Analysis of Elevated Temperature Flow Behavior of 316LN Stainless Steel Under Compressive Loading

To understand the flow behavior of a newly developed austenitic stainless steel 316LN with 0.14 wt% nitrogen, isothermal compression tests have been carried out in the hot working domain. From the analysis of flow behavior, it is observed that the nitrogen enhanced steel, in its hot working domain exhibits strain and strain rate hardening with thermal softening. The flow behavior analysis also demonstrates the coupled effect of strain–temperature and strain rate temperature on the flow stress. To depict the flow behavior of the material, strain compensated Arrhenius (SCA) equation and Model D8A have been used. The SCA predicts the flow curves with an average absolute error of 9.27% and a correlation coefficient of 0.977, whereas the prediction by Model D8A gives the average absolute relative error as 10.86% with a correlation coefficient of 0.966. For high temperature and intermediate strain rate, both Model D8A and SCA equation depict the flow behavior of 316L (0.14)N SS with good correlation and generalization. However, at low temperature and high strain rate domain, both the models are unable to depict the behavior; this is attributed to the fact that the material shows two slope behaviors where the constants have been calculated assuming a linear relationship between stress and strain rate.     

Static Recrystallization Behavior of 316LN Austenitic Stainless Steel

The static recrystallization of 316LN austenitic stainless steel was studied by double-pass hot compression tests on a Gleeble-3500 thermomechanical simulator. The specimens were compressed at the deformation temperatures of 950, 1050, 1150 °C, strain rates of 0.01, 0.1, 1s^?1, strains of 0.1, 0.15, 0.2, and intervals of 1 — 100 s. The results show that the volume fraction of static recrystallization of 316LN increases with the increase of deformation temperature, strain rate, strain and interval, which indicates that static recrystallization occurs easily under the conditions of higher deformation temperature, higher strain rate and larger strain. Deformation temperature has significant influence on static recrystallization of 316LN. The volume fraction of static recrystallization could easily reach 100% at higher deformation temperatures. By microstructure analysis, it can be concluded that the larger the volume fraction of static recrystallization, the more obvious the grain refinement. The static recrystallization activation energy of 317 882 J/mol and the exponent n of 0.46 were obtained. The static recrystallization kinetics was established. The predicted volume fraction of static recrystallization is in good agreement with the experimental results.     

Uniaxial Time-Dependent Ratcheting of SS304 Stainless Steel at High Temperatures

The uniaxial time-dependent strain cyclic behaviors and ratcheting of SS304 stainless steel were studied at high temperatures （350 ℃ and 700 ℃）. The effects of straining and stressing rates, holding time at the peak and/or valley of each cycle in addition to ambient temperature on the cyclic softening/hardening behavior and ratcheting of the material were discussed. It can be seen from experimental results that the material presents remarkable time dependence at 700 ℃, and the ratcheting strain depends greatly on the stressing rate, holding time and ambient temperature. Some significant conclusions are obtained, which are useful to build a constitutive model describiog the time-dependent cyclic deformation of the material.     

Ductile Fracture Prediction of 316LN Stainless Steel in Hot Deformation Process

A ductile fracture criterion of 316LN stainless steel, combined with the plastic deformation capacity of ma- terial and the stress state dependent damages, was proposed to predict ductile fracture during hot deformation. To the end, tensile tests at high temperatures were first performed to investigate the fracture behavior of 316LN stain- less steel. The experimental results show the variation of the critical fracture strain as a function of temperature and strain rate. Second, the criterion was calibrated by using the upsetting tests and the corresponding numerical simula- tions. Finally, the proposed fracture criterion was validated by the designed tests and the corresponding finite ele- ment （FE） simulation. The results show that the criterion can successfully predict the onset of ductile fracture at ele- vated temperatures.     

