High Temperature Gas Nitriding Research Articles

Fabrication and mechanical properties of Ti2AlN/TiAl composite with continuous network structure

Nitrogen was introduced into TiAl pre-alloyed powder using high-temperature gas nitriding, and Ti2AlN/TiAl composites with a continuous network structure of the reinforced phase were prepared via spark plasma sintering. The results show that the hardness of the composite is significantly higher than that of TiAl alloy, and increases with the increase of nitriding time. The strengthening effect is originated from the synergistic effect of the solid-solution strengthening caused by the nitriding of the powder, the continuous network of the Ti2AlN phase with high hardness and elastic modulus, and the increase of dislocation density. Additionally, the compressive strength of the Ti2AlN/TiAl composites is lower than that of the TiAl alloy, which is related to a part of Ti2AlN particles that are directly formed after nitriding and excessive reinforcement content.

The effect of cold rolling and high-temperature gas nitriding on austenite phase formation in AISI 430 SS

Austenitic stainless steel is the most commonly used material in the production of orthopedic prostheses. In this study, AISI 430 SS (0.12 wt. % C; 1 wt. % Si; 1 wt. % Mn; 18 wt. % Cr; 0.04 wt. % P and 0.03 wt. % S) will be modified by creating austenite and removing its ferromagnetic properties via the high-temperature gas nitriding process. Cold rolling with various percentage reduction (30, 50, and 70 %) was followed by gas nitriding at a temperature of 1200 °C with holding times of 5, 7, and 9 hours, then quenching in water was carried out on as-annealed AISI 430 SS. The formation of the austenite phase was examined by XRD (x-ray diffraction). The microstructure and element dispersion were observed using SEM-EDS (scanning electron microscope-energy dispersive spectrometry), whereas the mechanical properties after gas nitriding and water quenching were determined by Vickers microhardness testing. At all stages of the gas nitriding process, the FCC iron indicated the austenite phase was visible on the alloy's surface, although the ferrite phase is still present. The intensity of austenite formation is produced by cold rolling 70 % reduction with a 5-hour gas nitriding time. Furthermore, the nitrogen layer was formed with a maximum thickness layer of approximately 3.14 µm after a 50 % reduction in cold rolling and 9 hours of gas nitriding process followed by water quenching. The hardness reached 600 HVN in this condition. This is due to the distribution of carbon that is concentrated on the surface. As the percent reduction in the cold rolling process increases, the strength of AISI 430 SS after gas nitriding can increase, causing an increase in the number of dislocations. The highest tensile strength and hardness of AISI 430 SS of 669 MPa and 271.83 HVN were obtained with a reduction of 70 %.

Correlation between microstructure and micro-hardness of 316L nitrided austenitic stainless steel

This paper presents the effect of duration of solution nitriding towards microstructure and microhardness of AISI 316L stainless steel. The AISI 316L stainless steel was exposed to high temperature gas nitriding process at 1200 °C by using 50% ammonia gas and 50% nitrogen gas for different durations of 8, 15 and 20 hours. The nitrided austenitic stainless steel then underwent several analyses and tests to find the effect of different durations of nitriding towards its microstructures and microhardness. From the experiments conducted to the nitrided samples, it was found that the higher duration of nitriding resulted in the increase of microhardness of the samples due to the increase of nitrogen content in the stainless steel. However, there was no effect found on the microstructure of the nitrided samples. It was noted that the highest hardness was obtained from the nitrided sample of 20 hours that has 245.1 HV which is about 70% higher than the as-received sample.

