Tensile Fracture Behavior of 316L Austenitic Stainless Steel Manufactured by Hot Isostatic Pressing

Herein we investigate how the oxygen content in hot isostatically pressed (HIP’d) 316L stainless steel affects the mechanical properties and tensile fracture behavior. This work follows on from previous studies, which aimed to understand the effect of oxygen content on the Charpy impact toughness of HIP’d steel. We expand on the work by performing room-temperature tensile testing on different heats of 316L stainless steel, which contain different levels of interstitial elements (carbon and nitrogen) as well as oxygen in the bulk material. Throughout the work we repeat the experiments on conventionally forged 316L steel as a reference material. The analysis of the work indicates that oxygen does not contribute to a measureable solution strengthening mechanism, as is the case with carbon and nitrogen in austenitic stainless steels (Werner in Mater Sci Eng A 101:93–98, 1988). Neither does oxygen, in the form of oxide inclusions, contribute to precipitation hardening due to the size and spacing of particles. However, the oxide particles do influence fracture behavior; fractography of the failed tension test specimens indicates that the average ductile dimple size is related to the oxygen content in the bulk material, the results of which support an on-going hypothesis relating oxygen content in HIP’d steels to their fracture mechanisms by providing additional sites for the initiation of ductile damage in the form of voids.     

Influence of Temperature and Strain Rate on Tensile Deformation and Fracture Behavior of P92 Ferritic Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10^?5 to 1.26 × 10^?3 s^?1 over a temperature range of 300 K to 923 K (27 °C to 650 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of P92 ferritic steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The fracture mode remained transgranular. The steel exhibited serrated flow, an important manifestation of dynamic strain aging, along with anomalous variations in tensile properties in terms of peaks in flow stress/strength and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. At high temperatures, the rapid decrease in flow stress/strength values and work hardening rate, and increase in ductility with increase in temperature and decrease in strain rate, indicated the dominance of dynamic recovery.     

Ultrasonic Characterization of Strain Hardening Behavior in AISI 316L Austenitic Stainless Steel

The present work is aimed at characterizing the strain hardening behavior of AISI 316L austenitic stainless steel using ultrasonic velocity measurements. For this purpose, microstructural studies and ultrasonic velocity measurements were carried out on the samples deformed to different levels of strain at room temperature. Strikingly, the ultrasonic velocity?Cstrain plot of the alloy exhibited a three-stage behavior that was similar to the strain hardening rate?Cstrain response of the alloy. At strains lower than about 0.06 (stage A), a falling regime of velocity was observed that was related to the increase of dislocations density. This stage was followed by a stage of a nearly constant velocity (stage B). The initiation of this stage was concurrent with the onset of deformation twinning in the microstructure. Beyond a strain of about 0.2, the second falling regime of velocity (stage C) was developed. The occurrence of this stage was attributed to the difficulty of new twins formation with an increasing strain.     

316L奥氏体不锈钢的氮合金化

采用金相显微镜、XRD、拉伸试验机及高低温冲击试验机等,并结合Thermo-calc软件计算研究了氮对316L奥氏体不锈钢微观组织、析出相、力学性能和耐点蚀性能的影响.结果表明:氮合金化能够抑制316L不锈钢中σ相和Chi相的析出,增加Cr2N的析出倾向,对奥氏体晶粒细化不明显;氮的添加能够提高316L不锈钢的室温强度和-100℃以上温度的夏比冲击功,降低-100℃以下的夏比冲击功,但对室温拉伸塑性影响不明显.此外,氮能够改善316L不锈钢的耐点蚀能力.     

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.     

Nanograined/Ultrafine-Grained Structure and Tensile Deformation Behavior of Shear Phase Reversion-Induced 301 Austenitic Stainless Steel

We describe here an electron microscopy study of shear reversion-induced nanograined/ultrafine-grained (NG/UFG) structure and evolution of tensile strained microstructure in metastable type 301 austenitic stainless steel. The NG/UFG structure with grain size in the range of 200 to 500 nm was obtained by severe cold deformation and controlled annealing in the narrow temperature range of 973 to 1073 K (700 to 800 °C). The different stages of annealing involve the following: (a) transformation of strain-induced martensite to highly dislocated lath-type austenite, (b) formation of dislocation-cell structure and transformation to recovered austenite structure with defect-free subgrains, and (c) coalescence of subgrains to form a NG/UFG structure concomitant with a completely recrystallized structure, and consistent with martensitic shear-type phase reversion mechanism. The optimized cold working and annealing treatment resulted in NG/UFG material with a high yield strength (~1000 MPa) and high ductility (~30 pct) combination. Multiple deformation mechanisms were identified from postmortem electron microscopy examination of tensile strained NG/UFG 301 austenitic stainless steel and include dislocation glide and twinning. The evidence of heterogeneous nucleation of overlapping stacking faults and partial dislocations points toward deformation     

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.     

高应变率下00Cr23Ni4N双相不锈钢的热变形行为

 采用Gleeble-3800热力学模拟试验机对00Cr23Ni4N双相不锈钢进行了高温压缩试验,研究了其在900～1150℃、5～50s-1条件下的热变形行为,并利用Sellars双曲正弦模型建立了峰值流变应力与Zener-Hollomon（Z参数）之间的关系。研究结果表明,00Cr23Ni4N双相不锈钢的高温流变应力随变形温度的升高、应变速率的减小显著降低;在变形温度为1100～1150℃,材料均表现出良好的热加工性能;通过回归分析,00Cr23Ni4N双相不锈钢的应力指数为2.6,热变形激活能为263.4kJ/mol,Z参数能较好地描述该钢种的流变行为。     

High Temperature Deformation Behavior of a Fe-25Ni-20Cr (Wt Pct) Austenitic Stainless Steel

Metallurgical and Materials Transactions A - Structural components of Gen-IV nuclear reactors are expected to endure higher operating temperatures for longer service life. Therefore, understanding...     