Synthesis of Processable Polyaniline and its Anticorrosion Performance on 316LN Stainless Steel

The soluble polyaniline (PANI) salts were synthesized by chemical-oxidative polymerization of aniline using sodium dodecylbenzene sulfonic acid, camphor sulphonic acid and hydrochloric acid as dopants and ammonium per sulphate (APS) as an oxidant. The synthesized PANI were characterized by X-ray diffraction, scanning electron microscope, Fourier transform-infrared spectroscopy and thermogravimetric analysis. It is found that the yield of PANI, electrical conductivity, solubility and thermal stability varies with the dopants and the stoichiometrics of monomer/dopant ratios. The PANI doped sample follows the solubility order as: PANI–SDBS > PANI–CSA > PANI–HCl. Thermal stability of PANI–CSA is grater than PANI–SDBS and PANI–HCl. XRD pattern shows the higher crystallinity for HCl and CSA doped PANI while SDBS doped PANI shows the poor crystallinity. SEM study revealed the partially crystalline and agglomerated morphology of the polymer. According to the results, the corrosion rate or corrosion current of PANI–SDBS salts coated samples varies with molar ratios of SDBS to aniline and the optimum molar ratio of PANI–SDBS coating to achieve the best anticorrosive performance on 316LN SS is 2.5 mol mol^?1.     

氮对316LN奥氏体不锈钢力学性能和耐蚀性的影响

试验用316LN钢(/%:0.015C、0.65Si、0.90Mn、17.3Cr、12.8 Ni、2.6Mo、0.018~0.200N)由50 kg真空感应炉冶炼,破真空后加入氮化铬铁,铸锭锻成Φ20 mm棒材和热轧成4 mm板材,并分别经1 100℃30 min和10min水淬固溶处理。研究结果表明,316LN不锈钢每增加0.010%的氮,抗拉强度提高9 MPa,屈服强度提高7 MPa;伸长率降低0.55%,氮含量对断面收缩率没有影响,约保持在72.5%;氮强烈提高316LN不锈钢的耐点腐蚀性能,每增加0.010%的氮,其点蚀击穿电位提高7 mV;添加适宜的氮(0.079%N),可以改善316LN不锈钢的耐晶间腐蚀性能,过高的氮含量(超过0.120%N)对晶间腐蚀性能有害。     

316LN高氮超低碳奥氏体不锈钢的热变形行为和组织演变

试验的316LN钢（/%:0.011C,0.49Si,1.52Mn,0.016P,0.001S,16.65Cr,13.43Ni,2.18Mo,0.153N）为7.2 t铸锭初轧开坯并锻造成的Φ180 mm圆钢。利用Gleeble-3800热模拟试验机研究了316LN钢在应变速率0.1~50 s^-1、1050~1200℃时的热变形行为和组织演变规律,并通过线性回归分析建立了316LN钢的本构方程。结果表明,变形温度升高有利于动态再结晶的形核,并在热变形过程中促进动态再结晶扩展;热变形过程材料常数α为0.0061,应力指数n为5.5436,表观激活能550.512 kJ/mol,动态再结晶晶粒数量随着变形温度的增加而增多,到1 200℃变形时发生了完全的动态再结晶。     

On Characterizing a Composite Microstructure in 316LN Stainless Steel Weld Metal and a New Damage Micromechanism During Creep

The austenitic stainless steel weld metal fabricated by multipass welding exhibits a composite microstructure. Microstructural characterization of the weld metal revealed that there are two distinct regions on either side of the weld-pass interface. The variations in dislocation substructure and delta ferrite morphology are the two microstructural attributes which delineate these regions. The generation of subsequent thermal cycles during the fabrication of multipass weld joint is the paramount factor influencing the formation of the composite microstructure. During creep exposure, the extent of creep cavitation and propagation varies substantially in these two regions due to differences in their microstructures. This results in preferential damage during creep exposure of austenitic stainless steel weld metal.     

Influence of Inclusion and Specimen Orientations on Intergranular Corrosion Testing of AISI 316LN Stainless Steel

Fabricated components must be free from sensitization for using these in critical applications in aggressive environments. During fabrication of a hollow bar from solid bar, deep hole drilling was employed which introduces residual stresses. Stress-relieving heat treatment was employed by heating the hollow bar from room temperature to 1,065°C @ 40°C/h and soaking at 1,065°C for 1 h followed by cooling @ 40°C/h as well as 70°C/h. To detect the susceptibility to IGC, specimens were taken from both circumferential direction as well as longitudinal direction and subjected to ASTM A 262 Practice E test. In U bend, the specimens from the circumferential direction failed whereas longitudinal specimens did not fail. However specimens of both orientations showed Step structure in Practice A test indicating that no carbide has nucleated during the stress-relieving heat treatment ensuring that the cooling rates are faster than the critical cooling rates and the material is not susceptible to IGC. EDAX studies indicated the presence of numerous MnS inclusions enriched in chromium which might have led to chromium depletion around the inclusions resulting in poor passivity at these locations. This study presents the influence of orientation of MnS inclusions in causing failure in U bend test. The need to select specimens of correct orientation during IGC testing is emphasized in this investigation.     

