Plasma Carburizing Research Articles

Plasma carburizing with surface micro-melting

This paper presents carburizing the surface of 20 low carbon steel using electric arc and graphite prior. A carbon black solution was prepared with graphite powder and sodium silicate in water. A detailed analysis of the phase structure and the distribution profile of the sample hardness after plasma treatment were given. The hardened layer consists of three different zones: 1 – the cemented layer (thin white zone) on the surface, 2 – heat-affected zone (darkly etching structure), 3 – the base metal. The experimental result shows that the various microstructures and micro-hardness profiles were produced depending on the type of graphite coating (percentage of liquid glass) and processing parameters. The experiment proved that the optimum content of liquid glass in graphite coating is 50–87.5%. If the amount of liquid glass is less than 50%, adhesion to metal is insufficient. If liquid glass content is more than 87.5%, carburization of a metal surface does not occur. A mixture of the eutectic lamellar structure, martensite and austenite was obtained by using graphite prior with 67% sodium silicate and the levels of the hardness layer increased to around 1000 HV. The thickness of the cemented layer formed on the surface was around 200 μm. It is hoped that this plasma surface carburizing treatment could improve the tribological resistance properties.

Combined surface hardening and laser patterning approach for functionalising stainless steel surfaces

The paper reports a laser patterning method for producing surfaces with dual scale topographies on ferritic stainless steel plates that are hardened by low temperature plasma surface alloying. Nitrogen and carbon based gasses were used in the alloying process to obtain surface layers with an increased hardness from 172 HV to 1001 HV and 305 HV, respectively. Then, a nanosecond infrared laser was used to pattern the plasma treated surfaces and thus to obtain super-hydrophobicity, by creating cell- or channel-like surface structures. The combined surface hardening and laser patterning approach allowed super-hydrophobic surfaces to be produced on both nitrided and carburised stainless steel plates with effective contact angles higher than 150°. The hardened layers on nitrided samples had cracks and was delaminated after the laser patterning while on plasma carburised samples remained intact. The results showed that by applying the proposed combined approach it is possible to retain the higher hardness of the nitrided stainless steel plates and at the same time to functionalise them to obtain super-hydrophobic properties.

A Methodology for Fabrication of Highly Pitting Corrosion-Resistant Type 304 Stainless Steel by Plasma Carburizing and Post-Pickling Treatment

Low temperature plasma carburizing was performed to improve the pitting corrosion resistance of Type 304 stainless steel. The optimal composition of the plasma gas and post-treatment were then examined. Carburizing was carried out for 18 ks at 470°C in H2-CH4, Ar-H2-CH4 or Ar-CH4 plasma, and carburized layers ca. 9 μm in thickness with an average ca. 1.3 mass% carbon concentration were obtained. However, it was found that Cr23C6 was precipitated in the carburized layer and intergranular corrosion occurred under anodic polarization in 0.1 M NaCl. The precipitation behavior of Cr23C6 differed according to the plasma gas composition: H2 gas was found to suppress the size of Cr23C6. No Cr23C6 was precipitated in the inner part of the carburized layer formed in the H2-CH4 plasma, and no stable pit was initiated on the inner parts of the carburized layer under anodic polarization in 0.1 M NaCl. Pickling treatment using HF-HNO3 was applied to remove the Cr-depleted zones in the outer part of the plasma carburized layer. The Type 304 stainless steel after the plasma carburizing in the H2-CH4 atmosphere and subsequent the HF-HNO3 pickling exhibited no pitting corrosion under anodic polarization even in 3 M NaCl at 25°C.

