X-ray photoelectron spectroscopic analysis of oxidized Fe-16Cr-16Ni-2Mn-1Mo-2Si austenitic stainless steel

Depth profile analysis (argon ion etching/X-ray photoelectron spectroscopy) was conducted on a series of Fe-16Cr-16Ni-2Mn-1Mo-2Si austenitic stainless steel samples oxidized at 973 and 1073 K with exposure times of 25, 100, 193, 436 and 700 h. Surface and near surface rearrangement following oxidation resulted in a region of high Cr concentration on all oxidized samples. Temperature and time dependence to O₂ penetration depth was observed. In general, O₂ penetration depth was found to increase with increasing exposure up to 436 h. No increase in depth was observed between 436 and 700 h exposure time.     

Abnormal magnetic behaviour of powder metallurgy austenitic stainless steels sintered in nitrogen

The magnetic response of AISI 304L and AISI 316L obtained through powder metallurgy and sintered in nitrogen were studied. AISI 304L sintered in nitrogen showed a ferromagnetic behaviour in as-sintered state while AISI 316L was paramagnetic. After solution annealing both were paramagnetic. Magnetic behaviour was analysed by using a vibrating sample magnetometer, a magnetic ferritscope and magnetic etching. A microstructural characterization was performed by means of optical metallography, X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive analysis of X-rays (EDS). Some samples when needed were submitted to aged heat treatments at 675 and 875 °C for 90 min, 4, 6, 8 or 48 h. The main microstructural feature found was the presence of a lamellar constituent formed by nitride precipitates and an interlamellar matrix of austenite and/or ferrite. The abnormal magnetic response was explained based on this.     

Elevated temperature dry sliding wear behavior of nickel-based composite coating on austenitic stainless steel deposited by a novel central hollow laser cladding

In order to improve the high-temperature wear resistance of austenitic stainless steel, a wear resistant composite coating reinforced with hard (Cr,Fe)₇C₃ carbide and toughened by ductile γ-(Ni,Fe)/(Cr,Fe)₇C₃ eutectic matrix was fabricated by a novel central hollow laser cladding technique. The constituent phases and microstructure as well as high-temperature tribological behaviors of the Ni-based coating were investigated, respectively, and the corresponding wear mechanisms were discussed. It has been found that the composite coating exhibits superior wear resistance than substrate either at ambient or high temperatures. The coating shows better sliding wear resistance at 600 °C than 300 °C owing to high-temperature stability of the reinforced carbide and polishing effect as well as formation of continuous lubricious films, which implied it has large potential industrial applications at relatively higher temperatures.     

Evolution of magnetic phase at low aging temperature in a heavily cold-drawn stainless steel fiber

The evolution of the magnetic phase upon aging at 300–520 °C in a heavily cold-drawn AISI 316L austenitic stainless steel fiber was studied using thermomagnetic analysis (TMA) and magnetic force microscopy with a heating stage. An increasing trend of magnetization from 50 °C to around 470 °C in the heating curves of TMA in austenitic stainless steels after a cold-drawing process was observed. No significant M_s temperature signal in the TMA curve at cooling indicated an increase in magnetization upon cooling period without significant phase transformation. A series of in situ magnetic force microscopy observations reveal a growth of the magnetic domain structure after aging at 300 °C for 2.5 h. Results show that the ferromagnetic increase during aging at lower annealing temperature resulted from the growth of martensite.     

Initial oxidation of duplex stainless steel

Three different techniques were used to produce thin oxide layers on polished and sputter-cleaned duplex stainless-steel samples. These samples were exposed to 10⁻⁵ mbar of pure oxygen inside the vacuum chamber, exposed to ambient conditions for 24 h, and plasma oxidized. The oxide layers thus produced were analysed using XPS depth profiling in order to determine the oxide layers’ compositions with depth. We found that all the techniques produce oxide layers with different traces of metallic components and with the maximum concentration of chromium oxide shifted towards the oxide-layer-bulk-metal interface. A common characteristic of all the oxide layers investigated is a double-oxide stratification, with regions closer to the surface exhibiting higher concentrations of iron oxide and those more in-depth exhibiting higher concentrations of chromium oxide. A simple non-destructive Thickogram procedure was used to corroborate the thickness estimates for the thinnest oxide layers.     

