Formation of ultrafine grained microstructure in the austenitic stainless steel and its impact on tensile properties

Commercial grade AISI 316L austenitic stainless steel was heavily cold rolled to 90% of thickness reduction. The cold rolled material was subjected to repetitive annealing treatment for short duration of 45-60 s at various temperatures. The microstructure of the cold rolled and after annealing was studied by optical as well as transmission electron microscope. The microstructural examination of the specimens after repetitive annealing process revealed the formation of ultrafine grain size microstructure. It was also noted that depending on the processing condition the grain size distribution varied widely. The tensile testing of the annealed specimen showed that the yield strength increased by 4-5 times that of the coarse grained material. However, a loss in the strain hardening ability was observed in these specimens. A good combination of yield strength and ductility for ultrafine grained stainless steel as compared to the coarse grained material could be obtained by the optimization of the microstructure.     

不锈钢激光-MAG复合焊接头微区性能研究

激光复合焊可用于不锈钢焊接,但激光复合焊接头热影响区小,组织变化梯度大.研究接头微区性能可以确定接头薄弱环节,为焊接接头的工艺评定和断裂分析提供理论依据.为此,本文对4 mm厚SUS301L-HT不锈钢进行激光-MAG复合焊接,采用维氏硬度、微型剪切和微拉伸等试验,研究了焊接接头焊缝、热影响区及母材的微区力学性能,并结合金相、断口扫描等分析了各微区力学性能的差异.结果表明：焊缝区域组织主要为柱状奥氏体树枝晶+少量的δ铁素体;母材的剪切强度和抗拉强度最高,分别为560和1 066 MPa,其次为复合焊接头热影响区,焊缝区域最差,接头硬度分布规律与各微区强度变化趋势一致;运用数学方法,得出了接头微拉伸强度与微型剪切强度、硬度之间关系的经验公式.接头各微区剪切断口和拉伸断口SEM分析呈现典型的韧性断裂特征.     

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.     

Determination of high-temperature elastoplastic properties of welded joints by indentation test

Abstract

The stress and strain fields formed in the heat-affected zone (HAZ) of welds in pipes in thermal power plants has recently attracted attention as a key plant management issue. The microstructure in the HAZ changes locally as a result of the thermal history imposed by the welding procedure; these changes are manifested in the form of a stress–strain response different from that of the base metal. In the present study, a method based on indentation tests is proposed to estimate the variation of yield stress over the HAZ. The well-known indentation tests to infer the stress–strain curve from indentation load and penetration depth measurements are familiar. However, there are difficulties in applying this approach at high temperatures. An alternative procedure based on impression size is proposed and applied to evaluate the variation of mechanical properties of Grade 122 steel over the HAZ in a welded part at 873?K.     

Improved creep-resistance of austenitic stainless steel for compact gas turbine recuperators

Abstract

Primary surface recuperators (PSRs) are compact heat-exchangers made from thin-foil type 347 austenitic stainless steel, which boost the efficiency of land-based gas turbine engines. Compact recuperators are also an essential technology for some new microturbines. Solar turbines uses foil folded into a unique corrugated pattern to maximize the primary surface area for efficient heat transfer between hot exhaust gas on one side, and the compressor discharge air on the other side of the foil. Allegheny-Ludlum produces 0.003–0.004 inches thick foil for a range of current turbine engines using PSRs that operate up to 660°C. One goal of this team-effort project is to modify the processing to enable improved creep resistance of such 347 stainless steel foils at 650–700°C. Laboratory-scale processing modification experiments recently have demonstrated that dramatic improvements can be achieved in the creep resistance of such typical 347 stainless steel foils. The modified processing enables fine NbC carbide precipitates to develop during creep at 650–700°C, which provides strength even with a fine grain size. Such improved creep-resistance allows greater flexibility in optimizing the cost-performance relationship as increased demands are placed on the PSR at higher operating temperatures. The next challenges are to better understand the nature of the improved creep resistance in these 347 stainless steel foil, and to achieve similar improvements with scale-up to commercial foil production.     

