Experimental and numerical modelling of the residual stresses induced in orthogonal cutting of AISI 316L steel

Residual stresses in the machined surface layers are affected by the cutting tool, work material, cutting regime parameters (cutting speed, feed and depth of cut) and contact conditions at the tool/chip and tool/workpiece interfaces. In this paper, the effects of tool geometry, tool coating and cutting regime parameters on residual stress distribution in the machined surface and subsurface of AISI 316L steel are experimentally and numerically investigated. In the former case, the X-ray diffraction technique is applied, while in the latter an elastic–viscoplastic FEM formulation is implemented. The results show that residual stresses increase with most of the cutting parameters, including cutting speed, uncut chip thickness and tool cutting edge radius. However, from the range of cutting parameters investigated, uncut chip thickness seems to be the parameter that has the strongest influence on residual stresses. The results also show that sequential cuts tend to increase superficial residual stresses.     

Residual stress driven creep cracking in AISI Type 316 stainless steel

Specially designed AISI Type 316H austenitic stainless steel 25 mm thick compact tension specimens have been plastically deformed to produce significant tensile hydrostatic residual stresses at the notch root at mid-thickness. These specimens were thermally exposed at 550 °C for 4500 hours in order to study elevated temperature creep relaxation of residual stress and the development of reheat cracking creep damage. Residual strains within the specimens were measured using diffraction techniques before and after thermal exposure. A three-dimensional finite element model was developed both to predict the residual stress within the specimens before and after thermal exposure. No reheat cracking was found near surface, but due to the reduced creep ductility with increasing hydrostatic stress, significant creep cavitation was found mid-thickness. A previously developed creep damage model was applied to predict the onset of reheat cracking. Good correlation has been found between measurements and finite element predictions of strain and stress before and after thermal exposure. The extent of creep damage has also been assessed through destructive examination, providing validation for the creep damage prediction model.     

3D modeling of residual stresses induced in finish turning of an AISI304L stainless steel

This paper addresses the development of a new methodology predicting residual stresses induced in finish turning of a AISI304L stainless steel. A hybrid approach combining experimental results and a numerical model is applied. The model simulates the residual stresses generation by applying equivalent thermo-mechanical loadings onto the machined surface without modeling the chip removal process, which enables rapid calculation. The shape and the intensity of equivalent thermo-mechanical loadings are identified through experimental measurements. Friction tests enable to model the thermal and mechanical loadings along the tool-workmaterial interface. Orthogonal cutting tests provide thermal and mechanical loadings below the primary and third shear zone. This model has already been presented in several papers, but only in a 2D configuration. The objective of this paper is to transfer this hybrid approach into a 3D configuration, which is closer to a concrete longitudinal turning operation. Based on this new model, the paper aims at investigating the interactions between each revolution. It is shown that around five revolutions are necessary to reach a steady state. Finally numerical results are compared with experimental measurements obtained by X-Ray diffraction. It is shown that residual stresses cannot be considered as homogeneous over the surface due to tool's feed. Additionally, the X-Ray beam is much too large to be able to quantify this heterogeneity. Based on average numerical values coherent with average values obtained by X-Ray diffraction, it is shown that the numerical model provides consistent results compared to experimental measurements for a large range of cutting speed and feed.     

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.     

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.     

Corrosion and wear behaviors of boronized AISI 316L stainless steel

In this study, the effects of a boronizing treatment on the corrosion and wear behaviors of AISI 316L austenitic stainless steel (AISI 316L) were examined. The corrosion behavior of the boronized samples was studied via electrochemical methods in a simulation body fluid (SBF) and the wear behavior was examined using the ball-on-disk wear method. It was observed that the boride layer that formed on the AISI 316L surface had a flat and smooth morphology. Furthermore, X-ray diffraction analyses show that the boride layer contained FeB, Fe₂B, CrB, Cr₂B, NiB, and Ni₂B phases. Boride layer thickness increased with an increasing boronizing temperature and time. The boronizing treatment also increased the surface hardness of the AISI 316L. Although there was no positive effect of the coating on the corrosion resistance in the SBF medium. Furthermore, a decrease in the friction coefficient was recorded for the boronized AISI 316L. As the boronizing temperature increased, the wear rate decreased in both dry and wet mediums. As a result, the boronizing treatment contributed positively to the wear resistance by increasing the surface hardness and by decreasing the friction coefficient of the AISI 316L.     

