On the influence of microstructure and carbide content of steels on the electrochemical dissolution process in aqueous NaCl‐electrolytes

The electrochemical dissolution behaviour of armco‐iron and of the steels C15, C45, C60 and 100Cr6 in concentrated sodium chloride media has been investigated. Anodic metal dissolution experiments have been carried out using the flow channel cell (parallel plate reactor), the rotating cylinder electrode (RCE) and the capillary cell. The microstructure of the steel has been varied through variation of carbon content and heat treatment (e.g. soft annealed with globular carbides or pearlitic). Current‐efficiency values have been obtained by gravimetric measurements in the current‐density range from i = 5 to 60 A/cm². For the soft annealed steels, the divalent ferrite dissolution in combination with electroless cementite removal dominates. For the pearlitic steels, the occurrence of oxygen evolution at electronically conductive metal carbides or trivalent ferrite dissolution, depending on the current density applied, was detected. Microstructure dependent potentiostatic current transients and potentiodynamic polarization curves have been presented. Polarization resistances, R_pol, were measured in dependence on NaCl concentration and the applied anode potential. For pearlitic steels (with carbon contents ≥ 0.45%) R_pol exceeds that of the analogous soft annealed steels. The topographies of the steel surfaces after anodic dissolution show microscopic structures, based on inert metal carbides, which are the result of preferential ferrite dissolution. Qualitative metal dissolution models explain the electrochemical dissolution behaviour of soft annealed and pearlitic steels on the basis of the formation of solid films at the substrate surfaces and recognizing the role of the inert metal carbides in the steel matrix. In these models, the role of a polishing layer forming between the solid particles has been taken into account.     

Oxidation of ultra low carbon and silicon bearing steels

Oxidation tests were carried out in samples from an ultra low carbon and two silicon bearing steels to determine the distribution and morphology of the oxide species present. The ultra low carbon steel was oxidized for short periods of time within a chamber designed to obtain thin oxide layers by controlling the atmosphere, and for longer times in an electric furnace; the silicon steels were reheated only in the electric furnace. The chamber was constructed to study the behaviour encountered during the short period of time between descaling and rolling in modern continuous mills. It was found that the oxide layers formed on the samples reheated in the electric furnace were made of different oxide species. The specimens treated in the chamber had layers made almost exclusively of wustite. Selected oxide samples were studied by scanning electron microscopy to obtain electron backscattered diffraction patterns, which were used to identify the oxide species in the layer.     

Thermo-mechanical modelling of residual stresses induced by martensitic phase transformation and cooling during quenching of railway wheels

A finite element (FE) method was used to study the formation of residual stresses in low carbon bainitic-martensitic rail wheels. The FE model combines a commercially available heat treatment software DANTE to the finite element analysis software ABAQUS. Material data which include thermo-mechanical properties and kinetics of phase transformations for low carbon bainitic-martensitic (LCBM) steels were obtained from dilatometry experiments and added to DANTE material library. The results showed that quenching conditions can be designed to promote the development of compressive residual stresses in the rim of LCBM rail wheels making it possible to produce LCBM steel rail wheels, which have superior properties compared to conventional pearlitic steels.     

Microstructure-property relationships in pearlitic eutectoid and hypereutectoid carbon steels

Fully pearlitic steels are of great importance in a number of extremely demanding structural applications, in large part because of their combination of strength and toughness. Strength and toughness are controlled by the microstructures developed in pearlitic steels, especially interlamellar spacing, pearlite colony size, and prior austenite grain size. This article reviews the effects of these microstructural features on the yield strength and toughness of fully pearlitic steels, the importance of hypereutectoid alloy compositions for increasing the strength of fully pearlitic steels.     

Atom probe study on the ductility of drawn pearlitic steels

The microstructural change with increased strain in dry- and wet-drawn pearlitic steels was investigated by atom probe field ion microscopy. The comparison of the measured ductility with atom probe results suggests that the local increase of carbon concentration in the lamellar ferrite within the steels is the main reason for the degradation of ductility.     

