Evaluation of variation of tensile strength across 316LN stainless steel weld joint using automated ball indentation technique

Tensile strength variation across 316LN stainless steel fusion welded joint comprising of base metal, deposited weld metal and heat affected zone (HAZ) has been evaluated by Automated Ball Indentation (ABI) technique. Automated Ball Indentation tests were conducted on the various zones of the steel weld joint at 300, 523 and 923?K. The flow curves obtained from ABI results were consistent with corresponding conventional uniaxial tensile test results. The HAZ exhibited higher tensile strength than the other regions of the steel weld joint at all investigated temperatures. The ratio of ultimate tensile strength to yield stress (YS), which represents the work hardening behaviour, increased with an increase in temperature for the base metal and HAZ; whereas it remained nearly the same for the weld metal.     

Mechanical properties characterisation of welded joint of austenitic stainless steel using instrumented indentation technique

The instrumented indentation test is a promising nondestructive technique for evaluating mechanical properties of metallic materials. In this study, the localised mechanical properties of welded joint of 304 austenitic stainless steel were characterised with the instrumented indentation test. The single V-groove welded joint was produced using the electric arc welding. A series of instrumented indentation tests were carried out at different regions, including base material, weld zone and heat-affected zone (HAZ). A soft zone regarding strength properties was found in the coarse-grain HAZ. The results show that the HAZ has the lowest yield strength and tensile strength (263.6 MPa, 652.5 MPa) compared with the base material (307.4 MPa, 807.9 MPa) and the weld zone (285.6 MPa, 702.1 MPa). In addition, characterisations of microstructure, microhardness and conventional tensile tests have been performed for comparison. The results reveal that the localised mechanical properties of welded joint of austenitic stainless steel can be represented effectively with the instrumented indentation technique.     

Creep properties and design curves for nitrogen enhanced grade of 316LN stainless steel

Abstract

316L(N) stainless steel (SS) containing 0·02–0·03 wt-% carbon and 0·06–0·08 wt-% nitrogen is the principal material for the high temperature structural components of the prototype fast breeder reactor in India. In order to increase the economic competitiveness of sodium cooled fast reactors (SFRs), there is a strong desire to increase the design life from the current level of 40 years to at least 60 years for the future reactors. As a part of the efforts to develop materials with superior mechanical properties suitable for longer design life, the influence of nitrogen at concentrations higher than 0·07 wt-%, on the high temperature mechanical properties of type 316L(N) SS is being studied. Four heats of 316L(N) SS, containing 0·07, 0·11, 0·14 and 0·22 wt-% nitrogen have been evaluated extensively in terms of their tensile, creep, low cycle fatigue and creep fatigue interaction properties. Based on these studies, the nitrogen content has been optimised at 0·14 wt-%. This nitrogen enhanced grade of steel (NE316LN SS) was found to have significantly better tensile, creep, low cycle fatigue and creep-fatigue properties as compared to the PFBR grade of 316L(N) SS. This paper presents the influence of nitrogen on the creep deformation, damage and fracture behaviour of NE316LN SS. Design of high temperature SFR components is made on the basis of RCC-MR design code. The creep properties of NE316LN SS have been analysed in terms of the procedures for generation of the design code. Time–dependent design curves have been generated.     

Evaluation of tensile properties of austenitic stainless steel 316L with linear hardening by modified indentation method

Abstract

The instrumented indentation test has been widely used for the non-destructive evaluation of the tensile properties of metal materials. The true stress–strain curve, yield strength and tensile strength can be obtained by this method. In the present study, a new modified indentation algorithm was used to determine the tensile properties of austenitic stainless steel 316L by taking into account its linear hardening characteristic. As received 316L was solution treated at four different temperatures in the range of 1223–1473K for 2 h followed by water quenching. The effect of solution treatment temperature on the tensile properties of 316L was investigated by the instrumented indentation test using the modified indentation algorithm. Results reveal that the new modified indentation algorithm can be used to estimate the tensile properties of austenitic stainless steel with linear hardening.     

