Hydrogen-enhanced localization of plasticity in an austenitic stainless steel

Microscopic observations and the results of static strain aging, stress relaxation, and strain rate change tests on 310s stainless steel foils, with and without hydrogen, have been presented to complement the stress-strain curves in a previous article. The hydrogen-free specimens showed minute yield points during static strain aging, while the hydrogen-containing specimens demonstrated “preyield microstrain. ” Thermal activation analysis of the strain rate change and stress relaxation plots led to the conclusion that the activation area for dislocation motion is decreased by hydrogen. Microstructural examination with the scanning electron microscope (SEM) revealed extensive strain localization, while transmission electron microscopy (TEM) studies showed microtwinning and austenite faulting in hydrogenated specimens tested at room temperature. The relation of hydrogen-induced changes in plastic deformation to hydrogen embrittlement is discussed. Formerly Graduate Student, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign.     

Study of Cu-rich phase precipitation in austenitic antibacterial stainless steel

Cu precipitation behaviors in two Cu-bearing austenitic antibacterial stainless steels,type 304 and type 317L,were systematically studied by using relatively simple methods for materials analysis,including micro-hardness,electrical resistivity,electrochemical impedance spectroscopy,X-ray diffraction and differential scanning calorimetry.The results indicated that after aging at elevated temperature,the micro-hardness, electrical resistivity,electrochemical impedance and lattice constant of the steel were all varied at different degrees due to the precipitation and growth of Cu-rich phases.The results also showed that the heat evolution during the process of Cu precipitation could be sensitively detected by means of differential scanning calorimetry,obtainning the starting temperature,peak temperature,peak area of the Cu-rich precipitation,and even the activation energy by calculation.The results confirmed that the Cu-rich phased precipitation in the Cu-bearing austenitic antibacterial stainless steel should be a thermal activation process controlled by Cu diffusion.All the materials analysis methods used in this study can be more simple and effective for application in R & D of the Cu-bearing antibacterial stainless steels.     

Influence of shock prestraining on the formation of shear localization in 304 stainless steel

Adiabatic shear localization was studied in both annealed (as-received) and shock-prestrained 304 stainless steels (304 SS). A forced shear technique was used to probe the evolution of shear localization under high-strain-rate loading using a compression split-Hopkinson pressure bar (SHPB) and hatshaped specimens. The shearing responses of the shock-prestrained steel at high strain rate indicate that they exhibit a higher initial yield strength but a lower work-hardening rates than those observed in the annealed steel. The shock-prestrained specimens, having achieved an unstable condition at a smaller plastic displacement and a lower flow stress, were seen to exhibit distinct sharp-edged bands. Corresponding comparisons between the mechanical responses and the deformed microstructures demonstrate that loading-path dependency exerts a strong influence on shear-band formation. The shock-prestrained 304 SS displayed a higher incidence of deformation twins, slip bands, and dislocation debris prior to the shear tests, this substructure suppressed further dislocation storage (work hardening) and thereby facilitated the propensity for shear localization.     

Irradiation-produced defects in austenitic stainless steel

Metallurgical and Materials Transactions B - The microstructure of annealed AISI Type 304 and type 316 stainless steels has been characterized by transmission electron microscopy as a function of...     

Surface segregation in austenitic stainless steel

We report in this paper a study of surface segregation in austenitic stainless steel. Auger electron spectroscopy was used to measure segregation as a function of time and temperature. We have found that P, N, S, Cr, and Ni will all segregate to the surface. However, their presence on the surface often depends on the competitive and attractive interactions between the various elements. We show that thermodynamic data on ternary liquid iron alloys are quite valuable in predicting these interactions. We also discuss possible applications of these studies.     

Carbon paraequilibrium in austenitic stainless steel

Carburization of austenitic stainless steels under paraequilibrium conditions—i.e., at (low) temperatures where there is essentially no substitutional diffusion—leads to a family of steels with remarkable properties: enhanced hardness, resulting in improved wear behavior, enhanced fatigue, and corrosion resistance, and with essentially no loss in ductility. These enhanced properties arise from an enormous carbon solubility, which, absent carbide formation, is orders of magnitude greater than the equilibrium solubility. Using interaction parameters from the latest CALPHAD assessment of the Fe-Cr-Ni-carbon system, the authors have calculated the equilibrium and paraequilibrium carbon solubility in a model Fe-18Cr-12 Ni (wt pct) austenitic steel (essentially a model 316L composition), as well as the carbon solubility in this austenite when paraequilibrium carbide formation occurs (i.e., when carbides form in a partitionless manner). For temperatures in the range 725 to 750 K, the calculations predict a paraequilibrium carbon solubility of ～5.5 at. pct. Carburization of 316L stainless steel at these temperatures, however, results in significantly higher concentrations of carbon in solid solution—up to 12 at. pct. Much better agreement with experimental data is obtained by calculating the paraequilibrium carbon solubility using Wagner interaction parameters, taken from the most comprehensive experimental study of this system. The discrepancy between the two predicted solubilities arises because the CALPHAD Cr-carbon interaction parameters are not sufficiently exothermic at the low temperatures used for paraequilibrium carburization. After multiple paraequilibrium carburization cycles, carbide formation can occur. The carbides that form under these conditions do so in a near-partitionless manner (there is modest Ni rejection to the austenite/carbide interface) and have an unusual stoichiometry: M₅C₂ (the Hägg or η carbide).     

