Propagation of small surface cracks in stainless steel at high temperature

Growth behavior of small surface cracks in stainless steel is experimentally examined at 500°C and room temperature. The crack is confirmed to maintain a semi-elliptic shape during its growth. Cyclic deformation behavior of the material is investigated to evaluate elasto-plastic fracture mechanics parameters. It is found that crack growth rates have a good correlation with the strain intensity factor range ΔK and J-integral range ΔJ.     

常温制备抗菌不锈钢的研究

抗菌不锈钢是目前材料领域研究的热点.本文以AgNO3、ZnSO4和TiO2为主要原料,制备含有Ag 、Zn2 和TiO2的浸泡液.在常温条件下,将普通不锈钢浸入浸泡液中,制备出表面含有抗菌离子的新型抗菌不锈钢,并对其抗菌性能和持久性进行了表征,结果表明,在经过50次的反复冲洗及NaCl和CH3COOH溶液浸泡,其对金黄色葡萄球菌的平均抑菌率为99.48%,对大肠杆菌的平均抑菌率为99.09%.     

Yttrium implantation effect on 304L stainless steel high temperature oxidation at 1000°C

Scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDXS) and in situ X-ray diffraction techniques were carried out to observe the oxide scale evolutions of yttrium implanted and unimplanted commercial 304L stainless steels during and after their high temperature oxidation at 1000°C for 100 h. Our results clearly demonstrate that yttrium implantation promotes a faster oxide scale growth and the formation of a more uniform chromia layer due to a higher chromium selective oxidation compared to unimplanted 304L stainless steel. Moreover, the presence of yttrium also leads to the formation of an enriched silicon layer at the metal-oxide interface limiting the growth of iron-based oxides which were not detected (even during cooling) in the case of yttrium implanted samples. These results allow to understand the low weight gain of yttrium implanted 304L stainless steel observed by thermogravimetry and underline the beneficial effect of yttrium implantation on the 304L oxidation resistance at high temperature.     

Fabrication of stainless steel dewars for experiments at low temperature

A basic stainless steel dewar design is described which can be used in a variety of experimental situations. With a 63.5 mm ($\text{2}\text{1}\text{2}\text{″}$) diameter neck, the dewar holds 4.8 litres of liquid helium for a period of 8 days. A large multi-purpose dilution refrigerator which has evolved from the basic design is also described. Liquid helium consumption of the dilution refrigerator is 18 1/week.     

Mechanical behaviour of high nitrogen stainless steel reinforced conductor for use in pulsed high field magnets at cryogenic temperature

The use of copper--high nitrogen stainless steel macrocomposites for applications such as pulsed high magnetic field coils is demonstrated in this contribution. The macrocomposite reveal an enhancement of up to 40% in strength when cooling from 300 K down to liquid nitrogen temperature. An ultimate tensile strength of 1.53 ± 0.017 GPa is reached with a 3 mm × 2 mm wire with 52 vol.-% Cu and a logarithmic drawing strain of η=2.8 at liquid nitrogen temperature. Compared to the low nitrogen stainless steel macrocomposites in use up to now, which have an UTS of 1.02 ± 0.017 GPa and reveal an enhancement of 28% in strength when cooling down, these values related to high nitrogen stainless steel macrocomposites bear great improvements.     

常温下赋予不锈钢抗菌性能的实验研究

以AgNO3为主要原料制备了含有Ag^ 的冷敷液，在常温条件下，采用将冷敷液涂抹在不锈钢表面上或将不锈钢浸入冷敷液中的方法，制备了出表面含有抗菌Ag^ 离子的新型抗菌不锈钢。用扫描电镜(SEM)、X射线衍射(XRD)和原子吸收光谱(AAS)进行表征，表征结果表明，不锈钢表面形成一层含有柠檬酸银和过氧化银的薄膜，Ag^ 还渗透到不锈钢的内部。还对此材料的的抗菌性能进行了奎因实验，实验结果显示，抗菌不锈钢在接种金黄色葡萄球菌和大肠杆菌后培养24h后，不锈钢表面的菌落数均为0。     

Strain-rate effect on high temperature low-cycle fatigue deformation of AISI 304L stainless steel

The effect of strain rate on the behaviour of high temperature low-cycle fatigue is investigated for AISI 304L stainless steel. Regardless of the test temperature of 873 or 973 K, the fatigue life is saturated in the strain-rate range of slower than 4 × 10^–3 sec^–1. Also it is interesting to note that serrated flow, which is evidence of the occurrence of dynamic strain ageing, is clearly observed in the load-elongation hysteresis loops for strain rates that are slower (at 873 K) and faster (at 973 K) than 4 × 10^–3 sec^–1. Since the combination of temperature and strain rate is concerned with the phenomenon of dynamic strain ageing, it is considered that the above-mentioned saturated fatigue life at 873 K is caused by dynamic strain ageing and that the hardening effect due to dynamic strain ageing abnormally increases the fatigue life. However, even though the behaviour of fatigue life under strain rates slower than 4 × 10^–3 sec^–1 at 973 K has nothing to do with the dynamic strain ageing, it has been found that the failure life is also saturated in this slower strain-rate range. This behaviour is considered to be caused by the effect of creep, because the deformation under the low strain-rate activates the recovery process and as a result it causes saturation of the inelastic strain range.     