Effect of notch root radius on tensile behaviour of 316L(N) stainless steel

Abstract

Type 316L(N) stainless steel (SS) is used as the major structural material for high temperature components of sodium cooled fast reactors. The influence of notch root radius on the tensile behaviour of 316L(N) SS under multi-axial stress state was investigated. Double U-notches with five different kinds of notch geometry were incorporated symmetrically into the tensile testing specimens by changing the notch root radius while keeping the gross diameter, net diameter and notch depth as the same for all the notches. The notch root radius was varied as 0·25, 0·5, 1·25, 2·5 and 5 mm. Tensile tests were carried out on the notched specimens at room temperature (298 K) and at 923 K at a constant strain rate of 3×10^?3 s^?1. The tensile strength and yield strength of notched specimen of 316L(N) SS increased with decrease in notch radius at both the temperatures and the notch severity was less pronounced at high temperature. The fractured notch surface was analysed using scanning electron microscope and unfractured notch was sliced along the axis and observed under optical microscope. Finite element analysis was performed on the models of notched specimens with various notch root radii. These results showed that Von Mises equivalent stress which was derived from triaxial stresses decreased with decrease in notch radius. The shift of location of peak values of maximum principal stress and hydrostatic stress towards the axis of the specimen, leading to formation of cracks, occurred at a lower nominal stress when the notch radius was increased.     

An assessment of mechanical properties of P92 steel weld joint and simulated heat affected zones by ball indentation technique

Mechanical properties of P92 steel weld joint fabricated by shielded metal arc welding were evaluated using ball indentation (BI) technique. Microstructure of the P92 weld joint consisted of the weld metal, coarse grain region, fine grain region, intercritical region and base metal. The individual microstructural regions of the heat affected zones (HAZs) were separately prepared by heat treating the steel at particular temperatures. Ball indentation and uniaxial tensile tests were carried out across the weld joint and on the simulated HAZ microstructures at temperatures of 300 K (27 °C) and 623 K (350 °C). The tensile strengths gradually decreased from weld metal to the base metal with trough in the intercritical region (ICR) of the joint and simulated at 1173 K (900 °C) steel. The formation of coarser M₂₃C₆ precipitates and sub-grain formation with reduced dislocation density led to soften the ICR/simulated at 1173 K (900 °C) steel than the other regions of the joint. The variation of mechanical properties across the joint was comparable with variation of hardness and microstructural constituents across the joint.     

Impression creep deformation behaviour of 316LN stainless steel

The impression creep deformation behaviour of 316LN SS was investigated from microstructure, substructure, microhardness and profilometry studies of the creep deformed region. Impression creep tests were conducted on 316LN SS in the temperature range of 923–973?K, at different punching stresses in the range of 472–760?MPa. The impression creep deformation was characterised by a hemispherically shaped plastic zone which developed around the indentation. The study revealed the distinct regions under the punch undergoing deformation to different extents. The deformation was found to occur predominently on (111) planes. The dislocations in the highly deformed region were well dispersed in the matrix. The size of the plastic zone was estimated to be ～1·5 times the diameter of the indenter based on the microhardness and profilometry studies. The critical spacing to be maintained between the adjacent indentations was estimated to be >5 times the diameter of indenter.     

Indentation technique for evaluation of hot hardness and creep of SS 316 end-plug welds of nuclear,fuel pins

The high-temperature mechanical properties of SS 316 end-plug welds of nuclear fuel pins were studied using a hot hardness tester. The hardness was measured as a function of temperature on the base metal, weld pool and heat-affected zone from ambient temperature to 1273 K. Hardness was also measured as a function of dwell time from 873 to 1173 K to evaluate the activation energy for creep. The activation energy of the weld pool was found to be higher than that of the base metal. The indentation technique is very suitable for the evaluation of creep properties of very small components.     

