Strain-rate effects on the mechanical behavior of the AISI 300 series of austenitic stainless steel under cryogenic environments

A series of uni-axial tensile tests were carried out under various low temperatures and strain-rate ranges for AISI 300 austenitic stainless steel. The strain-rate dependencies of the materials under investigation were evaluated at temperatures ranging from ambient to cryogenic. Non-linear mechanical behavior such as phase transformation, discontinuous yielding and micro-damage of four kinds of commercial stainless steel-based material were quantitatively investigated by measuring transformation induced plasticity (TRIP) and threshold strain for 2nd hardening. In this study, the main properties of each material were analyzed and compared based on the conditions of strain-rates and temperature. Test results showed that all the test materials were strongly dependent on temperature and strain rate. It is expected that the findings in this study could be used for the cryogenic design and further research of structure materials under cryogenic environments.     

304不锈钢箔材在不同应变速率下的拉伸性能研究

304不锈钢是一种常用的奥氏体不锈钢.在拉伸应变过程中,应变速率的变化会诱发马氏体转变量和转变速率,以及内部组织滑移线、位错、层错、形变孪晶密度的转变量和转变速率的不同,从而表现出不同的应变硬化行为.本文针对0.1 mm厚度304奥氏体不锈钢箔材,从断后伸长率,断面收缩率,屈服强度,抗拉强度及硬化指数5个方面,研究了室温条件下不同应变速率对其拉伸性能的影响.实验结果表明：马氏体转变理论同样适用于304奥氏体不锈钢箔材, 且0.1 mm厚度304不锈钢存在“越薄越脆,越小越强”的尺寸效应现象;同时,0.1 mm厚度304奥氏体不锈钢箔材拉伸力学性能随应变速率的变化主要表现在以下几方面：断后延伸率和断面收缩率均随着应变速率的增加而降低;低应变速率时,随着应变速率的增加屈服强度增大,而抗拉强度随应变速率的提高呈现减弱的相反规律;高应变速率下,304奥氏体不锈钢的强度主要由材料本身性能决定,应变速率的改变对强度的影响较小;准静态低应变速率下,硬化指数随应变速率增大而升高,较高应变速率下,硬化指数与应变速率变化无关.     

循环荷载下奥氏体型和双相型不锈钢材料本构关系研究

为研究循环荷载下不锈钢材料的本构关系,对奥氏体型S30408不锈钢和双相型S220503不锈钢材料进行了单调拉伸和大应变超低周循环加载试验。采用三种常用的单调拉伸本构模型对所得应力-应变曲线进行拟合,得到相应单调荷载下材料本构参数;采用Ramberg-Osgood本构模型对循环骨架曲线进行拟合,得到材料循环强化参数;利用Chaboche塑性本构模型,标定了两种材料的循环本构参数。结果表明:在单调拉伸荷载下,G-R-O本构模型更适用于拟合不锈钢材料的单调拉伸本构;在循环荷载下,不锈钢材料滞回曲线饱满,且随着应变增大,两种材料在加载后期均表现出了明显的循环强化现象;Ramberg-Osgood本构模型对骨架曲线拟合较好,有限元计算结果和试验滞回曲线吻合度高;表明该文标定出的强化参数、循环本构参数可用于结构体系地震响应分析之中,为准确分析不锈钢结构在地震作用下的受力性能提供参考。     

奥氏体不锈钢滞回本构模型研究

准确的材料滞回本构模型是保证弹塑性地震反应预测准确性的基本前提,如果本构模型选取不当,会对计算结果产生较大影响。为此该文提出了奥氏体不锈钢考虑循环强化作用的单轴滞回本构模型,包括骨架准则及滞回准则。建立数学模型描述奥氏体不锈钢在循环荷载作用下的受力性能。根据提出的理论模型并利用ABAQUS用户材料子程序UMAT,采用Fortran语言二次开发了能够进行循环荷载下奥氏体不锈钢计算分析的程序。通过与试验结果进行对比,表明提出的模型能够准确描述奥氏体不锈钢材料的滞回行为,兼顾计算精度和效率,为奥氏体不锈钢结构体系强震分析提供有力工具。     

