Carbon Content In Solid Solution Research Articles

Effect of pre-existing martensite within the microstructure of nano bainitic steel on its mechanical properties

Partial quenching below the martensite start temperature accelerates the subsequent bainite transformation in nanostructured bainitic steels. However, it is essential to clarify how the microstructural variations affect the resultant mechanical properties in these materials. This article aims to study the strength-ductility combinations in nanobain after introducing partial martensite within the microstructure. Two series of heat treatments were designed after preliminary dilatometry tests. First set of samples were directly transformed to bainite at 250 and 300 °C and for the second set, bainite transformation has been implemented after partial quenching at 150, 140 and 120 °C to obtain different amounts of pre-existing martensite. Mechanical properties have been described according to the microstructural evolutions by optical, field-emission gun scanning electron microscopy and XRD tests. It has been shown that pre-existing martensite changed the amount of high carbon retained austenite and increased both carbon content in solid solution and dislocation density in austenite. Interfacial autocatalytic transformation of bainite approved during which bainite started to form from pre-existing martensite/austenite interfaces. Microstructural changes gave rise to strength promotion and reduction of the elongation and impact energy even if still an acceptable balance of strength-elongation-impact energy could be obtained.

Giant Snoek peak in ferrite due to carbon-carbon strain interactions

Internal friction measurements are commonly used to quantify solute carbon in body-centered iron via the Snoek relaxation effect. For low-carbon steels, the height of the Snoek peak is proportional to the carbon content in solid solution. Our simulations show that highly carbon-supersaturated ferrite violates this linear relationship and exhibits a giant Snoek peak around a specific temperature-dependent carbon content. This effect is related to the carbon-carbon strain interaction, also responsible for the Zener order-disorder transition. We provide analytic formulae allowing the quantitative analysis of experimental results of mechanical spectroscopy on supersaturated ferrite. Our conclusions rely on Monte Carlo simulations carried out at the atomic scale and on the mean-field elastochemical model, using no adjustable parameter.

Criterial Parameters for Achieving the BH Effect in Ultralow-Carbon Steels for Deep Drawing

Key technological parameters of the stable production of interstitial free (IF) grade steel sheet with the bake hardening (BH) effect have been studied. The features of ultralow-carbon IF grade steels and IF steels with the BH effect, as well as the process flowchart for hardening IF-BH steel during hot drying of a car body after painting are described. Data concerning the chemical composition of IF and IF-BH steels produced at Russian and foreign enterprises are presented. The causes of slip band defects in steel sheet occurring when the yield plateau is observed in the tensile diagram of steel samples are analyzed. Requirements for the shelf life of IF-BH steel grades are presented and analyzed. A calculation formula of effective carbon content in steel is proposed based on the content of total carbon, niobium, titanium and nitrogen in steel. The range of effective carbon content in steel required for achieving the optimal BH effect magnitude is presented based on the papers published earlier. The industrial IF-BH steel production results have been analyzed for Russian enterprise conditions. In addition, the following criteria are recommended: the optimum range of carbon content in solid solution, the recommended maximum total concentrations of carbon and nitrogen in steel, and the BH effect magnitude that provides a high yield of IF-BH steel sheet fit for production. The calculations of various IF-BH steel microalloying with titanium and niobium have been performed. Criteria have been formulated that for obtaining a given BH effect magnitude in cold-rolled ultralow-carbon steels. A two-stage scheme for microalloying with titanium and niobium is proposed. The grain size influence in sheet steel upon the presence and BH effect magnitude, as well as upon the presence or absence of a yield plateau observed in the sheet metal’s tensile diagram in the initial state, is described.