Passivity and its breakdown on low temperature plasma carburized AISI 204Cu stainless steel in chloride‐containing solution

The passivity and passivity breakdown of low temperature plasma carburized AISI 204Cu stainless steel were studied in borate buffer solution (pH 8.4) in the absence and presence of 150 mM KCl, performing potentiodynamic polarization, potentiostatic transients, and electrochemical impedance spectroscopy (EIS) measurements complemented with scanning electron microscopy (SEM). In the absence of chloride ions, the surface treatment exercise a beneficial role in the passivating characteristics of AISI 204Cu. Plasma carburizing significantly modifies the electrochemical response of 204Cu steel resulting in a more stable passive film. In the presence of chloride ions, both the untreated and plasma carburized specimen experience localized corrosion (pitting). However, plasma carburizing increases the breakdown potential and the extent of passivation domain, indicating improved resistance to initiation of pitting corrosion. Furthermore, EIS measurements performed at the breakdown state evidenced higher Rct and lower depression angle values for the carburized specimen compared to the untreated alloy, confirming its better localized corrosion behavior. The high amount of interstitial carbon is believed to be responsible for the enhanced protective function of passive film which exerts an effective inhibiting action towards the localized corrosion process.

Diffusion behavior of carbon and its hardening effect on plasma carburized M50NiL steel: Influences of treatment temperature and duration

Diffusion behavior of carbon and its hardening effect on plasma carburized M50NiL steel were investigated systematically. The plasma carburizing was carried out in a mixture of gaseous acetone (30%) and hydrogen (70%) at temperatures of 400, 450, 500 and 550°C for durations of 4, 8 and 12h, respectively. Influences of treatment temperature and duration on properties of the carburized layer were investigated with optical microscope, atomic force microscope, X-ray diffractometer, micro- and nano-indenters, and pin-on-disc tribometer. Results of the tests indicate that cementite is the main phase in the surface layer of specimens carburized at 400 and 450°C, while that in those carburized at 500 and 550°C is mainly the carbon expanded martensite. The specimens with cementite in the surface layer show lower surface roughness, higher surface hardness and thus lower wear rate. The diffusion behavior of carbon and resultant microstructure evolution mechanism were studied, indicating that plasma carburizing is a diffusion-controlled process. Activation energies of carbon diffusion into martensite and cementite were calculated, which are 64.5kJ·mol−1 and 120.7kJ·mol−1, respectively. The information on the activation energies provides basic data for tuning and predicting the surface properties of plasma carburized M50NiL steel.

New plasma carburizing method

The paper deals with a new plasma-assisted carburizing method and sodium silicate-based production of innovative carbon coating for carburizing. Microstructure and micro-hardness of a carburized layer were studied. It was found that during plasma impact for 0.1–1 sec the surface is carburized to white iron concentration. Microstructure and metal surface characteristics subjected to plasma carburizing were studied. Basic parameters of the carburized layer were determined: carburized layer depth is 35–250 μm, microhardness goes up to 12000 MPa.

Wear resistance of AISI 304 stainless steel submitted to low temperature plasma carburizing

Abstract Despite the AISI 304 stainless steel has high corrosion/oxidation resistance, its tribological properties are poor, being one of the barriers for use in severe wear applications. Thus, there is a wide field for studying technologies that aim to increase the surface hardness and wear resistance of this material. In this work, hardness and wear resistance for AISI 304 stainless steel submitted to the thermochemical treatment by low temperature plasma carburizing (LTPC) in a fixed gas mixture composition of 93% H2 and 7% CH4 are presented. Through the evaluation of the carburizing layers, it was possible to observe a substantial improvement in tribological properties after all temperature and time of treatment. This improvement is directly related to the increase of the process variables; among them temperature has a stronger influence on the wear resistance obtained using LTPC process.

Scratch resistance of low-temperature plasma nitrided and carburized martensitic stainless steel