STRUCTURE AND MECHANICAL PROPERTIES OF AUSTENITIC Cr – Ni – Ti STEELS AFTER HIGH-TEMPERATURE NITRIDING

Alloying of corrosion-resistant austenitic steels with nitrogen is widely used in production to stabilize austenite and to improve the strength and other properties of the metal. The possibility of alloying titanium-containing steels with nitrogen by introducing nitrogen into the melt is not possible, as it causes formation of the coarse defects in steel during casting and solidification of the metal (twisting of the peel, large nitride inclusions, accumulations of nitrides, etc.). The method of high-temperature gas nitriding can be alternative to liquid-phase nitriding for alloying austenitic titanium-containing chromium-nickel steels with nitrogen in order to increase their strength properties. In this work, we investigated the possibility of increasing the strength characteristics of thin-sheet austenitic corrosion-resistant Cr – Ni – Ti (Kh18N12T type) steel, containing 1.5 % and 3 % of titanium, through the use of solid-phase high-temperature nitriding. The nitriding was carried out at a temperature of 1000 – 1100 °С in an atmosphere of pure nitrogen for 5 or 8 hours. The average mass fraction of nitrogen in the samples after nitriding for 5 hours was 0.6 % and 0.7 % for the steels with 1.5 and 3 % of titanium, respectively, and after nitriding for 8 hours – 0.8 % and 0.9 %. It was shown that high-temperature nitriding followed by annealing provides a significant (by 2 – 3 times) increase in the metal strength characteristics compared with the state before nitriding, but reduces the ductility. Ductility of the steel is restored during final processing. For Kh18N12Т type steel with 1.5 % of titanium, an increase in the yield strength is obtained – by 3.3 times (from 180 to 600 MPa), strength – by 1.8 times (from 540 to 970 MPa), with a relative elongation of 28 %. An additional increase in strength properties was not found for the steel with 3 % titanium. The obtained results show the possibility of obtaining thin-sheet titanium-containing high-nitrogen steel (or products from it, for example, thin-walled pipes) by applying solid-phase high-temperature nitriding.

Enhanced Tribological, Electrochemical, and Biocompatibility Properties of Ti6Al4V Alloy Through Gas Nitriding and CN Coating Deposition

The tribological properties, electrochemical corrosion resistance, and biocompatibility performance of Ti6Al4V alloy were improved through a duplex surface treatment process consisting of high-temperature gas nitriding followed by amorphous carbon nitride (CN) coating deposition. The tribological properties of the specimens were evaluated using a reciprocating sliding wear tester, while the electrochemical properties were examined by potentiodynamic polarization tests. Finally, the biocompatibility of the samples was investigated by means of cytotoxicity tests performed using mouse leukemic monocyte macrophage cells (Raw 264.7). The duplex treatment significantly improved the tribological performance of the Ti6Al4V sample. In particular, compared to an untreated Ti6Al4V sample, the duplex surface treatment (CN-NTs) reduced the wear depth by around 55 times when sliding against a 316L counterbody and 16 times when sliding against a Ti6Al4V counterbody. Of the various specimens, the CN-T sample showed the best corrosion resistance, while the CN-NT sample showed the best biocompatibility performance.

Modification of Low-Alloy Steel Surface by High-Temperature Gas Nitriding Plus Tempering

The low-alloy steel was nitrided in a pure NH3 gas atmosphere at 640 ~ 660 °C for 2 h, i.e., high-temperature gas nitriding (HTGN), followed by tempering at 225 °C, which can produce a high property surface coating without brittle compound (white) layer. The steel was also plasma nitriding for comparison. The composition, microstructure and microhardness of the nitrided and tempered specimens were examined, and their tribological behavior investigated. The results showed that the as-gas-nitrided layer consisted of a white layer composed of FeN0.095 phase (nitrided austenite) and a diffusional zone underneath the white layer. After tempering, the white layer was decomposed to a nano-sized (α-Fe + γ′-Fe4N + retained austenite) bainitic microstructure with a high hardness of 1150HV/25 g. Wear test results showed that the wear resistance and wear coefficient yielded by the complex HTGN plus tempering were considerably higher and lower, respectively, than those produced by the conventional plasma nitriding.

