Nitriding in cathodic cage of stainless steel AISI 316: Influence of sample position

Cathodic cage plasma nitriding is a new growth technique based on multiple hollow cathode effects. The samples are kept at a floating potential inside a cage that acts as a cathode and shields the samples from the cathodic potential. The aim of this work is to perform a systematic study of the properties of nitrided layers as a function of the distance from the nitriding sample surface to the cage wall using this technique. Cylindrical austenitic stainless steel AISI 316 samples were placed in different positions on an alumina plate inside the cathodic cage. The nitrided samples were characterized by optical microscopy, X-ray diffraction and microhardness measurements. The results show that the temperature inside the cage is nearly uniform and that the nitrided layers possess good physical properties and uniformity. Therefore, the differences in the nitrided layer thicknesses obtained at different positions must be due to particle flow reaching the sample surface, depending on how far the sample is from the cage walls.     

Effect of a nanoparticle-filled lubricant in turning of AISI 316L stainless steel (SS)

Release of heat and generation of friction associated with machining operation ever posture a problem which not only reduce the tool life but also impair the quality of the product. Nano cutting fluids play a significant role in machining operations and impact tool life and quality of work. In the present work, tool flank wear is analyzed during turning AISI 316L Stainless steel (SS) under a nano cutting environment. Experiments are conducted by turning of AISI 316L SS under wet machining with and without multiwalled carbon nanotube (MWCNT) inclusions in the conventional lubricant. The second order quadratic models were developed to predict tool wear using response surface methodology (RSM) based D-optimal design. Machining parameters such as speed, feed rate, and depth of cut are chosen as numerical factors and the type of lubricant is considered as the categorical factor. The results show that the influence of the feed rate is more significant while machining the AISI 316L SS with a whisker reinforced ceramic insert. The addition of MWCNTs in SAE20W40 enhances the tool performance with their enhanced penetration. After turning experiment, a scanning electron microscope (SEM) with energy dispersive X-ray (EDS) was used to investigate the tool wear.     

Experimental analysis of magneto rheological abrasive flow finishing process on AISI stainless steel 316L

In this experimental study, the surface quality of AISI stainless steel 316L was improved to a nano-level surface finish by means of magneto rheological abrasive flow finishing process. In order to determine the effect of input process parameters toward the responses such as final surface roughness (SR) and material removal rate (MRR), response surface model was built up and optimal parameters were found using the desirability analysis. Based on the experimental design, 20 experiments were conducted and the minimum SR and maximum MRR obtained are 53.46?nm and 1.757?mg/s, respectively, and their optimized values are 53.10?nm and 1.817?mg/s. By using the regression equations obtained for SR and MRR as input, an evolutionary optimization algorithm called as firefly algorithm has been utilized where the required surface finish was constrained as ≤60?nm and the optimized results were confirmed by means of validation experiments. The obtained results depict that the voltage to the electromagnet plays a most significant role to produce minimum SR and maximum MRR. Moderate and least significant contributions are given by the hydraulic pressure and number of cycles, respectively, toward the responses.     

Fatigue crack initiation and crystallographic growth in 316L stainless steel

A mesoscale model of fatigue crack formation and stress–strain behavior in crystalline alloys entitled Sistaninia–Niffenegger Fatigue (SNF) model is applied to AISI 316L austenitic stainless steel. An inelastic hysteresis energy criterion in conjunction with continuum damage modeling provides a strong tool for studying the behavior of the austenitic steel under cyclic loading. The model predictions are validated against fatigue experimental data. The results show that this microstructural-based modeling approach is capable for predicting the behavior of the steel even under complex loading conditions. It can reproduce and help to understand well known fatigue experimental facts, e.g. the effect of grain size and initial defects, by considering the anisotropic behavior of crystalline materials at the level of the microstructure.     

Modifying the properties of AISI 316L steel by glow discharge assisted low-temperature nitriding and oxynitriding

Apart from titanium, its alloys and CoCrMo alloys, austenitic steels are widely used in medical applications. In order to improve the frictional wear resistance of these steels, they are subjected to various surface treatments such that the good corrosion resistance of the steels is preserved.The paper analyzes the structure and phase composition of AISI 316L steel after subjecting it to low-temperature nitriding and oxynitriding under glow discharge conditions. The treatments produced diffusion-type surface layers composed of nitrogen-expanded austenite (known as the phase S, i.e. supersaturated solution of nitrogen in austenite) with a thin surface layer of chromium nitride (CrN) zone (after nitriding) or chromium oxide (Cr₂O₃) zone (after oxynitriding). It has been shown that the treatments substantially increase the hardness and frictional wear resistance of the steel without degrading its good corrosion resistance (examined in the Ringer physiological solution at a temperature of 37 °C).     