Correlation of Electrochemical Noise Parameters with Degree of Sensitization in AISI Type 316LN Stainless Steel

Degree of sensitization in 316LN stainless steel (SS) specimens sensitized at 898, 923 and 948 K for 500 h was obtained using double loop electrochemical potentiokinetic reactivation (DLEPR) technique as 7.4, 14.5 and 9.3% respectively. The sensitized specimens were pulsed polarized so that only Cr-depleted regions of the sensitized grain boundaries contributed to the electrochemical noise (EN) study. The DOS values evaluated from DLEPR technique were correlated with the EN parameters viz. standard deviation of current, σ_I and characteristic charge, q and characteristic frequency, f _n , obtained from shot noise analysis in order to assess the extent of intergranular corrosion (IGC) attack in 316LN SS. The plot of σ_I versus time showed highest σ_I values for the specimen sensitized at 923 K for 500 h, indicating high grain boundary dissolution and hence, severe IGC attack, whereas the specimen sensitized at 898 K for 500 h showed the least σ_I values indicating lower dissolution and least IGC attack. 316LN SS specimen heat-treated at 948 K for 500 h showed intermediate grain boundary dissolution rate. The charge q, determined from σ_I versus time plot showed a good correlation (>99%) with the DOS values obtained from DLEPR experiments. The power spectral density values of the current signal in the frequency independent region were found to have excellent correlation with these observations. The above findings were further supported by scanning electron microscopic examination which showed an increase in grain boundary width in the sensitized specimens when the heat-treatment temperature was raised from 898 to 923 K and on further increasing the temperature to 948 K, a marginal decrease in the grain boundary width was observed.     

Effect of Arc Welding Processes on the Weld Attributes of Type 316LN Stainless Steel Weld joint

The present study aims at understanding the effect of various arc welding processes on the evolution of microstructure, mechanical properties, residual stresses and distortion in 9 mm thick type 316LN austenitic stainless steel weld joints. Weld joints of type 316LN stainless steel were fabricated by three different arc welding processes which were commonly employed in the nuclear industry. All the weld joints passed radiographic examination. Microstructural characterization was done using optical and scanning electron microscope. Volume fraction of δ-ferrite was lowest in the A-TIG weld joint. The A-TIG welded joint exhibited adequate strength and maximum impact toughness values in comparison to that of weld joints made by SMAW and FCAW processes. The A-TIG weld joint was found to exhibit lowest residual stresses and distortion compared to that of other welding processes. This was attributed to lower weld metal volume and hence reduced shrinkage in the A-TIG weld joint compared to that of weld joints made by FCAW and SMAW processes which involved v-groove with filler metal addition. Therefore, type 316LN stainless steel A-TIG weld joint consisting of lower δ-ferrite, adequate strength, high impact toughness, lower residual stresses and distortion was suited better for elevated temperature service compared to that of SMAW and FCAW weld joints.     

Characteristics of Mechanical Properties and Microstructure for 316L Austenitic Stainless Steel

 A comparative study on mechanical properties and microstructure of 316L austenitic stainless steel between solution treated specimen and hot rolled specimen was conducted. After a specimen was subjected to solution treatment at 1050 ℃ for 6 min, its mechanical properties were determined through tensile and hardness tests. Based on the true stress vs true strain and engineering stress vs engineering strain flow curves, the work hardening rate has been explored. The results show that the solution treated specimen has an excellent combination of strength and elongation, and that this steel is easy to work-hardening during deformation. Optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffraction examinations were conducted, these reveal that twins in 316L austenitic stainless steel can be divided into suspended twin and transgranular twin which have different formation mechanisms in growth, and that the deformation induced martensite nucleated and grown in the shear band intersections can be observed, and that the fracture surfaces are mainly composed of dimples and exhibit a tough fracture character.     

