology for the replacement of Cr<Superscript>6+</Superscript> plating for application to vehicle engine shafts

Plasma nitrocarburizing and post-oxidation treatments were performed to improve the wear and corrosion resistance of S45C steel. Plasma nitrocarburizing was conducted for 3 h at 570°C in a nitrogen, hydrogen and methane atmosphere to produce the ε-Fe₂₋₃(N,C) phase. It was found that the compound layer produced by plasma nitrocarburising was predominantly composed of the ∈-phase with traces of the γ′-Fe₄(N,C) phase. The thickness of the compound layer was approximately 12 μm and the diffusion layer was approximately 300 μm in thickness. Plasma post oxidation was performed on nitrocarburized samples with various oxygen/hydrogen ratios at a constant temperature of 500°C for 1 h. The very thin magnetite (Fe₃O₄) layer 1 μm to 2 μm in thickness on top of the compound layer was obtained by plasma post oxidation. It was also confirmed that further improvement of the corrosion characteristics of the nitrocarburized compound layer was possible with an application of the superficial magnetite layer. Finally, throttle valve shafts of S45C steel were treated under optimum plasma processing conditions. Accelerated life time test results using a throttle body assembled with a shaft treated by plasma nitrocarburising and post oxidation showed that plasma nitrocarburizing and plasma post-oxidation processes could be a viable technology in the very near future in place of Cr⁶ plating.     

The microstructure and properties of 17-4PH martensitic precipitation hardening stainless steel modified by plasma nitrocarburizing

17-4PH martensitic precipitation hardening stainless steel was plasma nitrocarburized at 430 °C and 460 °C for 8 h. The nitrocarburized layers were characterized by optical microscope, scanning electron microscope, X-ray diffractometer, microhardness tests, pin-on-disc tribometer and the anodic polarization method in a 3.5% NaCl solution. The results show that the microstructure of plasma nitrocarburized layer is characterized by a compound layer with no evident diffusion zone. The phases in the 430 °C treated layer are mainly of γ′-Fe₄N, nitrogen and carbon expanded martensite (α′_N), and some incipient CrN phases. When the temperature increases up to 460 °C, there is no evidence of α′_N phase. The processes of bulk precipitation hardening and surface treatment by plasma nitrocarburizing can be successfully combined in a single-step process on this steel. The hardness of modified layer can reach up to 1186HV, which is 3 times higher than that of untreated steel. The wear and corrosion resistance of the specimens can be apparently improved by plasma nitrocarburizing. The 460 °C/8 h treated specimen has the best wear and corrosion resistance in the present test conditions.     

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.     

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.     

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.     

Electrochemical studies of hot corrosion of type 347H stainless steel

Abstract

Electrochemical studies of the hot corrosion of AISI SA 213 TP 347H stainless steel have been carried out in a mixture of 80 wt-% V₂O₅ + 20 wt-% Na₂SO₄. The range of temperatures was 540–680°C at intervals of 20 K and the techniques employed included corrosion potential, Tafel polarisation, and electrochemical noise measurements. At 620°C the corrosion potential, measured against a platinum reference electrode (PRE), decreases from ?350 mV to ?480 mV and remains at this level during the first 8 h. Using Tafel polarisation, it was found that, with change in the temperature from 540 to 680°C, the corrosion potential decreased on initial heating to 600°C and then increased again at higher temperatures, the corrosion rate increasing continuously with increasing temperature. However, at constant temperature (620°C) the corrosion rate increased with time during the first 8 h, after which it decreased and reached a steady state after 27 h, probably owing to the formation of a surface film. Electrochemical noise measurements, of both voltage and current noise, indicated a combination of general corrosion, probably owing to the formation of a surface layer, and localised corrosion in the grain boundaries.     