Thermal and mechanical stability of retained austenite surrounded by martensite with different degrees of tempering

The mechanical and thermal stability of austenite in multiphase advanced high strength steels are influenced by the surrounding microstructure. The mechanisms underlying and the relations between thermal and mechanical stability are still dubious due to the difficulty of isolating other factors influencing austenite stability. In this work, martensite/austenite microstructures were created with the only significant difference being the degree of tempering of the martensite matrix. Hence, the effect of tempering in martensite is isolated from other factors influencing the stability of austenite. The thermal stability during heating of retained austenite was evaluated by monitoring phase fractions as a function of controlled temperature employing both dilatometry and magnetometry measurements. The mechanical stability was studied by performing interrupted tensile tests and determining the remaining austenite fraction at different levels of strain. The thermal stability of this remaining austenite after interrupted tests was studied by subsequent reheating of strained specimens. The results are evidence for the first time that thermal recovery of martensite during reheating assists austenite decomposition through shrinkage and softening of martensite caused by a reduction of dislocation density and carbon content in solid solution. This softening of martensite also leads to a subsequent reduction of austenite mechanical stability. Additionally, remaining austenite after pre-straining at room temperature was thermally less stable than before pre-straining, demonstrating that plastic deformation has opposing effects on thermal and mechanical stability.

Comment on “Origin of low-temperature shoulder internal friction peak of Snoek–Köster peak in a medium carbon high alloyed steel” by Lu et al. [Solid State Communications 195 (2014) 31

We want to discuss the interpretation of low-temperature shoulder internal friction peak of Snoek-Koster peak (LTS-SK). Lu et al. (2015) [1] attributed it to the interaction between the carbon atoms and twin boundaries in martensite. Nevertheless, the decrease of the amplitude of LTS-SK peak due to carbon segregation is correlated with the interstitial carbon content in solid solution in martensite (Hoyos et al., 2015 [2]). Therefore, this peak can also be attributed to the presence of an internal friction athermal background, which is proportional to the concentration of interstitial carbon in solid solution (Tkalcec et al., 2015 [2,3]). In addition, they used an alloyed steel, in which epsilon carbide precipitated above of the LTS-SK peak temperature. As this behavior cannot be generalized for carbon and high alloyed steels, the carbide precipitates could made an additional contribution to the internal friction. (C) 2015 Elsevier Ltd. All rights reserved.

Comparative study and quantification of cementite decomposition in heavily drawn pearlitic steel wires

Heavily cold-drawing was performed on a pearlitic steel wire and on an ultra-low carbon (ULC) steel wire in order to highlight and quantify the microstructural changes caused by this type of deformation. Both global techniques (thermoelectric power, electrical resistivity, internal fiction background) and local techniques (Atom Probe Tomography) were combined for this study. It was shown that two distinct stages have to be taken into account during the cold-drawing of pearlitic steels. The first stage (below a true strain of 1.5) was attributed mainly to the lamellar alignment, while the second stage (above a true strain of 1.5) was unambiguously interpreted as being due to a gradual enrichment of the carbon content of ferrite arising from the strain induced cementite decomposition. The carbon content in solid solution in ferrite was assessed as a function of the true strain. All the techniques showed that this carbon content exceeds the solubility limit of carbon in the ferrite above a true strain of 2.2. A correlation between the increase in the carbon content of ferrite and the increase in yield strength was also highlighted. Moreover, a scenario was proposed to explain the microstructural changes caused by drawing.

Multistage strain hardening through dislocation substructure and twinning in a high strength and ductile weight-reduced Fe–Mn–Al–C steel

We investigate the kinetics of the deformation structure evolution and its contribution to the strain hardening of a Fe–30.5Mn–2.1Al–1.2C (wt.%) steel during tensile deformation by means of transmission electron microscopy and electron channeling contrast imaging combined with electron backscatter diffraction. The alloy exhibits a superior combination of strength and ductility (ultimate tensile strength of 1.6GPa and elongation to failure of 55%) due to the multiple-stage strain hardening. We explain this behavior in terms of dislocation substructure refinement and subsequent activation of deformation twinning. The early hardening stage is fully determined by the size of the dislocation substructure, namely, Taylor lattices, cell blocks and dislocation cells. The high carbon content in solid solution has a pronounced effect on the evolving dislocation substructure. We attribute this effect to the reduction of the dislocation cross-slip frequency by solute carbon. With increasing applied stress, the cross-slip frequency increases. This results in a gradual transition from planar (Taylor lattices) to wavy (cells, cell blocks) dislocation configurations. The size of such dislocation substructures scales inversely with the applied resolved stress. We do not observe the so-called microband-induced plasticity effect. In the present case, due to texture effects, microbanding is not favored during tensile deformation and, hence, has no effect on strain hardening.

