Determination of susceptibility to intergranular corrosion and electrochemical reactivation behaviour of AISI 316L type stainless steel

In this study, double loop electrochemical potentiokinetic reactivation (DLEPR) test was applied to determine the degree of sensitization in 316L type stainless steel, where obtained results were correlated with revealed microstructures after oxalic acid test and weight loss measurements of Streicher and Huey acid tests. Best agreement was provided with test parameters which are 1 M H₂SO₄ and 0.005 M KSCN at 0.833 mV/s scan rate at 30 °C. Specimens were classified structurally as absence of chromium carbides - step, no single grain completely surrounded by carbides - dual and one or more grain completely surrounded by carbides - ditch, in the as-etched structure, if the I_r:I_a (×100) ratios were obtained to be between 0 and 0.2, 0.2 and 5.0 and 5.0 and higher, respectively. It was also found that at high KSCN concentrations, reactivation current profile skewed to higher potentials where this was attributed the formation of metastable pits, during the anodic scan of the test procedure.     

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.     

Corrosion behaviour of a dissimilar joint TIG weld between austenitic AISI 316L and ferritic AISI 444 stainless steels

The AISI 444 stainless steel (SS) has become an option to substitute the AISI 316L SS because of its low cost and satisfactory corrosion resistance. However, the use of AISI 444 alloy tubes in heat exchangers causes the welding of a dissimilar joint. The aim of this study was evaluate the corrosion resistance of the tube-to-tubesheet welded by a TIG process composed of AISI 316L and AISI 444. Preparation of samples was executed through replication of tube-to-tubesheet joints. In order to test the corrosion resistance of the welded joint, the following tests were applied: sensitisation, mass loss from room temperature up to 90 °C and electrochemical corrosion tests in 0.5 mol/L HCl and 0.5 mol/L H₂SO₄ electrolytes. The results have shown that the dissimilar joint suffers galvanic corrosion with increased degradation of the heat-affected zone of the AISI 444 tube. Nevertheless, the mechanisms of localised corrosion (pit and intergranular) were more active in the AISI 316L alloy. It is concluded that the dissimilar joint showed better corrosion resistance than the welded joint composed solely of AISI 316L at temperatures up to 70 °C, as the conditions observed in this work.     

316奥氏体不锈钢低温气体渗碳层组织与强化性能

采用自主研发的低温气体渗碳技术对AISI316奥氏体不锈钢进行处理,目的是增强耐磨性且不损害其耐蚀性。对低温气体渗碳层显微组织、硬度梯度、耐蚀性和耐磨性进行分析。结果表明:低温气体渗碳层硬度梯度变化与其组织和碳浓度有一定关系,随渗碳层深度的不同表现出不同的组织和性能。低温气体渗碳处理前后AISI316奥氏体不锈钢的磨损机制由粘着磨损转变成磨粒磨损,S相是提高耐磨性的主要因素,470℃时表现出较好的耐磨性,其耐蚀性基本保持不变。     

AISI 201奥氏体不锈钢低温离子渗碳

利用低温等离子体辉光放电技术对AISI 201奥氏体不锈钢进行低温离子渗碳(DCPC)处理,处理后的不锈钢表面可以形成一层无碳化铬析出的碳的过饱和固溶体(SC相)。由于渗入钢中的过饱和碳原子引起奥氏体晶格发生畸变,结果使渗层的硬度和耐蚀性都有较大幅度的提高。     

316L奥氏体不锈钢低温气体渗碳后的表面特性

提出了一种新型的奥氏体不锈钢低温气体渗碳工艺,采用该工艺对316L奥氏体不锈钢进行不同时间的渗碳处理,并对渗碳后不锈钢的表面特性进行了分析和探讨。结果表明,渗碳后试样表面晶格发生膨胀,形成了15~35 μm的扩张奥氏体层,该层具有高残余压应力、高硬度、高含碳量及良好的韧性,且层中无碳化铬析出,说明该工艺兼顾了奥氏体不锈钢的表面强化与抗腐蚀性能;同时,渗碳时间对表面的强化效果影响显著,表面残余应力、硬度及碳含量等随渗碳时间延长均增加。     

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.     

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.     

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.     

