Dynamic Strain Aging Region Research Articles

Dynamic strain aging-mediated temperature dependence of ratcheting behavior in a 316LN austenitic stainless steel

Tensile tests and ratcheting tests were performed on a 316LN austenitic stainless steel at 300 °C, 400 °C, 500 °C, and 600 °C, respectively, to study the influence of dynamic strain aging (DSA) on deformation behavior and microstructural evolution. The DSA temperature range in present work was determined to be ≥ 400 °C in both uniaxial tests and ratcheting tests. Especially in the ratcheting tests, a sudden drop in ratcheting strain accumulation as well as in steady ratcheting strain rate was evident when the temperature raising from 300 ° to 400 °C. While within the DSA temperature region, the ratcheting behavior was insensitive to the temperature. Microstructural examinations revealed dislocation cells and tangles in the sample ratcheted at 300 °C (free of DSA), while complex dislocation structure in the samples ratcheted at ≥ 400 °C (with DSA) that was consisted mainly by high-density dislocation walls, planar slip bands, and Lomer-Cottrell locks. This clearly indicates a change in predominant ratcheting deformation mechanism from cross-slip at DSA-free temperature region to planar-slip at DSA region. It was further demonstrated that, within the temperature range from 400 °C to 600 °C, the DSA-induced hardening effect balanced with the thermally-activated softening effect and this balance led to temperature-insensitive ratcheting behavior as experimentally observed. Finally, the DSA-affected dislocation structure was quantitatively evaluated in the ratcheted samples, with an aim in correlating to the ratcheting behavior.

Effect of Formability Process Parameters in Dynamic Strain Ageing Region of Austenitic Stainless Steel

The consumption of Austenitic Stainless-Steel ASS 304 has been increasing recently in the outer body of Automobileapplications. Sheet Metal Forming is one of the processes used to convert ASS 304 sheet into an end product. ASS 304 hasgood ductility with the rise in temperature up to 450oC and later dynamic strain aging (DSA) in the temperature range 450-600oC. Formability studies of ASS 304 in the DSA region and the effect of process parameters like anisotropy and strainrate are investigated in the present paper. Stretch forming Investigations have been carried out at a temperature of 500oC intwo quasi-static strain rate ranges 1x 10-4 s-1 and 1x 10-1 s-1 concerning three orientations parallel (R0), diagonal (R45) andTransverse(R90) to Rolling Direction. The experiments were carried out at both the necking and fracture regions. Deformedspecimens' major and minor strains are measured to plot Fracture Forming limit Diagrams, strain distribution, and limitingDome height. Particular attention is given to the evaluation of strain rate and orientation of the sheet concerning rollingdirection on formability.

Dynamic strain aging and serrated flow behavior of Cr-Ti-B low carbon steel during warm deformation

The compression behavior of Cr-Ti-B low carbon steel was studied over the temperature range from 250 to 650 °C at three strain rates: 0.01 s−1, 0.1 s−1, and 1 s−1. Moreover, the serrated plastic flow was analyzed during compression, which revealed that type A serrations occurred at high strain rates, while types B or B + C serrations formed at low strain rates. The temperature range (250–350 °C) of negative strain rate sensitivity (NSRS) was slightly lower than that (300–400 °C) of dynamic strain aging (DSA). A pronounced increase of peak stress was observed at 350 °C deformation temperature and a 1 s−1 strain rate, which was attributed to the interactions between the interstitial atoms or atmospheres and movable dislocations. The average diffusion activation energy (73 kJ/mol) at low temperature (250–350 °C) also indicated that the serration mechanism was associated with the diffusion of interstitial atoms. The serration behavior at high temperatures (550–650 °C) was attributed to the pinning of dislocations by precipitates. The steel tended to exhibit planar slip of dislocations in the DSA region, and dynamic recovery was dominant during deformation at 550 °C and above.

