Effect of post weld heat treatment on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel weldments

The effect of postweld heat treatment (PWHT) on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel joints with ER316LMn filler material was investigated. PWHT aging was performed for 1 h at four different temperatures of 600 °C, 760 °C, 870 °C and 920 °C, respectively. The microstructure revealed the sigma phase precipitation occurred in the weld metals heat-treated at the temperature of 870 °C and 920 °C. The PWHT temperatures have the less effect on the tensile strength, and the maximum tensile strength of the joints is about 630 MPa, reaching the 95% of the base metal, whereas the elongation is enhanced with the rise of PWHT temperatures. Meanwhile, the sigma phase precipitation in the weld metals reduces the impact toughness.     

轧制工艺对316L不锈钢组织和耐蚀性能的影响

为探索改善不锈钢耐腐蚀性能的途径,对316L不锈钢施加相同变形量的同步轧制和异步轧制,利用X射线衍射、透射电镜观察、电化学测量和扫描电镜表面观察研究了轧制工艺对钢的显微组织和腐蚀性能的影响.结果表明,经过异步轧制后显微组织中出现大量孪晶界,优化了晶界结构,在酸性介质中的晶间腐蚀敏感性明显减轻;而经过同步轧制后,样品呈现出高位错密度的显微组织,在酸性介质中的耐腐蚀性能降低.异步轧制后耐蚀性能得到改善是由于大量孪晶界的形成优化了晶界结构.     

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 pre-strain on tensile behaviour of 316L austenitic stainless steel

ABSTRACT

The effect of pre-strain on tensile behaviour of 316L austenitic stainless steel was investigated, focusing on strain rate sensitivity, temperature sensitivity and strain hardening. Experimental data showed that strain rate sensitivity, temperature sensitivity and strain hardening were weakening with pre-strain. Meanwhile, the variation of microstructure with pre-strain was observed by optical microscopy, scanning electronic microscopy and X-ray diffraction. Then, the mechanical properties of pre-strained material were correlated with the increase in dislocation density and mechanical twinning with pre-strain. Finally, an improved Arrhenius-PS model considering the effect of pre-strain was developed.

This paper is part of a thematic issue on Nuclear Materials.     

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.     

烧结气氛对MIM316L不锈钢致密化和尺寸精度的影响

用注射成形方法(MIM)制备了316L不锈钢样品,并在Ar、Ar H2、N2、N2 H2和低真空5种气氛下对样品进行了烧结,讨论了烧结气氛对合金致密化、力学性能和尺寸精度的影响.结果表明:烧结气氛的露点显著影响着合金的致密化和最终力学性能;烧结气氛中的H2可以脱去合金中的碳来影响致密化和力学性能;用注射成形获得的最终样品尺寸精度受注射、脱脂和烧结工序的影响;在采用溶剂脱脂时,3个工序对尺寸精度的影响由大至小依次为烧结、注射和脱脂.在不同气氛下,3个工序对尺寸精度的影响相对稳定.     

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.     

316L不锈钢冠脉支架制造工艺的研究

研究了激光精细雕刻冠状动脉血管支架的制造工艺,对冠脉支架结构设计原则、激光精细雕刻加工参数、热处理工艺进行了探讨.并给出一种冠脉支架结构,进行切割实验,分析了影响切口质量因素及真空热处理工艺、电化学工艺对冠脉支架样品质量的影响.     

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.     

Effect of laser-scan strategy on microstructure and fatigue properties of 316L additively manufactured stainless steel

The particular roles of grain morphology and defects, controlled using laser-scan strategies, on the mechanical properties and the fatigue behavior of 316L stainless steel are investigated. Microstructural characterization and X-ray tomography analysis was performed to understand the genesis of polycrystalline microstructure and defects. Tensile and fatigue tests were performed to analyze the effect of defect population and microstructural properties on plasticity and damage mechanisms during monotonic and cyclic loading. The effect of the grain-size and shape and type of defect was carefully investigated to evaluate the mechanisms driving the mechanical behavior under quasi-static and fatigue loading. It is shown that the laser-scan strategy determines the anisotropy in the plane perpendicular to the building direction. Moreover, contrary to the existing literature, for 316L obtained by AM, the grain size and shape does not affect the mechanical properties, and LoF defects drive the fatigue life, independent of the defect/grain size ratio.     

