Non-equilibrium grain-boundary segregation of phosphorus in an Ni-Cr steel

In order to confirm the segregation characteristic of phosphorus in Ni-Cr steel at grain boundary, a grain boundary segregation kinetic curve of phosphorus is obtained using Auger electron spectroscopy. The results show that a peak concentration of phosphorus appears at about 60 min during isothermal ageing at 600 °C after quenching from 1000 °C. The boundary concentration of phosphorus decreases with decreasing (increasing) ageing time, when the ageing time is shorter (longer) than 60 min. These results are analyzed by the model of non-equilibrium grain boundary segregation, indicating that this peak is corresponding to the critical time for phosphorus non-equilibrium grain boundary segregation.     

Nuclei for heterogeneous formation of graphite spheroids in ductile cast iron

Abstract

The crystal structure and composition of nuclei for graphite spheroids in a ductile cast iron containing small amounts of magnesium and traces of aluminium have been studied in a transmission electron microscope equipped with EDX and parallel electron energy loss spectrometer and in an electron microprobe analyser. The particles were identified as Al–Mg–Si nitrides, having a trigonal superlattice crystal structure derived from a hexagonal AlN type fundamental cell. The superlattice can be indexed according to a hexagonal Bravais lattice, with parameters a=0.544 nm and c=0.482 nm. The parameters of the fundamental cell are a _f=0.314 nm and c _f=0.482 nm, deviating only 1–3% from the parameters of hexagonal AlN. Based on the compositional analysis, the chemical formula of the nitride particles is suggested to be AlMg_2.5Si_2.5N₆.     

Cobalt intergranular segregation in WC-Co composites

Interfaces between carbide grains in the tungsten carbide-cobalt composite have been considered. Different techniques, such as transmission electron microscopy and energy dispersive X-ray analysis have been used to characterize the orientation relationship, the nature of the planes and the chemical composition of the grain boundaries.The cobalt concentration at WC-WC grain boundaries was determined by X-ray energy selective analysis in the TEM. Cobalt profiles were performed across low-angle grain boundaries, coincidence and general grain boundaries. Cobalt segregation was found whenever dislocations were imaged in the grain-boundary plane of a low-energy grain boundary. The segregation value was compared with the segregation ratio measured in special grain boundaries characterized by a coincidence site lattice.     

磷的非平衡晶界偏聚现象及对回火脆性的影响

为确定304L钢中P的偏聚特征,分析了Briant在俄歇能谱(AES)下得到的磷晶界偏聚浓度.研究表明,经1100℃固溶1h、水淬后的304L不锈钢,不仅在700℃恒温时效过程中出现磷的晶界偏聚浓度峰值,并在500、550、600、650、700℃分别恒温时效100h后,600℃处发现磷的晶界偏聚浓度峰值.分析发现,磷在304L钢中发生了非平衡晶界偏聚.通过对磷非平衡晶界偏聚恒温动力学特征的讨论,提出了回火脆性的非平衡晶界偏聚机理.     

Effects of trace impurities on the adherence of oxide to ultra low carbon steel

The effects of trace impurities on the adherence of oxides to ultra low carbon steels were investigated. Three steels, of differing chromium, aluminium, silicon, nitrogen and oxygen content, were oxidized at 850 K in 10% CO₂-90% N₂ gas and then oxidized at 770 K in 10% CO₂-10% O₂-80% N₂ gas. Surface analysers, i.e. a Mössbauer spectrometer, Auger electron spectrometer and/or ion micro analyser, were utilized in the present study because of the very thin layer of oxides formed (1.0 to 2.0 µm). The following results were obtained. The oxide on the Al-containing steel spalled at the outer-inner oxide interface, where aluminium was enriched and many cavities appeared. The inner oxide layer, oxidized by the transport of CO₂ or O₂ gases along the outer layer grain boundaries or micropores, became thick. This generated a stress at the outer-inner layer interface with the resultant formation of cavities. On the other hand, the oxide on the Cr-containing steel showed good adherence to the metal. The inner layer, enriched with chromium, did not become thick and had no cavities at the interface. The addition of chromium to the Al-containing steel resulted in good oxide adherence because chromium acted as a barrier to the aluminium enrichment.     

