Plasma Carburizing Research Articles

AISI 420 martensitic stainless steel low-temperature plasma assisted carburizing kinetics

The present paper reports the experimental results on martensitic stainless steel low-temperature plasma assisted carburizing kinetics. The treatments were carried out using a gas mixture of 99.5% (80% H2+20% Ar)+0.5% CH4 at temperatures of 623, 673, 723 and 773K and for times of 4, 8, 12 and 16h. The peak voltage and the pulse period of the pulsed DC power supply were kept constant at 700V and 240μs, respectively. Temperature was controlled by adjusting the duty cycle. The treated samples were characterized by confocal laser scanning microscopy, X-ray diffractometry and microhardness measurements. Results indicate that low-temperature plasma carburizing is a diffusion controlled process. The calculated activation energy for outer and diffusion layer growth was 29 and 85kJmol−1, respectively. An apparently precipitation-free layer can be produced only when the processing temperature is sufficiently low (≤723K) and time sufficiently short (≤12h).

Effects of plasma carburizing and DLC coating on friction-wear characteristics, mechanical properties and fatigue strength of stainless steel

This study was conducted to investigate the effects of hybrid surface treatment on the friction-wear characteristics, mechanical properties and fatigue strength of austenitic stainless steel JIS SUS316. In hybrid surface treatment, after plasma carburizing as a pre-treatment had been performed at 773K for 43.2ks (12h), DLC (diamond-like carbon) coating was performed. The formed duplex layer had the DLC layer (thickness 1.6–1.8μm) outermost and the hardened layer (thickness 90μm) below it. The DLC layer was markedly effective in decreasing the friction coefficient and improving wear resistance. At the same time, the durability of the DLC layer was improved by the existence of the hardened layer. Since no marked change in the microstructure of the substrate occurred through hybrid surface treatment, the mechanical properties were almost unchanged. Furthermore, fatigue strength was greatly improved by hybrid surface treatment (improvement rate 53%) because the generation of fatigue cracks from the surface was strongly suppressed by the duplex layer.

Low-temperature plasma carburizing of AISI 420 martensitic stainless steel: Influence of gas mixture and gas flow rate

Low-temperature plasma carburizing was studied aiming to determine the effect of the gas mixture and flow rate on the surface properties of AISI 420 martensitic stainless steel samples. Plasma carburizing was carried out for gas mixtures of 20% Ar+80% H2 comprising CH4 contents between 0.25 and 1.00%, and gas flow rates ranging from 1.67×10−6 to 6.68×10−6Nm3s−1. The modified layers were characterized by confocal laser scanning microscopy, X-ray diffractometry and microhardness measurements. The plasma was also characterized by optical emission spectrometry. Results indicate the presence of a hard and thin outer layer and a carbon-enriched martensite diffusion layer. It is shown that gas mixture composition plays an important role in the process kinetics. Spectroscopic characterization of the glow discharge shows that the variation of the CH4 content in the gas mixture leads to a variation of the emission lines intensity but does not significantly alter the relative peak intensities. It suggests a variation on the plasma density and no significant variation on the active species. It also indicates that, for the studied conditions, the emission spectroscopy cannot be applied as a tool for process control.

Modification of S phase on austenitic stainless steel using fine particle shot peening

Hardening of the S phase by fine particle shot peening was investigated. The S phase created by carburizing has an austenite phase expanded by supersaturated diffusion of interstitial carbon. The austenite phase has good strain hardenability. Therefore, the expanded austenite structure is also expected to have it. With plasma carburizing for 14.4ks, S phase of ca. 15μm was produced by carbon diffusion on the austenitic stainless steel surface substrate. Fine particle shot peening was conducted on the S-phase surface with two different grits and different shot speeds controlled using air pressure of 0.1–0.9MPa. Then the layer hardness was measured and the microstructures of specimens were analyzed. The carbon concentration profiles were also measured using glow discharge optical emission spectrometry. Results showed that the S phase can be hardened without cracking by shot peening, and that the origin of the hardness increase is strain hardening of the S phase itself.

Tribocorrosion behavior of low temperature plasma carburized stainless steel

Low temperature plasma carburizing is a plasma surface engineering technique developed for austenitic stainless steels to achieve combined improvements in tribological and corrosion resistant properties. However, until recently very little work has been conducted to study the response of carburized stainless steel to combined actions by wear and electrochemical corrosion. In this paper, the tribocorrosion behavior of low temperature plasma carburized AISI 316L stainless steel under unidirectional sliding in 1M H2SO4 and 0.5M NaCl solutions is compared. It is found that in both test solutions, the carburized layer exhibits much better resistance to material removal than the untreated specimen at anodic potentials, but its effectiveness in improving wear resistance at open circuit and cathodic potentials is less significant. Particularly, in the pitting environment containing 0.5M NaCl, numerous corrosion pits are formed inside the wear track on the untreated specimen at potentials much lower than the pitting potential, while no corrosion pits are observed inside the wear track on the carburized specimen at anodic potentials as high as 1000mV(SCE). Further experiments conducted in 1M H2SO4 solution at various contact loads and frequencies reveal that the carburized layer has a much better ability to repassivate than the untreated specimen. The much improved tribocorrosion behavior of the carburized layer can thus be attributed to its high hardness, good corrosion resistance and excellent repassivation ability.