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。     

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.     

Notch fatigue behaviour of low-temperature gaseous carburised 316L austenitic stainless steel

ABSTRACT

The influence of low-temperature gaseous carburisation on notch fatigue behaviour of 316L steel under cyclic axial loading was investigated. After carburisation, the carburised case was well distributed at the surface region and was not influenced by the notch geometry. Low-temperature carburisation considerably enhanced the notch fatigue performance, which led to 32% and 44% increase in the endurance limits for the specimens with stress concentration factors K _t?=?1.91 and 3.91, respectively. The notch sensitivity of 316L steel reduced after carburisation. Irrespective of the applied stress amplitude, the fatigue crack nucleation sites were always at the notch root surface for the untreated specimens. For the carburised specimens, fatigue cracks nucleation changed from surface at high-level stress to subsurface at low-level stress.     

水导激光加工对316L不锈钢微观形貌的影响规律

水导激光加工是一项利用水光纤将激光引导到材料加工表面的新颖加工技术,具有几乎无微裂纹、热影响区小、无污染、重熔层少、加工精度高和光束平行等优点。为研究不同水导激光加工工艺参数对微观形貌的影响,探索水导激光与物质的相互作用机理。本文采用自主研发的水导激光加工系统对316L不锈钢薄片试件进行切槽和打孔实验;使用Zeiss Vert.A1金相显微镜观察加工试件的二维形貌;使用Leica DVM6超景深显微镜和Bruke Contour Elite I白光干涉仪观察试件的三维微观形貌。实验结果表明：无论是对试件进行切槽还是打孔实验,均会在加工区域产生一定宽度的沉积层,且沉积层的大小不随加工时间和加工次数变化,其宽度约为13.5 µm;通过观察试件加工区域的二维形貌,发现打孔试件的dr和切槽试件的wl也不随加工试件和加工次数变化;通过观察切槽试件加工区域的三维形貌,其截面呈倒梯形。     

沉积效率对激光近净成形316L不锈钢组织及性能的影响

利用激光近净成形技术进行了316L不锈钢单道多层结构的制备。在相同的激光功率及扫描速度条件下,通过调整送粉率及层间提升量实现了不同沉积效率的成形,并讨论了不同沉积效率下成形结构的微观组织及力学性能特征。结果表明,在一定的激光功率及扫描速度条件下,随着送粉率及层间提升量的提升,沉积效率由初始工艺参数条件下的12.14 mm3/s提高至22.62 mm3/s,提高了86.3%。同时成形单位有效体积消耗的激光能量由初始工艺参数条件下的98.84 J/ mm3降低至53.06 J/mm3,能量利用效率提高了46.32%。成形结构的微观组织呈柱状枝晶形态,随着沉积效率的提高,枝晶长度呈明显的增大趋势。性能检测结果显示,不同沉积效率下的样件力学性能保持了较高的一致性,并未随沉积效率的提高而降低,成形样件抗拉强度及屈服强度分别稳定在510 MPa与290 MPa,延伸率稳定在40%左右,显微硬度也未出现明显波动,处于180 HV,均达到了锻造的同等水平。该研究表明,在一定的工艺参数范围内,可以实现沉积效率及力学性能的协同优化,达到低能耗高效制备高性能零件的效果。     

Detection and analysis of microbiologically influenced corrosion of 316 L stainless steel with electrochemical noise technique