Influence of nitrogen on tensile properties of 316LN SS

Abstract

The influence of nitrogen on tensile properties of 316L stainless steels has been studied for nitrogen levels of 0·07, 0·11, 0·14 and 0·22 wt-%. Tensile tests have been carried out at several temperatures in the range 300–1123 K. Nitrogen was found to be beneficial for tensile strength at all the test temperatures. Yield strength and ultimate tensile strength were found to increase linearly with increase in nitrogen content at all the test temperatures. Tensile ductility showed a non-monotonic variation with nitrogen content and test temperature. Equations have been developed to predict yield strength and ultimate tensile strength of 316L stainless steel as a function of nitrogen content and tensile test temperature.     

Plastic deformation and fracture behaviors of nitrogen-alloyed austenitic stainless steels

The plastic deformation and fracture behaviors of two nitrogen-alloyed austenitic stainless steels, 316LN and a high nitrogen steel (Fe–Cr–Mn–0.66% N), were investigated by tensile test and Charpy impact test in a temperature range from 77 to 293 K. The Fe–Cr–Mn–N steel showed ductile-to-brittle transition (DBT) behavior, but not for the 316LN steel. X-ray diffraction (XRD) confirmed that the strain-induced martensite occurred in the 316LN steel, but no such transformation in the Fe–Cr–Mn–N steel. Tensile tests showed that the temperature dependences of the yield strength for the two steels were almost the same. The ultimate tensile strength of the Fe–Cr–Mn–N steel displayed less significant temperature dependence than that of the 316LN steel. The strain-hardening exponent increased for the 316LN steel, but decreased for the Fe–Cr–Mn–N steel, with decreasing temperature. Based on the experimental results and the analyses, a modified scheme was proposed to explain the fracture behaviors of austenitic stainless steels.     

Numerical simulation with finite element and artificial neural network of ball indentation for mechanical property estimation

A combined mechanical property evaluation methodology with ABI (Automated Ball Indentation) simulation and Artificial Neural Network (ANN) analysis is evolved to evaluate the mechanical properties for Carbon Manganese Steel (SA-333 Grade-6) and Stainless Steel (SS-304LN). The experimental load deflection data is converted into meaningful mechanical properties for these materials and their evaluated property is verified with experimental tensile specimen results. An ANN database is generated with the help of contact type finite element analysis by numerically simulating the ABI process for various magnitudes of yield strength (σ _yp ) (200 MPa–400 MPa) with a range of strain hardening exponent (n) (0.05–0.5) and strength coefficient (K) (600 MPa–1600 MPa). For the present problem, a ball indenter of 1.57 mm diameter having Young’s modulus higher than test piece is used to minimize the error due to indenter deformation. Test piece dimension is kept large enough in comparison to the indenter configuration in the simulation to minimize the deflection at the outer edge of the test piece. Further, this database after the neural network training; is used to analyse measured material properties of different test pieces. The ANN predictions are reconfirmed with contact type finite element analysis for an arbitrary selected test sample. The methodology evolved in this work can be extended to predict material properties for any irradiated nuclear material in the service. Extensions of the ABI tests and the associated database analysis could lead to evaluation of the indentation energy to fracture needed for the structural integrity assessment of aged components.     

Behavior of AISI 316L Steel Subjected to Uniaxial State of Stress at Elevated Temperatures

This paper presents an experimental investigation on an AISI 316L stainless steel regarding mechanical properties and short uniaxial creep tests at elevated temperatures. The short time creep tests were carried out under different but constant stresses. The obtained data of ultimate tensile strength, yield strength, creep curves and effects of elevated temperatures on mechanical properties were presented. For a selected rheological model,material parameters were obtained. As a justification, such rheological model is implemented in the finite element procedure for an uniaxially stressed specimen in selected environmental conditions.     