Temperature and strain rate dependent constitutive model of TRIP steels for low-temperature applications

The aim of this study was to develop a constitutive model that takes into consideration the strain rate and temperature dependent characteristics of TRIP steels, in conjunction with a damage mechanics approach. The martensitic-transformation-induced strain hardening of type 300 series austenitic stainless steels at low temperature is a remarkable phenomenon. From a mechanical point of view, the temperature and strain rate play critical roles in such material nonlinearities. A series of tensile tests for 304L ASS, which is representative of TRIP steels and cryogenic materials, were conducted at various temperatures and strain rates. The experimental results revealed nonlinear material characteristics of TRIP at low temperature and were simulated by the proposed numerical model. A strain-induced martensitic transformation model was implemented in a unified viscoplastic constitutive equation. The damage evolution equation was also incorporated into the proposed constitutive model to simulate material degradation. Using a series of finite element analyses, quantitative verification of the proposed numerical technique was carried out by comparing the experimental and numerical results.     

Impact behavior of different stainless steel weldments at low temperatures

A comparative study was made of the fracture behavior of austenitic and duplex stainless steel weldments at cryogenic temperatures by impact testing. The investigated materials were two austenitic (304L and 316L) and one duplex (2505) stainless steel weldments. Shielded metal arc welding (SMAW) and tungsten inert gas welding (TIG) were employed as joining techniques. Instrumented impact testing was performed between room and liquid nitrogen (?196 °C) test temperatures. The results showed a slight decrease in the impact energy of the 304L and 316L base metals with decreasing test temperature. However, their corresponding SMAW and TIG weld metals displayed much greater drop in their impact energy values. A remarkable decrease (higher than 95%) was observed for the duplex stainless steel base and weld metals impact energy with apparent ductile to brittle transition behavior. Examination of fracture surface of tested specimens revealed complete ductile fracture morphology for the austenitic base and weld metals characterized by wide and narrow deep and shallow dimples. On the contrary, the duplex stainless steel base and weld metals fracture surface displayed complete brittle fracture morphology with extended large and small stepped cleavage facets. The ductile and brittle fracture behavior of both austenitic and duplex stainless steels was supplemented by the instrumented load–time traces. The distinct variation in the behavior of the two stainless steel categories was discussed in light of the main parameters that control the deformation mechanisms of stainless steels at low temperatures; stacking fault energy, strain induced martensite transformation and delta ferrite phase deformation.     

The strain induced martensite transformation in austenitic stainless steels: Part 1 – Influence of temperature and strain history

Abstract

The strain induced martensite transformation in austenitic stainless steels is of considerable interest, because it results in materials with attractive combinations of strength and ductility. The present work examines the mechanical response for a variety of strain and temperature paths, and relates these to microstructural observations. New evidence of the detailed transformation sequence is presented, along with direct evidence of codeformation of the austenite and martensite. Using different deformation temperature sequences enables the transformation to be changed from one that is heterogeneous to one that propagates axially along the sample. The strain hardening that occurs due to combined plasticity and martensitic transformation results in high kinematic hardening that is revealed by microstructural observations here, and which are linked directly to the mechanical response of these materials described in Part II of the present work.     

Effect of strain rate on microstructures and mechanical properties of Fe–18Cr–8Ni steel

The effect of strain rate on deformation microstructures and mechanical properties of Fe–18Cr–8Ni austenitic stainless steel was investigated at strain rates of from 10^?3 to 100?s^?1. The results indicated that the deformation mechanism of steel changes from transformation induced plasticity (TRIP) to TRIP?+?twinning induced plasticity (TWIP) effect when the strain rate is increased from 10^?3 to 100?s^?1. The yield strength of steel increases gradually with strain rate increased, while the tensile strength and elongation first decreases and then increases slowly. The changes in tensile strength and elongation are due to the change of deformation mechanism with the strain rate increased.     

