Influence of plasma nitriding on hydrogen environment embrittlement of 1.4301 austenitic stainless steel

The aim of this work was to study if hydrogen environment embrittlement of DIN 1.4301 austenitic stainless steel can be suppressed by a nitrided surface. DIN 1.4301 was plasma nitrided in a N₂/H₂ discharge. Nitriding produced 3-layered structure consisting of a γ_N top layer, an intermediate γ/γ_C-layer and a diffusion layer. It is assumed that the γ_C phase was formed due to the decomposition of CO originating from the reactor walls and the subsequent incorporation of C into the material. The γ_C phase is characterized by distinct XRD peaks and carbon contents between 0.5 and 4 wt.% as well as nitrogen contents between 0.5 and 8 wt.%. Plastic deformation of the plasma nitrided specimen showed cracks and some delamination of the γ_N layer, whereas the γ/γ_C-layer behaved in a very ductile manner. Even at a plastic deformation of 35% no cracks or any other damage was visible. A tensile test in gaseous hydrogen showed severe embrittlement of the unnitrided steel and the nitrided steel with a γ_N layer. No cracks were observed in areas where just the γ/γ_C-layer was present.     

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.     

On the formation of expanded austenite during plasma nitriding of an AISI 316L austenitic stainless steel

Plasma nitriding of an AISI 316L austenitic stainless steel at low (400 °C) and high temperatures (550 °C) was performed under different nitriding gas mixtures. Nitrided surfaces were characterized by XRD using the Rietveld method. Expanded austenite “γ_N” with a special triclinic (t) crystalline structure was formed during the low-temperature nitriding treatment. Minor volume fractions of Fe₃N, Fe₄N and Cr₂N nitrides were also found. The expanded austenite phase showed a distortion ε of the lattice angles due to a very high nitrogen content dissolved in austenite, supersaturating the solid solution and leading to a 10% lattice distortion and to high compressive residual stresses at the surface.After nitriding the specimens at 550 °C the case was composed primarily by a high volume fraction of Fe₄N, Cr₂N and CrN nitrides, leading to a low distortion of the parent austenitic phase, maintaining the original cubic lattice.     

温度对AISI304奥氏体不锈钢离子渗氮的影响

对AISI304奥氏体不锈钢进行脉冲电流辉光离子渗氮处理,在不同处理温度(480 ℃、520 ℃、580 ℃)下渗氮8 h后,获得了一定厚度的渗氮层.通过对渗层进行金相分析和硬度测试表明,随着渗氮温度升高,渗层厚度增大,显微硬度先增大后减小.综合温度对渗层厚度与显微硬度的影响,AISI304奥氏体不锈钢卡套辉光离子渗氮温度可采用520 ℃,渗氮后渗层厚度为90 μm,显微硬度为1317 HV0.1.     

Modeling of nitrogen penetration in polycrystalline AISI 316L austenitic stainless steel during plasma nitriding

The nitrogen depth profile in polycrystalline AISI 316L austenitic stainless steel after plasma nitriding at temperatures around 400 °C is analyzed by the “trapping-detrapping” model. This model considers the diffusion of nitrogen under the influence of trap sites formed by local chromium atoms. Nitrogen depth profiles in polycrystalline AISI 316L steel simulated on the basis of this model are in good agreement with experimental nitrogen profiles. The enhanced nitrogen diffusivity as well as a plateau-type shape of nitrogen depth profile can be explained. The nitrogen diffusion coefficient at 400 °C is found to be D = 4.81 × 10⁻¹² cm²/s and the diffusion pre-exponential factor D₀ (0.837 × 10⁻³ cm²/s) and detrapping activation energy E_B (0.28 eV) were deduced from fitting experimental data. It is known that the nitrogen penetration depth (and nitrogen diffusivity) depends on the crystalline orientation and a tentative to take into account this anisotropy effect and describe nitrogen depth profiles in polycrystalline AISI 316L steel is proposed by using different diffusion coefficients characteristic for each crystallite orientation.     