Morphology and composition of nanoscale surface oxides on Fe–20Cr–18Ni{1 1 1} austenitic stainless steel

Surface oxidation ranging from initial stages to the onset of passive oxide layer formation have been investigated on Fe–20Cr–18Ni{1 1 1} single crystal surface by X-ray photoelectron spectroscopy (XPS). Surface segregation of the alloying elements and the morphology of the surface oxide nanostructure were characterized quantitatively by inelastic electron background analysis. Our results demonstrate that by increasing the oxidation temperature the relative concentrations of Fe²⁺ and Fe³⁺ cations increase due to their enhanced mobility. Higher temperature also improves the mobility of chromium, thus enhancing its segregation to the oxygen-rich surface and thereby reinforcing the passive layer on the alloy. This is in agreement with the results showing the sudden decrease in oxide film thickness at the oxidation temperatures exceeding 600 K. Additionally, a pronounced segregation of metallic nickel is found in the interface between the surface oxide layer and the bulk alloy.     

Grain refinement and strengthening of austenitic stainless steels during large strain cold rolling

The microstructure/texture evolution and strengthening of 316?L-type and 304?L-type austenitic stainless steels during cold rolling were studied. The cold rolling was accompanied by the deformation twinning and micro-shear banding followed by the strain-induced martensitic transformation, leading to nanocrystalline microstructures consisting of flattened austenite and martensite grains. The fraction of ultrafine grains can be expressed by a modified Johnson-Mehl-Avrami-Kolmogorov equation, while inverse exponential function holds as a first approximation between the mean grain size (austenite or martensite) and the total strain. The deformation austenite was characterised by the texture components of Brass, {011}<211>, Goss, {011}<100>, and S, {123}<634>, whereas the deformation martensite exhibited a strong {223}<110> texture component along with remarkable γ-fibre, <111>∥ND, with a maximum at {111}<211>. The grain refinement during cold rolling led to substantial strengthening, which could be expressed by a summation of the austenite and martensite strengthening contributions.     

Low-nickel stainless steel passive film in simulated concrete pore solution: A SIMS study

Low-nickel and AISI 304 austenitic stainless steel (SS) passive films were studied using secondary ion mass spectrometry (SIMS). An alkaline Ca(OH)₂ saturated test solution containing different chloride additions was used at room temperature. The passive film formed consists mainly of an inner chromium-rich oxide layer and an outer iron-rich oxide layer. The chemistry of the passive film depends strongly on the chloride content in the alkaline solution. Under these exposure conditions nickel was detected in the outer part of the oxide, whereas chloride ions were not found in the passive film for either the low-nickel or AISI 304 SS alloys.     

Morphological and compositional analysis of passive film on austenitic stainless steel in nitric acid medium

Passive film properties of type 304L stainless steel in nitric acid medium are investigated in both ex situ and in situ conditions. Ex situ results revealed that variation in passive film morphology occurs depending upon the concentration and time of immersion. In situ surface morphological investigation showed formation of platelet like structures at lower concentrations (0.1 M, 0.5 M), and towards higher concentration (0.6 M, 1 M) the platelet like structures got agglomerated, homogenized and started depleting from the surface leading to opening up of oxide boundaries. Compositional analysis of the passive film revealed duplex nature at lower concentration consisting of hydroxide and oxide layer, and with increasing concentration oxide layer predominates over the surface.     

Geometric dynamic recrystallization of austenitic stainless steel through linear plane-strain machining

ABSTRACT

Type 316L austenitic stainless steel was severely plastically deformed at room temperature using linear plane-strain machining in a single pass that imparted shear strains up to 2.2 at strain rates up to 2?×?10³ s^?1. The resulting microstructures exhibited significant grain size refinement and improved mechanical strength where geometric dynamic recrystallization was identified as the primary microstructural recrystallization mechanism active at high strain rates. This mechanism is rarely observed in low to medium stacking fault energy materials. The critical stress required for twin initiation is raised by the combined effects of refined grain size and the increase in stacking fault energy due to the adiabatic heating of the chip, thus permitting geometric dynamic recrystallization. The suppression of martensite formation was observed and is correlated to the significant adiabatic heating and mechanical stabilisation of the austenitic stainless steel. A gradient of the amount of strain induced martensite formed from the surface towards the interior of the chip. As the strain rate is increased from 4?×?10² s^?1–2?×?10³ s^?1, a grain morphology change was observed from a population of grains with a high fraction of irregular shaped grains to one dominated by elongated grain shapes with a microstructure characterised by an enhanced density of intragranular sub-cell structure, serrated grain boundaries, and no observable twins. As strain rates were increased, the combination of reduction in strain induced martensite and non-uniform intragranular strain led to grain softening where a Hall-Petch relationship was observed with a negative strengthening coefficient of ?0.08?MPa m^1/2.     

