Creep deformation and fracture behavior of types 316 and 316L(N) stainless steels and their weld metals

The creep properties of a nuclear-grade type 316(L) stainless steel (SS) alloyed with nitrogen (316L(N) SS) and its weld metal were studied at 873 and 923 K in the range of applied stresses from 100 to 335 MPa. The results were compared with those obtained on a nuclear-grade type 316 SS, which is lean in nitrogen. The creep rupture lives of the weld metals were found to be lower than those of the respective base metals by a factor of 5 to 10. Both the base and weld metals of 316L(N) SS exhibited better resistance to creep deformation compared to their 316 SS counterparts at identical test conditions. A power-law relationship between the minimum creep rate and applied stress was found to be obeyed for both the base and weld metals. Both the weld metals generally exhibited lower rupture elongation than the respective base metals; however, at 873 K, the 316 SS base and weld metals had similar rupture elongation at identical applied stresses. Comparison of the rupture lives of the two steels to the ASME curves for the expected minimum stress to rupture for 316 SS base and weld metals showed that, for 316L(N) SS, the specifications for maximum allowable stresses based on data for 316 SS could prove overconservative. The influence of nitrogen on the creep deformation and fracture behavior, especially in terms of its modifying the precipitation kinetics, is discussed in light of the microstructural observations. In welds containing δ ferrite, the kinetics of its transformation and the nature of the transformation products control the deformation and fracture behavior. The influence of nitrogen on the δ ferrite transformation behavior and coarsening kinetics is also discussed, on the basis of extensive characterization by metallographic techniques.     

Effects of temperature and strain rate on tensile properties and activation energy for dynamic strain aging in alloy 625

Alloy 625 ammonia cracker tubes were service exposed for 60,000 hours at 873 K. These were then subjected to a solution-annealing treatment at 1473 K for 0.5 hours. The effects of temperature and strain rate on the tensile properties of the solution-annealed alloy were examined in the temperature range of 300 to 1023 K, employing the strain rates in the range of 3×10⁻⁵ s⁻¹ to 3×10⁻³ s⁻¹. At intermediate temperatures (523 to 923 K), various manifestations of dynamic strain aging (DSA) such as serrated flow, peaks, and plateaus in the variations of yield strength (YS) and ultimate tensile strength (UTS) and work-hardening rate with temperature were observed. The activation energy for serrated flow (Q) was determined by employing various methodologies for T<823 K, where a normal Portevien-Le Chatelier effect (PLE) was observed. The value of Q was found to be independent of the method employed. The average Q value of 98 kJ/mol was found to be in agreement with that for Mo migration in a Ni matrix. At elevated temperatures (T≥823 K), type-C serrations and an inverse PLE was noticed. The decrease in uniform elongation beyond 873 K for 3×10⁻⁵ s⁻¹ and 3×10⁻³ s⁻¹ and beyond 923 K for 3×10⁻⁴ s⁻¹ strain rates seen in this alloy has been ascribed to reduction in ductility due to precipitation of carbides and δ phase on the grain boundaries.     

Effect of Strain Rate on the Dynamic Recrystallization Behavior in a Nitrogen-Enhanced 316L(N)

In this paper, the effect of strain rate (in the domain of 0.001 to 10 s^?1) on dynamic recrystallization (DRX) kinetics in a nitrogen-enhanced 316L(N) austenitic stainless steel during high temperature [≥1123 K (≥850 °C)] deformation is reported. In the low strain rate domain (i.e., <0.1 s^?1), the DRX is predominantly governed by higher growth of DRX grains resulting in a higher DRX fraction and larger DRX grain size. On the other hand, DRX at higher strain rates (i.e., ≥1 s^?1) is mainly controlled by higher nucleation resulting in higher DRX fraction with a finer grain size. In the intermediate strain rate regime of 0.1 s^?1, sluggish kinetics of DRX is observed since neither the nucleation nor the growth of DRX grains is predominant. The annealing twinning event, which may accelerates the DRX kinetics, is also observed to occur more frequently during the low and high strain rate deformations.     