Corrosion behaviour of 316L stainless steel manufactured by selective laser melting

This work investigates the electrochemical behaviour of an AISI 316L stainless steel produced by selective laser melting (SLM) and compares its behaviour with that of wrought stainless steel with similar chemical composition. The SLM stainless steel specimens are tested in the as‐produced condition without stress relief or recrystallization heat treatments. The electrochemical tests are carried out in two electrolytes: 3.5 wt% NaCl solution with neutral pH and with pH of 1.8. At the macroscale, the microstructure of the SLM specimens is determined by the laser scanning pattern and displays an overlapping network of melt pools. At the microscale, the SLM specimens exhibit a cellular/columnar dendritic structure with submicrometric cell size. Electrochemical measurements highlight a more extended passive range for SLM stainless steel in both neutral and acid electrolytes indicating higher protective properties of the oxide film on SLM specimens. In contrast to the wrought material, the refined microstructure of the SLM specimens promotes a very shallow morphology of attack without deep penetration in the bulk.     

Surface microstructure of 316L austenitic stainless steel by the salt bath nitrocarburizing and post-oxidation process known as QPQ

Systematic materials characterization of the Quench-Polish-Quench complex salt bath heat-treatment process (QPQ) surface modified 316L steel was investigated. The results reveal that the nitro-carburized sample surfaces consist of Cr₂N/γ´-Fe₄N and CrN/γ-Fe, while the post-oxidized sample surfaces are comprised of CrO₃, (Fe₃O₄)/ε-Fe₂(N,C), γ′-Fe₄N and S(γ_N)/CrN/α-Fe. Nuclei ε-Fe₂N_1 − x accumulates at the interface between oxide layer and nitride compound layer by the help of post-oxidation. The diffraction peak lines of S-phase (γ_N) move gradually towards higher diffraction angles as increasing depth of 15 μm to 35 μm. An increased content of oxygen is recorded in the post-oxidized surface layer down to the depth of approximately 15 μm, a small peak concentration of carbon occurs at the front of the nitrided layer. Micro-hardness of the post-oxidized samples reaches about 1300 HV_0.1 near the surface region and then reduces sharply across the case/substrate interface.     

Electrochemical and XPS studies of AISI 316L stainless steel after electropolishing in a magnetic field

The thermodynamic stability and corrosion resistance of surface oxide layer are the most important features of stainless steels. Electrochemical polishing (EP) is the most extensively used surface technology for austenitic stainless steels. We have modified this surface technology by introducing a magnetic field to the system. With this new process called the magnetoelectropolishing (MEP) we can improve metal surface properties by making the stainless steel more resistant to halides encountered in a variety of environments.In this paper, the corrosion research results are presented on the behaviour of the most commonly used material - medical grade AISI 316L stainless steel. The corrosion investigations have been concerned on the open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and polarisation curves studies in the Ringer’s body fluid under room temperature (25 °C). The X-ray photoelectron spectroscopy (XPS) was performed on 316L samples after three treatments: MP - abrasive polishing (800 grit size), EP - conventional electrolytic polishing, and MEP - magnetoelectropolishing. The comparison of the corrosion behaviour of the stainless steel’s surface after these processes was also carried out. The purpose of XPS studies was to reveal the surface film composition and the reason of this modified corrosion behaviour. It has been found that the proposed MEP process modifies considerably the composition of the surface film and improves the corrosion resistance of the same 316L SS studied.     

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.     

Improvement of pitting corrosion resistance of AISI 444 stainless steel to make it a possible substitute for AISI 304L and 316L in hot natural waters

The pitting corrosion resistance of AISI 444, 304L and 316L stainless steels in two tap waters with different chloride concentrations at 80 °C was studied. Cyclic potentiodynamic polarization (CPP) tests were carried out starting from E_corr ? 30 mV until the current density reached 0.1 mA/cm² (scan rate 0.166 mV/s); the scan was then reversed and continued until new passivity conditions were achieved. The corrosion potential was measured before the polarization experiments. From the E‐log i plots, the values of pitting and protection potential were obtained; from these potentials, the perfect and the imperfect passivity regions were defined to compare the corrosion resistance of the studied steels. CPP tests were performed both on as received stainless steel samples and on samples submitted to different cleaning–passivation treatments to improve their corrosion resistance. The results indicate that, for industrial production, AISI 444 stainless steel can substitute the more expensive AISI 304L or 316L after a cleaning–passivation treatment that reduces the presence of inclusions.

