Continuous Cooling Transformation Diagrams Research Articles

Evaluation of Mechanical Properties and Microstructures of Molybdenum and Niobium Microalloyed Thermomechanically Rolled High-Strength Press Hardening Steel

This article studied the effect of molybdenum and niobium on the microstructures and mechanical properties of laboratory control rolled steels based on grade 22MnB5. The constructed continuous cooling transformation diagrams revealed that an addition of Mo significantly increased the hardenability. Especially in the case of austenite compressed below its recrystallization temperature, an Mo addition delayed ferrite and bainite formation, and it increased martensite hardness. Laboratory hot-rolling experiments with a finish rolling temperature of 850°C produced a flattened pancaked austenite. After hot rolling and direct quenching, samples were rapidly re-austenitized at 900°C followed by water quenching to simulate an actual press hardening process. Especially in the case of Nb-Mo steel, a strongly pancaked austenitic structure was achieved after hot rolling, which led to a fine, uniform prior austenite grain structure after re-austenitization and quenching. The Nb-Mo steel had a tensile strength > 1500 MPa and ~ 11% total elongation combined with good impact toughness, which can be considered excellent for this type of press hardening steel.

Development of Alloyed Pipe Steel Composition for Oil and Gas Production in the Arctic Region

Gas and oil pipelines for the Arctic region must sustain low temperatures and high internal pressures of a corrosive active medium. Generation of new steel compositions for oil and gas pipelines is an urgent issue in order to ensure their high reliability. Low-carbon steels with a ferritic–pearlitic structure are normally used in pipe production, but they are unable to cope with increased market demands. The main objective of this study is to investigate the influence of the cooling rate on the structure and morphological characteristics of bainite, which determines the exploitation properties of the pipe steels. Dilatometric tests were carried out using a Gleeble 3500 complex. Optical microscope and scanning electron microscope analysis, with the computer image analysis system Thixomet PRO, were used to study the microstructure of steel. Hardness was measured in accordance with ASTM E-384. Morphological characteristics of bainite components that were formed at various cooling rates from 0.05 to 100 °C/s were defined. The novel result of this study is the continuous cooling transformation diagram of undercooled austenite of the steel containing 0.062% C; 1.80% Mn; 0.120% Mo; 0.032% Cr, 0.90% Ni and other elements (Al, Cu, V, Nb, Ti).

Investigation and Numerical Simulation of the High Strength Silicon Steel Phase Transformations and Microstructure

Abstract High strength silicon steel implemented for manufacturing engineering products, such as aircraft components and various mining equipment, was under consideration. To obtain data for the numerical simulation of the studied steel heat treatment, the following investigations were conducted: (1) plotting of the continuous cooling transformation diagram; (2) identification of the steel microstructure constituents and the temperature ranges of their formation; (3) determination of the heat transfer coefficients of the industrial cooling media for the heat treatment (oil and still air) by the analysis of the cooling curves. The algorithm was developed for quantifying the amount of the microstructure constituents after continuous cooling to the room temperature at the rates 0.025°C/s–75°C/s. The obtained experimental data was implemented in the commercial software package to simulate the heat treatment process of the steel parts. The simulated data (microstructure distribution over the cross-section of the steel parts) was verified using the microstructure investigation of the heat-treated steel parts. A good agreement of the data was obtained.

Retained austenite phase detected by Mössbauer spectroscopy in ASTM A335 P91 steel submitted to continuous cooling cycles

Samples of ASTM A335 P91 steel submitted to continuous cooling at different rates were analyzed in the form of foils and powders by means of Mössbauer spectroscopy. The Continuous Cooling Transformation (CCT) diagram of steel ASTM A335 P91 displays two basic microstructural domains at low temperatures – ferritic and martensitic – whose limits depend on the austenite holding temperature, the precise chemical composition and the cooling conditions from the austenite mother phase. Under certain conditions, the martensitic transformation may not be completed, leading to a final microstructure with a non-negligible percentage of the austenite phase retained in a metastable state. This retained austenite could be detrimental for the mechanical properties of the steel.Mössbauer analysis suggested that powdering process promotes the retained austenite transformation to martensite; in particular, in the present case, all the austenite transformed into martensite during powdering. Foil samples instead displayed retained austenite whose relative fraction was determined as a function of the cooling rate. At the same time, the carbon content of retained austenite was estimated for the faster cooled samples; a preliminary explanation for the observed trends is given.

