A parametric study of CaCO3 scaling in AISI 316 stainless steel tubes

 The formation of undesirable layer of deposits on the heat-transfer surface is defined as fouling. These deposits present a major problem in the operation and maintenance of heat exchangers, particularly in cooling-water systems. It has been generally observed that the deposits in such systems consist mainly of calcium carbonate (CaCO₃), which has inverse solubility characteristics. An experimental study was carried out to determine the effect of tube surface temperature, Reynolds number, tube diameter and salt concentration on the growth of CaCO₃ scale. In this paper, effects of some of these parameters on fouling growth are discussed. The effect of CaCO₃ concentration on the scale growth is compared with the ionic diffusion model presented by Hasson. The variation of the fouling thickness along the length of the heat exchanger is also illustrated. Furthermore, dimensionless parameters are introduced to present the fouling resistance data collected during the experimental study. Received on 14 June 2000     

Influence of surface treatments on the total normal emittance of AISI316 stainless steel

Total normal emittance measurements have been performed on AISI 316 L samples in temperature range 400–1050 K. Two sets of mirror finished samples, with the same average roughness, have been obtained by means of polishing wheels and metallographical procedure and then etched with different acid mixtures.Emittance tests were carried out on all samples oxidized at 1050 K for a period of 3 and 14 hours.Experimental results show a remarkable influence of surface finishing on the total normal emittance of oxidized samples. The influence of surface finishing becomes negligible when an electropolishing process is made after etching.

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Einfluß der Oberflächenbehandlung auf den normalen Gesamtemissionsgrad von rostfreiem Stahl AISI 316

Zusammenfassung Der normale Gesamtemissionsgrad an Proben von AISI 316 L wurde für Temperaturen von 400 bis 1060 K gemessen. Zwei Sätze von spiegelblanken Proben mit gleicher Rauhigkeit wurden mit Hilfe von Polierscheiben oder durch eine metallographische Prozedur vorbehandelt und dann mit verschiedenen Säuremischungen geätzt. An allen oxidierten Proben wurde der Emissionsgrad bei 1050 K über 3 und 14 Stunden gemessen. Die Versuche zeigen einen starken Einfluß der Oberflächenbehandlung auf den Emissionsgrad der oxidierten Proben, der jedoch zu vernachlässigen war, wenn die Proben nach dem Ätzen elektropoliert wurden.

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Research sponsored by C.N.R.-Consiglio Nazionale delle Ricerche, Roma, Italy     

单个弹丸撞击316L不锈钢引起的变形场

运用金属材料表面纳米化试验机对单个弹丸撞击316L不锈钢表面进行了撞击实验;采用激光共聚焦显微镜观察了弹坑的三维形貌,测量不同振动频率下弹坑的直径及离面位移;采用云纹干涉法对弹坑周围的面内应变场进行测量,并分析振动频率及撞击方式对弹坑尺寸、塑性应变大小以及塑性应变区范围的影响;采用有限元方法对单个弹丸垂直撞击试件表面的应变场进行数值模拟,与实验结果进行比较,分析了弹坑周围残余应力的分布。结果表明:随振动频率的增加,弹坑直径和离面位移都增加,频率在50~55Hz,弹坑直径有突变,离面位移和振动频率呈线性关系;振动频率越大,塑性应变越大,塑性应变分布范围均大于弹坑直径的2倍;同一振动频率下弹丸垂直撞击比倾斜撞击的塑性应变大,而塑性应变分布范围相差不大;面内残余应变场的数值模拟结果和实验结果吻合较好,最大误差小于10%。     

High temperature sensitivity of notched AISI 304L stainless steel tests

Experiments were designed to determine the failure characteristics of AISI 304L stainless steel under different stress triaxialities and temperatures up to 70% of melt. The data show that as temperature increases the displacement to failure of notched tensile specimens increases. The complex interaction of deformation mechanisms, such as twinning and dynamic recrystallization, appears to negate the damage accumulation at higher temperatures. Microstructural analyses and finite element simulations indicate that voids nucleate, grow, and coalesce more rapidly as temperature and triaxiality increase. Finite element simulations were performed to analyze temperature dependence on the Cocks–Ashby void growth model. The finite element simulations qualitatively show a double-knee that was observed in the notched experimental specimens after loading. The combined experimental–numerical study indicates that failure can be defined at several points in the notch tests when: (1) macrovoids starts to form, (2) the load drop-off occurs, and (3) total perforation of the specimen occurs. These three points occur simultaneously in ambient conditions but occur at different displacements at higher temperatures.     