Design of a new Ni-free austenitic stainless steel with unique ultrahigh strength-high ductility synergy

A conceptual approach was used to design a new Ni-free austenitic stainless steel with a unique combination of ultrahigh strength and ductility. The concept was based on the alloying of the 0.05C–18Cr–12Mn (wt.%) steel by 0.39%N and heavy warm rolling (84% reduction) at 1173 K (900 °C) to achieve the yield strength of minimum 1 GPa and high tensile strength and elongation due to a proper stability of the austenite as a result of the optimized stacking fault energy (SFE). The yield strength of 1010 MPa, tensile strength of 1150 MPa and high fracture strain of 70% were measured for the steel designed. Dislocation and solid solution hardening mechanisms are introduced as the main contributors for the ultrahigh yield strength of the steel. The strain hardening is gradual and the hardening rate reaches a high level of ∼2400 MPa at a high true strain of 40% due to slow α′-martensitic transformation and mechanical twinning. Consequently, the ductility of the designed steel is excellent.     

Microstructures and mechanical properties of welded joints of novel 3Cr pipeline steel using an inhouse and two commercial welding wires

The welded joints of the novel 3Cr pipeline steel were fabricated via the gas tungsten arc welding (GTAW) technique using an inhouse welding wire labeled as R01 and two kinds of commercial wires (H08Cr3MoMnA and TGS-2CML). Microhardness, impact toughness and tensile properties of the joints were measured, and microstructure characteristics were observed by scanning electron microscopy (SEM). The results show that under selected welding procedure, the joints of R01 can achieve quite good mechanical properties without preheating and post weld heat treatment (PWHT). After thermal refining, elongation (15.2%) doubled and met the DNV-OS-F101 standard. For low carbon or super low carbon pipeline steels such as 3Cr steel, the revised formula with the carbon applicable coefficient (A(c)) was quite good for predicting the maximum hardness in heat affected zone (HAZ). Compared with these two selected commercial wires, the inhouse welding wire R01 can provide the highest cost-performance ratio.     

Mechanical properties of austenitic stainless steel with high niobium contents

To better understand the influence of the microstructural characteristics on the mechanical properties of Fe-Cr₂₂-Ni₂₅-Nb_x austenitic stainless steel (ASS-Nb_x), the mechanical properties were investigated. Nb addition was conducted with four different amounts: 0, 0.29, 0.58 and 0.86%. With the increasing Nb content, the mean grain size for the Fe-Cr-Ni-based alloys decreased, while the size of Nb precipitate increased. Owing to the different microstructural characteristics, their mechanical properties were altered. The highest tensile strength was obtained for ASS-Nb_0.29 alloy. However, with the increasing Nb content in ASS-Nb_0.29–0.86 alloys, the tensile strength decreases despite the grain refinement. The mechanical properties of the ASS-Nb alloys were influenced by the Nb precipitation hardening and the grain boundary cohesive strength, arising from the size of Nb precipitations.     

A simulation of Cr depletion in austenitic stainless steel with cellular automaton

A cellular automaton modeling was developed to investigate the growth kinetics of Cr-rich carbides’ precipitation and the three dimensional distribution of Cr concentration. The effects of solution treatment conditions and sensitization temperatures on Cr depletion were studied in detail. The results indicated that the amount of precipitate increased as sensitization temperature increased, decreasing along with the solution treatment temperature. Furthermore, the changing tendency of the precipitate’s amount was in accordance with that of the degree of sensitization (DOS) obtained from the samples treated at the corresponding conditions.     

The inhibition effect of some pyrimidine derivatives on austenitic stainless steel in acidic media

5-Benzoyl-4-(substitutedphenyl)-6-phenyl-3,4-dihydropyrimidine-2(1H)-(thio)ones (DHPMs) (I and II) were synthesized using the Biginelli three component cyclocondensation reaction of an appropriate β-diketone, arylaldehyde, and (thio) urea. The effect of these corrosion inhibitors on the corrosion of austenitic stainless steel in 0.5 M H₂SO₄ has been studied by electrochemical methods using Tafel plot, linear polarization and electrochemical impedance spectroscopy at 298 K. The inhibition efficiencies obtained from all the methods employed are in good agreement. The adsorption of the DHPMs onto the stainless steel surface was found to follow Langmuir and Dubinin–Radushkevich adsorption isotherm models. Negative values of ΔG_ads in the acidic media ensured the spontaneity of the adsorption process. Results show DHPM I to be the best inhibitor with a mean efficiency of 91% at 2 × 10⁻³ M additive concentration.     