Augmentation of crevice corrosion resistance of medical grade 316LVM stainless steel by plasma carburising

Precipitate free carbon S-phase was produced on the surface of AISI 316LVM medical grade austenitic stainless steel with the use of low temperature direct current and active screen plasma carburising. The treated and untreated alloy was characterised and tested for pitting and crevice corrosion resistance. From this work it can be concluded that when compared to the untreated material, both treatments augment the pitting and crevice corrosion resistance. Using an active screen set-up results in a better surface composition and a higher crevice corrosion resistance than that produced using the direct current plasma carburising treatment.

The Corrosion Behavior of Carburized Aluminum Using DC Plasma

Because of the outstanding properties of aluminum, it is widely used in today's advanced technological world. However, its insufficient wear resistance limits its use for commercial and industrial applications. In this study, we performed DC diode plasma carburizing of aluminum in the gas composition of CH4–H2 (20–80%) and at a temperature of about 350°C for 4 and 8 hours. The corrosion properties of the untreated and plasma-carburized samples were evaluated using anodic polarization tests in 3 N HCl solution according to ASTM: G5-94. The metallurgical characteristics were then investigated using XRD and SEM. The results showed that the carburizing process improves the corrosion resistance of treated specimens at low temperature.

Corrosion behaviour of carbon S-phase created on Ni-free biomedical stainless steel

Following the success of forming a carbon S-phase (expanded austenite) surface layer on medical grade Ni-free austenitic stainless steel by DC plasma carburising, the established commercial carburising process Kolsterising® was performed on both Ni-containing (AISI 304) and Ni-free austenitic stainless steels. While the Ni-containing stainless steel responded very well to Kolsterising®, the Ni-free alloy did not. The carbon absorption and the hardness of the Kolsterised® Ni-free alloy are inferior to Kolsterised® AISI 304 Ni-containing stainless steel, however, the hardness of the untreated Ni-free alloy was doubled by Kolsterising®. The response of both Kolsterised® Ni-free and Ni-containing alloys to pitting, crevice corrosion and intergranular corrosion resistance was similar. From this work it can be concluded that the Kolsterised® austenitic stainless steels do not suffer from intergranular corrosion but are susceptible to intragranular pitting when tested in boiling sulphuric acid and copper sulphate solution. It was also observed that Kolsterising® improves significantly the pitting and crevice corrosion resistance of the alloys used in this study.

Tribological Behavior of Ti<sub>2</sub>AlNb Alloy by Surface Plasmas Carburized

In order to improve the wear resistance of Ti2AlNb orthorhombic alloy, the carburized coating has been successfully prepared on Ti2AlNb alloy by double glow plasma technology. The microstructure, chemical composition and phase structure o f the carburized coating were analyzed by means of SEM, EDS and XRD. The Micro hardness of the alloy layer was measured, and the bond strength of carburized coating with substrate was also evaluated by scratch test. The tribological behaviors of layer were investigated by friction-wear test. The results show that a carburized layer is formed which is about 40μm and the content of C gradient decrease after double glow plasma carburizing, the Micro hardness of the layer is 1051 HV0.1 and the bond of the coating is 62N, the tribological behavior of layer is much better than matrix.

Low temperature plasma carburizing of martensitic stainless steel

Abstract The present work reports results of successful low temperature plasma carburizing of martensitic stainless steel carried out in dc abnormal glow discharge. Very promising results are presented, which are confirmed by microstructural analysis, XRD and microhardness profiles of the treated samples.