Influence of chemical composition and processing conditions on interstitial content of cold rolled ferritic steels

Determining the content of solute elements in cold rolled ferritic steels constitutes an important aspect in metallurgy, since the interstitial elements have a harmful effect on the anisotropy properties (i.e. texture) of the final product, usually steel sheets for automotive applications. Both the carbon and nitrogen contents in solid solution depend on the chemical composition and thermomechanical parameters, such as the coiling temperature (CT) and the cold reduction degree. In the present paper, thermoelectric power measurements technique has been applied to study the relationship between the process variables such as CT and cold rolling reduction, and the amount of interstitial elements in solid solution. It was shown that at high CTs the value of the thermoelectric power only depends on the carbon content in solid solution, meanwhile at low CTs, a second contribution should be considered due to the presence of nitrogen in solid solution.

Comparison of the evaluation of the carbon content in solid solution in extra-mild steels by thermoelectric power and by internal friction

The comparison of the evaluation of the carbon content in solid solution in extra-mild steels by thermoelectric power (TEP) and by internal friction (IF) showed the existence of a population of carbon atoms, which is not detected by IF but is visible by TEP. This population, which contributes to the strain ageing, could be constituted of carbon atoms in interaction with substitutional atoms.

Effect of residual austenite on the mechanical properties of carburized steels

The effect of residual austenite on the mechanical properties of carburized steels was investigated by controlling residual austenite content during the specimen preparation stage and then conducting metallographic, layer-by-layer chemical and phase chemical, and x-ray analysis on carburized specimens of 25KhGT, 25KhGM, and 20KhN2M steels. Bending resistance, impact strength, and fatigue limit were also determined. Bench tests were also carried out on gear wheels. A directly proportional relationship was found between the carbon content in solid solution and residual austenite. An extreme relationship was established between the mechanical properties of the steels and residual austenite content in the surface layers with the attainment of maximum properties at content values of 35 to 50% for residual austenite.

濃厚塩化マグネシウム水溶液中におけるオーステナイト・ステンレス鋼の応力腐食割れに及ぼす窒化物および炭化物形成元素の影響

Effect of less than one per cent of aluminum, titanium, vanadium, zirconium, niobium or uranium and heat treatment on the stress corrosion cracking (SCC) of 18Cr-20Ni or 18Cr-16Ni steel was investigated in boiling magnesium chloride solution at 151°C.The results indicate that the above alloying elements have no appreciable influence on the SCC resistance of stable austenitic stainless steel if they do not form nitride or carbide. The addition of aluminum, titanium, zirconium, niobium or uranium improved the SCC resistance of the nitrogen contaminated steel, possibly by decreasing the nitrogen content in solid solution, while vanadium, a weak nitride former, showed no effect. If carbide is precipitated, the SCC life is lowered since the carbon content in solid solution is decreased. It was indicated that aluminum, which forms nitride but does not react with carbon in solid solution, and uranium could be useful alloying elements in improving the SCC resistance of the air-melted stable austenitic stainless steel which might be contaminated with nitrogen.

ON THE MECHANISM OF THE INTERNAL FRICTION PEAKS ASSOCIATED WITH THE STRESS-INDUCED DIFFUSION OF CARBON IN FACE-CENTERED CUBIC ALLOY-STEELS

摘要 Abstract 作者及机构信息 葛庭燧；钱知强 1. 中国科学院金属研究所 Authors and contacts Ge Ting-sui;Qian Zhi-qiang 参考文献 [1] 施引文献