Some elements on multiaxial behaviour of 316 L stainless steel at room temperature

This paper presents some experimental results on 316 L stainless steel at room temperature. Tubular specimens are subjected to axial and torsional strain controlled loadings. The main investigation deals with the additional hardening due to multiaxiality: it is present every time that the strain (or stress) path is non-proportional (two-level tests in tension and torsion, circular or square paths in the strain plane).

In order to characterize the amount of hardening, two-dimensional simulations on the volume element are performed with a classical constitutive model with isotropic and non-linear kinematic hardening.     

EXAFS investigation of low temperature nitrided stainless steel

Low temperature nitrided stainless steel AISI 316 flakes were investigated with EXAFS and X-ray diffraction analysis. The stainless steel flakes were transformed into a mixture of nitrogen expanded austenite and nitride phases. Two treatments were carried out yielding different overall nitrogen contents: (1) nitriding in pure NH₃ and (2) nitriding in pure NH₃ followed by reduction in H₂. The majority of the Cr atoms in the stainless steel after treatment 1 and 2 was associated with a nitrogen–chromium bond distance comparable to that of the chemical compound CrN. The possibility of the occurrence of mixed substitutional–interstitial atom clusters or coherent nitride platelets in nitrogen-expanded austenite is discussed.

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Constitutive equations to predict high temperature flow stress in a Ti-modified austenitic stainless steel

The experimental stress–strain data from isothermal hot compression tests, in a wide range of temperatures (1123–1523 K) and strain rates (10⁻³–10² s⁻¹), were employed to develop constitutive equations in a Ti-modified austenitic stainless steel. The effects of temperature and strain rate on deformation behaviors were represented by Zener-Holloman parameter in an exponent type equation. The influence of strain was incorporated in the constitutive analysis by considering the effect of strain on material constants. The constitutive equation (considering the compensation of strain) could precisely predict the flow stress only at 0.1 and 1 s⁻¹ strain rates. A modified constitutive equation (incorporating both the strain and strain rate compensation), on the other hand, could predict the flow stress throughout the entire temperatures and strain rates range except at 1123 K in 10 and 100 s⁻¹. The breakdown of the constitutive equation at these processing conditions is possibly due to adiabatic temperature rise during high strain rate deformation.     

Phase transformations in plasma source ion nitrided austenitic stainless steel at low temperature

Plasma source ion nitriding has emerged as a low-temperature, low-pressure nitriding approach for low-energy implanting nitrogen ions and then diffusing them into steel and other alloys. In this work, 1Cr18Ni9Ti (18–8 type) austenitic stainless steel was treated at a process temperature from 280 to 480 °C under an average nitrogen implantation dose rate (nitrogen ion current density) of 0.44–0.63 mA cm^–2 during a nitriding period of 4 h. The nitrided surfaces of the stainless steel were characterized using Auger electron spectroscopy, electron probe microanalysis, glancing angle X-ray diffraction, and transmission electron microscopy. Below 300 °C, a high nitrogen f.c.c. phase (_N) and an ordered f.c.c. phase () mixed phase and a _N and a nitrogen-induced martensite (_N) mixed phase were obtained respectively under lower and higher nitrogen implantation dose rates. In the range of 300–450 °C a single _N phase was observed under various nitrogen implantation dose rates. Above 450 °C, the decomposition of the _N phase to a CrN phase with a b.c.c. martensite was obtained. Phase states and phase transformations in the plasma source ion nitrided 1Cr18Ni9Ti stainless steel at the low process temperatures are dependent on all the process parameters, including process temperature, nitrogen implantation dose rate, nitrogen ion energy, and processing time, etc.. The process parameters have significant effects on the formation and transformation of the various phases.     

Microstructural thermostability of high nitrogen austenitic stainless steel

A high nitrogen austenitic stainless steel, Fe–24Mn–18Cr–3Ni–0.62N, was isothermally annealed from 750 to 900 °C for different times to investigate the thermostability of its microstructures. Results show that the precipitates were Cr₂N and initially formed along the grain boundary. The time–temperature–precipitation diagram was established, according to which the critical cooling rate of this material with less than 0.1–0.5 vol.% of precipitated Cr₂N was specified as 30 °C/min. The microhardness of the matrix in samples annealed at different temperatures decreased with the diffusion of nitrogen from the matrix to grain boundaries.     

Deformation response of grain boundary networks at high temperature

The deformation response of random grain boundary networks as a function of temperature and strain rate is explored using molecular dynamics atomistic simulations and an embedded atom method interatomic potential. We find that deformation at higher temperatures promotes both dislocation emission and grain boundary accommodation processes. The results allow estimating the activation energies and volumes for the deformation process. We find activation energy values for the deformation process similar to those for grain boundary diffusion and activation volumes consistent with an atomic shuffling mechanism. Our results suggest a picture of the deformation process as governed by the combination of the applied stress and thermally activated processes.     