A new constitutive model of austenitic stainless steel for cryogenic applications

A series of uniaxial tensile test under cryogenic temperature was carried out for AISI 304 and 316 austenitic stainless steels (ASS) in this study. Typical non-linear hardening phenomena under the cryogenic environment, such as transformation induced strain hardening and threshold strain for the 2nd hardening, has been observed in a quantitative manner.The important factors affecting the non linear material behavior of austenitic stainless steel including phase transformation, discontinuous yielding and micro-damage are modeled using constitutive equations system based on strain decomposition at the small strain formulation. A strong nonlinearity of strain hardening is described using the coupling of modified Bodner’s plasticity model and phase transformation induced strain model. The strain (threshold strain for onset of 2nd hardening) dependent plasticity model was proposed in the hardening function of Bodner’s model. In order to explicitly express the phase transformation induced strain, TI model (Tomita and Iwamoto model [Y. Tomita, T. Iwamoto, Constitutive modeling of TRIP steel and its application to the improvement of mechanical properties, International Journal of Mechanical Sciences 37 (1995) 1295–1305.]), which is a function of accumulated plastic strain and volume fraction factor of martensite, is selected in this study.Also the unified damage model, which can be connected with elasto-plastic constitutive equation developed in this study, is suggested, and the utility of proposed model was validated by the comparison between experiments and numerical evaluations.     

In situ studies of evolution of microstructure with temperature in heavily deformed Ti-modified austenitic stainless steel by X-ray diffraction technique

The mechanism of the evolution of the deformed microstructure at the earliest stage of annealing where the existence of the lowest length scale substructure paves the way to the formation of the so-called subgrains, has been studied for the first time by X-ray diffraction technique. The study has been performed at high temperature on heavily deformed Ti-modified austenitic stainless steel. Significant changes were observed in the values of the domain size, both with time and temperature. Two different types of mechanism have been proposed to be involved during the microstructural evolution at the earliest stages of annealing. The nature of the growth of domains with time at different temperatures has been modelled using these mechanisms. High-resolution transmission electron microscopy has been used to view the microstructure of the deformed and annealed sample and the results have been corroborated successfully with those found from the X-ray diffraction techniques.     

Study on hot workability and optimization of process parameters of a modified 310 austenitic stainless steel using processing maps

To investigate the optimized hot deformation parameters of a modified 310 austenitic stainless steel, the hot compression tests were performed using a Gleeble 3500 thermal simulator. The hot deformation behavior and hot workability characteristics were investigated in a temperature range of 800–1100 °C and a strain rate range of 0.1–10 s⁻¹. The hot processing maps of the tested steel were developed based on the dynamic material model (DMM), from which the safe deformation regions and instable deformation regions were determined. The corresponding microstructural and hardness evolutions during deformation were analyzed in detail. It was found that the deformation in the safe regions was beneficial to dynamic recovery (DRY) and dynamic recrystallization (DRX), while the deformation in unstable region would lead to flow instability, kink boundaries and grain growth. Near 950 °C, the energy dissipation rates were unusually lower, and the hardness of the deformed sample exhibited a significant increase, as a result of strain-induced precipitation. Coupled with the microstructure analysis and processing map technology, the workability map was schematically plotted and the optimal working conditions were determined. Such conditions were: temperatures in the range of 1075–1100 °C and strain rates in the range of 0.5–1.7 s⁻¹. These conditions are critical to attain an excellent homogeneous microstructure with fine grains after deformation for the modified 310 austenitic stainless steel.     

Study of hydrogen effusion in austenitic stainless steel by time-resolved in-situ measurements using neutron radiography

The purpose of the present study was to show the feasibility of measuring hydrogen effusion in austenitic stainless steel (1.4301) using neutron radiography at the facility ANTARES of the research reactor FRM II of the Technische Universität München. This method is appropriate to measure in-situ hydrogen effusion for hydrogen concentrations as small as 20 ppm_H. Experiments were carried out in the temperature range from room temperature up to 533 K. The measurement principle is based on the parallel comparison of electrochemically hydrogen charged specimen with hydrogen-free reference specimen at the same temperature. This allows the determination of the hydrogen concentration in the specimens as a function of time and temperature. Separate hot carrier gas extraction experiments using the same temperature-time profiles as the radiography experiments have been used to calibrate the grey values of the neutron transmission images into hydrogen concentrations. It can be stated that the hydrogen effusion correlates with the specimen temperature.     