Concerning the strength of dynamic strain aging in zirconium

Tensile tests on high purity (6 x 10^-4 oxygen equivalent) and commercial purity (6 x 10^-3 oxygen equivalent) zirconium were performed between 77 and 1000 K in order to evaluate dynamic strain aging. A comparison with earlier data from two equivalent titanium compositions yielded the following; reducing the interstitial concentration to the zone refined iodide level removes most evidence of strain aging in both zirconium and titanium. At this impurity concentration, zirconium also shows a greatly reduced thermally activated flow stress component. This was not observed in titanium. At the commercial purity level, both metals exhibit strain aging phenomena. These are much weaker, however, in Zr than in Ti. Even at this impurity level Zr does not exhibit a strain aging yield point, the Portevin-Le Chatelier effect or a well defined work hardening rate peak. All of these latter are found in commercial purity Ti. Several other aspects of DSA, while observed in Zr, are less pronounced than in Ti. The principal interstitial impurity in these materials is oxygen. Oxygen in solid solution strongly increases the (c/a) ratio of Ti, but has little effect on this ratio in Zr. This distortion of the Ti hcp lattice may account, in part, for the greater strength of DSA in this metal. A. M. GARDE, formerly a Post Doctoral Fellow at the University of Florida. E. AIGELTINGER, formerly a Post Doctoral Fellow at the University of Florida. B. N. WOODRUFF, formerly a student at the University of Florida.     

Concerning the strength of dynamic strain aging in zirconium

Tensile tests on high purity (6 × 10⁻⁴ oxygen equivalent) and commercial purity (6 × 10⁻³ oxygen equivalent) zirconium were performed between 77 and 1000 K in order to evaluate dynamic strain aging. A comparison with earlier data from two equivalent titanium compositions yielded the following; reducing the interstitial concentration to the zone refined iodide level removes most evidence of strain aging in both zirconium and titanium. At this impurity concentration, zirconium also shows a greatly reduced thermally activated flow stress component. This was not observed in titanium. At the commercial purity level, both metals exhibit strain aging phenomena. These are much weaker, however, in Zr than in Ti. Even at this impurity level Zr does not exhibit a strain aging yield point, the Portevin-Le Chatelier effect or a well defined work hardening rate peak. All of these latter are found in commercial purity Ti. Several other aspects of DSA, while observed in Zr, are less pronounced than in Ti. The principal interstitial impurity in these materials is oxygen. Oxygen in solid solution strongly increases the(c/a) ratio of Ti, but has little effect on this ratio in Zr. This distortion of the Ti hcp lattice may account, in part, for the greater strength of DSA in this metal. Formerly a Post Doctoral Fellow at the University of Florida. Formerly a Post Doctoral Fellow at the University of Florida. Formerly a student at the University of Florida.     

Manifestations of dynamic strain aging in soft-oriented NiAl single crystals

The tensile and compressive properties of six NiAl-base single-crystal alloys have been investigated at temperatures between 77 and 1200 K. The normalized critical resolved shear stresses (CRSS/E) and work-hardening rates (θ/E) for these alloys generally decreased with increasing temperature. However, anomalous peaks or plateaus for these properties were observed in conventional purity (CPNiAl), Si-doped (NiAl-Si), C-doped low Si (UF-NiAll), and Mo-doped (NiAl-Mo) alloys at intermediate temperatures (600 to 1000 K). This anomalous behavior was not observed in high-purity, low interstitial material (HP-NiAl). Low or negative strain-rate sensitivities (SRS) also were observed in all six alloys in this intermediate temperature range. Coincident with the occurrence of negative strain-rate sensitivities was the observation of serrated stress-strain curves in the CPNiAl and NiAl-Si alloys. These phenomena have been attributed to dynamic strain aging (DSA). Chemical analysis of the alloys used in this study suggests that the main specie responsible for strain aging in NiAl is C but indicate that residual Si impurities can enhance the strain aging effects. The corresponding dislocation microstructures at low temperatures (300 to 600 K) were composed of welldefined cells. At intermediate temperatures (600 to 900 K), either poorly defined cells or coarse bands of localized slip, reminiscent of the vein structures observed in low-cycle fatigue specimens deformed in the DSA regime, were observed in conventional purity, Si-doped, and in Mo-doped alloys. In contrast, a well-defined cell structure persisted in the low interstitial, high-purity alloy. At elevated temperatures (≥1000 K), more uniformly distributed dislocations and sub-boundaries were observed in all alloys. These observations are consistent with the occurrence of DSA in NiAl single-crystal alloys at intermediate temperatures.     