Martensitic stainless steel is a potential material class for selection in some tribological systems, particularly those subjected to wear by hard particles and corrosion. To improve its mechanical resistance, without loss of corrosion performance, surface treatments such as low-temperature nitriding and carburizing have been systematically employed. In this context, this paper studied martensitic stainless steel samples subjected to different plasma-assisted treatments, in the case the plasma nitriding and plasma carburizing. The treatments were carried out at temperatures of 300, 350 or 400°C, for different times, aiming to compare the scratch resistance of the treated surfaces. Specimens were initially characterized using X-Ray diffractometry, micro and nanohardness measurements. Scratch resistance was performed using the constant load mode for two levels, 8 and 15N. All worn tracks were evaluated using optical interferometry, to get information about the widths and depths of track profiles. Worn surfaces were analysed using scanning electron microscope, revealing tensile cracks on the surface of nitrided samples. The friction coefficient and the wear resistance were analysed and related to the geometry of scratches, which was associated to the micro-mechanism of wear. Despite the high hardness and low friction coefficient achieved using nitriding treatment, the global performance of carburized samples could be considered as more suitable, because the carburized case significantly minimized the cracks formation. Therefore, plasma carburized martensitic stainless steel can be considered an adequate combination for corrosion environments subjected to wear.

Effect of Plasma Carburizing Treatment on Pitting Corrosion Resistance of Type 304 Stainless Steel

Pitting corrosion resistance of stainless steels was improved by plasma carburizing at low temperatures. From anodic polarization, carburized stainless steels showed higher pitting potential than that of untreated stainless steels, and no stable pit was generated. Inclusions acted as initiation sites for metastable pits, but no stable pit was observed on the carburized stainless steels. It is likely that active dissolution of steel matrix was suppressed by the interstitial carbon.

Effect of shot peening and treatment temperature on wear and corrosion resistance of sequentially plasma treated AISI 316L steel

Austenitic stainless steels exhibit excellent corrosion resistance but relatively poor wear resistance. Previous investigations have demonstrated that surface treatments consisting of plasma carburizing and plasma nitriding were able to successfully increase the wear resistance of austenitic stainless steels. In this work, the effect of a prior shot peening (SP) process on wear and corrosion resistance of sequentially plasma carburized and plasma nitrided AISI 316L austenitic stainless steel was investigated. Triode plasma carburizing (TPC) and triode plasma nitriding (TPN) were sequentially carried out at two temperatures: 400°C and 475°C. SP processing prior to sequential plasma treatments led to a significant increase in the near-surface hardness. The sequential plasma treatment at 475°C in combination with a SP pre-treatment promoted a further increase in thicknesses of carburized and nitrided layers, leading to a greater hardening depth. The best wear resistance exhibited by austenitic AISI 316L samples subjected to SP and sequential plasma processing at higher temperature could be attributed to their high surface hardness and greater treatment depth. However, this treatment at higher temperature yielded CrN precipitates that impaired the corrosion resistance in aerated 0.5M H2SO4 aqueous solution. Electrochemical tests also revealed that the use of shoot peening prior to sequential plasma treatments at 475°C could partially counteract the deleterious reduction in corrosion resistance promoted by chromium nitride precipitation at this higher processing temperature. Although the precipitation of chromium nitrides at higher plasma processing temperatures promoted a reduction in corrosion resistance in such acidic environment, results indicate that austenitic stainless steels modified by shot peening (SP) followed by sequential plasma treatments at high processing temperatures could be potentially used in applications where high wear resistance and moderate corrosion resistance in specific environments are required.

Plasma carburizing for improvement of Ni-Fe cathodes for alkaline water electrolysis

Electrodeposited Ni-Fe-C alloys have high electroactivity for hydrogen evolution reaction (HER) in alkaline water electrolysis. In the present work carbon was introduced into Ni and Ni–Fe alloys by plasma treatment in CH4+H2 gas mixture at 470°C. Despite of a very low solubility of carbon in nickel, carbon entered into nickel to the depth of about 0.5μm. In high-Fe alloys it formed a carbide layer about 2-μm thick and increased surface hardness. Electrochemical measurements in 25wt.% KOH at 80°C showed that carburization resulted in an improvement of catalytic activity toward HER, especially of Ni and 1Ni-Fe. Carburization also increased the resistance to corrosion during cathodic polarisation and under open-circuit conditions. XPS surface analysis showed that after corrosion the oxide content on carburized materials was significantly lower than that on untreated materials. It is suggested that the enhanced electroactivity of plasma carburized cathodes was due mainly to the enlargement of the surface area of disintegrated material. Also, a catalytic effect might be exerted by carbon-metal particles.