316L不锈钢随动强化模型改进研究

本文对316L不锈钢进行了单轴与多轴非比例路径下的应力控制棘轮试验,考察了应力幅值、平均应力和加载历程对棘轮特性的影响。同时进行了应变控制循环试验以研究材料的应力松弛特性。试验结果表明轴向棘轮效应在对称剪切荷载下效果明显,同时棘轮应变随应力幅值和平均应力的增加而增加。研究了Chen-Jiao随动强化模型与Jiang-Sehitoglu随动强化模型采用的单轴与多轴参数对背应力分量增量方向的影响,将Chen-Jiao模型中的多轴系数替换为界面饱和率,并在此基础上引入新的参数对塑性模量系数进行修正,计算结果表明修正后的模型能提升应力控制下多轴棘轮的预测精度,并能很好的预测应力松弛现象,表明了新模型的正确性与有效性。     

Sequential creep-fatigue interaction in austenitic 316 L-SPH stainless steel

Stainless steel type 316 L-SPH are predamaged by fatigue from 20 to 70% of fatigue life or creep load from the primary to tertiary range. Their subsequent fatigue and creep properties are investigated at 600°C for different fatigue saturation and creep stresses. Results were evaluated by the time and cycle fraction rule.In this way, information can be gained on the effect of very long intermittent hold times for low cycle fatigue and on transitory loads under normal service at elevated temperatures. In the cases of creep-fatigue and fatigue-creep sequence of preloading, the subsequent resistance of the specimens remained unchanged, if not improved. Thin-foil specimens were also made to reveal that the dislocation substructure developed during preloading was quickly erased and replaced by that corresponding to the subsequent load type. More specifically, grain boundary cavitation did not occur until creep loadapplied well beyond the end of the secondary stage. Hence, preloading by creep up to approximately 80% of rupture life does not affect the subsequent fatigue properties. On the other hand, significant surface cracks were found in the prefatigued specimens after more than approximately 50% of life. The subsequent creep damage was found to be localized at the tip of the crack and the remaining creep life was fatigue damage with long intermittent hold times is less severe than continuously repeated loading.     

CaCO3 scaling in AISI 316 stainless steel tubes – effect of thermal and hydraulic parameters on the induction time and growth rate

Calcium Carbonate (CaCO₃) is predominantly present in cooling water which is commonly used as a coolant in many industrial processes. It has inverse solubility characteristics i.e., it is less soluble in warm water, resulting in the deposition of scale on heat transfer surfaces. An experimental study was carried out to determine the effect of tube surface temperature, Reynolds number, tube diameter and salt concentration on the induction time of CaCO₃ scaling. It was observed that tube surface temperature, Reynolds number and tube diameter had no effect on the onset time of scaling, whereas salt concentration and tube surface roughness had a profound influence on the induction period. The data collected from the experiments were used to develop dimensionless fouling resistance models for estimation and prediction purposes. Received on 22 December 1997     

Numerical simulation of laser ultrasonic detection of the surface microdefects on laser powder bed fusion additive manufactured 316L stainless steel

A numerical model is presented in this article to investigate the interactions between laser generated ultrasonic and the microdefects (0.01 to 0.1 mm), which are on the surface of the laser powder bed fusion additive manufactured 316L stainless steel. Firstly, the influence of the transient sound field and detection positions on Rayleigh wave signals are investigated. The interactions between the varied microdefects and the laser ultrasonic are studied. It is shown that arrival time of reflected Rayleigh (RR) waves wave is only related to the location of defects. The depth can be checked from the feature point Q, the displacement amplitude and time delay of converted transverse (RS) wave, while the width information can be evaluated from the RS wave time delay. With the aid of fitting curves, it is found to be linearly related. This simulation study provides a theoretical basis for quantitative detection of surface microdefects of additive manufactured 316L stainless steel components.     