AISI316L 강에 저온 플라즈마침탄 및 DLC 복합 코팅처리 시 처리온도에 따른 표면특성평가

A low temperature plasma carburizing process was performed on AISI 316L austenitic stainless steel to achieve an enhancement of the surface hardness without degradation of its corrosion resistance. Attempts were made to investigate the influence of the processing temperatures on the surface hardened layer during low temperature plasma carburizing in order to obtain the optimum processing conditions. The expanded austenite (<TEX>${\gamma}_c$</TEX>) phase, which contains a high saturation of carbon (S phase), was formed on all of the treated surfaces. Precipitates of chromium carbides were detected in the hardened layer (C-enriched layer) only for the specimen treated at <TEX>$550^{\circ}C$</TEX>. The hardened layer thickness of <TEX>${\gamma}_c$</TEX> increased up to about <TEX>$65{\mu}m$</TEX> with increasing treatment temperature. The surface hardness reached about 900 <TEX>$HK_{0.05}$</TEX>, which is about 4 times higher than that of the untreated sample (250 <TEX>$HK_{0.05}$</TEX>). A minor loss in corrosion resistance was observed for the specimens treated at temperatures of <TEX>$300^{\circ}C{\sim}450^{\circ}C$</TEX> compared with untreated austenitic stainless steel. In particular, the precipitation of chromium carbides at <TEX>$550^{\circ}C$</TEX> led to a significant decrease in the corrosion resistance. A diamond-like carbon (DLC) film coating was applied to improve the wear and friction properties of the S phase layer. The DLC film showed a low and stable friction coefficient value of about 0.1 compared with that of the carburized surface (about 0.45). The hardness and corrosion resistance of the S phase layer were further improved by the application of such a DLC film.

Surface Polishing of Plasma Carburized Austenitic Stainless Steel

After being plasma carburized, the surface of AISI 316 austenitic stainless steel was covered with a layer of compact black thin-film. The surface polishing treatment, electrochemical brightening was carried out to remove the black thin-film and renew the original color of the stainless steel. The surface appearance, microstructures, micro hardness, surface roughness and corrosion resistance of the samples treated by electrochemical brightening process were analyzed. Experimental results show that the electrochemical brightening treatment can remove the black thin-film from and restore the original color of the plasma carburized stainless steel. After electrochemical brightening treatment, the thickness and surface hardness of the carburized layer were all decreased little, but the corrosion resistance was improved significantly. And the surface roughness (Ra) was lower than before. Therefore, the electrochemical surface brightening treatment can be used to improve the surface quality of the austenitic stainless steel treated by plasma carburizing.

GDA and ToF_SIMS of Plasma Carburizing Layer on Pure Titanium with Hydrogen-Free

A triple-cathode plasma surface alloying technology has been invented, which has been used to carburize on a surface of pure titanium in an operation of hydrogen-free. A plasma carburized layer has been obtained on a sample of pure titanium, which features were examined with GDA and ToF_SIMS. The result is pointed that this kind of carburized layer possesses some special and interesting characteristics.

Dry Sliding Wear of Active Screen Plasma Carburised Austenitic Stainless Steel

Low temperature diffusion treatments with nitrogen and carbon have been widely used to increase the tribological performance of austenitic stainless steels. These processes produce a layer of supersaturated austenite, usually called expanded austenite or S-phase, which exhibits good corrosion and wear resistance. The novel active screen technology is said to provide benefits over the conventional DC plasma technology. The improvements result from the reduction in the electric potential applied to the treated components, and the elimination of such defects and processing instabilities as edge effects, hollow cathode effects and arcing. In this study, AISI 316 coupon samples were plasma carburised in DC and active screen plasma furnaces. The respective layers of carbon expanded austenite were characterised and their tribological performance was studied and compared. Detailed post-test examinations included SEM observations of the wear tracks and of the wear debris, EDX mapping of the wear track, EBSD crystal orientation mapping of the cross sections of the wear tracks, and cross-sectional TEM. Based on the results of wear tests and post-test examinations, the wear mechanisms involved are discussed.

EBSD and AFM observations of the microstructural changes induced by low temperature plasma carburising on AISI 316

Low temperature plasma carburising (LTPC) has been increasingly accepted as a hardening process for austenitic stainless steels because it produces a good combination of tribological and corrosion properties. The hardening mechanism is based on the supersaturation of the austenitic structure with carbon, which greatly hardens the material, significantly expands the fcc unit cell, produces high levels of compressive residual stresses and, ultimately, leads to the occurrence of deformation bands and rotation of the crystal lattice. The microstructural changes introduced during plasma carburising have a significant impact on the mechanical, tribological and corrosion performance and, for this reason, the microstructure of expanded austenite or S-phase has been extensively studied. However, modern surface characterisation techniques could provide new insights into the formation mechanism of S-phase layers. In this work, backscattered electron diffraction and atomic force microscopy were used to characterise the surface layers of expanded austenite produced by LTPC in an active screen furnace. Based on the experimental results, the plastic deformation, its dependence on crystallographic orientation, the evolution of grain boundaries, and their effects on mechanical, tribological and corrosion properties are discussed.