Polarization Corrosion Test of Austenitic Stainless Steel 316L after High Temperature Gas Nitriding

Austenitic stainless steel 316L is ones of popular metallic biomaterial for implant due to the good mechanical properties and corrosion resistance. However at corrosion media, highly stressed and oxygen-depleted environment, it still susceptible to corrosion. Various methods have been developed to enhance the corrosion resistance of stainless steel such as high temperature gas nitriding. High temperature gas nitriding for 316L has been successfully done. This process is conducted at temperature 1050, 1100 and 1200oC for 15 and 30 minutes before quenching in the water. Polarization corrosion test showed that the corrosion resistance was significant improved for treatment at temperature 1050oC and holding time 15 minutes.

Evaluation The Effect Of HTGN Treatments On The Corrosion Resistance and Magnetic Properties Of Austenitic Stainless Steel 316L And 316LVM

High temperature gas nitriding (HTGN) is the new methods to enhanced the properties of stainless steel. The HTGN process is able to diffuse the nitrogen atom into stainless steel. Increasing the nitrogen concentration produces higher corrosion resistance. Stainless steel for implant and medical devices such as 316L and 316LVM not only have to high corrosion resistance but also have to magnetic properties stabilities. Evaluation of corrosion and magnetic properties for austenitic stainless steel 316L and 316LVM after HTGN treatments was succesfully done. The corrosion resistance not only significantly increases but the stability of austenite phase is also increases. Therefore HTGN treatments is suitabe for improving the corrosion resistance for 316L and 316LVM which used as implant material.

Development and microstructure characterization of single and duplex nitriding of UNS S31803 duplex stainless steel

In this work, the development of a duplex nitriding (DN) surface treatment combining High Temperature Gas Nitriding (HTGN) and Low Temperature Plasma Nitriding (LTPN) is reported. The microstructure and hardness variation of the duplex treated steel is compared with the properties obtained during single HTGN and single LTPN of UNS S31803 stainless steel. Single LTPN of UNS S31803 was carried out in an Active Screen Plasma Nitriding reactor, at 400°C for 20h, in a 75% N2+25% H2 atmosphere. Single HTGN of UNS S31803 was carried out at 1200°C, under a 0.1MPa high purity N2 gas atmosphere, during 8h. The developed duplex nitriding (DN) surface treatment consists of a combination of both, HTGN and LTPN treatments, carried out in the same conditions described above.The microstructure of the as received material was composed by ferrite and austenitic stringers, aligned in the rolling direction. The results showed that LTPN of the UNS S31803 duplex stainless steel promotes the formation of a duplex modulated structure composed by 2.5μm thick, 1510±52HV hard, expanded ferrite (αN) regions, and 3.0μm thick, 1360±81HV hard, expanded austenite (γN) regions on ferrite and austenite grains, respectively. Intense coherent ε-Fe3N nitride precipitation inside expanded ferrite was observed. ε-Fe3N nitrides precipitated with an orientation relationship [111] αN//[120] ε-Fe3N, leading to increased microhardness of the expanded ferrite regions. After the first step of the duplex nitriding treatment (HTGN) a 550μm thick, 330HV hard, nitrogen rich, fully austenitic layer formed at the surface of the specimens, by transformation of ferrite stringers into austenite. The second nitriding step (LTPN) led to the formation of a homogeneous expanded austenite layer, 1227±78HV on top of the thick fully austenitic layer, formed during the first step. The duplex treatment resulted in a more homogeneous, precipitate-free, microstructure and a better transition between the mechanical properties of the hardened outermost layer and the softer substrate.