Parametric study of multiphase TRIP steels undergoing cyclic loading

The behavior of multiphase steels assisted by transformation-induced plasticity (TRIP steels) undergoing low cycle, fully-reversed strain-controlled deformations is studied by means of numerical simulations based on micromechanical models. The ferritic phase is simulated using a single-crystal elasto-plasticity model for BCC crystals whereas the austenitic phase, which may transform into martensite, is simulated by a crystallographic phase transformation model coupled to a single-crystal elasto-plasticity model for FCC crystals. The influence of the TRIP mechanism on the overall behavior of the steel is investigated for selected variations of microstructural properties such as phase morphology, local carbon concentration in the austenite and austenitic grain size. The results of the simulations show a strong initial hardening associated to the martensitic transformation in accordance with experimental results. The simulations indicate an asymmetric hardening behavior under extension and contraction, particularly for large austenitic volume fractions and lower carbon concentrations.     

03Cr21Ni14Mn5Mo3N不锈钢动态力学性能和本构关系

以准静态压缩性能作为参考,通过气枪式霍普金森压杆试验,研究了03Cr21Ni14Mn5Mo3N不锈钢在常温20℃和低温-40℃两种环境条件下的动态力学性能,并基于Johnson-Cook本构模型建立了03Cr21Ni14Mn5Mo3N不锈钢本构关系。结果表明,准静态及高应变率下,03Cr21Ni14Mn5Mo3N不锈钢的强度均随着温度的降低呈升高趋势;与准静态相比,03Cr21Ni14Mn5Mo3N不锈钢在高应变率下显示出明显的应变率硬化效应;在1 500/s~4 000/s高应变率范围内,随应变率的增加,钢板动态强度略有增加,但应变率硬化效应不明显。      

Cyclic deformation behavior of austenitic Cr–Ni-steels in the VHCF regime: Part II – Microstructure-sensitive simulation

In Part I – Experimental study, the cyclic deformation behavior of two austenitic stainless steel grades (AISI 304, AISI 316 L) were experimentally investigated at low stress amplitudes in the very high cycle fatigue (VHCF) regime. The observations indicate that during VHCF the metastable austenitic stainless steel (304 grade) performs a pronounced localization of plastic deformation in shear bands followed by a deformation-induced martensitic phase transformation. The 316 grade undergoes only a very limited local plastic deformation in shear bands with almost no phase transformation. Consequently, both materials exhibit distinctly different cyclic softening and hardening characteristics during VHCF. In order to provide a more detailed knowledge about the individual deformation mechanisms and their effect on the cyclic softening and hardening behavior the experimental study is extended by microstructure-sensitive modeling and simulation. Two-dimensional (2-D) microstructures consisting of several grains are represented using the boundary element method and plastic deformation within the microstructure is considered by a mechanism-based approach. Specific mechanisms of cyclic plastic deformation in shear bands and deformation-induced martensitic phase transformation – as documented by experimental results and based on well-known model approaches – are defined and implemented into the simulation. The fatigue behavior at low stress amplitudes observed in experiments can be well represented in simulations so that the underlying model helps to understand the cyclic deformation behavior of austenitic stainless steels at low stress amplitudes in the regime of VHCF strength. In a comparative study based on the resonant behavior the effect of certain deformation mechanisms on the global cyclic softening and hardening characteristics is pointed out for both materials.     