Duplex surface treatment of an AISI 316L stainless steel; microstructure and tribological behaviour

Austenitic stainless steel AISI 316L is used in several industrial applications, mainly due to its excellent corrosion resistance; however, its low hardness and poor wear performance impose strong limitations in many cases. A combination of DC-pulsed plasma nitriding and plasma assisted PVD coating as a surface treatment has been shown to improve the material fatigue and wear resistance without affecting the corrosion performance. In the present work a duplex treatment, consisting of a plasma nitriding at 673 K for 20 h and a subsequent coating with a TiN layer was applied to an AISI 316L steel. The microstructure obtained as well as the tribological behaviour was extensively studied. Wear tests were performed in rolling-sliding condition under different loads (490, 1225 and 1960 N). Different wear mechanisms were observed depending on the normal applied load. Analysis and discussion of the wear test results showed that the combination of the two processes, plasma nitriding and plasma assisted PVD coating, improves considerably the wear resistance of the AISI 316L. At low applied loads, the duplex treatment improved significantly the wear resistance during the sliding/rolling contact, i.e. only abrasion was observed. However, upon increasing the applied loads fatigue and delamination wear mechanism appeared. In the case of the highest applied load, delamination was the main wear mechanism observed in the tested samples.     

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.     

304奥氏体不锈钢低温盐浴渗氮处理

采用430℃低温盐浴对304奥氏体不锈钢进行渗氮处理,研究了渗氮时间对渗氮层组织和性能的影响。利用XRD衍射仪、光学显微镜、表面显微硬度计和带能谱仪(EDS)的扫描电镜(SEM)分别分析渗氮层的相组成、厚度、表面硬度和显微组织。结果表明:304奥氏体不锈钢在430℃渗氮不同时间后,渗氮层厚度和表面硬度都随着时间的延长而增加。渗氮时间为1 h时,渗氮层仅为单一的S相,随着渗氮时间的增加,渗氮8 h时开始有少量CrN生成,渗氮16 h时,渗氮层由大量CrN+S相两相混合。用电化学极化的方法评价耐蚀性能的结果表明:盐浴渗氮处理后耐Cl-点蚀性能得到了一定的改善,在430℃渗氮4 h,其耐蚀性能是最好的,优于没经过渗氮的试样,而在所有的渗氮试样中,渗氮8 h、16 h的试样耐点蚀性能较差。     

离子溅射在奥氏体不锈钢离子渗氮中的应用

利用自制的直流脉冲离子渗氮设备采用加强离子溅射预处理方法对奥氏体不锈钢进行了离子渗氮,并与普通的离子渗氮方法进行对比.结果表明,通过加强溅射的方法得到的试样表面硬度在1 200 HV0.5以上,耐磨性能提高了4～5倍,硬度梯度变得更为平缓.     

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

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

Intensified plasma-assisted nitriding of AISI 316L stainless steel

In the present study, processing of AISI 316L stainless steel (316ss) has been conducted by intensified plasma-assisted processing (IPAP). The processing parameters (bias voltage, current density, chamber pressure and substrate temperature) of IPAP have been varied in an effort to determine which conditions lead to the formation of a single-phase structure, ‘m’ phase, and evaluate the properties of this phase. The structural characteristics of the nitrided layers produced by IPAP have been investigated by X-ray diffraction analysis. Nanoindentation experiments have been performed over cross-section to determine hardness and elastic modulus profiles. Dry sliding wear and potentiodynamic aqueous corrosion experiments have been conducted to characterize 316ss nitrided by IPAP. IPAP has been successful in producing single-phase m with high hardness and in shorter processing time compared to diode plasma nitriding. The IPAP produced single-phase nitrided layer was found to possess higher hardness (fourfold increase over the unprocessed alloy), excellent wear and corrosion resistance.     