Effect of Plasma Carburizing Treatment on Pitting Corrosion Resistance of Type 304 Stainless Steel

Stainless steels suffer from pitting in corrosive environment. Pitting corrosion resistance of stainless steels is known to be improved by carburizing at low temperatures.1,2)Plasma carburizing is suitable for stainless steel, since it is possible to keep low temperatures during the treatments to prevent sensitization. In this study, we analyzed the effect of plasma carburizing treatments on the pitting corrosion resistance of type 304 stainless steel. Specimen used was Type 304 stainless steel. Chemical composition in mass% was C: 0.036, Si:0.61, Mn:0.82, Cr:17.92 and Ni:7.93. The specimen was heat-treated at 1373 K and quenched in water. Before plasma carburizing, the surface of specimen was polished using SiC grinding paper and 1μm diamond paste. Plasma carburizing was carried out at 573 K or 743 K under the atmosphere of CH4:H2:Ar = 1.5:13.5:15 (sccm). The total pressure was kept at 550 Pa (at 573 K) or 650 Pa (at 743 K). The analysis of lattice parameters and phases was conducted using X-ray diffraction (XRD). The surface composition was analyzed by grow discharge optical emission spectroscopy (GD-OES). A scanning transmission electron microscope (STEM) equipped with an X-ray energy dispersive spectroscopy (EDS) system was used to identify the microstructure of the carburized specimen. In the electrochemical measurement, the surface of specimen was coated using epoxy resin and paraffin without the electrode area (ca. 0.05 mm2 or 100 mm2). The counter electrode was a Pt plate. The reference electrode was an Ag/AgCl electrode (3.33 M KCl). Anodic polarization measurements were conducted in 0.1 M NaCl (298 K).The potential scan rate was 23 mV min-1. After polarization, the specimen surface was observed using confocal laser scanning microscope and a field emission scanning electron microscope (FE-SEM). Figure 1a shows the XRD patterns of the carburized and untreated specimens. The lattice parameter of the austenitic phase was changed from 0.3599 nm to 0.3688 nm. And also, after the carburization, the austenitic phase was partly transformed into martensite. Figure 1b shows the GD-OES depth profile of the relative concentration of C, Ni, Cr and O in the specimen treated at 743 K. The thickness of the carburized layer was determined to be around 16 μm. The average of relative concentration of C was 2.5 mass%. Figure 1c shows the anodic polarization curves of specimens. In the case of untreated specimen, pit was initiated around 0.55 V, but the large current increases were observed at lower potentials for the carburized specimens. Figure 1d shows the surface appearance of the electrode area after the polarization. The intergranular corrosion occurred on the surface of the carburized stainless steel at 743 K. From STEM observation, Cr23C6 was precipitated after the plasma carburizing treatments. References 1) F.J.Martin, E.J.Lemieux, T.M.Newbauer, R.A.Bayles, P.M.Natishan, H.Kahn, G.M.Michal, F.Ernst, and A.H.Heuer, Electrochem.Solid-State Lett,10,C37(2007) 2) A.Chiba, S.Shibukawa, I.Muto, T.Doi, K.Kawano, Y.Sugawara, and N.Hara, J. Electrochem. Soc, 162,C270-C278(2015) Figure 1

Dry sliding behavior (block-on-ring tests) of AISI 420 martensitic stainless steel, surface hardened by low temperature plasma-assisted carburizing

This paper reports on the dry sliding behavior (block-on-ring tests) of Low-Temperature Plasma Carburized (LTPC) AISI 420 martensitic stainless steel, treated under different times (4–16h) and temperatures (350–500°C), so as to identify the relationships between microstructure, micro-hardness and tribological behavior. In particular, LTPC was effective in decreasing wear when treatment conditions led to the highest diffusion layer thickness combined with absence of a thick outer layer (i.e. at 450°C for a treatment time of 12h). The dominant wear mechanism was mild tribo-oxidation, with the protective oxide tribolayer being partly removed by abrasive debris in samples treated at 500°C.