Creep mechanisms in a vanadium alloyed austenitic stainless steel

In certain alloys it is possible to stop any movement of the original or strain introduced dislocations by a strong, heterogeneous nucleation of alloying elements. Diffusion will be the only active process left to contribute to the straining during a high temperature creep test. The dislocation structure generated before or during the nucleation process resists the deformation for a very long time and therefore forbids a true secondary structure to develop. A creep resistant steel, UHB SS 724 LN of the type AISI 316, has been intensively studied for the internal structure by TEM through all the three classical stages. This steel appears to have a critical stress above which dislocations at grain boundaries are generated in the first few seconds of the testing. The secondary creep stage is significant by showing an extremely low minimum creep rate at stresses below the critical one contrary to the creep rate predicted by the Coble model. The steel has a very developed tertiary creep stage, and observations of the dislocations generated by the initial straining indicate that the dislocations during this stage climb away from the precipitated particles. The contribution from void growth to the tertiary stage will probably still exist, and this stage therefore has at least two contributions.     

Creep mechanisms in a vanadium alloyed austenitic stainless steel

In certain alloys it is possible to stop any movement of the original or strain introduced dislocations by a strong, heterogeneous nucleation of alloying elements. Diffusion will be the only active process left to contribute to the straining during a high temperature creep test. The dislocation structure generated before or during the nucleation process resists the deformation for a very long time and therefore forbids a true secondary structure to develop. A creep resistant steel, UHB SS 724 LN of the type AISI 316, has been intensively studied for the internal structure by TEM through all the three classical stages. This steel appears to have a critical stress above which dislocations at grain boundaries are generated in the first few seconds of the testing. The secondary creep stage is significant by showing an extremely low minimum creep rate at stresses below the critical one contrary to the creep rate predicted by the Coble model. The steel has a very developed tertiary creep stage, and observations of the dislocations generated by the initial straining indicate that the dislocations during this stage climb away from the precipitated particles. The contribution from void growth to the tertiary stage will probably still exist, and this stage therefore has at least two contributions.      

Nonmetallic inclusions in vacuum-treated austenitic stainless steel

The nonmetallic phase in 08X18H10T and 03X18H10 vacuum-treated austenitic stainless steel is investigated by means of a scanning electron microscope. The composition of the nonmetallic inclusions in cast samples of metal from the casting ladle, the vacuum unit, and the continuous-casting machine and samples of cold-rolled sheet (thickness 1.2 mm) is studied. In 08X18H10T steel samples from the tundish of the continuous-casting machine, the nonmetallic phase consists mainly of titanium-nitride aggregations, the crystals of which contain oxides of aluminum and titanium; a few of the nonmetallic inclusions are small globules with a titanium-oxide shell. In cold-rolled sheet, the nonmetallic inclusions are distributed over the whole cross section and consist of small (5–6 μm) crystals of titanium nitrides. Cast samples of 03X18H10 austenitic titanium-free steel are relatively free of nonmetallic inclusions. The nonmetallic phase consists mainly of small inclusions of alumocalcium silicate. In cold-rolled sheet (thickness 1.2 mm), the nonmetallic phase consists of a few very small globules (2–3 μm). The results are compared with the composition of the nonmetallic phase in analogous steel with no vacuum treatment.     

奥氏体耐热不锈钢的焊接实践

通过对奥氏体耐热不锈钢的焊接性分析，提出奥氏体耐热不锈钢焊接应采取的焊接工艺及应注意的问题．     

Hydrogen attack in an austenitic stainless steel

A titanium stabilized 321 grade austenitic stainless steel was exposed to hydrogen at 14 MPa pressure and at 873 K. Hydrogen attack occurred in the form of small bubbles at alloy-carbide-matrix interfaces. The damaged regions were studied by transmission electron microscopy, and the gas composition in the bubbles was analyzed. The results are discussed in terms of a thermodynamic analysis of possible hydrogen attack reactions.     