Optimization of the surface mechanical strength of AISI 316L physically vapour deposited nitrogen-doped coatings on AISI 316L substrates

High load Vickers indentation, scratch testing under constant and progressive loading as well as pin-on-disc wear testing were used to assess the overall strength of coated specimens. In scratch testing, the major damage mechanism was found to be brittle, by thickness cracking of the coating with growth defects the dominant crack intiators. No adhesive failure was observed within the load range considered (0–200 N, 1.58 mm WC ball as pin).

Under high load, WC ball-on-disc wear testing with significant plastic deformation, analogous brittle cracking mechanisms were observed. The coating process parameters optimized simultaneously with respect to scratch and wear resistance were the percentage of reactive nitrogen in the plasma, related to the level of nitrogen-doping and hardness of the coating, and the substrate bias voltage. During wear testing, an adhesive transfer layer built up on the WC ball during the run-in phase was found to have a detrimental influence on the long-term wear rate.     

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.     

离子渗碳温度对316L不锈钢渗层组织和性能的影响

利用低温离子渗碳技术.在不同温度下对AISI 316L奥氏体不锈钢进行渗碳处理.利用光学显微镜、显微硬度计、XRD以及电化学测试技术研究了渗碳温度对不锈钢表面显微组织和性能的影响.结果表明,渗碳温度显著影响AISI 316L奥氏体不锈钢渗碳层的组织结构与性能.渗碳温度在400～550℃之间时,可以获得无碳化物析出的、具有单一γ_c相结构的渗碳层;渗碳温度在550℃时,渗碳层为γ相+Cr_(23)C_6+Cr_7C_3+Fe_3C+Fe_2C的混合组织.渗碳层的厚度与硬度均随渗碳温度的升高而增加.550℃是AISI 316L奥氏体不锈钢中铬的碳化物析出的临界温度.为了避免铬的碳化物析出而降低不锈钢的耐蚀性能.奥氏体不锈钢渗碳必须在低于550℃的渗碳温度下进行.     

Prediction of pitting corrosion of surface treated AISI 316L stainless steel by artificial neural network

Abstract

This paper presents an artificial neural network based solution method for modelling the pitting resistance of AISI 316L stainless steel in various surface treated forms. Surface treatment is a promising technique for improving the corrosion resistance of stainless steels. In this study, cyclic polarisation tests were performed before and after surface treatment. Experimental results were modelled by the neural network. The artificial neural network model exhibited superior performance based on the fitness of the observed versus predicted data. The results showed that the predicted data from the neural network model were considerably similar to the experimental data. The model has been saved and can easily be used to predict the corrosion in different surface treatment methods.     

Anti-corrosion characteristics of nitride-coated AISI 316L stainless steel coronary stents

The corrosion resistance of TiN and TaN coatings deposited on AISI 316L stainless steel thread-coiled coronary stents by pulsed bias arc ion plating is evaluated by electrochemical methods in deaerated Tyrode's simulated body fluids (37 ± 1 °C). The free corrosion potential of the TaN-coated stents is found to be nobler than that of the TiN-coated and uncoated stents throughout most of the immersion time. The potentiodynamic polarization test results indicate that the TaN coatings offer better passivation stability and anti-breakdown performance. The longer-term 6-month immersion tests disclose slight localized corrosion on the surface of both coatings, but no film delamination or large area pitting can be observed indicating reasonably good corrosion resistance after the long period.     

Hydrogen-assisted fatigue crack growth of AISI 316L stainless steel weld

The fatigue crack growth behaviors of AISI 316L stainless steel (SS) welds in air and gaseous hydrogen were evaluated, and further compared with the base plate. In air, the fatigue crack growth rate (FCGR) of the weld after heat-treatment at 1050 ^oC/1 h was similar to that of the base metal. Furthermore, all specimens became susceptible to hydrogen-accelerated crack growth. Mainly quasi-cleavage fracture related with the strain-induced martensite accounted for the accelerated crack growth in hydrogen. A smaller amount of martensite in the weld was responsible for the decreased susceptibility to hydrogen-enhanced fatigue crack growth relative to the base metal.     

Electrochemical behavior of SUS316L stainless steelafter surface modification

1　INTRODUCTIONAsametallicimplantdevice ,coronarystentsneedgoodmechanicalperformance ,wonderfulanti corrosionbehavioraswellasbloodcompatiblecapabil ityforinvivoenvironments.Nowadaysthestentsaremadebyausteniticstainlesssteel,cobalt basedalloy ,titaniumanditsalloy[1] .Amongthesematerials ,SUS316Lstainlesssteelisthemostcommonusedonebecauseofitscheapcost ,goodmechanicalbehaviorandlowcorrosionrate .Itdoeshowever ,frequentlycorrodeinthebody ,andreleasesomenoxiousionssuchasCr,Ni,Moions .Theio…     