Preparation of Fe2B boride coating on low-carbon steel surfaces and its evaluation of hardness and corrosion resistance

Fe₂B coating was prepared on low-carbon steel by surface alloying. A series of experiments were carried out to examine some surface properties of boride coating. The surface heat treatment of coated low-carbon steel was performed at 700 °C, 800 °C and 900 °C for 2 h, 4 h, 6 h and 8 h under hydrogen atmosphere. The boride coating was revealed by XRD analysis and the microstructure of the boride coating was analyzed by scanning electron microscopy (SEM). Depending on the temperature and time of the process, the hardness of the borided low-carbon steel ranged from 99 to 1100 HV. The hardness showed a maximum (about 1100 HV) at 900 °C for 8 h. The corrosion resistance of the borided samples was evaluated by the Tafel polarization and electrochemical impedance spectroscopy (EIS). Shift in the corrosion potential (E_corr) towards the noble direction was observed, together with decrease in the corrosion current density (I_corr), increase in the charge transfer resistance (R_ct) and decrease in the capacitance (C_c), which indicated an improvement in corrosion resistance with increasing temperature and time of the treatment.     

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.     

A study of the corrosion properties of plasma nitrocarburized and oxidized AISI 1020 steels

Plasma nitrocarburized AISI 1020 steels were oxidized for 15, 30 and 60 min to evaluate their corrosion and microstructural properties. After plasma nitrocarburizing for 3 h at 570°C in a gas mixture comprising 85 vol.% N₂, 12vol.% H₂ and 3 vol.% CH₄, the compound layer composed of ɛ-Fe_2–3(N,C) and γ’-Fe₄(N,C) phases and the diffusion layer above the matrix were observed. The top oxide layer, consisting mainly of magnetite (Fe₂O₄) and hematite (Fe₂O₃) phases, forms after post-oxidation treatment at 500°C. However, the oxide layer was severely degraded by spallation as a result of increases in post-oxidizing time. The difference in corrosion resistance should be attributed to the thickness of the top oxide layer, which was governed by post-oxidizing time.     

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

The oil-quenched 30CrMnSiA steel specimens have been pulse plasma-nitrided for 4 h using a constant 25% N₂-75% H₂ gaseous mixture. Different nitriding temperatures varying from 400 to 560 °C have been used to investigate the effects of treatment temperature on the microstructure, microhardness, wear, and corrosion resistances of the surface layers of the nitrided specimens. The results show that significant surface-hardened layer consisting of compound and diffusion layers can be obtained when the oil-quenched steel (α′-Fe) are plasma-nitrided at these experimental conditions, and the compound layer mainly consists of ε-Fe_2-3N and γ′-Fe₄N phases. Lower temperature (400-500 °C) nitriding favors the formation of ε-Fe_2-3N phase in surface layer, while a monophase γ′-Fe₄N layer can be obtained when the nitriding is carried out at a higher temperature (560 °C). With increasing nitriding temperature, the compound layer thickness increases firstly from 2-3 μm (400 °C) to 8 μm (500 °C) and then decreases to 4.5 μm (560 °C). The surface roughness increases remarkably, and both the surface and inner microhardness of the nitrided samples decrease as increasing the temperature. The compact compound layers with more ε-Fe_2-3N phase can be obtained at lower temperature and have much higher wear and corrosion resistances than those compound layers formed employing 500-560 °C plasma nitriding.     

Martensitic stainless steel modified by plasma nitrocarburizing at conventional temperature with and without rare earths addition

17-4PH Martensitic stainless steel was plasma nitrocarburized at conventional temperature (560 °C) with and without rare earths (RE) addition. The surface treated layers were characterized by optical microscope, scanning electron microscope equipped with an energy dispersive X-ray analyzer, X-ray diffraction and microhardness test. The wear and corrosion behavior of the modified specimens was studied respectively using pin-on-disc tribometer and anodic polarization tests. The results show that rare earths atoms can diffuse into the surface of the stainless steel. The microstructures of all modified layers are characterized by a compound layer containing three distinct zones but without an evident diffusion zone. The phases on all modified surface layers are mainly of θ-Fe₃C and CrN. It is exciting that the hardness profile of the modified layer is decreased gradually, which is rarely found in plasma nitriding of the stainless steel. The friction coefficient of a specimen can be dramatically decreased by plasma RE nitrocarburizing, whereas the corrosion resistance is deteriorated. In contrast, the corrosion resistance of a plasma nitrocarburized specimen is enhanced but the friction coefficient is not improved as much as that of a plasma RE nitrocarburized one.     

