Artificial neural networks for pitting potential prediction of resistance spot welding joints of AISI 304 austenitic stainless steel

The aim of this work is to predict the influence of welding process on pitting corrosion behaviour (PCB) of resistance spot welding (RSW) joints of AISI 304 austenitic stainless steel (ASS). PCB is estimated from the pitting potential (E_pitt), which is predicted from three RSW parameters. The prediction is carried out by artificial neural networks (ANNs) because the phenomena that relate the heat generated in RSW process to the PCB are highly complex and nonlinear. The study of PCB of RSW joints of ASS by ANNs is a novel research field in which this work makes a significant contribution.     

The effects of pH on the repassivation of 304L stainless steel

The rate of anodic film growth on freshly generated 304L stainless steel surfaces is presented as a function of electrode potential and pH. For all pH values between 0 and 14 the kinetics are controlled by ion migration under high electric field, with two distinct sequential kinetic regions. The high field parameters are presented, and show that in the early stages the film grows with a lower activation energy than in the later stages.     

Acoustic emission during pitting corrosion of 304 stainless steel

Acoustic emission (AE) during pitting corrosion of 304 stainless steel (304 SS) in H₂SO₄ solutions with different pH values and Cl⁻ concentrations was studied. Two types of AE signals are detected in all solutions. Each type of signals is characterized by AE parameters (rise time, counts number, duration and amplitude) and waveform carefully. It is believed that the hydrogen bubbles evolution inside the pits is the AE source.     

The pitting resistance of AISI 316 stainless steel passivated in diluted nitric acid

The pitting resistance of AISI 316 stainless steel after passivation in diluted nitric acid was studied in comparison with that of non-passivated specimens. The passivation treatment increased the pitting potential but decreased the resistance to crevice corrosion under open circuit conditions in aerated sea water. Immersion in the nitric acid solution was found to remove the sulphide inclusions from the metal surface, thus eliminating the most susceptible sites for attack. In the absence of sulphide particles pitting nucleated at aluminium-rich oxides.     

Phase formation during solidification of AISI 304 austenitic stainless steel

Following casting of AISI 304 strip, both cellular and skeletal ferrite were observed within individual austenite grains. It is proposed that solidification commences with primary austenite, γ_p, to produce cellular ferrite and further solidification results in the formation of ferrite but its subsequent epitaxial transformation to austenite at the γ_p interface produces skeletal ferrite in addition to cellular ferrite.     

Corrosion and wear of 304 stainless steel using a scratch test

A scratch test developed by the U.S. Bureau of Mines was used to wear the surface of 304 stainless steel. Corrosion of the freshly exposed surface was measured by the charge density consumed in reforming the passive film. Wear was measured by integrating the area between the profile of the scratch and the original surface. Neither scratch hardness (resistance to plastic deformation) nor wear were affected by the presence or absence of the passive film. For rubbing wear, a mode where the surface is plastically deformed and no debris is formed, the passive film remained adherent to the surface.     

Fatigue life improvements of the AISI 304 stainless steel ground surfaces by wire brushing

The surface and subsurface integrity of metallic ground components is usually characterized by an induced tensile residual stress, which has a detrimental effect on the fatigue life of these components. In particular, it tends to accelerate the initiation and growth of the fatigue cracks. In this investigation, to deliberately generate compressive residual stresses into the ground surfaces of the AISI 304 stainless steel (SS), wire brushing was applied. It was found that under the experimental conditions selected in this investigation, while the surface roughness was slightly improved by the brushing process, the surface residual stress shifted from a tensile stress (σ_‖=+450 MPa) to a compressive stress (σ_‖=−435 MPa). On the other hand, the work-hardened deformation layer was almost two times deeper after wire brushing. Concerning the fatigue life, an improvement of 26% in terms of endurance limit at 2×10⁶ cycles was realized. Scanning electron microscope (SEM) observations of the fatigue fracture location and size were carried out to explain the fatigue life improvement. It was found that the enhancement of the fatigue strength could be correlated with the distribution and location of the fatigue fracture nucleation sites. Concerning the ground surfaces, it was seen that the fatigue cracks initiated at the bottom of the grinding grooves and were particularly long (150–200 μm). However, the fatigue cracks at the brushed surfaces were shorter (20–40 μm) and appeared to initiate sideways to the plowed material caused by the wire brushing. The results of the wire-brushed surface characterization have shown that significant advantages can be realized regarding surface integrity by the application of this low-cost process compared to shot peening.     