Multiscale approach with RSM for stress–strain behaviour prediction of micro-void-considered metal alloy

This paper presents the concept of using a representative volume element (RVE) in a multiscale approach to predict the macroscopic stress–strain behaviour of a cast SS316L specimen under tension up to the point prior to necking. RVE models with various micro-void spatial configurations were built, and the effects of micro-voids and strain rate on the material properties (e.g., yield strength, ultimate tensile strength (UTS), ultimate tensile strain and strain hardening coefficient) were analysed. The spatial configuration of the micro-voids inside the cast SS316L specimen was acquired by the X-ray CT scanning system and each micro-void in the gauge length part was converted into a matching RVE model in the finite element (FE) analysis. Response surface methodology (RSM) was employed to investigate the effect of RVE configurations, i.e., the size of the RVE and the shape and spatial location of the micro-voids, on the material properties (yield strength and UTS) of the cast SS316L specimen at the macroscopic level, and then the optimal levels of the RVE configuration were determined. The stress–strain curve from the simulation did show a good agreement with the experimental results and hence the proposed concept was verified.     

Fracture and strength properties of selected austenitic stainless steels at cryogenic temperatures

Austenitic stainless steels have an excellent combination of mechanical and physical properties for load-bearing structures of large superconducting magnets for plasma containment in magnetic fusion experiments. To assess their relative suitability fracture toughness, fatigue crack growth, and tensile properties data for five austenitic steels at 295, 76, and 4 K have been obtained. The steels were AISI 304, 316, 304LN, and 316LN, and an Fe-21cr-12Ni-5Mn alloy with a higher nitrogen content than the other four grades. The two principal findings were the systematic variation of yield strength with nitrogen content and a systematic inverse correlation between fracture toughness and yield strength. Data from previous studies are reviewed which confirm the trends of the present data.     

Information relevant for the design of structure: Ferritic – Heat resistant high chromium steel X10CrAlSi25

In order to determine the behavior of the X10CrAlSi25 steel at room and elevated temperatures, a number of uniaxial tests were performed using a modern computer controlled material testing machine. Based on these tests, two types of their responses were considered. The first type of responses refers to the material properties presented in the form of engineering stress–strain diagrams. From these diagrams it is possible to derive and consequently to determine tensile strength, yield strength and a Modulus of elasticity. The second type of responses refers to creep behavior presented in the form of creep curves. Based on these curves, creep resistance of the considered material can be derived. Besides, the Charpy impact tests were performed with a Charpy impact machine to define Charpy impact energy as the basis for calculating fracture toughness. Considering tensile strength (584 MPa/20 °C) and yield strength (487 MPa/20 °C), it is visible that both of them are decreased when temperature is increased and fairly low strength levels at high temperature (tensile strength: 29 MPa/800 °C; yield strength: 26 MPa/800 °C) are measured. According to performed creep tests it is visible that this material does not belong to the materials resistant to creep.     

Tensile behaviour of 316LN stainless steel at elevated temperatures

Abstract

The tensile deformation behaviour of 316LN stainless steel was investigated from ambient temperature up to 1000°C. The hardness and microstructure of area near tensile fracture were characterised. The results show that the engineering stress increases smoothly with engineering strain when the tensile temperature is at 400°C or below, while the plastic deformation stage displays a serrated/jerky flow at 600°C. At tensile temperatures of 800°C or above, the plastic deformation stage is dramatically prolonged. The deformation mechanisms of 316LN stainless steel are proposed to be sliding and twinning at 400°C or below, tangle dislocations due to cross-slipping at 600°C, dynamic recovery at 700°C, and dynamic recrystallisation at 800°C or above. The finding provides useful guidelines for the processing and service of 316LN stainless steel components at high temperatures.     

Mechanical test on the ITER TF jacket

This paper focuses on mechanical tests on the ITER TF jacket 316LN stainless steel material. During manufacture the conductor will be compacted, spooled and aged at approximately 650 °C after cable insertion. Therefore, sample jackets were prepared under compaction, stretching and annealing in order to simulate the manufacturing process of TF coils. The mechanical properties of the given material were measured at 4.2 K and 300 K. Young’s modulus, yield strength (0.2% offset) and elongation are reported. SEM images showed that the TF jacket had more pronounced ductile behavior. The elongation at failure is more than 30% at cryogenic temperature, showing that the TF jacket has high mechanical performance. It is concluded that the results complies with ITER requirements.     