Comparison of SS 304 LN and 316 LN as choice of structural material for primary pipe in fast breeder reactors

In pool type FBRs primary sodium pumps (PSP), operating in parallel, circulate the sodium through the core to remove the nuclear heat. The pumps suck the sodium from cold pool and supply it to a spherical header at the bottom; subsequently the sodium flows through pipes from the spherical header into the grid plate, before entering the core subassemblies. Under normal operating condition, each pipe is subjected to 0.8 MPa internal pressure of sodium flowing at 670 K. As the pipe is located in the cold pool, it is at an isothermal temperature of 670 K during normal operation. The pipe is subjected to hot shock during two thermal transients Viz. offsite power failure (160 times) and loss of Steam water system (47 times). Both these events lead to Safety Grade Decay Heat Removal (SGDHR) operation and the cold pool reaches 791 K during offsite power failure and 816 K during loss of steam water system. Even though the normal operating temperature for the primary pipes conveying sodium from spherical header to grid plate is below creep regime, creep damage occurs due to the governing hot shocks discussed above. One of the criteria for selecting the material is the creep fatigue damage for the longer design life (40 years for the Prototype Fast Breeder Reactor under construction and 60 years for the future reactors). The material choice for out of core reactor assembly components are generally austenitic stainless steels (SS316LN or SS304LN) except for the top shield. In this paper, the choice between SS 304 LN Vs SS 316 LN for primary sodium pipe with respect to the governing failure mode of creep-fatigue interaction is addressed.     

Impact Response and Microstructural Evolution of 316L Stainless Steel under Ambient and Elevated Temperature Conditions

The impact response and microstructural evolution of 316L stainless steel are examined at strain rates ranging from 1?×?10³ to 5?×?10³?s^?1 and temperatures between 298?K and 1073?K (25?°C and 800?°C) using a split Hopkinson pressure bar and transmission electron microscopy (TEM). The results show that the flow behavior, mechanical strength, and work-hardening properties of 316L stainless steel are significantly dependent on the strain rate and temperature. The TEM observations reveal that the dislocation density increases with increasing strain rate but decreases with increasing temperature. Moreover, twinning occurs only in the specimens deformed at 298?K (25?°C), which suggests that the threshold stress for twinning is higher than that for slip under impact loading. Finally, it is found that the volume fraction of transformed ???? martensite increases with increasing strain rate or decreasing temperature. Overall, the results suggest that the increased flow stress observed in 316L stainless steel under higher strain rates and lower temperatures is determined by the combined effects of dislocation multiplication, twin nucleation and growth, and martensite transformation.     

On the Relationship Between Cyclic Deformation Behavior and Slip Mode in 316LN Stainless Steel with Varying Nitrogen Content

The relationship between cyclic deformation, slip-mode and dislocation structures is investigated in 316LN stainless steel (with 0.07–0.22 wt% Nitrogen) subjected to low cycle fatigue at temperatures in the range 300–873 K and at a 0.6 % strain amplitude. Irrespective of the nitrogen content, cyclic softening/saturation occupied a large fraction of fatigue life at temperatures <773 K. The end-of-life dislocation structures (e.g. dislocation cells, planar slip-bands) characterizing the cyclic softening/saturation belong to wavy/mixed/planar slip-modes of deformation. On the other hand at temperatures ≥773 K, similar dislocation structures are noticed to be associated with significant cyclic strengthening with limited softening. The differences in the above deformation behavior is found to be controlled not by the nature of slip-mode but by the consequences of dynamic strain aging occurrence (e.g. significant cyclic strengthening and pronounced serrations) which are noticed to vary in the temperature range 573–873 K. Maximum fatigue life is observed at 0.11–0.14 wt% N that induced mixed mode of deformation.     

新型奥氏体耐热不锈钢的高温持久性能

以0Cr21Ni11SiNRE为对象,研究了该种新型奥氏体耐热不锈钢高温持久性能,分析了持久强度σ与持久寿命τ及Larson-Miller参数P之间的关系,并以此为基础预测了该钢种在750～927℃,103h下的持久强度。结果表明,在本试验条件下,0Cr21Ni11SiNRE的蠕变机制大体相同,其持久强度σ与持久寿命τ的对数,L-M参数P呈线性关系,其中,σ与P之间关系可表示为σ=-20.66×10-3P+589.25(2.33×104P2.7×104);通过回归计算得到其蠕变激活能Q=548kJ/mol,激活体积分数V=940mm3/mol。     

Influence of Laser Peening With and Without Coating on the Surface Properties and Stress Corrosion Cracking Behavior of Laser-Welded 304 Stainless Steel

Metallurgical and Materials Transactions A - This study aims to investigate the effect of laser peening with (LP) and without coating (LPwC) process on subsurface microstructural evolution,...