激光选区熔化316L不锈钢凝固特征与微观组织结构

对不同工艺参数下激光选区熔化(Selective Laser Melting, SLM)成形316L不锈钢微观组织结构进行表征,研究不同工艺参数下SLM成形316L不锈钢微观组织结构演化规律、单熔化道凝固特性。结果表明,SLM成形316L不锈钢具有跨尺度、非均质凝固组织特征,包括微米尺度柱状晶粒、小角晶界、熔池界面和纳米尺度亚结构。单熔化道的稳定成形是三维块体成形的基础,熔化道稳定性由激光工艺参数与金属粉体物理特性共同决定。不同的激光工艺参数显著影响SLM成形316L不锈钢微观组织结构,通过改变激光参数可实现微观组织结构的调控,在不同的激光逐层旋转角度下,SLM成形316L不锈钢晶粒尺寸随着扫描间距的增大而增大。强制定向热流使得外延生长机制主导凝固晶粒的生长,在不同的激光工艺参数下,沿增材方向的柱状晶粒形貌普遍存在。     

Microstructural investigation on strengthening mechanisms of AISI 304L austenitic stainless steel during cryogenic deformation

Abstract

In the present paper, microscopy techniques and mechanical tests were used to investigate in detail the strengthening mechanisms of an AISI 304L austenitic stainless steel during cryogenic deformation. The strain hardening rate–strain response of the alloy indicated three distinct regimes of hardening, being very similar to that previously reported for other low stacking fault energy alloys. The hardening rate initially decreased up to a strain of ～6% (stage I). Then, a second stage of increasing hardening rate began (stage II). At strains larger than ～25%, stage III with decreasing hardening rate followed. It was suggested that the formation of ?-martensite and α′-martensite together is responsible for the appearance of stage II. The high strain hardening values of the alloy in stage II were related to the increased fraction of α′-martensite and dislocation pile-ups behind the Lomer–Cottrell locks. The appearance of stage III was attributed to the difficulty of α′-martensite nucleation and ease of dislocation cross-slip at higher strains.     

Viscoplastic behaviour of stainless steel AISI 316L under cyclic loading conditions

Summary Experimental results on stainless steel AISI 316L under cyclic loading conditions, at room temperature, showing dependence of the consolidation stress on strain rate are obtained and used for the calibration of a viscoplastic numerical model based on total strain and overstress. An explicit dependence for the evolution of the nonlinear viscosity function on cycle number and strain rate on the one hand, and of the equilibrium stress-strain diagram origin shift at loading reversal on cycle number, have been obtained and the calibrated model is found to yield results which are in very good agreement with the experimental data.     

Production of nano/ultrafine grained AISI 201L stainless steel through advanced thermo-mechanical treatment

In recent years, great attention has been focused on the development of nanostructured stainless steels to improve their mechanical properties. This work reports the formation of nano/ultrafine grain structure in the AISI 201L stainless steel using advanced thermo-mechanical treatment. The cast specimens were first homogenized and then hot forged to provide a suitable microstructure prior to the treatment. Cold rolling was carried out with the reductions of 10-95% followed by annealing at temperature of 850 °C for 15-1800 s. X-ray diffraction, optical and scanning electron microscopy, and tensile and hardness tests were used to characterize the processed specimens. The results showed that the nanocrystalline austenitic structure with a grain size of about 65 nm was obtained by annealing at 850 °C for 30 s after the cold reduction of 95%. The yield strength, total elongation and hardness of this specimen were measured as 1485 MPa, 33% and 386 Vickers, respectively.     

Selective dissolution of austenite in AISI 304 stainless steel by bacterial activity

Selective attack in an AISI 304 stainless steel weld metal has been developed after three months in service in well water. Welding zones showed a severe corrosive attack that in some cases led to the steel perforation. Optical and scanning electron microscopy (SEM/EDX) revealed a selective attack. An in-depth analysis showed indications of microbiological activity which could be responsible of the severe attack.     

Experimental investigation on corrosion and hardness of ion implanted AISI 316L stainless steel

The AISI 316L stainless steel has been widely used both in artificial knee and hip joints in bio-medical applications. In the present study AISI 316L SS was implanted with two different ions: nitrogen and helium at 100 keV with a dose of 1 × 10¹⁷ ions/cm² at room temperature. The crystallographic orientation and surface morphology were studied using X-ray diffraction (XRD) and scanning electron microscope (SEM). The effects of ion implantation on the corrosion performance of AISI 316L stainless steel was evaluated in 0.9% NaCl solution using electro chemical test both on the virgin and implanted samples. The subsequent Tafel analysis shows that the ion implanted specimens were more corrosion resistant when compared to the bare specimens. Microhardness was also measured by Vickers method by varying the loads. The results of the studies indicated that there was a significant improvement in both corrosion resistance and hardness of implanted samples.     