Deformation behavior and microstructure/texture evolution of an annealed 304 AISI stainless steel sheet. Experimental and micromechanical modeling

Different tensile tests are performed at −60 °C, on a 304 AISI stainless steel, to produce strain induced α′ martensite. Using EBSD analysis and X-ray diffraction methods, the corresponding texture and microstructure evolutions is studied, as well as the γ → α′ transformation mechanism. It is shown that α′ martensite is associated with {1 1 1}_γ faulted bands, partially ε bands, as often reported in the literature for low SFE stainless steels. As a consequence, these deformation bands play a key role in the nucleation and growth of α′ martensite. A micromechanical model is applied to simulate the deformation behavior of the stainless steel as well as the corresponding transformation kinetic and γ texture evolution. Different simulations are proposed, using various assumptions for the γ → α′ transformation mechanism. The simulation data are in better agreement with the experimental ones, when the γ → α′ transformation strain is derived from the Bowles–MacKenzie theory using the (1 1 1)_γ[−1 2 −1]_γ stacking fault shear system as lattice invariant shear.     

On the ballistic resistance of double-layered steel plates: An experimental and numerical investigation

Civil and military ballistic protection systems often consist of thin, high-strength steel plates. Such plates may either be monolithic or layered with or without spacing. The idea of using layered plates instead of a monolithic one in order to increase the ballistic perforation resistance is not new, and the effect of using targets made up of several thinner plates has been investigated in the literature for a long time. However, results by various authors are contradicting and detailed experimental and numerical work is still required.In the present study, the ballistic perforation resistance of double-layered steel plates impacted by blunt and ogival projectiles was investigated both experimentally and numerically. In the tests, 12 mm thick (monolithic or layered) targets of Weldox 700 E were impacted using a gas-gun at sub-ordnance velocity, and the ballistic limit velocity of the different target combinations was obtained. In general, good agreement was obtained between the numerical simulations and the experimental results. It was found that in the case of blunt projectiles a large gain in the ballistic limit is offered by double-layered systems. These advantages seem to disappear when ogival projectiles are used. However, the main conclusion from both the experimental and numerical studies is that the overall protection level, i.e. the minimum ballistic limit velocity obtained independently of projectile nose shape, seems to increase significantly by double-layering the target.     

An experimental and numerical study of the localization behavior of tantalum and stainless steel

In general, the shear localization process involves initiation and growth. Initiation is expected to be a stochastic process in material space where anisotropy in the elastic–plastic behavior of single crystals and inter-crystalline interactions serve to form natural perturbations to the material’s local stability. A hat-shaped sample geometry was used to study shear localization growth. It is an axi-symmetric sample with an upper “hat” portion and a lower “brim” portion with the shear zone located between the hat and brim. The shear zone length is 870–890 μm with deformation imposed through a split-Hopkinson pressure bar system at maximum top-to-bottom velocity in the range of 8–25 m/s. We present experimental results of the deformation response of tantalum and 316L stainless steel samples. The tantalum samples did not form shear bands but the stainless steel sample formed a late stage shear band. We have also modeled these experiments using both conductive and adiabatic continuum models. An anisotropic elasto-viscoplastic constitutive model with damage evolution was used within the finite element code EPIC. A Mie-Gruneisen equation of state and the rate and temperature sensitive MTS flow stress model together with a Gurson flow surface were employed. The models performed well in predicting the experimental data. The numerical results for tantalum suggested a maximum equivalent strain rate on the order of 7 × 10⁴ s⁻¹ in the gage section for an imposed top surface displacement rate of 17.5 m/s. The models also suggested that for an initial temperature of 298 K a temperature in the neighborhood of 900 K was reached within the shear section. The numerical results for stainless steel suggest that melting temperature was reached throughout the shear band shortly after peak load. Due to sample geometry, the stress state in the shear zone was not pure shear; a significant normal stress relative to the shear zone basis line was developed.     

On the experimental determination of yield surfaces and some results of annealed 304 stainless steel

Factors affecting the experimental determination of yield surfaces are discussed. They include the elastic moduli and the zero offset strain, the strain domain used to determine the yield stress, the probing path, and the strain rate of probing. To obtain yield surfaces consistently, it is necessary to account for these factors. The initial and subsequent yield surfaces of annealed AISI type 304 stainless steel have been experimentally determined in the axial-torsional stress space. Three loading paths have been studied. They are a pure axial path, a pure torsional path, and a proportional axial-torsional path. Each path includes loading, unloading, reloading, and the cyclically steady state.     