Grooving corrosion of electric-resistance-welded oil well casing of J55 steel

Constant potential polarization approach was used to investigate the grooving corrosion performance of electric resistance welded oil well casings of J55 steels with different carbon content. The grooving corrosion sensitivity coefficient, α, depends intensively on the carbon content, microstructure of the steels and post-weld heat treatment. The steel with higher carbon content and fibrous pearlitic microstructure has higher grooving corrosion susceptibility. Post-weld heat treatment is shown to decrease the α value to some extent. The test solution and temperature have no apparent effects on the grooving corrosion sensitivity coefficient. The reasons for the grooving corrosion are discussed from the microstructure.     

Influence of copper on the structure and mechanical properties of pearlitic steels

The structure and mechanical properties of pearlitic steels, which contain ~0.6% carbon and copper in the amount of 1.25 and 1.4%, have been studied in the states immediately after the pearlitic transformation (with different rates of cooling) and after tempering at 500°C. It has been established that tempered pearlitic steel with copper is 10–15% stronger than the steel of similar composition without copper. The strengthening of copper-containing pearlitic steel after tempering is caused by the precipitation of copper particles 5–20 nm in size in the ferritic regions of pearlite and in grains of free ferrite.     

中高碳钢珠光体球团尺寸的快速精确统计方法

研究了显示中高碳钢珠光体球团界的多种试验方法。结果表明:金相腐蚀方法能快速显示出珠光体球团界;电子背散射技术(EBSD)方法显示出的晶界较为复杂,不能精确统计珠光体球团尺寸。在中高碳车轮钢中,应用此金相腐蚀方法,充分显示出了珠光体球团界,且界线清晰,可以保证进行精确定量统计。统计结果表明,珠光体球团尺寸大小与生产工艺有关,球团尺寸减小,韧性提高。此金相腐蚀方法可以应用在其它中高碳珠光体钢中,而不需对腐蚀方法进行较大调整。     

Determination of trapping parameters and the chemical diffusion coefficient from hydrogen permeation experiments

An improved diffusion theory accounting for trapping effects is applied to evaluation of hydrogen permeation experiments performed for pure iron and pearlitic and martensitic steels. The trapping parameters as molar volume and depth of traps are determined by fitting experiments by simulations based on the theory. The concentration-dependent chemical diffusion coefficient of hydrogen is extracted indicating that the trapping effect on diffusion in pure iron and pearlitic steel is negligible. However, it is significant for martensitic steel, for which the chemical diffusion coefficient cannot be considered as concentration-independent as it is established in current standards.     

THEORETICAL PREDICTION OF THE KINETICS CURVES OF PEARLITIC TRANSFORMATION IN HYPO-PROEUTECTOID STRUCTURAL STEELS

1.IntroductionDuringthepastseveraldecades,researchonpeaxlitictransformationhasbeenfocusedonpearlitegrowthmechanism.In1938,Mehl[IJsuggestedthatthegrowthrateofpearlitedependsoncarboncontentgradientandcarbondiffusionrate.In1946,basedonthegeneraltheoryofdiffusion,Ze.e.[2]proposedanequationtodescribethepearlitegrowthrate.Soonafterwards,Zenner,BrandtandWagner[3]wereawareofthefactthattheGibbsThomsoncapillarityaffectsboththe7--aand7-Fe3Cboundaxiesandrevisedthekineticsequation.Lateronsin1957,based…     

Oxidation of Pearlitic Steels in Nitrite Solutions

The effect of treatment parameters on the protective properties of oxide coatings, which are obtained on pearlitic steels in solutions of nitrous acid and nitrites is studied. The optimal parameters of pearlitic steel oxidation are determined. It is shown that, when NH₄NO₂ is used as the passivating agent, the passivation technology can be waste-free.     