连铸过程铸坯应变对碳氮化铌析出行为的影响

通过应变诱导析出模型分析了连铸过程铸坯应变对碳氮化铌析出的影响,结果表明:拉速的改变对碳氮化铌的最快析出时间具有一定的影响,在同样的应变及应变温度下,拉速越快碳氮化铌最快析出时间越短;在连铸过程的矫直段及弯曲段,由于铸坯应变及应变速率较小,在研究连铸过程碳氮化铌析出行为时铸坯应变对碳氮化铌析出行为的影响可以忽略.     

Process Parameter Optimization of AISI 316L(N) Weld Joints Produced using Flux-Cored Arc Welding Process

Bead on plate welds were carried out on AISI 316L(N) austenitic stainless steel using flux cored arc welding process. The bead on plates weld was conducted as per L₂₅ orthogonal array with statistical design of experiment technique. In this paper, the welding parameters will be optimized based on the weld bead geometry such as depth of penetration, bead width and weld reinforcement. Grey relational analysis and desirability approach are used to optimize the input parameters like wire feed rate, voltage, travel speed and torch angle while considering the multiple output variables simultaneously. Confirmation experiment has also been conducted to validate the optimized parameters.     

Low cycle fatigue and creep-fatigue interaction behaviour of 316L(N) stainless steel and its welds

High temperature low cycle fatigue (LCF) is influenced by various time dependent processes such as creep, oxidation, phase transformations and dynamic strain ageing (DSA) depending on test conditions of strain rate and temperature. In this paper the detrimental effects of DSA and oxidation in high temperature LCF are discussed with reference to extensive studies on 316L(N) stainless steel. DSA has been found to enhance the stress response and reduce ductility. It localizes fatigue deformation, enhances fatigue cracking and reduces fatigue life. High temperature oxidation accelerates transgranular and intergranular fatigue cracking during long hold time tests in austenitic stainless steel. In welds, microstructural features such as presence of coarse grains and formation of brittle phases due to transformation of δ ferrite during testing influence crack initiation, propagation and fatigue life.     

The effect of dynamic annealing on dynamic strain aging phenomena in commercial purity titanium

A study has been made of the effect of dynamic annealing, associated with climb, on the dynamic strain aging phenomena in commercial purity titanium. In speciemens deformed at a strain-rate of 10⁻⁴ s⁻¹ with an average grain diam of about 17 μm, dynamic annealing starts to become important at the temperature of the major work hardening peak maximum. Metallographic evidence supports a conclusion that the shape of the high temperature side of the peak is determined largely by climb controlled processes. Reducing the grain size to 6 μm lowers the temperature, at which dynamic annealing becomes significant, enough so that the work hardening peak, the “blue-brittle” ductility minimum and the yield stress plateau are either nearly eliminated or greatly reduced in importance.     

The activation energy for creep of columbium (Niobium)

The activation energy for creep of nominally pure columbium (niobium) was determined in the temperature range 0.4 to 0.757T_M by measuring strain rate changes induced by temperature shifts at constant stress. A peak in the activation energy vs temperature curve was found with a maximum value of 160 kcal/mole (672 kJ/mole). A pretest heat treatment of 3000F (1922 K) for 30 min (1800 s) resulted in even higher values of activation energy (>600 kcal/mole, 2520 kJ/mole) in this temperature range. The activation energy for the heat-treated columbium (Nb) could not be determined near 0.5T_M because of unusual creep curves involving negligible steady-state creep rates and failure at < 5 pct creep strain. It is suggested that the anomalous activation energy values and the unusual creep behavior in this temperature range are caused by dynamic strain aging involving substitutional atom impurities and that this type of strain aging may be in part responsible for the scatter in previously reported values of activation energy for creep of columbium (Nb) near 0.5TM.     