Uniaxial ratcheting of SS304 stainless steel at high temperatures: visco-plastic constitutive model

The uniaxial ratcheting of SS304 stainless steel at high temperatures (300, 600 and 700 °C) were analyzed experimentally, and described by a cyclic constitutive visco-plasticity model. The rate dependence of the material was accounted for by introducing a viscous term. The cyclic hardening and cyclic flow behavior of the material under asymmetrical stress-controlled cycling were described by the evolution rules of kinematic hardening back stress and isotropic deforming resistance. Under the isothermal condition, temperature effect was included by terms involving temperature in the evolution equations of isotropic deforming resistance. The effect of load history on ratcheting was also considered by introducing a fading memory function of the maximum inelastic strain amplitude and isotropic deformation resistance. After the material constants were determined from the experimental data, the uniaxial ratcheting of SS304 stainless steel was numerically simulated and compared with the corresponding experimental results at high temperatures. The predicted results agree well with the experimental ones.     

An experimental study of uniaxial creep,cyclic creep and relaxation of aisi type 304 stainless steel at room temperature

Following previous work (Krempl, 1979), a servocontrolled testing machine and strain measurement at the gage length were used to study the uniaxial rate(time)-dependent behavior of AISI Type 304 stainless steel at room temperature. The test results show that the creep strain accumulated in a given period of time depends strongly on the stress-rate preceding the creep test. In constant stress-rate zero-to-tension loading the creep strain accumulated in a fixed time-period at a given stress level is always higher during loading than during unloading. Continued cycling causes an exhaustion of creep ratchetting which depends on the stress-rate. Periods of creep and relaxation introduced during completely reversed plastic cycling show that the curved portions of the hysteresis loop exhibit most of the inelasticity. In the straight portions, creep and relaxation are small and there exists a region commencing after unloading where the behavior is similar to that at the origin for virgin materials. This region does not extend to zero stress.The results are at variance with creep theory and with viscoplasticity theories which assume that the yield surface expands with the stress. They support the theory of viscoplasticity based on total strain and overstress.     

Microscale prediction of deformation in an austenitic stainless steel under uniaxial loading

In this study, the deformation behaviour of polycrystalline austenitic 316H stainless steel under uniaxial loading is investigated by means of in-situ neutron diffraction (ND) measurement and crystal plasticity-based finite element (FE) modelling. Data have been obtained for the macroscopic stress–strain response and the lattice strain evolution in the longitudinal and transverse direction relative to the uniaxial loading axis. Comparison between the model predictions and the ND measurements suggests that in most cases the FE model can predict the lattice strain evolution at the microscale and capture the general trends observed in the experiments. Both ND measurements and FE modelling simulations identify little effect of micromorphology effect on the longitudinal lattice strain evolution, while the transverse lattice strain response appears to be sensitive to the microstructure, in particular the initial crystallographic orientation of the material.     

Multi-layer film flow down an inclined plane: experimental investigation

We report the results from an experimental study of the flow of a film down an inclined plane where the film itself is comprised of up to three layers of different liquids. By measuring the total film thickness for a broad range of parameters including flow rates and liquid physical properties, we provide a thorough and systematic test of the single-layer approximation for multi-layer films for Reynolds numbers \(Re = \rho Q/\mu \approx 0.03 - 60\) . In addition, we also measure the change in film thickness of individual layers as a function of flow rates for a variety of experimental configurations. With the aid of high-speed particle tracking, we derive the velocity fields and free-surface velocities to compare to the single-layer approximation. Furthermore, we provide experimental evidence of small capillary ridge formations close to the point where two layers merge and compare our experimental parameter range for the occurrence of this phenomenon to those previously reported.     

Strain rate sensitivity of flow stress at low temperatures in 304n stainless steel

Strain rate and temperature effects on the flow stress of 304N stainless steel were investigated using data obtained from torsion tests on thin-walled tubular specimens, at given strain rates of between 10⁻³ and 10⁻¹ with temperatures ranging from 83 to 296 K. Initially, the apparent strain rate sensitivity of the flow stress, obtained at test temperatures of 83, 153, 213, and 296 K, was examined. The strain dependence and the strain rate dependence of the mechanical threshold stress (the flow stress at O K) at strains less than 0.1 and at strain rates of 2.4 × 10⁻³ and 8.3 × 10² s⁻¹, is also discussed. It was concluded from the experimental results that the mechanical threshold stress depends not only on the strain but also on the strain rate.