基于时间反转MUSIC的奥氏体不锈钢超声成像分析

奥氏体不锈钢管道广泛应用于核电及某些化工领域等,但奥氏体不锈钢焊缝缺陷的检测仍然比较困难。在时间反转的基础上,采用DORT算法和MUSIC算法对较低信噪比(-5 dB和-10 dB)的缺陷定位进行仿真分析,在定制的奥氏体不锈钢对接焊缝试块上(300 mm×90 mm×30 mm),采用16阵元超声相控阵探头对缺陷进行检测,引入TR-MUSIC算法对缺陷回波信号进行频域分析。结果表明,低信噪比时,TR-MUSIC算法较TR-DORT算法能更好的抑制旁瓣,使信号能量能更好的聚焦于缺陷;在奥氏体不锈钢焊缝相距25 mm的Φ2mm缺陷(深度为50 mm)检测实验数据基础上,TR-MUSIC算法能够更准确定位缺陷。     

铸造奥氏体不锈钢中铁素体与奥氏体位向关系及其对声衰减的影响

研究了铸造奥氏体不锈钢中铁素体与奥氏体位向关系及其对超声散射衰减的影响.利用电子背散射衍射技术表征了两相的晶体取向及其位向关系,基于真实的铁素体形貌建立了二维声传播各向异性模型并利用时域有限差分法进行了计算,分析了不同位向关系、铁素体形貌特征对声衰减系数的影响规律并进行了实验验证.结果表明：铸造奥氏体不锈钢奥氏体晶粒中散布着形状复杂的铁素体,典型铁素体形貌为条状和岛状;铁素体与奥氏体的位向关系以Kurdjumov-Sachs关系为主,少量满足Nishiyama-Wassermann关系.对声传播过程进行计算,发现两相位向关系和铁素体形貌协同作用影响超声波传播,在较高检测频率（15 MHz）下对散射衰减的影响不能忽略.结合“原位”实验对奥氏体<101>柱状晶粒的声衰减影响因素进行了定量分析,发现对于单一铸造奥氏体晶粒,晶粒内部取向不均匀性、奥氏体-铁素体位向关系以及奥氏体晶粒内铁素体形态都是超声散射衰减的主要原因.     

Dependence of grain boundary character distribution on the initial grain size of 304 austenitic stainless steel

Abstract

In order to study the dependence of the grain boundary character distributions (GBCD) on the grain size, annealing treatment was carried out on 304 austenitic stainless steel with different initial grain sizes. The evolution of the GBCD was analysed by electron backscatter diffraction. The experimental results showed that abnormal grain growth (AGG) occurred when grain size was small. With a smaller initial grain size, the number density of abnormally large grains and the fraction of low-Σ CSL boundaries increased but the size of abnormally large grains decreased and the random boundaries presented a continuous network. With a larger initial grain size, the fraction of low-Σ CSL boundaries also increased as well as the size of abnormally large grains but the number density of abnormally large grains decreased and the connectivity of random boundary network was disrupted by low-Σ CSL boundaries, especially Σ3ⁿ (n = 1, 2, 3) boundaries. However, with a very large initial grain size, normal grain growth (NGG) occurred, which had no effect on the fraction of low-Σ CSL boundaries and the connectivity of random boundary network.     

Effects of cold rolling deformation on microstructure,hardness, and creep behavior of high nitrogen austenitic stainless steel

Effects of cold rolling deformation on the microstructure, hardness, and creep behavior of high nitrogen austenitic stainless steel （HNASS） are investigated. Microstructure characterization shows that 70% cold rolling deformation results in significant refinement of the microstructure of this steel, with its average twin thickness reducing from 6.4 μm to 14 nm. Nanoindentation tests at different strain rates demonstrate that the hardness of the steel with nano-scale twins （nt-HNASS） is about 2 times as high as that of steel with micro-scale twins （mt-HNASS）. The hardness of nt-HNASS exhibits a pronounced strain rate dependence with a strain rate sensitivity （m value） of 0.0319, which is far higher than that of mt-HNASS （m = 0.0029）. nt-HNASS shows more significant load plateaus and a higher creep rate than mt-HNASS. Analysis reveals that higher hardness and larger m value of nt-HNASS arise from stronger strain hardening role, which is caused by the higher storage rate of dislocations and the interactions between dislocations and high density twins. The more significant load plateaus and higher creep rates of nt-HNASS are due to the rapid relaxation of the dislocation structures generated during loading.     