Influence of sigma phase on corrosion behavior of 316L stainless steel in high temperature and high pressure water

In order to elucidate the impact of σ phase on the oxidation film formation and stress corrosion cracking (SCC) resistance of 316L stainless steel, corrosion, SCC and three-point bending tests were conducted and the microstructures of the σ phase in 316L safe-end pipes were characterized via optical microscopy, environmental scanning electron microscopy and scanning Kelvin probe force microscopy. The results indicated that the σ phase was detrimental to the SCC resistance of 316L in high temperature and high pressure environments and the existence of inherently hard and brittle σ phase could change the cracking mode.     

Effect of strain rate on sigma formation in ferrite-austenite stainless steel at high temperatures

Sigma precipitation in an Fe-19Cr-5Ni-2.7Mo ferrite -austenite duplex stainless steel has been investigated during and after uniaxial compression at 900° C. At this temperature ferrite and austenite phases are present in equivolume proportions (1:1 ). It was found that sigma precipitation was enhanced by hot working and the volume proportion of sigma decreased as the straining rate increased. Sigma nucleated at ferrite-austenite interfaces and grew into ferrite grains by the eutectoid reaction + . Transmission electron microscopy revealed that large sigma particles enhanced recrystallization of the austenite phase in duplex structures at high temperature.     

Study on microstructure of low carbon 12% chromium stainless steel in high temperature heat-affected zone

Coarsening, embrittlement and corrosion sensitization in high temperature heat-affected zone (HTHAZ) are the major problems when low carbon 12% chromium stainless steel is being welded, which induce deterioration of the impact toughness at low temperature and intergranular corrosion resistance. This study investigated the corresponding microstructures in HTHAZ with different chemical compositions and heat inputs through thermal simulation tests. There are several primary conclusions: (1) When ferrite factor (FF) is above 9.0, the microstructure in HTHAZ is fully ferrite or a small amount of martensite net likely distributing along delta ferrite grain boundaries. On the other hand, if FF is below 9.0, the martensite content increases with the decreasing of FF. (2) Heat input influences the microstructure of high FF steel in HTHAZ. The martensite content and its distribution of low FF steel are not sensitive to heat inputs, but the grain size grows up with the increase of heat inputs. (3) The coarse Ti-rich particles in low FF steels containing Ti can promote intragranular austenite formation inside delta ferrite resulting in packet morphology of martensite. On the other hand, martensite of low FF steels only stabilized with Nb is characterized by grain boundary allotriomorphs, Widmanstätten structures and secondary sawteeth. This martensite reticularly distributes along ferrite grain boundaries.     

Effect of high temperature annealing on texture and microstructure on an AISI-444 ferritic stainless steel

AISI 444 is a Mo-alloyed ferritic stainless steel which presents good naphthenic corrosion resistance, making it attractive for applications in petroleum refining plants; however, good formability is also important. To achieve good formability with this alloy the annealing process is crucial. The annealing temperature in ferritic stainless steel is usually around 850 °C, which falls in the range of sigma phase precipitation. A means to avoid this precipitation is to anneal at temperatures around 1000 °C followed by rapid cooling. Annealing at high temperatures can cause grain growth and carbide or nitride precipitation which can result in a reduction of room temperature toughness. In this paper, the rolling and recrystallization textures of AISI 444 steel were studied in samples cold rolled with different thickness reductions (30%, 60%, 80% and 90%) followed by annealing at 955, 980 and 1010 °C. Aspects of grain size and carbide precipitation after annealing were characterized using EBSD and AFM. The material drawability was analyzed through strain rate or Lankford (r) coefficients calculated from texture results.     

Detection of martensite transformation in high temperature compressively deformed austenitic stainless steel by magnetic NDE technique

The present work demonstrates that a magnetic non-destructive evaluation technique can be useful for detecting the presence and extent of ferromagnetic martensitic phase in high temperature deformed 304 austenitic stainless steel.A good correlation between the martensitic transformation and magnetic parameters; saturation magnetization, coercive force and magnetic susceptibility have been obtained. Saturation magnetization was increased depending on the volume percentage of martensite transformation. The volume percentage of martensite was found to be dependent on the temperature and level of plastic strain. At temperatures below 623 K, martensitic transformation was detected after deformation of 10 to 40% plastic strain. A massive increase in martensite phase was observed in the specimen deformed at RT to 40% plastic strain.Compressive deformation at RT formed thermodynamically more stable long and broad shape of martensite. But as the temperature of deformation increases lath shape gradually converted into needle shape.Coercive force was exclusively connected with size, shape and spatial distribution of martensite. For specimens deformed at 523 K coercive force were much higher than those of the specimens deformed at RT. Coercive force decreased remarkably at temperatures above 623 K. Formation of ferromagnetic martensite in a paramagnetic matrix was also accompanied by an increase in magnetic susceptibility. Low magnetic susceptibility at temperatures above 623 K was due to disappearance of martensitic phase.