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.     

Effects of Temperature and Strain Rate on Flow Behavior and Microstructural Evolution of Super Duplex Stainless Steel under Hot Deformation

Hot compression tests were carried out in the temperature range of 1 223-1 473 Kand strain rate range of0.01-30s-1 to investigate the flow behavior and microstructural evolution of super duplex stainless steel 2507(SDSS2507).It is found that most of the flow curves exhibit a characteristic of dynamic recrystallization(DRX)and the flow stress increases with the decrease of temperature and the increase of strain rate.The apparent activation energy Qof SDSS2507 with varying true strain and strain rate is determined.As the strain increases,the value of Qdeclines in different ways with varying strain rate.The microstructural evolution characteristics and the strain partition between the two constituent phases are significantly affected by the Zener-Hollomon parameter(Z).At a lower lnZ,dynamic recovery(DRV)and continuous dynamic recrystallization(CDRX)of the ferrite dominate the softening mechanism during the compression.At this time,steady state deformation takes place at the last stage of deformation.In contrast,a higher lnZ will facilitate the plastic deformation of the austenite and then activate the discontinuous dynamic recrystallization(DDRX)of the austenite,which leads to a continuous decline of the flow stress even at the last deformation stage together with CDRX of the ferrite.     

Microstructure and Property Evolution for Hot-Rolled and Cold-Rolled Austenitic Stainless Steel 316L

Grain boundary character distribution plays an important role in determining the functional and mechanical properties of polycrystalline materials. The aim of this work was to achieve improved coincident site lattice (CSL) fraction without increasing low angle grain boundary (LAGB) proportion. We utilized single-step thermo-mechanical processing route involving rolling followed by short heat treatment and compared the effect of rolling temperature. Our results indicated that rolling at elevated temperature led to significant increase in the fraction of special boundaries while keeping the fraction of LAGB very low, as desired. We conducted thermal stability of our sample-conditions at elevated temperatures for various lengths of time and found the microstructure of the samples to be stable up to 1000 °C. This study showed that even commercially suitable process (single step processing with short heat treatment duration) could lead to microstructure with considerable increase in CSL boundaries fraction, improved hardness values and good thermal stability.     

Impact of Low-Temperature Neutron Irradiation on Tensile Behavior of Base Metal and Electron-Beam Welded 316L Stainless Steel

Metallurgical and Materials Transactions A - A study of 316L type stainless steel in its base metal and electron-beam (e-beam) welded conditions was performed to observe the effects of...     

Modeling the Tensile Strain Hardening Behavior of a Metastable AISI 301LN Austenitic Stainless Steel Pre-strained in Compression

Metallurgical and Materials Transactions A - Boltzmann-type sigmoidal equations to model the tensile strain hardening and flow stress behavior of a metastable AISI 301LN austenitic stainless steel...     

Gas Tungsten Arc Welding of 316L Austenitic Stainless Steel with UNS S32205 Duplex Stainless Steel

In the present work, dissimilar welding between UNS S32205 duplex stainless steel (DSS) and 316L austenitic stainless steel (ASS) was performed by using gas tungsten arc welding and ER2209 filler at two different heat inputs (0.52 and 0.98 kJ/mm). Microstructures were characterized using reflected light optical microscope and scanning electron microscope. Micro-hardness and tensile properties were measured across the weld for both the heat inputs. The microstructure of the welded region was primarily austenitic (for both heat inputs) with Widmanstätten morphology. The grain size of the heat affected zone on DSS side was very large (~200 µm) for the high heat input sample with the presence of partially transformed austenite and acicular austenite. The precipitation of intermetallic phases and carbides was not observed for both the heat inputs. The proportion of ferrite in the weld metal (as measured by feritscope) was higher for the high heat input sample than the low heat input sample. During the tensile test, fracture occurred in 316L ASS base metal (because of its lower strength) in ductile manner. For high heat input welds, the impact tested sample showed the presence of fine spherical precipitates rich in Cr, Mn and Fe in the fracture surface of weld metal.     

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.     

304奥氏体不锈钢高温氧化行为研究

针对304热轧卷高温氧化缺陷问题,通过SEM,EPMA,XRD,EDS和XPS分析了304热轧卷氧化皮的成分和结构,研究了304的氧化行为,探讨了大生产过程中卷取温度对304氧化行为的影响.研究表明304奥氏体不锈钢热轧卷的氧化皮结构比较致密,主要成分为铁铬尖晶石(Fe3-yCryO4).304的抗氧化性较强,温度低于900℃时,氧化极为缓慢;温度高于900℃后,氧化稳步增加.适当降低卷取温度,有利于304氧化皮的去除.     

应变速率对高氮奥氏体不锈钢塑性流变行为的影响

研究了室温拉伸时应变速率对高氮奥氏体不锈钢18%Cr-18%Mn-0.65%N力学性能和塑性流变行为的影响。结果表明,随应变速率的升高,试验钢的屈服强度Rp0.2升高,而抗拉强度Rm及塑性略有降低;在各应变速率下,试验钢的塑性流变行为均可以用Ludwigson模型进行描述;应变速率的升高对试验钢流变方程参数的影响如下：1）强度系数K1、应变硬化指数n1和n2减小,试验钢的加工硬化能力降低;2）真实屈服强度TYS降低;3）瞬变应变εL减小,表明升高应变速率能够促进位错多系滑移和交滑移。