430不锈钢中TiN的形成和析出机理研究

从理论上研究了TiN在430铁素体不锈钢中的热力学和动力学形成条件.计算结果表明,微量的氮钛就可以从热力学上形成TiN,然而动力学分析表明,要使TiN形核析出,需要很高的氮和钛的浓度积或者借助其它形核外力;研究也表明,温度越低,TiN形成和析出越容易,为了使TiN顺利形核析出而又不在随后的凝固过程中长大,应该把TiN的开始形成温度控制在两相区的某个合适温度.     

节镍型奥氏体不锈钢中形变马氏体的产生和逆变规律

通过不同保温时间的退火处理,获得了具有不同晶粒度的节镍型奥氏体不锈钢试验材料.利用Gleeble-3000热模拟机进行不同变形程度、变形温度的冷加工,分析了变形程度、变形温度和原始晶粒度对形变马氏体含量的影响.对轧硬态节镍型奥氏体不锈钢进行不同温度和时间的热处理,研究了形变马氏体的逆变规律.结果表明,冷加工过程中,变形程度和变形温度对形变马氏体的产生有重要影响,而材料的原始晶粒度对形变马氏体含量没有显著影响.形变马氏体发生逆变的临界温度约为550℃,在800℃时,形变马氏体可以在20 s之内消除.     

Austenitic solidification mode in austenitic stainless steel welds

The micro- and macrostructures of about 50 different stainless welds of the AISI/AWS 300 series are analyzed. The results indicate that in welding condition corresponding to a typical SMA welding those and only those welds in which the ratio Cr_eq/Ni_eq≲1.48, where Ni_eq and Cr_eq are the nickel and chromium equivalents on the Schaeffler diagram, solidify with the austenite as the primary or leading phase and the delta ferrite, if any, formed from the rest melt between growing cells or cellular dendrites of the austenite. At room temperature these welds are characterized by a regular general microstructure, soft forms of the ferrite and relatively large compositional differences mainly caused by solidification. T. TAKALO, formerly Research Staff Member, University of Oulu     

奥氏体不锈钢伸长率的换算

对奥氏体不锈钢不同标距间伸长率的换算公式进行研究,结果表明：试验所得n值约为0．199,与标准中Olive公式的规定值存在一定的差异。     

Fatigue crack propagation in austenitic stainless steel weldments

The fatigue crack propagation rate (FCPR) in 316L austenitic stainless steel (ASS) and its weldments was investigated, at two loading amplitudes, 7 and 8.5 kN, under tension-tension mode. Two welding techniques, submerged arc welding (SAW) and manual arc welding (MAW), have been used. Magnetic δ-ferrite, depending upon Ni and Cr content in the metal, in the weld zone upon solidification was considered. The ferrite number (FN) of δ-ferrite formed in the SAW zone was much higher (maximum 9.6) compared to the corresponding value (maximum 0.75) in the MAW zone. A fatigue starter notch was positioned at different positions and directions with respect to the weld zone, in addition to the heat-affected zone (HAZ). Regions of high and low FCPRs as the fatigue crack propagated through and across the weld zone have been noticed. This is related to the direction of the tensile residual stresses present in weld zone, resulting from solidification of the weld metal. The FCPR was higher along through the HAZ and weld zone because of the microstructural change and direction and distribution of tensile residual stresses. The FCPR was much lower when crack propagated perpendicular to the weld zone, particularly in the case of SAW in which higher δ-ferrite volume fraction was noticed. A lower FCPR found across the weld zone, in both SAW and MAW, was accompanied by rubbed areas in their fractures.     

Ferritic-austenitic solidification mode in austenitic stainless steel welds

The macro-and microstructures of about fifty different stainless welds of the AISI/ AWS 300 series are analyzed. The results indicate that under conditions corresponding to a typical shielded metal arc (SMA) welding the welds with a ratio in the range 1.48≾Cr _eq /Ni _eq ≾1.95, where Ni _eq and Cr _eq are the nickel and chromium equivalents on the Schaeffler diagram, solidify in accordance with a duplex mode with the delta ferrite as the primary (leading) phase. The austenite forms between ferrite dendrites through a three-phase reaction between liquid, ferrite and austenite, and subsequently grows into the ferrite by either an equiaxial or an acicular mechanism, resulting in a drastic decrease in the volume fraction of the delta ferrite. The micro-structure at room temperature is characterized by a general irregularity and the varied morphology of the ferrite. The compositional differences observed at room temperature are a consequence both of the solidification and the solid state transformation. Formerly Research Staff Member, Laboratory of Physical Metallurgy, University of Oulu.