Brittle intergranular fracture at elevated temperatures in low–alloy steel

Abstract

Recent studies of stress-relief cracking in low-alloy steels have focused attention on a novel mode of brittle intergranular fracture which occurs at elevated temperatures (300–650°C) in hard, coarse–grained heat–affected–zone microstructures. Fracture initiates at stress concentrators such as sharp cracks or inclusions, and can propagate under static loading at rates of 10^?11?10^?5 ms^?1 to produce intergranular facets with very little associated plastic deformation. The stress-intensity parameter K has been used to characterize crack growth, and three regimes of behaviour have been observed: (i) a threshold region at growth rates of 10^?11?10^?10 m S^?1, (ii)a plateau region, in which growth rates are independent of K between 10^?10 and 10^?8 m S^?1, and (iii) a region of highly K-sensitive crack growth between 10^?8 and ^?5 m S^?1. Independent Auger electron spectroscopy analyses have demonstrated that sulphur segregates locally to the high-temperature crack tip, giving rise to the embrittlement of a limited area of grain boundary. Together with other presegregated solutes, this enables brittle fracture to occur at high temperature, and the transfer of sulphur to the crack tip controls the rate of crack growth. Two models describing crack-tip sulphur segregation are currently proposed. In the first model, a quantitative analysis demonstrates that the crack-tip stress field will drive undersize solute atoms such as sulphur to the physical crack tip. In the second, the intergranular crack is modelled as a sharp cavity. Grain-boundary sulphides which are exposed by cavity formation become unstable and dissolve, saturating the cavity surface with sulphur, which is then drawn into the tip as part of the cavity growth process.

MSTj77     

Tensile flow behavior of ultra low carbon,low carbon and micro alloyed steel sheets for auto application under low to intermediate strain rate

This paper is concerned with tensile characteristics of auto grade low carbon, ultra low carbon and micro alloyed steel sheets under low to intermediate strain rates ranging from 0.0007 to 250 s⁻¹. Experimental investigation reveals two important aspects of these steels under intermediate strain rate deformation. Firstly, the yield stress increases with strain rate in all these steels. Of course yield stress increment is higher for low carbon and ultra low carbon steel sheets. Secondly, the strain hardening rate drastically decreases with strain rate for low carbon and ultra low carbon steel sheets, whereas it remains steady for micro alloyed steel sheets. Based on these observations, a constitutive model has been proposed which predicts the strain rate sensitive flow behavior of all these grades within the strain rate range of automotive crash event.     

Gaussian process modelling of austenite formation in steel

Abstract

The present paper introduces the Gaussian process model for the empirical modelling of the formation of austenite during the continuous heating of steels. A previous paper has examined the application of neural networks to this problem, but the Gaussian process model is a more general probabilistic model which avoids some of the arbitrariness of neural networks, and is somewhat more amenable to interpretation. It is demonstrated that the model leads to an improvement in the significance of the trends of the Ac₁ and Ac₃ temperatures as a function of the chemical composition and heating rate. In some cases, these predicted trends are more plausible than those obtained with the neural network analysis. Additionally, it is shown that many of the trace alloying elements present in steels are irrelevant in determining the austenite formation temperatures.     

Non-equilibrium grain boundary segregation kinetics of phosphorus in 12Cr1MoV steel at different solution temperatures

The non-equilibrium grain boundary segregation (NGS) concentrations of phosphorus in an industrial steel, 12Cr1MoV, at different solution temperatures, 1050 and 1300°C, were measured. The NGS kinetics curves of phosphorus at the isothermal holding temperature of 540°C are given. These results provide direct support for a NGS kinetic model, and also show that the higher the solution temperature, the higher the segregation concentration of phosphorus at the grain-boundaries, and also the longer the critical time.     