     

Oxidation behavior of sintered 316L austenitic stainless steel-yttria composites with various additions

The oxidation behavior of sintered 316L austenitic stainless steel-Y₂O₃ composites and the effect of various additions, such as copper (up to 3 wt.%), bronze (up to 3 wt.%), phosphorus (up to 2 wt.%), and silicon (up to 5 wt.%) have been studied at 550°C inflowing oxygen at one atmosphere pressure for a maximum period of 9 hr. Among all the alloys studied, 316L-1% P showed excellent oxidation resistance. Under the general conditions, Cr₂O₃ is always the preferred protective scale. In general, Y₂O₃-containing composites showed greater oxidation as compared to 316L with other additions.     

Strength and ductility of 316L austenitic stainless steel strengthened by nano-scale twin bundles

By means of dynamic plastic deformation (DPD) with high strain rates, a bulk nanostructured 316L austenitic stainless steel consisting of nano-sized grains embedded with bundles of nanometer-thick deformation twins was synthesized. The average transverse grain size is ∼33 nm and the twin/matrix lamellar thickness is ∼20 nm. The nano-twin bundles constitute ∼24% in volume. The nanostructured samples exhibit a high tensile strength of ∼1400 MPa but a limited ductility with a uniform elongation of ∼2%. Subsequent thermal annealing of the as-DPD samples in a temperature range of 730-800 °C led to a single-phased austenite structure consisting of static recrystallized (SRX) micro-sized grains embedded with remaining nano-twin bundles and nano-grains. The annealed DPD samples exhibit an enhanced strength-ductility synergy and much more enhanced work-hardening rates than the as-deformed samples. Work-hardening rates of the annealed DPD samples can be even higher than that of the original CG sample. Tensile ductility was found to increase almost linearly with the volume fraction of SRX grains. A combination of 1.0 GPa tensile strength with an elongation-to-failure of ∼27% is achieved in the annealed DPD 316L stainless steel samples.     

Influence of plasma nitriding on plain fatigue and fretting fatigue behaviour of AISI 304 austenitic stainless steel

AISI 304 austenitic stainless steel samples were plasma nitrided at 420 °C for 6 h in vacuum atmosphere by glow discharge technique, in the presence of nitrogen gas. Plain fatigue and fretting fatigue tests were carried out on unnitrided and plasma nitrided samples. Plasma nitrided samples exhibited higher surface hardness, compressive residual stresses at the surface and lower surface roughness compared with unnitrided samples. However, plasma nitrided samples exhibited inferior plain fatigue and fretting fatigue lives compared with unnitrided samples. This was attributed to segregation of chromium at the grain boundaries of plasma nitrided specimens which might have weakened the regions near grain boundaries resulting in early crack initiation and accelerated crack propagation.     

固溶温度对316LN奥氏体不锈钢微观组织和高温力学性能的影响

对316LN奥氏体不锈钢进行了不同温度(1020、1050和1070℃)的固溶处理,利用电子万能试验机对316LN奥氏体不锈钢在300℃的高温环境下进行单轴拉伸试验,采用光学显微镜(OM)、透射电镜(TEM)和扫描电镜(SEM)对其微观组织和高温力学性能进行分析表征.结果 表明:随着固溶温度的升高,316LN奥氏体不锈...     

Rainbow fringes around crevice corrosion formed on stainless steel AISI 316 after ennoblement in seawater

The crevice corrosion occurrence probability of stainless steel (SS) AISI 316 was increased under ennoblement condition due to chemically added H₂O₂ into seawater. The H₂O₂ was used to simulate the important factor causing ennoblement in natural marine biofilm. Morphology of the crevice corrosion was observed using an incident‐light source microscopy. Some interesting “rainbow” fringes were observed around micro‐crevices. The mechanism was discussed from the ions diffusion and potential distribution during the crevice formation. This result shows that under ennoblement condition the colored fringe is a distinct characteristic of the morphology of localized corrosion for stainless steel.