Soft X-ray spectroscopic study on the electronic structure of WO3 thin films fabricated under various annealing temperature and gas flow conditions

The electrical properties of WO₃ thin films vary significantly depending on the growth conditions. In this work, the influence of O₂ gas on the band gap of WO₃ thin films during growth was investigated via electronic structure characterization using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES). A substantial decrease in the electrical conductivity of the WO₃ films was observed with an increase in the O₂ partial pressure during growth. Spectral differences in the peak energy and intensity were apparent for WO₃ films grown under only Ar and those grown in Ar:O₂. It is difficult to explain the acquired spectrum of WO₃ with oxygen defects through the rigid-band model in terms of the simple addition of electrons to the conduction band of WO₃. Our results show that an oxygen deficiency in WO₃ moves the conduction band to the Fermi edge.     

Analysis of oxide formation induced by UV laser coloration of stainless steel

Laser-induced coloration on metal surfaces has important applications in product identification, enhancing styles and aesthetics. The color generation is the result of controlled surface oxidation during laser beam interaction with the metal surfaces. In this study, we aim to obtain in-depth understanding of the oxide formation process when an UV laser beam interacts with stainless steel in air. The oxide layer is analysed by means of optical microscopy, scanning electron microscopy (SEM) and time-of-flight secondary ion mass spectrometer (TOF-SIMS). TOF-SIMS results clearly show the formation of duplex oxide structures. The duplex structure includes an inner layer of Cr oxide solution and an outer layer of Fe oxide solution. The oxide layer thickness increased as the results of Fe diffusion to surface during multiple laser scanning passes.     

Effect of silver on antibacterial properties of stainless steel

The microstructural variation and antibacterial properties of the AISI 304 stainless steel containing silver (Ag) element have been investigated by means of optical microscopy (OM), grazing incidence X-ray diffractometry (GIXRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometer (EDS). Furthermore, the antibacterial testing was performed according to JIS Z2801:2000 specification. As the alloy contained Ag elements, the microstructure of the alloys was a mixture of (α + γ + Ag-rich compound)-phases. The amounts of α phase and Ag-rich compound increased as Ag contents increased. The Ag-rich compound has FCC structure with the lattice parameter a = 0.251 nm. No precipitates were found within the matrix and grain boundaries in the present alloys after SHT. Moreover, when the alloy is added to Ag element, antibacterial property was seen obvious against E. coli. It has an AR nearly of 100%.     

Assessment of surface relief and short cracks under cyclic creep in a type 316LN austenitic stainless steel

Formation of surface relief and short cracks under cyclic creep (stress-controlled fatigue) in type 316LN stainless steel was studied at temperatures ranging from ambient to 923 K using scanning electron microscopy technique. The surface topography and crack distribution behaviour under cyclic creep were found to be strong functions of testing temperature due to the difference in strain accumulation. At 823 K, surface relief mainly consisted of fine slip markings due to negligible accumulation of strain as a consequence of dynamic strain ageing (DSA) which led to an increase in the cyclic life. Persistent slip markings (PSM) with distinct extrusions containing minute cracks were seen to prevail in the temperature range 873–923 K, indicating a higher slip activity causing higher strain accumulation in the absence of DSA. Besides, a large number of secondary cracks (both transgranular and intergranular) which were partially accentuated by severe oxidation, were observed. Extensive cavitation-induced grain boundary cracking took place at 923 K, which coalesced with PSM-induced transgranular cracks resulting in failure dominated by creep that in turn led to a drastic reduction in cyclic life. Investigations on the influence of stress rate were also carried out which underlined the presence of DSA at 823 K. At 923 K, lowering the stress rate caused further strengthening of the contribution from creep damage marked by a shift in the damage mechanism from cyclic slip to diffusion.     

奥氏体不锈钢窄间隙焊缝侧壁未熔合相控阵超声检测*

针对核电站主管道奥氏体不锈钢窄间隙焊缝侧壁未熔合超声检测困难的问题,该文开展基于电子背散射衍射技术的相控阵超声检测研究,并结合全聚焦方法和相位相干成像方法抑制结构噪声。利用电子背散射衍射技术建立壁厚69.5 mm,且同时包含母材和焊缝的奥氏体不锈钢窄间隙焊缝模型。沿焊缝熔合线设置深度26.5 mm、高度3.0 mm的侧壁未熔合,经过仿真优化确定了中心频率2.25 MHz、32阵元以及45°纵波楔块的相控阵超声检测参数。仿真和实验检测结果显示,侧壁未熔合检测信噪比相差不超过0.6 dB,验证了所建模型的有效性。在此基础上,利用全聚焦方法和相位相干成像方法削弱结构噪声,检测信噪比较相控阵扇扫描图像分别提升2.3 dB和4.7 dB,且侧壁未熔合深度与高度定量误差均不超过6.7%。