Self-lubricating and anti-corrosion amorphous carbon/Fe3C composite coating on M50NiL steel by low temperature plasma carburizing

Abstract Low temperature plasma carburizing was employed to fabricate a carburized layer containing an amorphous carbon/Fe 3 C composite coating on the M50NiL steel surface in gaseous acetone. The influence of gaseous acetone content on the carburized layer was investigated by optical microscope, scanning electron microscope equipped with energy dispersive X-ray analyzer, X-ray diffraction, Raman spectroscopy, microhardness tests, pin-on-disc tribometer and anodic polarization tests. A specific carburized layer mainly comprised of carbon expanded martensite (α′ C ) with good wear resistance and modest corrosion resistance was formed by plasma carburizing at 550 °C for 4 h. The amorphous carbon/Fe 3 C composite coating-containing carburized layer was obtained using gas mixture of acetone (0.3 L/min) and hydrogen (0.2 L/min). The coating showed lowest friction coefficient (about 0.3) and 99.0% reduction in wear rate compared with untreated specimen due to its lubricating effect. Moreover, it showed better corrosion resistance than untreated specimen owing to its chemical inertness. The promising results demonstrated that the improved wear and corrosion resistance could be realized simultaneously by introducing the composite coating with merits of self-lubricating and anti-corrosion.

Double glow plasma carburization on zirconium to improve surface hardness and wear resistance

Though some important progress in the excellent mechanical properties of zirconium alloys have been reported, their high surface hardness and good wear properties need to be explored further. In this work, a carburized layer was formed on the surface of commercially pure zirconium by a double glow plasma hydrogen-free carburizing technique. Commercial high-purity graphite was used as the carbon source material. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Vickers hardness test, friction and wear test were used to characterize the samples carburized. The carburized layer could be clearly observed under a microscope. XRD patterns indicate that the zirconium carbide phase is formed in the carburized layer. The surface hardness of the sample increases significantly after carburization. Friction and wear tests results show that wear resistance and friction coefficient of zirconium are improved considerably after carburization. Surface plastic deformation is arrested to a low extent in contrast with pure zirconium because of the presence of ZrC phases during the wear test. The results may provide new insight into methods for surface strengthening of zirconium alloys. A carburized layer was formed on the surface of commercially pure zirconium by a double glow plasma hydrogen-free carburizing technique. The carburized layer could be clearly observed under a microscope. X-ray diffraction (XRD) patterns indicate that the zirconium carbide phase is formed in the carburized layer. The surface hardness of the sample increases significantly after carburization. Friction and wear tests results show that wear resistance and friction coefficient of zirconium are improved considerably after carburization .

The Effects of Gas Compositions During Post Nitriding on the AISI 316L Stainless Steel after Plasma Carburizing

In this experiment, post-nitriding treatment was performed at 400℃ on AISI 316 stainless steel which was plasma carburized previously at 430℃ for 15 hours. Plasma nitriding was implemented on AISI 316 stainless steel at various gas compositions (25% N₂, 50% N₂ and 75% N₂) for 4 hours. Additionally, during post nitriding Ar gas was used with H2 and N2 to observe the improvement of surface properties. After treatment, the behavior of the hybrid layer was investigated by optical microscopy, X-ray diffraction, and micro-hardness testing. Potentiodynamic polarization test was also used to evaluate the corrosion resistance of the samples. Meanwhile, it was found that the surface hardness increased with increasing the nitrogen gas content. Also small percentage of Ar gas was introduced in the post nitriding process which improved the hardness of the hardened layer but reduced the corrosion resistance compared with the carburized sample. The experiment revealed that AISI 316L stainless steel showed better hardness and excellent corrosion resistance compared with the carburized sample, when 75% N2 gas was used during the post nitriding treatment. Also addition of Ar gas during post nitriding treatment degraded the corrosion resistance of the sample compared with the carburized sample.