Evaluation of tensile properties of austenitic stainless steel 316L with linear hardening by modified indentation method

Abstract

The instrumented indentation test has been widely used for the non-destructive evaluation of the tensile properties of metal materials. The true stress–strain curve, yield strength and tensile strength can be obtained by this method. In the present study, a new modified indentation algorithm was used to determine the tensile properties of austenitic stainless steel 316L by taking into account its linear hardening characteristic. As received 316L was solution treated at four different temperatures in the range of 1223–1473K for 2 h followed by water quenching. The effect of solution treatment temperature on the tensile properties of 316L was investigated by the instrumented indentation test using the modified indentation algorithm. Results reveal that the new modified indentation algorithm can be used to estimate the tensile properties of austenitic stainless steel with linear hardening.     

Novel plasma nitriding-oxidizing duplex treatment of AISI 316 austenitic stainless steel

In this work, plasma nitriding and plasma nitriding-oxidizing treatment have been performed on AISI 316 austenitic stainless steel. In order to evaluate its response to this treatment, their microstructures and wear resistance have been compared with conventional plasma nitrided. The treatment of plasma nitriding was performed at temperature of 450 °C for 5 h with gas mixture of N₂/H₂:1/3 whereas plasma nitriding-oxidizing was performed with the same parameters of plasma nitriding and temperature of 450 °C with gas mixture of O₂/H₂:1/5 for 15, 30 and 60 min. The structural, mechanical and tribological properties were analyzed using XRD, SEM, microhardness testing and pin-on-disk tribotesting. The results showed that oxidation treatment reduces wear resistance of plasma nitrided sample under high loads. Furthermore the tribological evaluation indicates that by increasing the oxidation time further reduction of wear resistance can be occurred. In addition, it was found that oxidation treatment after plasma nitriding provides an important improvement in the friction coefficient against a AISI 52100 steel pin and reduces surface roughness.     

The effect of repeated repair welding on mechanical and corrosion properties of stainless steel 316L

The purpose of this study is to evaluate changes in the mechanical, micro structural and the corrosion properties of stainless steel 316L under repeated repair welding. The welding and the repair welding were conducted by shielded metal arc welding (SMAW). The SMAW welding process was performed using E316L filler metals. Specimen of the base metal and different conditions of shielded metal arc welding repairs were studied by looking in the micro structural changes, the chemical composition of the phases, the grain size (in the heat affected zone) and the effect on the mechanical and corrosion properties. The microstructure was investigated using optical microscopy (OM) and scanning electron microscopy (SEM). The chemical composition of the phases was determined using energy dispersive spectrometry (EDS). The corrosion behavior in 1 M H₂SO₄ + 3.5% NaCl solution was evaluated using a potentiodynamic polarization method. Tensile tests, Charpy-V impact resistance and Brinell hardness tests were conducted. Hardness of the heat affected zone decreased as the number of repairs increased. Generally an increase in the yield strength (YS) and the ultimate tensile strength (UTS) occurred with welding. After the first repair, a gradual decrease in YS and UTS occurred but the values of YS and UTS were not less than values of the base metal. Significant reduction in Charpy-V impact resistance with the number of weld repairs were observed when the notch location was in the HAZ. The HAZ of welding repair specimen is more sensitive to pitting corrosion. The sensitivity of HAZ to pitting corrosion was increased by increasing the number of welding repair.     

Corrosion behavior of ion nitrided AISI 316L stainless steel

In the present work the corrosion susceptibility of ion nitrided AISI 316L stainless steel was investigated for two different nitriding times and compared with the corrosion susceptibility of the untreated material. Plasma nitriding for short times (30 min) produced the “S” phase or expanded austenite (γ_N), with a thickness of ∼ 5 μm and a micro-hardness of 1300-1400 HV_0.025 (6.5 times higher than the untreated material). Plasma nitriding for long times (6 h) resulted in the precipitation of iron and chromium nitrides.To evaluate the corrosion resistance of both untreated and nitrided samples, anodic potentiodynamic polarization curves and immersion tests were performed in 1 M NaCl at room temperature. It was found that the corrosion resistance depends on the nitriding time. Samples nitrided for half an hour developed a much better corrosion resistance - close to that observed in the untreated samples - than those nitrided for 6 h. Samples nitrided for half an hour showed high roughness probably due to the presence of sliding bands developed in the expanded austenite phase. These sliding bands provide appropriate sites for the developing of the corrosion process. This would explain the results obtained in the corrosion tests. Samples ion nitrided for 6 h showed a severe and massive surface damage due to corrosion.Ion nitriding of AISI 316L stainless steel for short periods of time (30 min in the present case) may be an interesting surface treatment process that efficiently improves the surface hardness of the steel with some reduction in its corrosion resistance.     