Comparative analysis of continuous cooling transformation behaviour in CGHAZ of API X-80 and X-65 line pipe steels

Continuous cooling transformation diagrams were obtained for a simulated coarse-grain heat-affected zone of API X-80 and X-65 steels using Gleeble-3800 thermo-mechanical simulator. Dilatometry results showed that variable cooling rates have significant effect on phase transformation behaviour and microstructure of the steels. For X-80 steel, a mixture of bainite/martensite at fast and medium cooling rates and ferrite/pearlite at slow cooling rates was obtained and confirmed by microhardness measurements between 340 and 240 HV, whereas, for X-65 steel soft phase such as ferrite was nucleated along with bainite at moderate (15–100 °C s−1) and pearlite at slow cooling rates (3–10 °C s−1). Dilatometry curve comprises inherent noise, and Loess computational filter was used to filter background noise in the curves. Martensite/bainite and ferrite/pearlite coexisted for most cooling rates, which made it difficult to distinguish transformation kinetics of individual phases. In the first derivative analysis d/dT {(ΔL/Lo)} of dilatometry curves, peaks of individual phases were clearly observed. Peaks of individual phase were separated according to their transformation temperature range, and peak rate temperatures were determined. It was confirmed that the first derivative d/dT {(ΔL/Lo)} analysis described phase transformation behaviour precisely and aided in identification of the transformed phases.

Investigation of Effects of SiO2 Content and Cooling Rate on Crystallization in Fe2O3-CaO-SiO2 System Using In Situ Confocal Laser Scanning Microscopy

The crystallization behaviors of the Fe2O3-CaO-SiO2 system in a non-equilibrium state with various chemical compositions and cooling rates were investigated by confocal laser scanning. Equilibrium samples were prepared in melt states and cooled at a rate of 1 K/min to produce samples in quasi-equilibrium states. The effect of SiO2 on the phase constitution was investigated using Image Analyzer, and the changes of the phase ratios were analyzed using activity values of Fe2O3 calculated using FactSage 7.1. The Continuous cooling transformation diagram of the Fe2O3-CaO-SiO2 system was obtained for various cooling rates. The temperatures at which the primary phase formed for various SiO2 contents and cooling rates differed from the equilibrium temperatures, and the phase-formation temperature ranges varied with the cooling rate. In addition, the reduction degree of each sample was measured to determine the effect of SiO2 content and cooling rate on the Fe2O3-CaO-SiO2 system. The reduction degree in the FeO3-CaO-SiO2 system is dominantly influenced by the SiO2 bond-phase morphology (CF, C2S, C3S, or glass) rather than the cooling rate.

Effect of simulated thermomechanical processing on transformation behavior and microstructure of 82B steel

The effects of loop-laying temperature and austenite deformation on the phase transformation behavior during continuous cooling, microstructure, and pearlite interlaminar spacing in 82B steels were investigated. Static and dynamic continuous cooling transformation (CCT) diagrams were measured with a Gleeble-3500 thermal simulator, and the mechanisms governing changes in the initial temperature, initial time, and duration of the phase transformation zone were also analyzed and discussed. The results show that CCT diagram shifted to the bottom right, the initial temperature of the phase transition decreased, the initial time of the phase transition increased, the duration of the phase transition increased, and the lamellar spacing of pearlite was finer as the loop-laying temperature increased. The initial phase transition time decreased, and the phase transition duration first reduced, then increased, and finally decreased in the static condition and in the dynamic condition at 850 °C as the cooling rate increased. Meanwhile, the phase transition duration continuously decreased in the dynamic condition at 900 °C. At a given loop-laying temperature, the lamellar spacing in pearlite was finer due to austenite deformation compared with the undeformed case. Compared with the results shown in the dynamic CCT diagram, the corresponding phase diagrams of the static CCT diagram slightly shifted to the bottom right. Moreover, there was a clear linear relationship between the reciprocal of the lamellar spacing in pearlite and the average undercooling degree in the phase transformation zone.

Influence of the volume of deposited weld metal on the service life of the vertical steel tank in the North

The structural and mechanical inhomogeneity of welded joints, design and technology defects, residual welding stresses and deformations introduced during the technological processes of constructing and repairing are common in welding vertical steel tanks. Hence, the problem of increasing the operational strength of welded joints is a crucial task. The present article describes the key stages of the structure formation of the metal of the heat-affected zone during the welding of low-alloyed steels used for tank manufacturing in the range of permissible cooling rates in the temperature range from 600 to 500°C. Based on the analysis of continuous cooling transformation diagrams and using the expression for finding the volume of deposited metal during welding in the range of permissible cooling rates, the weld deposit area was calculated. The hardness values are obtained using continuous cooling transformation diagrams for specific grades of steel. It has been established that the strength characteristics change depending on the volume of the deposited weld metal. We have provided graphs that allow determining the mechanical properties of the heat-affected zone metal by area of the weld deposit. The form of a weld is determined from the area found at the optimum rate of cooling temperature, which ensures acceptable cold resistance and strength of the heat-affected zone metal. Subsequently, the volume of deposited weld metal could significant influence the service life of the tanks, taking into account the stress concentrators.