Steam oxidation and surface hardness of power plant valve materials under the ultra super critical steam condition

Current supereritical steam power plants operate at 3,600 psi and 1,000°F. If the steam temperature is raised from 1,000 °F (538 °C) to 1,150 °F (621°C), the efficiency increases by 2%. Therefore, study on the high temperature corrosion of power plant materials under ultra-superciritical conditions (USC) is necessary to protect the plant from corrosion. In this study, valve materials of 17% Cr martensitic steels (17Cr steel), Incoloy 901 (1901) and their surface nitrided specimens were exposed to USC of 621 °C and 3600 psi (255 kg/cm²) steam for 200 °C, 400 °C, and 800 h. The oxidation of both 17Cr steel and 1901 under the USC for 800 h is very small due to the formation of a protective thin oxide layer formation on the surface. The USC oxidation of both nitrided specimens were increased due to the decomposition and formation of active nitrogen from the non protective nitrides such as Fe₄N, Fe_2–3N, and CrN. The oxidation of nitrided 17Cr steel (n17Cr steel) is about two times higher compared to nitrided 1901 (n1901). The surface hardness is improved by more than two times near the surface by nitriding, and the degradation of hardness by USC oxidation is rapid for n17Cr steel, but slow for n1901.     

Layer growth kinetics and wear resistance of martensitic precipitation hardening stainless steel plasma nitrocarburized at 460°C with rare earth addition

To study the effect of rare earth (RE) addition on low temperature plasma nitrocarburizing of martensitic precipitation hardening stainless steel, 17-4PH stainless steel was plasma nitrocarburized at 460 °C for different times with RE addition. The modified layers were tested by optical microscope, scanning electron microscope, X-ray diffraction, microhardness tester and pin-on-disc tribometer. The experimental results show that the layer depth of plasma RE nitrocarburized layer can be increased up to 56% compared with plasma nitrocarburizing without RE addition. Incorporation of RE element is beneficial to the formation of nitrogen and carbon expanded martensite (α′_N). The surface microhardness of plasma RE nitrocarburized layer can be increased to 1286 HV and higher up to 80 HV than that obtained from the conventional treated one. The friction coefficient of martensitic stainless steel can be dramatically decreased by low temperature plasma nitrocarburizing with RE addition, and the friction coefficient of the modified specimens decrease gradually with increasing process time in the present test condition.     

SUS201奥氏体不锈钢表面低温盐浴硬化处理

为了提高奥氏体不锈钢的表面硬度,在不降低不锈钢耐蚀性的前提下,采用盐浴氮碳共渗技术对SUS 201奥氏体不锈钢表面进行低温硬化处理,对不同处理温度和处理时间下硬化层的组织和性能进行研究.结果表明,处理温度和处理时间对硬化层的组织结构和性能都有很大的影响,只有在正确的工艺条件下,才能获得无氮(碳)化合物析出的硬化层,表面硬度可达1000HV0.025以上,而且还能提高不锈钢表面的耐蚀性能.     

腐蚀产物MgO对Fe基合金在固态和熔融NaCl-MgCl2 中高温耐蚀性能的影响

熔融氯化盐是下一代聚光式太阳能热发电站（第3代CSP）候选传热和储热介质,含MgCl₂的熔融氯化盐对金属传热管道和储热容器腐蚀后在其表面形成MgO,MgO对管道耐腐蚀性能影响尚不清楚。通过对比碳钢和3种Fe-Cr-Ni合金在固态（345 ℃）和熔融NaCl-MgCl₂（445和545 ℃）中的腐蚀行为,分析了MgO对4种试样在不同温度下的腐蚀行为机理。结果表明,在固态NaCl-MgCl₂中,碳钢表面MgO壳致密且连续,可以保护试样免受腐蚀。在熔融NaCl-MgCl₂中,4种试样表面也生成了致密的MgO壳,但它因热应力作用而开裂和剥落,熔融盐沿着氧化膜裂纹渗入MgO/基体界面,发生化学-电化学联合腐蚀反应,不能保护试样免受该熔盐腐蚀。     

Microstructure and Properties of SAE 2205 Stainless Steel After Salt Bath Nitrocarburizing at 450 °C