Modelling tail effects in creep-crack initiation and growth for 316L stainless steel

For complex loading history (creep and fatigue) applied to engineering components, assessment procedures generally estimate the crack initiation and growth by using the summation of continuous fatigue and pure creep crack growth rates. This text deals with the pure creep correlation established in laboratory tests and applied to components subjected to creep-fatigue loading. The trend of the creep opening displacement history superimposed onto the crack progress is sufficient to predict what kind of tail effect will occur when plotting ? vs. C*. The exponent of this correlation is demonstrated to be very close to unity, whatever creep stage is concerned. The contribution of either the material behaviour or the crack extension to the ? -C* correlation is discussed.     

Structure and wear resistance of the composite layers produced by glow discharge nitriding and PLD method on AISI 316L austenitic stainless steel

The rapid technical development enhances the demands on constructional materials in terms of their resistance to frictional wear, resistance to corrosion and erosion, high hardness, high tensile and fatigue strength. These demands can be satisfied by e.g. applying various surface engineering techniques that permit to modify the microstructure, phase and chemical composition of the surface layers of the treated parts. A prospective line of the development of surface engineering is the production of composite layers by combining various surface engineering methods. The paper presents the results of examinations of the phase composition and frictional wear resistance of the layers produced by hybrid processes, i.e. such that combined glow discharge assisted nitriding performed at 450 °C and 550 °C with a pulsed laser deposition of boron nitride coatings (PLD method). It has been shown that the boron nitride coatings formed on nitrided AISI 316L steel increase its frictional wear resistance.     

Microstructural creep damage in welded joints of 316L stainless steel

The present work has been undertaken to study creep damage in welded joints. The complex dual phase microstructure of 316L welds are simulated by manually filling a mould with longitudinally deposited weld beads. Most of the moulded specimens were then aged for 2000 hours at 600°C. High resolution scanning electron microscopy was extensively used to examine the microstructure of the welded material before and after ageing. Columnar grains of austenite constitute a matrix in which thin dendrites of δ-ferrite can be found. The ageing generates the precipitation of carbides, resulting in less transformation in the material. Smooth and notched creep specimens were cut from the mould and tested at 600°C under different stress levels. The creep life of the simulated welded material is shown to be lower than that of the base material. Microstructural observations reveal that creep cavities are preferentially located along the austenite grain boundaries. This analysis of intergranular damage on test specimens is conducted to obtain a predictive damage law which could be used to calculate the lifetime of welded joints.     

The effect of grain size and martensitic transformation on the wear behavior of AISI 304L stainless steel

In the present study, a combination of cold rolling and subsequent annealing was used to produce an AISI 304L stainless steel with different grain sizes (650 nm, 3 μm and 12 μm). Wear behavior of the steel was subsequently examined using dry sliding wear test under different loads of 10 N, 20 N and 30 N. Different microstructural characterizations were conducted on the samples. The results demonstrated that the ultra-fine grained steel (650 nm grain size) had better wear resistance under normal loads of 10 N and 20 N, whereas under the normal load of 30 N, it showed weak wear resistance as compared to the steel with larger grain size (3 μm and 12 μm). This behavior can be attributed to the amount of induced martensitic transformation formed during the wear test. This transformation was evaluated using XRD analysis and quantified by Ferritescope measurements. Wear mechanism was recognized as delamination in the early stages of the wear test and the mixture of delamination and abrasion for higher distances.     

Strain-rate effects on the mechanical behavior of the AISI 300 series of austenitic stainless steel under cryogenic environments

A series of uni-axial tensile tests were carried out under various low temperatures and strain-rate ranges for AISI 300 austenitic stainless steel. The strain-rate dependencies of the materials under investigation were evaluated at temperatures ranging from ambient to cryogenic. Non-linear mechanical behavior such as phase transformation, discontinuous yielding and micro-damage of four kinds of commercial stainless steel-based material were quantitatively investigated by measuring transformation induced plasticity (TRIP) and threshold strain for 2nd hardening. In this study, the main properties of each material were analyzed and compared based on the conditions of strain-rates and temperature. Test results showed that all the test materials were strongly dependent on temperature and strain rate. It is expected that the findings in this study could be used for the cryogenic design and further research of structure materials under cryogenic environments.