Surface nanocrystallization by surface mechanical attrition treatment and its effect on structure and properties of plasma nitrided AISI 321 stainless steel

A plastic deformation surface layer with nanocrystalline grains was produced on AISI 321 austenitic stainless steel by means of surface mechanical attrition treatment (SMAT). Low-temperature nitriding of SMAT and un-SMAT AISI 321 stainless steel was carried out in pulsed-DC glow discharge. The effect of SMAT pretreatment on the microstructure and properties of the stainless steel were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Vickers hardness tester and UMT-2MT tribometer. The results show that the plasma nitriding of AISI 321 steel can be enhanced considerably by means of SMAT process before nitriding, and a much thicker nitrogen diffusion layer with higher hardness was obtained for the SMAT samples when compared with un-SMAT samples. In addition, the wear resistance and load capacity of the nitrided layers on the SMAT samples was much higher than that of the un-SMAT samples due to the thicker S phase case and the gradient nitrogen diffusion layer.     

Effect of heating post-treatment on nitrided stainless steel

Although plasma nitriding has been applied successfully to increase the hardness of austenitic stainless steels, the process cycles are long due to the low nitrogen diffusion rate for these steels. An alternative to reduce the nitriding time is to perform a heating treatment after nitriding to prolong the diffusion process. In this work we investigate the properties of plasma nitrided AISI 316 stainless steel after heating post-treatments. The samples were nitrided at 823 K during 3 h. After nitriding, heating post-treatments were performed in a vacuum furnace. The influence of the heating time, ranging from 1 up to 16 h, and heating temperatures, varying from 732 up to 873 K, on the surface properties was investigated. The samples were characterized using microhardness testing, scanning electron microscopy and X-ray diffraction. The nitriding treatment results in a compound layer 44 μm thick with a hardness of 1434 HV_0.1, consisting predominantly of γ'-[Fe₄N] and CrN phases. As expected, an increase of the compound layer thickness and a decrease of the surface hardness with heating time were observed. However, the microhardness profiles show that beneath the surface the layer hardness increases for long treatment times. New phases as Fe₃O₄ and FeCr₂O₄ appear and grow with increasing heating time.     

Delayed cracking in plasma nitriding of AISI 420 stainless steel

The phenomenon of delayed cracking in nitrided layers after DC-plasma nitriding of AISI 420 steel has been observed by optical microscopy. Prior to the plasma treatment, the samples were austenitized at 1303 K for 30 min and then oil quenched. Two tempering conditions were assessed: one group was tempered at 673 K, while another group was tempered at 943 K.All samples were subjected to sputtering, in the plasma chamber, to remove the passive oxide layer, under a 1:1 Ar/H₂ atmosphere. Finally, specimens were plasma nitrided at 673 K for 20 h, with a 1/3: N₂/H₂ relation, at a pressure of 6.5 hPa and 700 V bias in the nitriding chamber.The nitrided layers were analyzed initially by X-ray diffraction (XRD). Detailed observations were conducted at frequent and regular intervals under optical microscopy (OM) and scanning electron microscopy (SEM) with secondary and back-scattered electrons detectors. The results revealed that after an incubation time, even without any external disturbance, cracks are formed and propagate in the nitrided layers. Both groups of samples were equally affected. The presence of precipitated particles and local residual stresses are possible causes of such a phenomenon.     

Coatings to reduce hydrogen environment embrittlement of 304 austenitic stainless steel

Electroplated Zn, Ni, Cu, Al, PVD-Ti-DLC and electroless NiP coatings as well as carbon, nitrogen and oxygen diffusion layers were investigated for their suitability to reduce hydrogen environment embrittlement (HEE) of 304 austenitic stainless steel. The mechanical stability of the coatings was evaluated by interrupted slow strain rate tensile testing. The Zn, Ni, Ti-DLC and NiP coatings as well as the oxygen diffusion layer cracked or delaminated at very low strains. The Cu coating was too thin and showed poorly coated areas. HEE of the underlying bulk material was not improved by any of these coatings because embrittlement always started at such coating imperfections. Al coatings showed a high ductility but could not reduce HEE of the underlying material due to a columnar structure at which the H₂ gas could get in direct contact to the substrate material. The carbon and nitrogen diffusion layers could not eliminate HEE of the 304 steel entirely but crack propagation was reduced in these layers.     