Inhibition of pitting corrosion of AISI 304 by organic compounds

The influence of N-lauroylsarcosine, laurylaminedicarboxylic acid, Na-dodecylbenzene sulphonate and Na-laurylsulphate on the pitting corrosion of AISI 304 stainless steel has been studied in 0.1 N NaCl solutions, buffered in the pH range 2.2 to 7.5, at 50°C. The steel pitting potentials have been determined by potentiostatic measurements and by a scratching electrode technique. The potential value at which active pits could be repassivated has been identified. The pitting incubation times have been determined on the steel electrodes prepassivated for different times at 0.5 V/Ag/AgCl in buffer solutions in the absence or in the presence of inhibitors. The overall results confirmed the inhibiting effect of the mentioned substances against the intiation of pitting, just emphasized by the means of potentiodynamic tests. On the contrary, the examined compounds do not show any effect on the pitting propagation. Some more information on the mechanism of action of the mentioned inhibitors are obtained from the incubation time values: in order to explicate their action they must either find sufficiently stable oxide film or can contribute to the stabilization of the passive film during its formation.     

Influence of peening on the corrosion properties of AISI 304 stainless steel

The effects of peening treatment on the microstructure and corrosion behavior of AISI 304SS were investigated. Shot and ultrasonic peening were performed on the austenitic stainless steel, and peened specimens were compared in terms of microstructure, surface roughness and corrosion resistance. Nano-sized grains, multi-directional mechanical twins and strain-induced martensite were formed on the surfaces, and the volume fraction of strain-induced martensite in the ultrasonically peened specimen was higher than that of the shot-peened specimen. The ultrasonically peened specimen which had smoother surface and contained more strain-induced martensite showed superior general and localized corrosion resistance to the as-received and shot-peened specimens.     

Magnetic characterization of cold rolled and aged AISI 304 stainless steel

Magnetic easy axis predicted by the orientation distribution of the maximum amplitude of magnetic Barkhausen emission (MBE), which is obtained by magnetization in radial directions from the center of the specimens has been applied to determine the magnetic anisotropy on cold rolled and aged 304 SS in two sets of specimen. The maximum of the MBE has been found to orient along the rolling direction (RD) compared to the transverse direction (TD), indicating the presence of magnetic easy axis along the rolling direction for both sets. The strain induced martensite phase transformation has been determined using X-ray diffraction technique. The orientation distribution function (ODF) analysis has been carried out to obtain the crystallographic texture with cold rolling. ODF analysis revealed the 110 texture as the major. The magnetic anisotropy factor has also been determined with cold deformation and noticed that the strength of magnetic anisotropy decreases above 50% deformation for both the sets. Results have been explained considering two competitive effects, formation of crystallographic texture in the martensite phase and presence of compressive residual stresses along RD during cold rolling.     

304不锈钢低温离子渗氮和氮碳共渗工艺

在430 ℃对AISI304奥氏体不锈钢分别进行离子渗氮(PN)、离子氮碳共渗(PNC)和离子氮碳共渗加离子渗氮(PNC+PN)处理.利用金相显微镜、辉光放电光谱仪、X射线衍射仪和显微硬度计测试了试样渗层的横断面形貌、渗层成分、相组成和力学性能.结果表明,AISI304奥氏体不锈钢在430 ℃进行硬化处理时,相对于PN处理,经PNC和PNC+PN处理可以获得更高硬度、更厚渗层,但表面耐腐蚀性下降,3种处理得到的渗层中C和N的最大含量分别出现在不同深度.     