Residual stress in low temperature carburised layer of austenitic stainless steel

Microstructure, elements concentration and residual stress of a low temperature carburised layer on 316L austenitic stainless steel were investigated by optical microscopy, electron probe microanalysis, nanoindentation and X-ray diffraction (XRD). The results show that surface carbon concentration and nanohardness increase significantly after low temperature carburisation in the mixture gas of 30?vol.-% CO–30?vol.-% H₂–40?vol.-%N₂ at 743?K for 20?h, while Young’s modulus keeps unchanged. A finite element model was proposed to simulate the nanoindentation of unstressed carburised layer based on the results of nanoindentation experiments. Combined with the experimental and simulation results, the residual stress was calculated based on Suresh model, which agrees well with the corrected data by XRD method. The surface displacement around indenter was discussed.     

一种新型核级奥氏体不锈钢焊接接头的晶间腐蚀研究

利用"不锈钢硫酸-硫酸铜腐蚀实验"、光学显微镜和扫描电镜等测试方法分别对未敏化和经敏化处理(650℃×100 h)的800H和新型Cr18Ni30Mo2Al3Nb合金焊接接头的抗晶间腐蚀性能进行研究,结果表明,800H和Cr18Ni30Mo2Al3Nb接头焊缝组织均为单一的奥氏体基体,800H合金中TiN缺陷处易引起点蚀,而Cr18Ni30Mo2Al3Nb无明显点蚀现象;对比腐蚀失重、腐蚀深度等实验结果,未敏化和敏化态Cr18Ni30Mo2Al3Nb焊接接头的抗晶间腐蚀性能明显优于相同状态的800H。     

奥氏体不锈钢焊缝超声回波信号的匹配追踪处理

研究了奥氏体不锈钢焊缝组织的金相显微结构及其对超声检测的影响,利用超声相控阵检测技术对定制的奥氏体不锈钢对接焊接接头对比试块中不同深度(10、30、50、70 mm)、φ2 mm×30 mm的横通孔缺陷进行了不同波型(横波和纵波)的检测,采用匹配追踪后处理方法对超声回波信号进行了处理。结果显示:奥氏体不锈钢焊缝组织结构复杂,晶粒粗大,各向异性明显,对超声检测产生严重的声能衰减,纵波检测奥氏体不锈钢焊缝中较深缺陷(50 mm)的能力强于横波检测,且匹配追踪对奥氏体不锈钢焊缝超声检测回波信号的处理不仅能有效抑制噪声信号、提高信噪比,还能提取出被淹没在噪声信号中的缺陷信号。     

Mechanical,electrochemical and magnetic behaviour of duplex stainless steel for biomedical applications

Mechanical, electrochemical and magnetic properties of duplex stainless steel were analysed to evaluate its use as biomaterial, comparing the results with those obtained for austenitic stainless steel. Yield and ultimate tensile strengths are almost twice in duplex stainless steel, being the values 870?MPa and 564?MPa, respectively. The electrochemical test revealed that this material has lower susceptibility to localised corrosion because of its greater passive range, 1?V from the open circuit potential, while the austenitic stainless steel exhibited a passive region of 0.370?V. Both steels behave as soft magnetic materials, however, duplex stainless steel has higher magnetic saturation and remanence, while austenitic stainless steel is more prone to heating when exposed to a magnetic field.     

Study on hot deformation behaviour of 316LN austenitic stainless steel based on hot processing map

Abstract

316LN is a type of austenitic stainless steel whose grain refinement only depends on hot deformation. The true stress–strain curves of 316LN were obtained by means of hot compression experiments conducted at a temperature range of 900–1200°C and at a strain rate range of 0·001–10 s^?1. The influence of deformation parameters on the microstructure of 316LN was analysed. Both the constitutive equation for 316LN and the model of grain size after dynamic recrystallisation were established, and the effect of different deformation conditions on the microstructure was analysed. The results show that the suitable working region is the one with a relatively higher deformation temperature and a lower strain rate, in which the dynamic recrystallisation is finely conducted. Moreover, the working region that should be avoided during hot deformation was indicated.