Effect of DC plasma nitriding temperature on microstructure and dry-sliding wear properties of 316L stainless steel

A wear resistant nitrided layer was formed on 316L austenitic stainless steel substrate by DC plasma nitriding (DCPN). The structural phases, micro-hardness and dry-sliding wear behavior of the nitrided layer were investigated by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS), micro-hardness tester and ring-on-block wear tester. The results show that a single expanded austenite layer (S-phase) and a single CrN nitride layer were formed at 400 °C and 480 °C, respectively. In addition, the S-phase layers formed on the samples exhibited the best dry-sliding wear resistance under ring-on-block contact configuration test. Wear of the untreated 316L was sever and characterized by strong adhesion, abrasion and oxidation mechanism, whilst wear of the DCPN-treated 316L was mild and dominated by plastic deformation, slight abrasion and frictional polishing.     

Fatigue behaviour of low temperature carburised AISI 316L austenitic stainless steel

Low temperature carburising (LTC) was applied to AISI316L austenitic stainless steel and its effect on microstructure and fatigue behaviour was investigated. LTC treatment enhances surface hardness and wear resistance of the steel without reducing its corrosion resistance. Surface hardness up to 1150 Vickers was achieved in the carburised layer, thanks to the formation of the so-called “S-phase”, a carbon-supersaturated austenite phase. The XRD evaluation of treated material verified expanded austenite with no evidence of carbide precipitation. Rotating bending fatigue tests showed that the low temperature carburising treatment enhances the fatigue strength of the 316L steel by 40% with respect to the untreated material due to the high residual stresses present in the treated layer. A major temperature increase was found testing the LTC specimens, with a peak value at the end of the test up to 600 °C. By air cooling the LTC specimens during the tests, a further increase of fatigue strength up to 70% was achieved with respect to the untreated material. Fatigue cracks in the surface-treated specimens always nucleated near the boundary between the carburised case and the core.     

离子渗氮对AISI304L不锈钢耐磨损性能的影响

对AISI 304L不锈钢在450和500 ℃离子渗氮处理进行了研究,分析了渗氮处理的析出相、硬度、厚度、冲蚀磨损和高压釜中微动磨损性能的变化。详细分析了渗氮试样在垂直粒子冲蚀下的耐冲蚀磨损性能,并探讨了其冲蚀磨损机制。结果表明：渗氮层有γ_N和CrN相析出;450和500 ℃下的渗层厚度分别为2和20 µm;渗氮层的硬度都高于1000 HK。渗氮试样在高压釜中的耐微动磨损性能有大幅度的提高,尤其是450 ℃离子渗氮处理的试样,比未处理试样提高了29.4倍。     

Corrosion behavior of martensitic and precipitation hardening stainless steels treated by plasma nitriding

Plasma nitriding is a well established technology to improve wear and corrosion properties of austenitic stainless steels. Nevertheless, in the case of martensitic stainless steels, it continues being a problem mainly from the corrosion resistance viewpoint.In this work, three high chromium stainless steels (M340, N695 and Corrax) were hardened by ion nitriding at low temperature, intending to preserve their corrosion resistance.Corrosion behavior was evaluated by CuSO₄ spot, salt spray fog and potentiodynamic polarization in NaCl solution. Microstructure was analyzed by optical microscopy, SEM (EDS) and glancing angle X-ray diffraction. All the samples showed an acceptable corrosion resistance in experiments with CuSO₄, but in salt spray fog and electrochemical tests, only Corrax showed good behavior. The poor corrosion performance could be explained by chromium carbides formed in thermal treatment stage in martensitic steels and chromium nitrides formed during nitriding, even though the process was carried out at low temperature.     

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

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