核电主管道用316LN不锈钢焊缝热老化脆化的趋势

开展了核电站主管道用316LN不锈钢焊缝在325、365和400℃下1.5万h的加速热老化试验,测量了不同热老化时间下焊缝的冲击性能和焊缝铁素体的纳米硬度。以纳米硬度作为热老化脆化参量,利用Arrhenius方程得出该焊缝热老化激活能约为93.1kJ/mol;并以焊缝室温冲击功为预测参数,通过拟合的方法获得了焊缝热老化脆化预测方程;利用热老化激活能和热老化脆化预测方程预测了主管道用316LN焊缝在服役温度下60年寿命期内的热老化脆化趋势。试验结果表明,随着热老化时间的增加,焊缝的冲击韧性显著下降,焊缝中铁素体纳米硬度快速增加。预测结果表明,在运行15年内主管道焊缝韧性迅速下降,在随后的运行过程中下降趋缓。     

316L不锈钢金属间相固溶过程中的微结构表征

利用扫描电镜、X射线衍射仪和电子背散射衍射仪等设备研究了热轧态316L不锈钢金属间相(σ相和χ相)在固溶过程中的微结构演变规律,同时,利用高温激光共聚焦显微镜对金属间相回溶的全过程进行了原位在线观察,确定了最佳固溶温度。结果表明,固溶过程中发生了γ+σ+χ→γ+σ+χ+α/FeCr→γ+α/FeCr的相变过程。金属间相的回溶从1033.1℃开始到1149.5℃结束,回溶时间为21s,消除金属间相的最佳固溶温度约为1150℃。固溶处理前,基体中分布的带状组织形态不规则,χ相和σ相的面积比分别为0.46%和0.94%;固溶处理后,带状组织形态一致,主要分布铁素体和FeCr相,而χ相和σ相不可见。     

Theoretical and experimental research of molybdenum oxide alloying for 316L stainless steel smelting process

Thermodynamic studies were carried out to investigate the effects of temperature, molten metal composition on the relationship among MoO3, CaMoO4 and ??Fe??, ??Mn??, ??C??, ??Si??, ??Cr?? during the AOD remelting process. The calculated results show that MoO3 and CaMoO4 can easily be reduced by its reactions with active alloying elements during the refining process of 316L stainless steel. First of all, the feasibility of molybdenum oxide alloying for 316L stainless steel smelting was proved theoretically. Then an industrial test of 316L stainless steel alloyed with molybdenum oxide was performed in a 180t AOD furnace. The results show that the molybdenum oxide has no influence on composition of inclusions in the steel and quality of the cold rolling plate. Above all, molybdenum oxide used as alloying material during AOD of 316L stainless steel is applied in TISCO factory, which can relieve the environmental contamination burden from ferro- molybdenum alloying during the AOD process.     

Influence of Strain Rate and Temperature on Tensile Deformation and Fracture Behavior of Type 316L(N) Austenitic Stainless Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10^?5 to 3.16 × 10^?3 s^?1 over the temperatures ranging from 300 K to 1123 K (27 °C to 850 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of nitrogen-alloyed low carbon grade type 316L(N) austenitic stainless steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The steel exhibited distinct low- and high-temperature serrated flow regimes and anomalous variations in terms of plateaus/peaks in flow stress/strength values and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. The fracture mode remained transgranular. At high temperatures, the dominance of dynamic recovery is reflected in the rapid decrease in flow stress/strength values, work hardening rate, and increase in ductility with the increasing temperature and the decreasing strain rate.     