Some factors exerting an influence on the coaxing effect of austenitic stainless steels

The present paper describes some factors exerting an influence on the coaxing effect of austenitic stainless steels. Particularly, the influence of prestrain was investigated in detail. The materials used were austenitic stainless steels, type 304 and 316. Type 304N2 was also used to examine the properties of the stabilized austenitic phase in type 304. Two types of rotating bending fatigue tests, i.e. the conventional constant amplitude tests and stress‐incremental tests, were performed using the specimens subjected to the several tensile‐prestrain levels. Under the constant amplitude tests, the fatigue strengths of type 304 and 316 increased with increasing prestrain. Under the stress‐incremental tests, type 304 showed a remarkable coaxing effect, where the fatigue failure stress significantly increased regardless of the prestrain level. The coaxing effect in the unprestrained specimens was larger than those of the prestrained ones. Type 304N2 showed lower coaxing effect than type 304. In addition, the strain‐induced martensitic transformation did not occur because of the higher stability of austenitic phase in type 304N2. In type 316, the coaxing effect was dependent on the prestrain level, i.e. below 15% prestrain the coaxing effect became smaller with increasing prestrain, whereas above 25% prestrain the coaxing effect reappeared. Based on the tests results, it was considered that the coaxing effect in austenitic stainless steel was due to the mechanisms such as work hardening, strain ageing and strain‐induced martensitic transformation. The contribution of these mechanisms to the coaxing effect was different among type 304, 304N2 and 316.     

Phase Composition of Mg‐PSZ in Manganese Alloyed TRIP‐Steel MMC Processed via Steel Casting and Conductive Sintering

Metal matrix composites (MMCs) containing TRIP‐steel/Mg‐PSZ were processed by steel casting and conductive sintering. The MMC was based on austenitic steel in the system Fe–Cr? Mn? Ni showing transformation induced plasticity (TRIP). Depending on the processing route the different phase composition of MgO partially stabilized zirconia (Mg‐PSZ) was investigated by scanning electron microscopy (SEM), energy dispersive X‐ray microanalysis (EDX) as well as electron backscatter diffraction (EBSD). The interactions between the alloying metals of austenitic stainless steel and the ceramic stabilizer (MgO) during processing lead to a significant change in the phase composition of the Mg‐PSZ.     

Numerical model to predict deformation of corrugated austenitic stainless steel sheet under cryogenic temperatures for design of liquefied natural gas insulation system

Austenitic stainless steel exhibits nonlinear hardening behavior at low temperature and under various strain rate conditions caused by the phenomenon of transformation-induced plasticity (TRIP). In this study, a uniaxial tensile test for 304L austenitic stainless steel was performed below ambient temperature (−163, −140, −120, −50, and 20 °C) and at strain rates (10⁻⁴, 10⁻³, and 10⁻² s⁻¹) to identify nonlinear mechanical characteristics. In addition, a viscoplastic damage model was proposed and implemented in a user-defined material subroutine to provide a theoretical explanation of the nonlinear hardening features. The verification was conducted not only by a material-based comparative study involving experimental investigations, but also by a structural application to the corrugated steel membrane of a Mark-III-type cargo containment system for liquefied natural gas. In addition, an accumulated damage contour was represented to predict the failure location by using a continuum damage mechanics approach.     

Effect of Strain Rate,Adiabatic Heating and Phase Transformation Phenomena on the Mechanical Behaviour of Stainless Steel

Abstract: The influence of strain rate on the stress–strain behaviour of an AISI 304 austenitic stainless steel sample was investigated. For this purpose, uniaxial tensile tests were performed at room temperature for different strain rates. Microstructural measurements of transformed martensitic phase as a function of plastic strain, and thermal analyses of the specimens were carried out as well. It was found that increasing the strain rate from 10^?4 to 10^?1 s^?1 leads to a 25% improvement in uniform elongation. Moreover, a ‘curve‐crossing’ phenomenon was observed for the hardening behaviour measured at different strain rates. These results were rationalized in terms of martensitic phase transformation suppressed by a temperature increase in the specimens deformed with high strain rates.     

A simple model of elastoplasticity for nonproportional cyclic loading

On the basis of the analysis of experimental data, we formulate requirements to the constitutive relations of plasticity under the conditions of complex cyclic loading. We propose a version of constitutive relations obtained by a simple generalization of the Mazing model to the three-dimensional case and introduction of a function of cyclic hardening. We also suggest a procedure for the identification of this function. According to the results of numerical analysis, this model adequately describes the main effects of cyclic plasticity for austenitic stainless steels. Perm State Technical University, Perm, Russia. Translated from Problemy Prochnosti, No. 1, pp. 15 – 24, January – February, 1998.     