The characteristics of the pitting corrosion of stainless steel above breakdown potential

The pitting corrosion of AISI 304 stainless steel is studied in 1 N H₂SO₄|;0.5 N NaCl at 25°C. With the potentiokinetic method, a linear relationship is obtained between the breakdown potential and the cubic root of the scanning rate. Extrapolating to zero scanning rate gives the steady state breakdown potential E_b = 596 mV(SHE). At constant potential above E_b, the pits density first increases with time and tends to a steady state value (S.S.D.P.) which is reached above 10 mA cm⁻². The average deepening rate of pits V_dp increases with E − E_b according to log V_dp = −7.5 + 2.89 log (E − E_b). The time dependance of current density J at constant potential above E_b follows the equation log J = a + b log t. The values of b are different below and above 10 mA cm⁻², confirming the change in mechanism of pit corrosion: no new pit is initiated above that value. It is suggested that it is possible to characterize the quality of passivating layers not only on the basis of E_b, but also in terms of S.S.D.P., V_dp and b at given values of E − E_b.     

低温冷却对AISI 304不锈钢铣削加工的影响

研究低温冷却对AISI304不锈钢铣削加工的影响。用内径为1mm的管子来喷淋液氮到铣刀、削口和材料界面起到低温冷却的作用。液氮的流量为5.2L/min;采用顺铣和逆铣2个铣削方向、干铣和低温冷却2种方式和80、120、160和200m/min 4种铣削速度。结果表明：低温冷却和铣削速度对切削力有影响;低温冷却过程中的切削力和扭矩比干铣过程中的高;切削力随着铣削速度的加快而增大。进入铣刀半径范围内的削屑影响低速顺铣的效果。     

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.     

Nitrogen interstitial diffusion induced decomposition in AISI 304L austenitic stainless steel

The nature of the near-surface γ_N phase produced by low-temperature (～400 °C) plasma-assisted nitriding of an austenitic stainless steel 304L is studied. A combination of global probes (X-ray diffraction, nuclear reaction analysis, glow discharge optical emission spectroscopy) and local probes (field ion microscopy, conversion electron Mössbauer, X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies) is employed to reveal the morphology, phase structure, atomic ordering and chemical state of the obtained γ_N phase. The results consistently reveal the heterogeneous nature of the nitrided layer consisting of nanometric CrN precipitates embedded in a Fe₄N-like matrix. The size of the precipitates is found to be larger at the surface than at the nitrided layer–steel interface. The precipitates have irregular, sphere-like shapes. Moreover, X-ray spectroscopic investigation revealed three different intermetallic distances and different chemical environments for Fe, Cr and Ni, accompanied by a large static disorder. These findings suggest that the presence of the interstitial N destabilizes the homogeneous element distribution in 304L even at such low temperatures. This leads to the segregation into Cr-rich zones that are coherent with the Fe₄N matrix. Possible atomistic decomposition mechanisms are discussed. Based on the heterogeneous nature of the γ_N phase revealed in 304L, an alternative view of its remarkable combination of properties such as large hardness, induced ferromagnetism and preserved corrosion resistance is considered.     

304不锈钢稳态点蚀生长动力学及孔内环境测定

利用电感耦合等离子体发射光谱法(ICP-OES)及三维显微镜测定了304奥氏体不锈钢在不同极化电位下生成的单个三维稳态蚀孔内部的腐蚀产物浓度,同时,基于扩散控制理论,根据菲克第一定律计算了蚀孔内部腐蚀产物浓度的理论值,并观测和计算了蚀孔形貌及孔内点蚀电流密度,分析了蚀孔生长速度与孔内腐蚀产物浓度、极化电位、蚀孔几何形貌...     

The effect of specimen size on the measured pitting potential of stainless steel

The pitting potential of stainless steel measured in acidic chloride solutions using conventional potential sweeping is a function of the size of the test specimen. The results presented below, measured as an average of many repeat experiments, show that the pitting potential falls with increase in surface area of the specimen.     

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.

     

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.     

AISI 304 stainless steel marking by a Q-switched diode pumped Nd:YAG laser

Laser marking tests were carried out on AISI 304 steel, using a Q-switched diode pumped Nd:YAG laser. The aim was to determine the correlation occurring between working parameters (i.e. pulse frequency, beam scanning speed, and current intensity) and resulting mark visibility. The latter was evaluated as the contrast index measured from digital images of the marks. To characterize mark features, its width and roughness were estimated, and analyses employing optical and scanning electron microscopy coupled with energy dispersive X-ray technique were carried out.