Abnormal ferrite in hyper-eutectoid steels

The microstructural characteristics of ultra-high carbon hyper-eutectoid Fe–C and Fe–C–Cu experimental steels have been examined after isothermal transformation in a range just beneath the eutectoid temperature. Particular attention was paid to the formation of so-called “abnormal ferrite”, which refers to coarse ferrite grains which can form, in hyper-eutectoid compositions, on the pro-eutectoid cementite before the pearlite reaction occurs. Thus it is confirmed that the abnormal ferrite is not a result of pearlite coarsening, but of austenite decomposition before the conditions for coupled growth of pearlite are established. The abnormal ferrite formed on both allotriomorphic and Widmanstätten forms of pro-eutectoid cementite, and, significantly, it was observed that the pro-eutectoid cementite continued to grow, despite being enclosed by the abnormal ferrite. Under certain conditions this could lead to the eventual formation of substantially reduced amounts of pearlite. Thus, a model for carbon redistribution that allows the pro-eutectoid cementite to thicken concurrently with the abnormal ferrite is presented. The orientation relationships between the abnormal ferrite and pro-eutectoid cementite were also determined and found to be close to those which have been reported between pearlitic ferrite and pearlitic cementite.     

AFM study of the early corrosion of a high strength steel in a diluted sodium chloride solution

The high strength steels employed as reinforcement in pre-stressed concrete structures are drawn wire steels of eutectoid composition with a pearlitic microstructure. This work is focused on the study, by atomic force microscopy, of the early stages of the corrosion of such steels as a consequence of their exposition to a sodium chloride solution. The obtained images show the pearlitic microstructure of the steel, with a preferential attack of the ferrite phase and the cementite acting as a cathode. The corrosion rate was determined by calculating the amount of material lost from a roughness analysis. The obtained results are in good agreement with the predictions of Galvelel’s theory, according to which the corrosion rate slows down as the pit depth increases.     

Investigation of Variables Affecting Impedance Plane in Eddy Current Testing of Carburized Steels

Impedance plane is one of the most important ways for presenting results in Eddy current testing, which includes major data for evaluation of results. In this study, the impedance plane was drawn for carburized steel with different surface carbon content. The influences of temperature, fill factor, and edge effect on impedance plane were investigated. The ability of Eddy current testing for determination of surface carbon content using normalized impedance was also shown. Results demonstrate a strong relationship between normalized impedance and surface carbon content (R ² = 0.82). Besides the effects of temperature, fill factor, and edge effect on determination of surface carbon content were investigated. The fill factor and temperature have the largest and the least effect on correlation coefficient between surface carbon content and impedance plane, respectively.     

The structure of the superheated region of the heat-affected zone of the welded joint in heat-resisting pearlitic steels

Investigations were carried out into the formation of Widmanstatten ferrite in the structure of the section of incomplete recrystallization of the heat-affected zone of welded joints in heat-resisting pearlitic steels in the conditions of heating during welding.     

Influence of nonmetallic inclusion characteristics on the mechanical properties of rail steel

An extensive investigation has been carried out on six commercial heats of pearlitic rail steel to study the influence of nonmetallic inclusion characteristics on the tensile, fatigue, and fracture toughness properties. The steels investigated were made through the basic oxygen furnace (BOF)-continuous casting route and rolled in the rail and structural mill into 90 kg/mm² ultimate tensile strength (UTS) grade rails. While tensile properties (yield strength [YS], UTS, and elongation) of the rail steels investigated were found to be insensitive to inclusion type and volume fraction at their present level (0.23 to 0.45%), the fracture toughness and high-cycle fatigue properties were found to be inclusion sensitive. The fracture toughness values of the steels were found to range between 42.33 and 49.88 MPa √m; higher values, in general, were obtained in heats exhibiting lower volume fractions (0.15 to 0.19%) of sulfide inclusions. The high-cycle fatigue limit, i.e., stress corresponding to 10⁷ cycles, was found to be higher in cleaner steels, particularly in those with lower volume fractions of oxide inclusions. This phenomenon was corroborated by scanning electron microscopy (SEM) observations of fracture surfaces, where oxide inclusions in particular were found to be instrumental in crack initiation. Although fatigue life did not show any direct correlation with the volume fraction of sulfides, elongated MnS inclusions were sometimes observed at crack initiation sites of fatigue-tested specimens.     