Microbiologically Influenced Corrosion (MIC) is a specific type of corrosion caused or promoted by microorganisms usually chemoautotrophs. In recent years, there has been growing interest in the exploitation of electrochemical noise technique to investigate and monitor biocorrosion. The advantages of Electrochemical Noise (EN) technique includes the possibility to detect and study the early stages of localized corrosion; however the comprehension of EN signals still remains very limited. In the present work an attempt has been made to analyze the current and potential noise records for type 316 L stainless steel (SS) specimen immersed in Iron oxidizing bacteria inoculated medium amended with different concentrations of NaCl. All the potential and current noise data collected in the time domain were transformed in the frequency domain, using MATLAB software. Shot noise parameters like frequency of corrosion events (f_n), average charge in each event (q), true coefficient of variation and noise resistance (R_N) were analyzed. Low frequency events and high charge were observed for the specimen after the exposure of 3 weeks in microbial medium with 1% NaCl when compared to control. It indicates that microbes can influence the pitting corrosion over the specimen which was also evidenced by Scanning Electron Microscope (SEM). In addition to this, the probabilistic failure model for MIC on 316 L SS was predicted using Weibull distribution.     

316L不锈钢加工硬化机制及孪生行为

对固溶处理后的316L不锈钢试样进行了拉伸实验,根据Ludwik真应力应变模型对拉伸实验曲线进行了非线性拟合,并用Crussard-Jaoul法计算和分析了Ludwik模型中的加工硬化指数（n）,同时通过对拉伸试样的微观组织观察,分析了316L不锈钢的加工硬化机制.实验结果表明：316L不锈钢在拉伸变形过程中加工硬化非...     

Dynamic strain aging effect on the fatigue resistance of type 316L stainless steel

Mechanism of dynamic strain aging (DSA) and its effect on the high-temperature low-cycle fatigue resistance in type 316L stainless steel were investigated by carrying out low-cycle fatigue tests in a wide temperature range from 20 to 650 °C with strain rates of 3.2×10⁻⁵–1×10⁻²/s. The regime of DSA was evaluated using the anomalous features of material behavior associated with DSA. The activation energies for each type of serration were about 0.57–0.74 times those for lattice diffusion indicating that a mechanism other than lattice diffusion is involved. It is reasonably concluded that the pipe diffusion of solute atoms along the dislocation core is responsible for DSA. Dynamic strain aging reduced the fatigue resistance by ways of multiple crack initiation, which comes from the DSA-induced inhomogeneity of deformation, and rapid crack propagation due to the DSA-induced hardening.     

Influence of prior cyclic hardening on high temperature deformation and crack growth in type 316L (N) stainless steel

For power generating equipment subjected to cyclic loading at high temperature, crack growth could arise from the combinations of fatigue and creep processes. There is potential for the material to undergo hardening (or more generally changes of material state) as a consequence of cyclic loading. Results of an experimental study to examine the influence of prior cyclic hardening on subsequent creep deformation are presented for type 316L(N) stainless steel at 600°C. Experiments were also carried out to explore creep crack growth at constant load, and crack growth for intermittent cyclic loading. For the as-received material there is substantial primary creep (hardening) at constant load, while for the cyclically hardened material at constant load the creep curves show recovery, and increasing creep rate with increasing time. Specimens subjected to prior cyclic hardening were also used for a series of creep and creep-fatigue crack growth tests. These tests demonstrated that there was accelerated crack growth compared to crack growth in as-received material.     

Effect of thermal ageing on creep and oxidation behaviour of Type 316H stainless steel

The UK has unique experience in operating high temperature civil nuclear power systems, known as advanced gas cooled reactors (AGRs). One of the primary challenges for extending the lifetime of the AGR power stations is to understand the interaction that occurs between the AGR CO₂ environment and creep-fatigue cracking behaviour. This is one of the life limiting degradation mechanisms for steel components within the reactor pressure vessel. This paper addresses the effect of thermal aging on material internal state that controls both the creep deformation and oxidation behaviour of Type 316H stainless steels when they are exposed at a simulated AGR environment. Experimental results from creep tests are discussed with respect to a multi-scale self-consistent model, while experimental results from oxidation tests are considered with respect to the application of measured short term data to predict the long term oxidation behaviour. Finally, the interaction between oxidation and creep and its impact on high temperature structural integrity of AGR nuclear systems are discussed.