Assessment of intergranular corrosion in AISI Type 316L stainless steel weldments

Abstract

This paper deals with the applicability of various techniques for the assessment and quantification of sensitisation in AISI Type 316L welds. Welded joints of AISI Type 316L stainless steel were aged at 973 K for periods of up to 200 h. The base and weld metal components of the aged joints were then assessed for susceptibility to sensitisation and intergranular corrosion (IGC) by using various tests specified by ASTM A262, Practices A and E, and ASTM G108 (the electrochemical potentiokinetic reactivation (EPR) test). The possibility of using eddy current testing (ECT) to detect sensitisation and IGC was also assessed. The use of ASTM A262 Practice A and E tests indicated sensitisation in base metal aged for 20 h and above. Aged weld metals showed no failure in these tests. Tensile tests on the weld joints before and after exposure to Cu–CuSO₄-H₂SO₄ solution did not indicate any differences in the tensile properties. Double loop EPR tests indicated a significant increase in the ratios of charge and peak current densities on reactivation to activation after aging the welded joint for 20 h and above. However, weld metal showed no change in the ratios of the above two parameters. Following doubts about the suitability of the EPR test for on line corrosion monitoring, the ECT technique was investigated in order to assess its suitability for the detection and quantification of sensitisation. The ratio of eddy current amplitudes after and before exposure to Cu–CuSO₄-H₂SO₄ solution was used as an assessment criterion. A significant increase in this ratio was observed on aging the base metal for more than 20 h. No significant change was observed in the ratios of eddy current amplitudes for weld metal. The ECT results correlated very well with the findings of the EPR and ASTM Practice E tests. This indicates that ECT holds promise as an on line monitoring tool for sensitisation and IGC.     

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.     

316L不锈钢表面激光熔覆Stellite-F合金层的电化学腐蚀行为

采用CO₂激光器在316L不锈钢表面熔覆Stellite-F合金,通过电化学方法研究了Stellite-F熔覆合金层在5wt%NaCl溶液中的腐蚀行为。结果表明:与316L不锈钢相比,与基材呈冶金结合的Stellite-F合金层自腐蚀电位较正、自腐蚀电流密度较小,无点腐蚀现象出现,耐海水腐蚀性能较好;研究发现,熔覆合金层物相主要由γ-Co固溶体与Ni₃C、Cr₂₃C₆碳化物构成,在盐溶液中其腐蚀失效机制为选择性腐蚀,即固溶体合金相作为阳极被腐蚀浸出,碳化物相得到阴极保护暴露析出,同时固溶体相中的Co、Ni组分优先被腐蚀浸出,其余组分形成海绵状结构。     

Evaluation of layer adhered on PCBN tools during turning of AISI D2 steel

Polycrystalline cubic boron nitride (PCBN) tools have high abrasion resistance and are thus suitable for application in the machining of steels with a high volume fraction of primary carbides in their microstructure. These tools are usually applied in the machining of steels with hardness above 45–50 HRC and in the case of application to steels with hardness below 45 HRC, the formation of an adhered layer on the rake face of the tools often occurs. This paper reports a study on the impact of the layer adhered on PCBN tools during the turning of AISI D2 steel, with 35 and 50 HRC. The microhardness and microstructure of the adhered material were determined, as well as the tool wear based on volumetric wear parameters. The layer adhered on the PCBN tool rake face has the same chemical elements as the machined steel alloy. Its microstructure is oriented in the direction of the chip flow and the primary carbides were fragmented. For the sample with 35 HRC the amount of material adhered (W_AM) on the rake face of the PCBN tool was approximately 360% higher than the steel with 50 HRC. The material layer adhered on the PCBN tool rake surface in the case of the 35 HRC steel acts as an edge (assuming the cutting function), while for the 50 HRC steel, the adhered layer intensifies the adhesion wear mechanism through spalling on the tool rake face. The results obtained provide important information for the selection of materials and grades for the development of new cutting tools.     

316L不锈钢强流脉冲电子束表面钛合金化及其耐蚀性

利用强流脉冲电子束对不锈钢表面进行了快速钛合金化。将精细钛粉预涂在基体表面后采用强流脉冲电子束对其进行后处理。在电子束对表面的快速加热熔化、混合及增强扩散效应的作用下,部分钛熔入基体表层形成一层富钛层。由于钛的添加有利于形成α相,合金层由α相和γ相混合组成。在模拟体液中的动态极化测试表明,316L医用不锈钢经强流脉冲电子束表面钛合金化后,其在模拟体液中的耐腐蚀性能获得了显著的提高。