Influence of Dynamic Strain Aging on Tensile Deformation Behavior of Alloy 617

Abstract To investigate the dynamic strain aging (DSA) behavior of Alloy 617, high-temperature tensile tests were carried out with strain rates variations of 10 −3 /s, 10 −4 /s, and 10 −5 /s from 24°C to 950°C. Five flow relationships, Hollomon, Ludwik, Swift, Ludwigson, and Voce, were applied to describe the tensile true stress–strain curves, and the DSA region was defined. In describing the tensile curves, Ludwigson's equation was superior to the other equations, and the DSA region was adequately defined by this equation as plateaus at intermediate temperatures from 200°C to 700°C. It was identified that Alloy 617 is dominated by three types of serrations, known as Types D, A+B, and C. The activation energy values for each serration type were obtained by the Arrhenius equation. By using the obtained activation energy values, the serrated yielding map and the DSA mechanism were drawn and manifested. In addition, the relationship between the tensile strength and strain rate at higher temperatures above 700°C was found to be closely related to the amounts of slip lines. In the scanning electron microscope (SEM) fractographs, there was a significant difference at the low, intermediate, and high temperatures, but almost the same to the three strain rates.

An exponential strain dependent Rusinek–Klepaczko model for flow stress prediction in austenitic stainless steel 304 at elevated temperatures

In this paper, to predict flow stress of Austenitic Stainless Steel (ASS) 304 at elevated temperatures the extended Rusinek–Klepaczko (RK) model has been modified using an exponential strain dependent term for dynamic strain aging (DSA) region. Isothermal tensile tests are conducted on ASS 304 for a temperature range of 323–923K with an interval of 50K and at strain rates of 0.0001s−1, 0.001s−1, 0.01s−1 and 0.1s−1. DSA phenomenon is observed from 623 to 923K at 0.0001s−1, 0.001s−1 and 0.01s−1. Material constants are calculated using data obtained from these tensile tests for non-DSA and DSA region separately. The predicted results from the RK model are compared with the experimental data to check the accuracy of the constitutive relation. It is observed that to find out the constants of this model, some initial assumptions are required, and these initial values affect the predicted values. Hence, Genetic Algorithm (GA) is used to optimize the constants for RK model. Statistical measures such as the correlation coefficient, the average absolute error and standard deviation are used to measure the accuracy of the model. The resulting values of the correlation coefficient for ASS 304 for non-DSA and DSA region using modified extended RK model are 0.9828 and 0.9701. This modified, extended RK model is compared with Johnson–Cook (JC), Zerilli–Armstrong (ZA) and Arrhenius models and it is observed that specifically in DSA region, the modified extended RK model gives highly accurate predictions.

Investigation of Material Model for Simulations of Deep Drawing in Dynamic Strain Aging Region

Discontinuous plastic flow in metals referred to as dynamic strain aging. These DSA appears during the plastic deformation of the metallic materials under certain temperatures and strain rates. Present work is aimed to find the best available material model for simulations of deep drawing in DSA region. The limiting drawing ratio and thickness distribution of drawn cup are the indicators of formability in deep drawing. Thickness distribution of the simulated cup at LDR in DSA region was compared with experimental values. It was observed that Barlat material model best simulates the deep drawing process in DSA region.

Formability and fracture studies of austenitic stainless steel 316 at different temperatures

Deep drawing is one of the most important sheet metal forming processes in automotive, aerospace and nuclear industries. In this process, the sheet metal blank is formed into a cup shape by an application of punch into the die. The present work is aimed at studying the formability and the nature of fracture for one of the important materials in industrial applications, austenitic stainless steel 316 at different temperatures. Circular blanks were deep drawn at room temperature, 150 and 300°C using a 20Ton hydraulic press coupled with a furnace and found that formability of the austenitic stainless steel 316 increased as the temperature was increased. This material underwent dynamic strain aging between 350 and 550°C. Fractured surface of the broken tensile test specimen at different regions were studied and analyzed using scanning electron microscope. It was observed that the nature of the fracture was brittle in dynamic strain aging region.