Surface hardening of age-hardenable Cu–Ti alloy by plasma carburization

Abstract Plasma carburization was performed on specimens of a commercially used age-hardenable Cu–Ti alloy, in order to improve its surface hardness and wear toughness. Specimens of a Cu–4 mol% Ti alloy were carburized in a methane gas glow discharge plasma at temperatures of 1073 K and 1123 K for 6 h. The plasma-carburization treatment resulted in the formation of two layers of crystalline compounds on the surfaces of the Cu–4 mol% Ti alloy specimens: (i) a TiC top layer containing 11 at.% oxygen and (ii) a Cu 3 Ti 3 O sublayer. The thicknesses of the TiC and Cu 3 Ti 3 O layers were approximately 100 nm and 900 nm, respectively, in the case of the specimen plasma-carburized at 1073 K. This led to significant improvements in the surface hardness and wear toughness of the specimen. It was interesting to note that the presence of a carbon-friendly alloying element (Ti) at a concentration of only 4 mol% resulted in the formation of a hard TiC layer, leading to the surface hardening of the Cu-based alloy. Furthermore, plasma carburization at a higher temperature of 1123 K resulted in the formation of thicker layers of hard TiC and Cu 3 Ti 3 O, causing a greater degree of surface hardening. Thus, it was demonstrated that plasma carburization is highly suitable for the surface modification of age-hardenable Cu–Ti dilute alloys.

Towards near-permanent CoCrMo prosthesis surface by combining micro-texturing and low temperature plasma carburising

An advanced surface engineering process combining micro-texture with a plasma carburising process was produced on CoCrMo femoral head, and their tribological properties were evaluated by the cutting-edge pendulum hip joint simulator coupled with thin film colorimetric interferometry. FESEM and GDOES showed that precipitation-free C S-phase with a uniform case depth of 10μm was formed across the micro-textures after duplex treatment. Hip simulator tests showed that the friction coefficient was reduced by 20% for micro-metre sized texture, and the long-term tribological property of microtexture was enhanced by the C-supersaturated crystalline microstructure formed on the surface of duplex treated CoCrMo, thereby enhancing biotribological durability significantly. In-situ colorimetric interferometry confirmed that the maximum film thickness around texture area was 530nm, indicating that the additional lubricant during sliding motion might provide exceptional bearing life.

Short-term creep properties of Ti-6Al-4V alloy subjected to surface plasma carburizing process

The aim of this study was to investigate the short-time creep behavior of Ti-6Al-4V by plasma carburizing, which was performed at 725°C for 6h in a 50% Ar – 45% H2 – 5% CH4 gas mixture. Nano and microhardness testing, optical microscopy, TEM, X-ray diffraction and optical profilometry were used to characterize the samples. Furthermore, short-term creep tests were performed under a constant tensile load in air at 600°C using a dead-weight-creep-rupture machine. The carburizing treatment resulted in a compound layer measuring approximately 1.7μm in thickness with a hardness of 815HV and a composition of TiC0.66. The creep properties of the “Widmanstätten+carburized” specimens were improved relative to those of untreated specimens. TEM and fracture analysis indicated creep deformation process attributed mainly to α phase deformation and fracture by intergranular decohesion.

Plasma Carburizing of Sintered Pure Iron at Low Temperature

In this work, plasma carburizing of sintered pure iron at low temperature was investigated. Samples were carburized under a dc abnormal glow discharge in two mixed atmospheres (5%CH4 + 95%H2 and 20%CH4 + 80%H2) at 500 °C and 700 °C for 3 and 6 hours. The influence of these parameters was investigated by Scan Electron Microscopy (SEM), X-ray Diffraction (DRX), Raman spectroscopy and microhardness measurements. There is a correlation between temperature and layer morphology, and a layer of polycrystalline cementite was obtained at 500 °C and 700 °C on a ferrite substrate. Furthermore, the carburized layer thickness increased with an increase in the treatment time and the methane percentage. The topography is very dependent on the treatment temperature.