Effect of tensile stress on the formation of S-phase during low-temperature plasma carburizing of 316L foil

Low-temperature plasma carburizing of austenitic stainless steel can produce a carbon-supersaturated austenite layer, the “S-phase”, on the surface, which has high hardness, excellent wear and fatigue properties, and good corrosion resistance. Although the S-phase was discovered some years ago, the basic understanding of S-phase formation remains incomplete. In this paper, the effect of tensile stresses (0–80 MPa) on the formation and stability of S-phase during carburizing of 316L stainless steel foils at 400, 425 and 450 °C for 10 h has been investigated for the first time. The microstructures were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy and the mechanical properties were evaluated by microhardness and tensile tests. The results showed that the in situ applied tensile stress effectively thickened S-phase layers. The calculated activation energy for carbon diffusion in 316L was reduced from 142.76 to 133.91 kJ mol −1 when a tensile stress of 40 MPa was applied. However, chromium carbides were formed in the outmost surface when the tensile stress exceeded 40 MPa. The results are discussed and explained through appropriate thermodynamic calculations.

Fretting Wear and Microstructure of Plasma Carburized TiAl Alloy

A carburized layer is fabricated in the surface of TiAl alloy using plasma carburization at 975 °C. Fretting wear test at ambient temperature was carried out to evaluate wear resistance of carburized TiAl. Glow discharge spectrum, X-ray diffraction and scanning electron microscopy equipped with energy dispersive spectrometry were used to characterize the microstructure of carburized TiAl and its property capability. The experimental results show that the carburized layer with about 5 μm in thickness is mainly composed of Ti2AlC and possesses better fretting wear resistances at ambient temperature. Precipitation of carbides in surface layer leads to a higher surface hardness and compressive stress, causing a surface strengthening of TiAl. Strengthening mechanism of carburized TiAl is also considered in this investigation.

Plasma Carburizing and Post-Treatment of Austenitic Stainless Steel

Plasma carburizing of AISI 316L austenitic stainless steel was carried out at low temperature to improve the surface hardness without degradation of its corrosion resistance. And the post-treatment, namely electrochemical surface brightening process was carried out to clear away a layer of thin black film on the plasma carburized samples and improve the surface quality of the hardened stainless steel. The surface appearance, roughness, microstructures, microhardness and corrosion resistance of the samples before and after brightening were analyzed and compared. Experimental results of plasma carburizing at low temperature showed that high-quality hardened layers can be produced at the appropriate process parameters.The electrochemical brightening process can be used as a post-treatment to restore the original color and further improve the corrosion resisitance of the plasma carburized stainless steel.

Effect of electrochemical potential on tribocorrosion behavior of low temperature plasma carburized 316L stainless steel in 1 M H 2SO 4 solution

Low temperature plasma carburizing is a surface engineering technique developed for austenitic stainless steels to achieve combined improvements in tribological and corrosion resistant properties. However, very little work has been conducted to study the response of carburized stainless steel to combined actions by wear and electrochemical corrosion. In the present work, experiments have been conducted to study the tribocorrosion behavior of low temperature plasma carburized AISI 316L stainless steel, under unidirectional sliding in 1 M H 2SO 4 solution, using a pin-on-disk tribometer integrated with a potentiostat for electrochemical control. Sliding wear tests were conducted under potentiodynamic and potentiostatic conditions at a wide range of applied potentials. It is found that the carburized layer exhibits much better tribocorrosion resistance than the untreated specimen at anodic potentials, but is not effective in improving wear resistance at cathodic potentials. The results are discussed in terms of the accumulation of the third body wear particles, material transfer and the contribution of mechanical wear and chemical wear to overall material removal by tribocorrosion.

AISI304L 강에 저온 플라즈마침탄 처리 시 처리조건에 따른 표면특성평가

A low temperature plasma carburizing process was performed to AISI 304L austenitic stainless steel to achieve the enhancement of surface hardness without a compromise in their corrosion resistance. Attempts were made to investigate the influence of the processing temperatures on the surface-hardened layer during low temperature plasma carburizng in order to obtain the optimum processing conditions. The expanded austenite (<TEX>${\gamma}C$</TEX>) was formed on all the treated surfaces. Precipitates of chromium carbides were detected in the hardened layer (C-enriched layer) only for the specimen treated at <TEX>$500^{\circ}C$</TEX>. The hardened layer thickness of <TEX>${\gamma}C$</TEX> increased up to about <TEX>$35\;{\mu}m$</TEX>, with increasing treatment temperature. The surface hardness reached about 1000 <TEX>$HK_{0.05}$</TEX>, which is about 4 times higher than that of the untreated sample (250 <TEX>$HK_{0.05}$</TEX>). Minor loss in corrosion resistance was observed for the specimens treated at temperatures of <TEX>$310^{\circ}C-450^{\circ}C$</TEX> compared with untreated austenitic stainless steel. Particularly, the precipitation of chromium carbides at <TEX>$500^{\circ}C$</TEX> led to a significant decrease in the corrosion resistance.