탄소강 배관 재료의 DSA 거동에 미치는 노치 영향 평가

본 연구에서는 탄소강 배관 재료의 동적변형시효 거동에 미치는 노치에 의한 응력과 변형 집중효과를 파악하기 위해서, 표준 시편과 노치 시편을 이용하여 다양한 온도와 시험 속도에서 인장시험을 수행하였다. 또한, 시편의 형상에 따른 응력과 변형률 상태의 차이를 파악하기 위해서, 각 시편에 대한 유한요소 시뮬레이션을 수행하였다. 노치 시편을 이용한 인장시험 결과에서도 표준 시편에서 관찰된 것과 같은 동적변형시효의 증거인 serration과 인장강도의 증가 현상이 관찰되었다. 동일한 시험 속도에서 인장시험이 수행된 경우에 표준 시편에 비해 노치 시편에서 동적변형시효 현상이 고온에서 나타났다. 유한요소 해석 결과에 의하면 표준 시편에 비해 노치 시편에서 동적변형시효가 고온에서 나타나는 것은 노치부에서 응력과 변형의 집중으로 인한 높은 변형률 속도에 기인하는 것으로 확인되었다.

Effect of temperature on flow and work hardening behavior of high bainite dual phase (HBDP) steels

High bainite dual phase steel has been subjected to tension test at different temperatures from 25 to 500 °C with strain rate of 4.6 × 10 −4 s −1 to investigate the effect of temperature on its mechanical properties. Stress–strain curves of steels showed serration flow at temperature range of 200–350 °C and smooth flow at the other temperatures. In agreement with previous studies on some steels, peaks in the variations of yield strength (YS) and ultimate tensile strength (UTS) and minima in ductility were observed at temperature range of 200–350 °C which are various manifestations of dynamic strain aging (DSA). It has been also found that ferrite volume fraction has no effect on the temperature range of serrated flow but work hardening decreased slightly with increasing ferrite volume fraction. The tensile test data were also analyzed in term of Hollomon equation and it was found that this steel has two work hardening stages. In this study the effects of temperature on Hollomon equation parameters and onset strain of stage II hardening were investigated. The observed peaks/plateaus in the variation of Hollomon equation parameters with temperature at the intermediate temperature range have been identified as the other manifestation of dynamic strain aging. The work hardening analysis of flow stress data revealed that, in the DSA region, the onset strain of stage II work hardening is athermal.

Hydrogen-involved tensile and cyclic deformation behavior of low-alloy pressure vessel steel

The temperature- and strain-rate-dependent tensile behavior of hydrogen-charged low-alloy pressure vessel steel ASTM A508 C1.3 has been investigated. The fatigue crack initiation and propagation behavior of the steel in high-temperature water environments has also been evaluated. It was found that hydrogen played significant roles in both tensile and cyclic deformation processes, especially in the temperature and strain-rate region of dynamic strain aging (DSA). The presence of hydrogen resulted in a distinct softening in tensile strength and a certain loss in tensile ductility in the DSA region. Remarkable degradation in fatigue crack initiation and propagation resistance in high-temperature water environments was observed in the DSA strain-rate region. Typical hydrogen-induced cracking features also appeared on the corresponding fatigue fracture surfaces. The interactions between hydrogen and DSA in tensile and cyclic deformation processes are discussed as well as their combined effects on the environmentally assisted cracking (EAC) mechanism of pressure vessel steels in high-temperature water environments.

Investigation of dynamic strain aging in SA106 Gr.C piping steel

Tensile and J- R tests were carried out under various temperatures and strain rates to understand the characteristics of dynamic strain aging (DSA) in SA106 Gr.C piping steel and to estimate the effect of DSA on the material properties for leak-before-break (LBB) analysis. The manifestations of DSA were clearly observed in the tensile properties. However, the magnitude of serration and strength increase by DSA was relatively small compared to similar grade carbon steels. This may be due to high manganese content in the material. The variation in strain rate did not have a significant effect on the σ ys at the nuclear reactor operating temperature which falls in DSA region. Ramberg-Osgood parameter n decreased in the region of DSA, and increased with increasing strain rate near the plant operating temperature. The influence of DSA manifested in Ramberg-Osgood parameter α was similar to that in σ ys. But, it was less clear than that on σ ys and parameter n. Crack initiation resistance, J i , and crack growth resistance, dJ da , in DSA region were about 30–40% lower than those at RT, and showed the negative loading rate dependence.