Magnetic Properties of Austenitic Stainless Steel 316l and 316lvm after High Temperature Gas Nitriding Treatment

Biometallic materials for implant devices not only have to good corrosion resistance but also stable nonmagnetic properties. Various method have been developed for enhanced the corrosion resistance i.e low temperature gas nitriding treatments. Unfortunatelly, low temperature gas nitriding produce weakly ferromagnetic due the presence of expanded austenitic phases. Another treatments methods which is capable for improvement the mechanical properties is high temperature gas nitriding. However, the evaluation of magnetic properties of austenitic stainless steel 316L and 316LVM not yet investigation. The evaluation of magnetic properties of austenitis stainless steel 316L and 316LVM after high temperature gas nitriding treatments have been succesfully done. The magnetic properties are evaluated by vibrating sample magnetometre (VSM) test. The magnetic properties such as magnetic remenance, magnetic saturation and magnetic permeability are improved. As treated 316L and 316LVM have more stable non-magnetic properties and they more safe and compatible for MRI test

EVALUASI PERLAKUAN NITRIDASI GAS TEMPERATUR TINGGI TERHADAP STRUKTUR MIKRO, KEKERASAN DAN KETAHANAN AUS BAJA TAHAN KARAT AUSTENIT 316LVM

High temperature gas nitriding for austenitic stainless steel 316LVM is successfully done. Specimens are treated at temperature 1323, 1373 and 1573 K for 15 minutes holding time at 0.3 atm nitrogen gas pressures. The effect of nitriding treatments on the hardness, wear resistance and microstructure are evaluated using Buehler microhardness tester, Ogoshi universal wear test machine and light optical microscop respectively. The results shows that hardness, wear resistance and grain size increase with treatments temperature.
 Keywords: high temperature gas nitriding, 316LVM, hardness, wear, grain size

HIGH TEMPERATURE GAS NITRIDING TREATMENT OF AISI 430 USING LOW AND HIGH PURITY NITROGEN GAS

The properties of stainless steels can be improved by high temperature gas nitriding (HTGN) treatment. The improving of their properties are obtained from nitrogen atom which diffuse into stainless steel. Nitrogen gas is the main source of nitrogen atom on the HTGN treatment. Generally, these treatment use high purity of nitrogen gas. The aim of this research is to investigate the effect of nitrogen gas purity on the HTGN treatment for AISI 430. Stainless steel AISI 430 plate 2 mm thick was processed by HTGN treatment. The specimens was exposed at nitrogen gas atmosphere at temperature 1200oC and held for 2 hours prior quenching in water. The treatment used industrial/welding grade (99.5%) as low nitrogen gas purity and ultra high purity (UHP) grade (99.999%) as high nitrogen gas purity. The vickers micro-hardness test was conducted to evaluate the hardness distribution from surface into middle section of the specimens before and after treatment. Light optical microscope was applied to examine the microstructure of specimens after treatment. Metallographic examination shows both treatments using low and high purity gas have the same grain size. However HTGN treatment using low purity of nitrogen gas produces hardness slightly lower than the high purity. This is due the high content of impurity of the low purity gas that reduces the partial pressure of nitrogen gas. Another effect of impurity is the reaction between nitrogen gas and its impurity especially oxygen gas. These reactions reduce the amount of free nitrogen atom which diffuses on the stainless steel.

고온 가스질화 된 STS 430 스테인리스강의 냉간 가공성에 미치는 항온변태 열처리 시간 변화의 영향

This study is to investigate the phase changes and cold workability after isothermal transformation at by using the high temperature gas nitrided (HTGN) STS 430 ferritic stainless steel specimens. The phase diagram of STS 430 steel obtained by calculation showed that the phase appeared at showed as with increasing nitrogen concentration. Also, the transformation of during heat treatment isothermally at , nitrogen pearlite with lamellar type was fully formed at the nitrogen permated surface layer for 10 hrs. However, this transformation was not completed for 1 hr, resulting nitrogen pearlite plus martensite. The cold rolled specimen of isothermally transformed at for 10 hrs after high temperature gas nitriding decreased the layer thickness of nitrogen pearlite inducing the deformation of hard phase. the dissolution rate of phase increased rapidly with increasing cold rolling ratio. Specimens with the microstructure of nitrogen pearlite (isothermally transformed at for 10 hrs) were possible to cold rolling without crack formation. However, the mixed structures of nitrogen pearlite + martensite (isothermally transformed at for 1 hr) were impossible to cold deformation without cracking.