Nitrocarburizing of the type SAE 2205 duplex stainless steel was conducted at 450 °C, using a type of salt bath chemical surface treatment, and the microstructure and properties of the nitrided surface were systematically researched. Experimental results revealed that a modified layer transformed on the surface of samples with the thickness ranging from 3 to 28 μm changed with the treatment time. After 2205 duplex stainless steel was subjected to salt bath nitriding at 450 °C for time less than 8 h, the preexisting ferrite zone in the surface transformed into austenite by active nitrogen diffusion. The main phase of the nitrided layer was the expanded austenite. When the treatment time was extended to 16 h, the preexisting ferrite zone in the expanded austenite was decomposed and transformed partially into ε-nitride precipitate. When the treatment time extended to 40 h, the preexisting ferrite zone in the expanded austenite was transformed into ε-nitride and CrN precipitate. Further, a large amount of nitride precipitated from preexisting austenite zone. The nitrided layer depth thickness changed intensively with the increasing nitriding time. The growth of the nitride layer takes place mainly by nitrogen diffusion according to the expected parabolic rate law. The salt bath nitriding can effectively improve the surface hardness. The maximum values measured from the treated surface are observed to be approximately 1400 HV_0.1 after 8 h, which is about 3.5 times as hard as the untreated material (396 HV_0.1). Low-temperature nitriding can improve the erosion/corrosion resistance. After nitriding for 4 h, the sample has the best corrosion resistance.     

Plasma nitriding of AISI 304 austenitic stainless steel with pre-shot peening

AISI 304 austenitic stainless steel was plasma nitrided at the temperature ranging from 410 to 520 °C with pre-shot peening. The structural phases, micro-hardness and electrochemical behavior of the nitrided layer were investigated by optical microscopy, X-ray diffraction, micro-hardness testing and anodic polarization testing. The effects of shot peening on the nitride formation, nitride layer growth and corrosion properties were discussed. The results showed that shot peening enhanced the nitrogen diffusion rate and led to a twice thicker nitrided layer than the un-shot peening samples under the same plasma nitriding conditions (410 °C, 4 h). The nitrided layer was composed of single nitrogen expanded austenite (S-phase) when nitriding below 480 °C, which had combined improvement in hardness and corrosion resistance.     

Effect of temperature on anodic behavior of 13Cr martensitic steel in CO2 environment

The corrosion behavior of 13Cr martensitic stainless steel in a CO₂ environment in a stimulated oilfield was studied with potentiodynamic polarization and the impedance spectra technique. The results showed that the microstructure of the surface scale clearly changed with temperature. This decreased the sensitivity of pitting corrosion and increased the tendency toward general (or uniform) corrosion. The capacitance, the charge transfer resistance, and the polarization resistance of the corrosion product scale decrease with increasing temperature from 90 to 120 °C, and thus the corrosion is a thermal activation controlled process. Charge transfer through the scale is difficult and the corrosion is controlled by a diffusion process at a temperature of 150 °C. Resistance charge transfer through the corrosion product layer is higher than that in the passive film.     

Electrochemical evaluation of the corrosion protectiveness and porosity of vacuum annealed CrAlN and TiAlN cathodic arc physical vapor deposited coatings

The corrosion behavior of cathodic arc physical vapor deposited CrAlN and TiAlN coatings were examined in 1 M HCl solution before and after vacuum annealing at 700, 800, 900, and 1000 °C. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) methods were used to study the corrosion behavior and porosity of the coatings in comparison with the bare steel substrate (304SS). Structural and mechanical characterization of the coatings were also conducted. It is found that with increasing annealing temperature, the mechanical properties of TiAlN increased due to age hardening caused by spinodal decomposition while the hardness of CrAlN decreased as result of relaxation. Similarly, EIS and PDP results revealed that the as‐deposited and annealed coatings offer higher corrosion resistance as compared to the bare 304SS substrate. The coatings susceptibility to corrosion is reduced after annealing as indicated by the increasing nobility of E_corr. Both PDP and EIS tests revealed that CrAlN coating annealed at 1000°C exhibited superior corrosion resistance properties. It is found that the reduced current density for CrAlN coating annealed at 1000°C was due to the reduction in the porosity. Annealed TiAlN coating follows similar behavior until an optimum annealing temperature of 800°C. Beyond this temperature, porosity enlargement and an increase in the number of pores subsequent to structural changes deteriorated the corrosion resistance of TiAlN coating.