Cyclic strain rate effect on martensitic transformation and fatigue behaviour of an austenitic stainless steel

In this study, the effect of strain rate on the cyclic behaviour of 304L stainless steel is investigated to unveil the complex interrelationship between martensitic phase transformation, secondary hardening, cyclic deformation and fatigue behaviour of this alloy. A series of uniaxial strain controlled fatigue tests with varying cyclic strain rates were conducted at zero and non‐zero mean strain conditions. Secondary hardening was found to be closely related to the volume fraction of strain‐induced martensite which was affected by adiabatic heating due to increasing cyclic strain rates. Tests with lower secondary hardening rates maintained lower stress amplitudes during cyclic loading which resulted in longer fatigue lives for similar strain amplitudes. Fatigue resistance of 304L stainless steel was found to be more sensitive to changes in strain rate than the presence of mean strain. The mean strain effect was minimal due to the significant mean stress relaxation in this material.     

On the interaction between transformation induced plasticity and the austenitic stainless steel anisotropy (AISI 304) under shear loading path

It is well known that for the AISI 304 austenitic stainless steel some parameters such as temperature, strain rate, material anisotropy and loading path are the main factors which strongly affect the kinetic of transformation induced plasticity (TRIP) in this material. In literature, tensile and compression tests represent the commonly experimental tools studied on this material. Under such type of loading, dissymmetry plastic behavior was obtained due to the martensitic kinetic evolution.The aim of the present work is to highlight the role of the TRIP phenomenon on the initial material anisotropy of the AISI 304 material using appropriate experimental framework. The cross-coupled effect of the phase transformation on initial anisotropy is studied through special loading test (simple shear test (SST)) conducted at various temperatures.     

奥氏体不锈钢晶粒细化对形变机制和力学性能的影响

利用相逆转变原理采用冷变形使得亚稳奥氏体转变为形变马氏体,采用不同温度和时间退火分别获得纳米晶/超细晶和粗晶奥氏体不锈钢。通过拉伸实验得到不同晶粒尺寸的奥氏体不锈钢力学性能,采用透射电镜观察形变组织结构并利用扫描电镜观察断口特征。结果表明:高屈服强度纳米晶/超细晶奥氏体不锈钢通过形变孪晶获得优良塑性;而低屈服强度的粗晶奥氏体不锈钢发生形变诱导马氏体效应,得到良好的塑性;两组具有不同形变机制的奥氏体不锈钢拉伸断口均为韧性断裂。形变机制由形变孪晶转变为形变诱导马氏体归因于晶粒细化导致奥氏体稳定性大幅度提高。     

Simulation of mechanical behavior of multiphase TRIP steel taking account of transformation-induced plasticity

Due to the effect of transformation-induced plasticity, multiphase low alloy TRIP steel exhibits an enhanced combination of strength and ductility. Moreover, volume fractions of the constituents in TRIP steel vary during its plastic deformation. In this paper, a constitutive model for mechanical behavior of multiphase TRIP steel is presented. In the model, TRIP steel microstructure is decomposed into four individual constituents: austenite, martensite, bainitic ferrite and ferrite. Mechanical behavior of each individual phase is described by using physically-based model. On the basis of introducing transformation-induced plasticity (TRIP) strain into decomposition of total strain, stress–strain relation of multiphase composite is obtained by using mixture rule and Iso-W hypothesis. Kinetics of strain-induced martensitic transformation is described by a generalized form of Olson–Cohen model, which takes into account temperature and stress state. Moreover, a new method to describe evolving grain size of retained austenite due to martensitic transformation is developed. On the basis of presented model, mechanical behavior of multiphase steel is simulated by using finite element method. Parameters of the model are calibrated for different mixture laws used in calculation. The simulated results have a good agreement with those observed in experiments.