不同填充材料下316LN/Inconel718异种激光焊接头的显微组织与力学性能

目的研究不同填充材料下316LN/Inconel 718异种激光焊接接头的显微组织、显微硬度及室温拉伸性能。方法分别对316LN/Inconel 718异种材料在不填充焊丝、填充ER316LMn焊丝和填充HGH4169焊丝的情况下进行激光对接试验。采用XJP-2C型倒置光学显微镜观察不同填充材料下接头的显微组织,401MVD型数显显微硬度计测量不同填充材料下接头显微硬度,WDW-100型万能电子试验机测量不同填充材料下接头的室温拉伸抗拉强度,最终,对不同填充材料下316LN/Inconel718激光焊接接头的显微组织和力学性能进行对比分析。结果不填充焊丝与填充ER316LMn焊丝时,可获得外观成形良好的焊接接头;填充HGH4169焊丝时,接头外观成形稍差,但力学性能较好;不填充焊丝时,焊缝组织主要为柱状树枝晶、胞状晶和等轴树枝晶,填充焊丝时,焊缝组织主要为柱状树枝晶和胞状晶。填充焊丝和不填充焊丝情况下,316LN侧熔合区均会产生分层现象,而Inconel 718侧熔合区分层现象则不明显;当填充HGH4169焊丝时,焊缝的显微硬度值与抗拉强度值最大,焊缝填充ER316LMn焊丝时次之,不填充焊丝时最小。接头抗拉强度最大值为764.59 MPa,接头断裂方式为典型的韧性断裂。结论填充焊丝较不填充焊丝时,接头的力学性能有所提高,且填充HGH4169焊丝时,接头的力学性能达到最佳,但焊缝的宏观成形难以控制。     

Mechanical Properties and Defective Effects of 316LN Stainless Steel by First-Principles Simulations

In current International Thermonuclear Experimental Reactor (ITER) design, the 316LN austenitic stainless steel (316LN SS) is used for first-wall/blanket structures. Thus, it is necessary to study the fundamental mechanical properties and irradiation effect of 316LN SS. A random solid solution model of Fe-Cr-Ni-Mn-Mo-Si alloy is used for describing 316LN SS. Using first-principles approaches, the elastic constants and ideal strength of the alloy were calculated. Such alloy exhibits good ductile behavior according to the theoretical values of Cauchy pressure and ratio of bulk modulus and shear modulus. Within the 256-atom supercell, inclusion of single vacancy defect further enhances the ductility of the alloy, and the existence of interstitial (Fe, H, He) atoms enhances the Young s modulus.     

Tensile,creep behavior and microstructure evolution of an as-cast Ni-based K417G polycrystalline superalloy

The Ni-based K417G superalloy is extensively applied as aeroengine components for its low cost and good mid-temperature (600–900 °C) properties. Since used in as-cast state, the comprehensive understanding on its mechanical properties and microstructure evolution is necessary. In the present research, the tensile, creep behavior and microstructure evolution of the as-cast K417G superalloy under different conditions were investigated. The results exhibit that tensile cracks tend to initiate at MC carbide and γ/γ′ eutectic structure and then propagate along grain boundary. As the temperature for tensile tests increases from 21 °C to 700 °C, the yield strength and ultimate tensile strength of K417G superalloy decreases slightly, while the elongation to failure decreases greatly because of the intermediate temperature embrittlement. When the temperature rises to 900 °C, the yield strength and ultimate tensile strength would decrease significantly. The creep deformation mechanism varies under different testing conditions. At 760 °C/645 MPa, the creep cracks initiate at MC carbides and γ/γ′ eutectic structures, and propagate transgranularly. While at 900 °C/315 MPa and 950 °C/235 MPa, the creep cracks initiate at grain boundary and propagate intergranularly. As the creep condition changes from 760 °C/645 MPa to 900 °C/315 MPa and 950 °C/235 MPa, the γ′ phase starts to raft, which reduces the creep deformation resistance and increases the steady-state deformation rate.