Microstructure and Hardness of High Temperature Gas Nitrided AISI 420 Martensitic Stainless Steel

This study examined the microstructure and hardness of as-received and nitrided AISI 420 martensitic stainless steels. High temperature gas nitriding was employed to treat the steels at 1200°C for one hour and four hours using nitrogen gas, followed by furnace cooled. Chromium nitride and iron nitride were formed and concentrated at the outmost surface area of the steels since this region contained the highest concentration of nitrogen. The grain size enlarged at the interior region of the nitrided steels due to nitriding at temperature above the recrystallization temperature of the steel and followed by slow cooling. The nitrided steels produced higher surface hardness compared to as-received steel due to the presence of nitrogen and the precipitation of nitrides. Harder steel was produced when nitriding at four hours compared to one hour since more nitrogen permeated into the steel.

Effective Duration of Gas Nitriding Process on AISI 316L for the Formation of a Desired Thickness of Surface Nitrided Layer

High temperature gas nitriding performed on AISI 316L at the temperature of 1200°C. The microstructure of treated AISI 316L samples were observed to identify the formation of the microstructure of nitrided surface layer. The grain size of austenite tends to be enlarged when the nitriding time increases, but the austenite single phase structure is maintained even after the long-time solution nitriding. Using microhardness testing, the hardness values drop to the center of the samples. The increase in surface hardness is due to the high nitrogen concentration at or near the surface. At 245HV, the graph of the effective duration of nitriding process was plotted to achieve the maximum depth of nitrogen diffuse under the surface. Using Sigma Plot software best fit lines of the experimental result found and plotted to find out effective duration of nitriding equation as Y=1.9491(1-0.7947 x ), where Y is the thickness of nitrided layer below the surface and X is duration of nitriding process. Based on this equation, the duration of gas nitriding process can be estimated to produce desired thickness of nitrided layer.

Inward diffusion of Al and Ti3Al compound formation in the Ti–6Al–4V alloy during high temperature gas nitriding

Microstructure and composition analysis was performed on the nitrided Ti–6Al–4V alloy by using coupled techniques of X-ray diffraction analysis and glow discharge spectroscopy. Titanium nitride (TiNx) appeared at the outmost surface, but the low solubility of Al in the TiNx phase made Al atom to move inward. Such Al-rich phase as Ti3Al compound was formed by Al accumulation at the boundary between TiNx and α(N)-Ti phase (N diffusion zone). Interestingly, the Ti3Al had very low concentration of N, and it may inhibit N atom from diffusing inward as a diffusion barrier. The flat region of V concentration profile with about 2at% also appeared in the region of α(N)-Ti phase because there was a solubility limit of less than 2at% V in the α-Ti phase (hcp).

Fe-20Mn-12Cr-1Cu 제진합금의 고온가스 질화처리

The microstructural changes of Fe-20Mn-12Cr-1Cu alloy have been studied during high temperature gas nitriding (HTGN) at the range of in an atmosphere of nitrogen gas. The mixed microstructure of austenite and -martensite of as-received alloy was changed to austenite single phase after HTGN treatment at the nitrogen-permeated surface layer, however the interior region that was not affected nitrogen permeation remained the structure of austenite and -martensite. With raising the HTGN treatment temperature, the concentration and permeation depth of nitrogen, which is known as the austenite stabilizing element, were increased. Accordingly, the depth of austenite single phase region was increased. The outmost surface of HTGN treated alloy at appeared Cr nitride. And this was in good agreement with the thermodynamically calculated phase diagram. The grain growth was delayed after HTGN treatment temperature ranges of due to the grain boundary precipitates. For the HTGN treatment temperature of , the fine grain region was shown at the near surface due to the grain boundary precipitates, however, owing to the depletion of grain boundary precipitates, coarse grain was appeared at the depth far from the surface. This depletion may come from the strong affinity between nitrogen and substitutional element of Al and Ti leading the diffusion of these elements from interior to surface. Because of the nitrogen dissolution at the nitrogen-permeated surface layer by HTGN treatment, the surface hardness was increased above 150 Hv compared to the interior region that was consisted with the mixed microstructure of austenite and -martensite.