Evolution of interface microstructure and strength properties in titanium – stainless steel diffusion bonded transition joints

Abstract

Diffusion bonding was carried out to produce transition joints between commercially pure titanium and 304 stainless steel at a temperature of 800°C for different times ranging from 30 to 180 min. in steps of 30 min under load in vacuum. The diffusion couples thus produced were studied using optical microscopy, scanning electron microscopy, and electron probe microanalysis to characterise the reaction layers formed in the diffusion zone. The chemical compositions of these layers indicate that intermetallics like σ phase, Fe₂ Ti, Cr₂ Ti, χ phase, FeTi, β-Ti, and Fe₂ Ti₄ O are formed in the reaction zone. The presence of these intermetallic compounds was also confirmed by the X-ray diffraction technique. Maximum bond strength of ~242 MPa was obtained for diffusion welded joints processed for 120 min. At this joining time, the plastic collapse of the surface asperities reaches near completion, favouring the interdiffusion of chemical species. Reduction in the bond strength of the transition joint processed for 180 min is due to the formation of a large volume fraction of voids in the reaction zone. Under tensile loading, failure takes place through α-Fe + χ phase mixture for transition joints processed in the time range of 30 – 90 min and through β titanium for joining times greater than 120 min.     

Fatigue damage of low amplitude cycles in low carbon steel

The influences of low load cycles on fatigue damage in 0.15% C steel (C15E, No. 1.1141) are investigated in the very high cycle fatigue regime using ultrasonic fatigue testing equipment. Constant amplitude (CA) endurance limits at limiting lifetime of 10⁹ cycles are determined in cyclic tension–compression and cyclic torsion tests. Non-propagating fatigue cracks are found in specimens subjected to cyclic torsion loading at the endurance limit. The endurance limit is considered as maximum stress amplitude where possibly initiated fatigue cracks do not propagate to failure. Two-step variable amplitude (VA) tension–compression endurance tests are performed with repeat sequences consisting of high stress amplitudes above the endurance limit and far greater number of cycles below. The measured lifetimes are compared with linear damage accumulation calculations (Miner calculations). If the high stress amplitude is more than approximately 13% above the CA endurance limit, detrimental influences of low load cycles and failures at low damage sums are found. If the high stress is less than 13% above the CA endurance limit, numerous low load cycles cause prolonged fatigue lifetimes and specimens can sustain large damage sums without failure. Two-step VA fatigue crack growth investigations show that load cycles below the threshold stress intensity accelerate crack growth, if the high stress intensity is 18% or more above the CA threshold stress intensity. In repeat sequences with high stress intensities 14% above threshold stress intensity, low load cycles decelerated and stopped fatigue crack growth. Low load cycles can reduce or prolong fatigue lifetimes of low carbon steel and one reason is the accelerated or retarded fatigue crack growth due to numerous low amplitudes, and the maximum load amplitude of a VA load sequence determines whether detrimental or beneficial effects prevail.     

Dynamic recovery and recrystallization in a cold rolled ultra low carbon steel plate: a neutron and X-ray diffraction study

The cold rolling of ultra low carbon steel is a complex process involving the formation of texture and the development of residual stresses. The mechanism for both texture and residual stress development is dislocation multiplication. Both texture and residual stresses can be determined by means of neutron and X-ray diffraction. In this paper, results of such measurements on a cold rolled plate (final thickness 10 mm) are presented along with theoretical predictions based on finite element calculations of the residual stress state. The results show that during cold rolling not only the formation and multiplication of dislocations play a role. It is seen that besides the creation of dislocations, two dislocation annihilation processes are involved, namely recovery and recrystallization. These happen despite the fact that the cold rolling takes place at relatively low temperatures (20相似文献   

Non-equilibrium grain boundary segregation of phosphorus under a high applied tensile stress in a 2.25Cr1Mo steel

Grain boundary segregation of phosphorus under a 350 MPa tensile stress at 520 °C in a 0.025 wt.% P-doped 2.25Cr1Mo steel, which has already been thermally equilibrated, is examined using Auger electron spectroscopy. The segregation of phosphorus during stress ageing has a non-equilibrium characteristic, which has two phosphorus segregation peaks over its equilibrium segregation level, one of which is mainly due to the vacancy–phosphorus complex effect and the other due to the diffusional creep effect.     