Structure and mechanism of formation of dissimilar welded joints in nuclear power plant made of austenitic and pearlitic steels

The structure and mechanism of the formation of welded joints in pearlitic and austenitic steels in power engineering structures are analyzed. The results of investigations of the crystal structure of metals and metallurgical processes taking place in the welding zone are presented. Recommendations are given for improving the welding technology and welding materials, and the chemical composition of the materials used in these applications is determined more accurately.     

The precipitation of copper in abnormal ferrite and pearlite in hyper-eutectoid steels

The precipitation of copper within abnormal ferrite and pearlitic phases in hyper-eutectoid Fe-C-Cu and Fe-C-Mn-Cu experimental steels has been examined, principally by transmission electron microscopy, in terms of morphology, mechanism and crystallography. Interphase precipitation of copper during austenite decomposition was found to be a primary precipitation mechanism. However, precipitation due to ageing was also observed in both abnormal and pearlitic ferrite. The latter case appeared either as uniformly distributed precipitates within the ferrite or in association with dislocations. Copper precipitates adopted either the Kurdjumov–Sachs or the Nishiyama–Wasserman orientation relationship with respect to the ferrite. Disc-shaped precipitates thought to form by ageing within abnormal and pearlitic ferrite develop on {001}_ferrite habit planes. Twinning observed within copper precipitates formed by interphase precipitation was attributed to a requirement to accommodate the growth of precipitates at the interphase boundary. Multi-twinned precipitates were found in pearlitic ferrite and may be evidence for an intermediate transitional state between b.c.c. and f.c.c. structure.     

Developments in HSLA steel products

The technology of microalloyed steels is expanding beyond its original emphasis on low-carbon, severely control-rolled strip and plate products. A variety of economical, high-strength, tough, as-rolled or as-forged microalloyed products are replacing more expensive heat-treated steels. Recrystallization-controlled rolling is being utilized to produce very fine ferrite grain sizes and good toughness in strip, plate and bar products processed with relatively high rolling temperatures. High-strength microalloyed long products such as railroad joint bars, truck frame rails and flat bars for truck trailer construction are replacing heat-treated parts. Microalloyed, medium-carbon forging steels are used extensively for automobile engine and suspension components. Fully pearlitic high-carbon rods are being microalloyed to enhance the properties of wire and springs.     

Effect of Rare Earth Elements on Transformation Behavior and Microstructure Characteristics in X80 Pipeline Steel

The studies of the rare earth elements (RE) in low carbon steels suggest that the RE inhibits the ferrite transformation, which is the same effect as Mo alloying in pipeline steels. The purpose of this work is to discuss the relationships between the RE microalloying and the microstructure in pipeline steels. The X80 pipeline steels with different RE and Mo additions have been produced by vacuum induction furnace. The Gleeble-2000 thermal simulator, optical microscopy, and scanning electron microscopy with EBSD have been used. The continuous cooling transformation (CCT) curve was obtained and analyzed, combined with the study of microstructure. The results indicate that the microstructure of thermal simulator test metal is characteristic of quasi-polygonal ferrite and bainite, and trace RE could significantly inhibit the transformation of quasi-polygonal ferrite. The 0.0040wt% content of RE plays the same role as 0.1 wt% content of Mo alloying in pipeline steels. What’s more, the fine bainite grained structure is obtained with RE microalloying. Theoretically RE could be employed in pipeline steels as microalloying, and a partial substitution of Mo by RE is possible.