Isothermal Heat Treatment of AISI 430 Ferritic Stainless Steel after High Temperature Gas Nitriding

Abstract It has been known that the ferritic stainless steel can be changed to martensitic stainless steel whennitrogen is added. However the high hardness of martensitic stainless steel prevents the plastic deformation. Inthis study, instead of martensite, the surface microstructure was changed into nitrogen pearlite to increase theplastic deformation easily by isothermal heat treatment after high temperature gas nitriding (HTGN) the AISI 430ferritic stainless steel. The isothermal treatment was carried out at 780 o C for 4, 6, and 10 hrs, respectively, afterHTGN treatment at 1100 o C for 10 hrs. The surface layer of isothermal-treated steel appeared nitrogen pearlitecomposed with fine chromium nitride and ferrite. Hence, the interior region that was not affected by nitrogen per-meation exhibited ferrite phase. When quenching the isothermal treated steel at 1100 o C, martensitic phaseformed at the surface layer. The hardness of surface layer of isothermal-treated steel and quenched steel mea-sured the value of 150~240 Hv and 630 Hv, respectively.(Received March 27, 2012; Revised April 13, 2012; Accepted April 20, 2012)Key words: AISI430 ferrite stainless steel, HTGN, Isothermal treatment, Nitrogen pearlite

Analysis of Magnetism in High Nitrogen Austenitic Stainless Steel and Its Elimination by High Temperature Gas Nitriding

Stable austenitic structure in medical stainless steels is basically required for surgical implantation. A weak magnetism was found in a high nitrogen nickel-free austenitic stainless steel for cardiovascular stent application. This magnetic behavior in high nitrogen stainless steel was investigated by optical microscopy, X-ray diffraction (XRD), electron probe microanalysis (EPMA) and superconducting quantum interference device (SQUID). The results showed that the magnetism came from the composition segregation of ferrite formation elements such as Cr and Mo in the steel and some delta-ferrites were locally formed during the pressurized electroslag remelting process. The magnetism of high nitrogen stainless steel could be eliminated by a proper high temperature gas nitriding (HTGN).

Critical investigation of high temperature gas nitriding of a PM tool steel

AbstractThe nitrogen uptake of high-alloyed steels, especially tool steels, from gaseous atmospheres at elevated temperatures is a well known effect. Besides, for instance, the nitrogenbased case hardening, one application arises in the field of powder metallurgy. Here, introducing nitrogen containing atmospheres during sintering of high-alloyed powder metallurgically produced tool steels could lead to an optimization of the sintering process and an improvement of the materials properties. In this context, several works deal with the application of computational thermodynamics for considering the nitrogen uptake. The scope of this contribution is to investigate in detail the agreement of calculated and experimentally found nitrogen contents. Therefore, one typical gas-atomized powder of a ledeburitic cold work steel was sintered at a temperature of 1230°C, varying the nitrogen partial pressure during sintering between 0.02 MPa and 0.2 MPa. The measured nitrogen contents are compared with values obtained from equilibrium calculations. Additionally, the distribution of nitrogen in the microstructure is investigated by time-resolved optical emission spectrometry and energy dispersive X-ray spectrometry measurements. The results exhibit good agreement between calculated and measured values for low partial pressures of less than 0.1 MPa and an increasing deviation for higher partial pressures. Furthermore, the transformation of vanadium-containing MC carbides to M(C, N) carbonitrides was verified.