Corrosion behaviour of boronized low carbon steel

In this paper we present the results of corrosion resistance of boronized 1020 steel in naphthenic acid and during high temperature oxidation. The boride layer, consisting of FeB in the outer region and Fe₂B in the inner zone, was found to be extremely resistant to naphthenic acid containing media, both in the liquid and vapour phase. During high temperature oxidation in air at 650°C a very thin, 10_M protective oxide layer was formed on the boronized steel, indicating that the boride layer acts as a barrier to oxygen and/or Fe diffusion. Finally both the morphology and distribution of FeB/Fe₂B layers changed during high temperature oxidation.     

Fatigue crack tip plastic zones in low carbon steel

Microscopic plastic zone parameters at the tips of fatigue cracks in low carbon steel, as derived by the X-ray microbeam technique, have been correlated with measurement of some of the same parameters by the electron channeling contrast technique. Good correlation has been obtained for the parameters common to both techniques. The results for low carbon steel are found to correlate well with fatigue crack plasticity in other metals.

---

Résumé On met en corrélation les paramètres de la microdéformation plastique à l'extrémité des fissures de fatigue dans l'acier doux, tels qu'ils se déduisent d'une technique de microbombardement par des rayons X, et la mesure de certains de ces paramètres par une technique d'émission électronique à contraste renforcé.Une bonne corrélation a été obtenue pour les paramètres qui sont communs aux deux techniques et les résultats pour l'acier doux sont en corrélation satisfaisante avec les caractéristiques de plasticité des fissures de fatigue dans d'autres métaux.

     

Properties of reacto-thermitically sintered stainless steel

Abstract

Reacto-thermitic sintering (RTS) is a novel process capable of producing near net shape powder metallurgical stainless steel components with negligible interconnected porosity at sintering temperatures of 1150°C. It utilises chemical reactions between oxides present on the surface of conventional stainless steel powders and small quantities of reactive additives to produce a transient liquid phase. This liquid can be frozen during cooling to consolidate the component without slumping. Previous work by the authors has studied the fundamental mechanisms underpinning the evolution of this liquid phase and its subsequent behaviour in eliminating porosity. The present paper investigates the effect of varying the quantity of reactive additive on the dimensional stability, mechanical properties, and corrosion resistance of stainless steel specimens produced by RTS in the light of this work.     

Statistical analysis of phosphorus segregation on intergranular fracture facets in pressure vessel steels

Abstract

A533B and C–Mn steels, widely used as nuclear pressure vessel steels, have been aged at 520°C after tempering at 650°C for various periods of time to produce different levels of embrittlement resulting from the segregation of P to grain boundaries. Metallographic observation and tensile test results showed that the embrittlement heat treatment did not have significant influence on the microstructures or tensile properties of the steels. P segregation at grain boundaries and on intergranular facets was investigated using field emission gun transmission electron microscopy and Auger electron spectroscopy. After such treatment, enhanced segregation was found to be a linear function of the square root of embrittling time. Statistical analysis of the AES measurements indicated that there is a minimum segregation level for intergranular fracture to occur.     

Phase identification of oxide scale on low carbon steel

Abstract

The classical model of iron oxide scale layers; wüstite, magnetite and hematite, is more complicated in reality and its properties change with the factors that affect their development, such as time, temperature, alloying elements in the steel and oxidation environment. An understanding of the oxide scale formation and its properties can only be achieved by careful examination of the scale microstructure. Throughout this study, electron back-scatter diffraction (EBSD) has proved to be a powerful technique for verifying the individual phases in the oxide scale accurately. The results show that EBSD can be used to give a complete characterisation of the oxide scale formed on low carbon steel surface. Microstructural features such as grain size, shape and grain boundary characteristics have been successfully determined and analysed.