Fabrication and Characterization of Nanostructured AACVD Thin Films on 316L SS as Surface Protective Layers in Simulated Body Fluid

Metallurgical and Materials Transactions A - The surface of implant materials is one of the most significant factors for controlling the interaction between biomaterials and bone tissues. Hence,...     

Stage I corrosion fatigue crack crystallography in austenitic stainless steel (316L)

The crystallographic and fractographic aspects of stage I corrosion fatigue of SS 316L have been studied with the aid of etch pitting techniques. Single edge notch specimens were fatigued at 123 Hz in desiccated air and near neutral (pH 5.5) aqueous environments of 1 M NaCl and 1 M NaCl + 0.01 M Na₂S₂O₃. The fractographic features were found to be independent of the testing conditions. The orientation of primary facets and the crystallographic directions of crack propagation were identified. Six combinations of crack plane and direction were observed with primary facet orientations of {111}, {110}, and {001}. The primary facet crystallography was shown to arise from microscale propagation along a single variant of {111} or alternating periods of microscale propagation along a conjugate pair of {111} variants. The mechanism of cracking was discussed and shown to be consistent with a model of restricted slip reversibility (RSR), where environmental interactions with emergent slip steps influence the amount of slip reversibility.     

Characterisation of 316L powder injection moulding feedstock for purpose of numerical simulation of PIM process

Abstract

Viscosity, specific heat and thermal conductivity of the standard feedstock of 316L stainless steel have been measured under the typical conditions of a real powder injection moulding (PIM) process. The viscosity was measured in a wide range of shear rates at four different temperatures. The experimental viscosity data were fitted into the Carreau-Yasuda model. Both specific heat and thermal conductivity were measured in the temperature range that overlaps the recommended processing range for the studied feedstock. It has been shown that at high cooling rates the transition temperature of the binder material is shifted towards lower temperatures. Tabulated values of thermal conductivity and specific heat for the studied feedstock are presented. The obtained data can be used for numerical simulation of the powder injection moulding process.     

Small Punch Creep Studies for Optimization of Nitrogen Content in 316LN SS for Enhanced Creep Resistance

Small punch creep (SPC) studies have been carried out to evaluate the creep properties of 316LN stainless steel (SS) at 923 K (650 °C) at various stress levels. The results have been compared with uniaxial creep rupture data obtained from conventional creep tests. The minimum deflection rate was found to obey Norton power law. SPC rupture life was correlated with uniaxial creep rupture life. The influence of nitrogen content on the creep rupture properties of 316LN SS was investigated in the range of 0.07 to 0.14 wt pct. SPC rupture life increased and the minimum deflection rate decreased with the increase in nitrogen content. The trends were found to be in agreement with the results obtained from uniaxial creep rupture tests. These studies have established that SPC is a fast and reliable technique to screen creep properties of different experimental heats of materials for optimizing the chemical composition for developing creep-resistant materials.     

High frequency stage I corrosion fatigue of austenitic stainless steel (316L)

High frequency (123 Hz) fatigue crack propagation studies were conducted under rising ΔK conditions (R-ratio = 0.22) on single edge notch specimens of austenitic stainless steel (type 316L) that contained an annealed precrack. Tests were conducted in near neutral (pH 5.5) solutions of 1 M NaCl and 1 M NaCl + 0.01 M Na₂S₂O₃ under potentiostatically controlled conditions and in desiccated air. Attention was directed primarily to the near threshold behavior and the stage I (crystallographic) region of cracking. Good mixing between the crack solution and bulk solution was obtained and crack retardation and arrest effects, due to surface roughness induced closure, were minimized at high anodic potentials by electrochemical erosion. Thermodynamic considerations showed that hydrogen played no role in fatigue crack propagation. Analysis of the results in terms of the estimated effective cyclic stress intensity, ΔK _eff, showed a systematic effect of potential on the average crack growth increment per cycle,da/dN. Anodic dissolution processes were considered to make an insignificant contribution toda/dN. A model was proposed for stage I fatigue cracking based on the effect of oxide nucleation rate on restricted slip reversal. The essential features of the model were considered to be relevant to cracking in aqueous environments and in desiccated air.