Development of Niobium Microalloyed Steel for Railway Wheel with Pearlitic Bainitic Microstructure

Heavy haul transportation (load over 30 tons/axle), as well as the axle load, has been more and more used in Brazil and worldwide. The stress generated in the wheel-rail contact with loads up to 30 tons/axle is around 760 MPa, which causes premature wear and cracks of conventional wheels (AAR (Association of American Railroads) class C). Microalloyed wheels are fundamental on heavy haul transport, whose main function is to combine high hardness, ductility, and yield strength of the material in order to prevent shelling. The main purpose of this research is to develop a new microalloyed wheel steel with niobium addition that meets all the requirements of the AAR class D material with mixed microstructure composed of pearlite and bainite. The 0.71C/0.020Nb steel developed in this study (Nb material) for railroad achieved the standards required for AAR class D in all mechanical properties, with fracture toughness higher than the usual vanadium microalloyed steel used in comparison. The Continuous Cooling Transformation (CCT) diagram showed the presence of bainite even at very low cooling rate, in the range between 0.3 - 2 °C/s. These cooling rates to form bainite are much lower than those observed in other steels with similar composition.

Prediction of microstructure composition in steel plate heated using high power Yb:YAG laser radiation

This work concerns numerical modelling and computer simulations of temperature field and phase transformations during Yb:YAG laser heating of sheets made of S355 steel. The distribution of laser power emitted by Trumpf laser head D70 is used in the analysis. The heat source is modelled on the basis of interpolation algorithms using geostatistical kriging method. Coupled heat transfer and fluid flow in the fusion zone are described respectively by transient heat transfer equation with convective term and Navier-Stokes equation. The kinetics of phase transformations and volumetric fractions of arising phases are obtained on the basis of Johnson-Mehl-Avrami (JMA) and Koistinen-Marburger (KM) models. Continuous Heating Transformation (CHT) diagram is used for heating process and Continuous Cooling Transformation (CCT) diagram is used for heated steel with the decomposition of final volume fractions of phases transformed form austenite dependant on cooling rates.

Continuous cooling transformation diagrams of HSLA steel for seamless tubes production

The CCT and two DCCT diagrams were constructed for the HSLA steel containing Cr, V, Nb, and N microadditions, taking into account industrial processing parameters of this material in a seamless tubes rolling mill. The typical finish hot rolling temperature of 1173 K was used for the construction of the standard CCT diagram and the effect of previous austenite deformation at this temperature was evaluated in the DCCT diagram. Another DCCT diagram was developed after heating at 1553 K, followed by plastic deformation at 1173 K. The prior austenite grain size in the hot rolled material after heating at 1173 K was approx. 10 ?m, the heating of the as-cast material at 1553 K resulted in the prior austenite grain size over 200 ?m. The effect of the previous austenite deformation after low-temperature heating on the CCT diagram was negligible. The high-temperature heating showed a great influence on the austenite decomposition processes. The Ferrite-start temperature was significantly reduced at high cooling rates and the preferred decomposition of coarse grained austenite to acicular ferrite suppressed the bainite transformation at medium cooling rates. The developed DCCT diagrams can be used for the prediction of austenite decomposition products during the cooling of the seamless tubes from the finish rolling temperature. The CCT diagram can be utilized for the quality heat treatment of tubes.

Effect of the Chemical Homogeneity of a Quenched and Tempered C-Mn Steel Pipe on the Mechanical Properties and Phase Transformations

Seamless steel pipes for application in the oil and gas industries are manufactured from quenched and tempered steel. Aiming to meet the required characteristics, it is necessary that the microstructure and the mechanical properties are homogeneous along the wall thickness of the pipes, which is even more critical for thick wall pipes. Considering the seamless steel pipe manufacturing process, any chemical segregation along the thickness could affect the phase transformation kinetics during heat treatments and, consequently, microstructure and mechanical properties. In this context, this pioneer work when evaluating the effect of the chemical homogeneity of a seamless pipe on the mechanical properties and phase transformations. The studied pipe was manufactured by a C-Mn steel designed for oil and gas industry applications. Two regions of the pipe wall were analyzed - a region close to the inner surface and another near the outer surface. The steel was subjected to quenching and tempering heat treatments. Scanning electron and optical microscopy techniques were used to characterize the resultant microstructures. Microhardness and Charpy impact tests were performed aiming to analyze the pipe mechanical behavior in the studied regions. In addition, dilatometric tests were performed in order to determine the continuous cooling transformation diagrams of these regions.

Austenite Decomposition and Precipitation Behavior of Plastically Deformed Low-Si Microalloyed Steel

The aim of the present study is to assess the effects of hot deformation and cooling paths on the phase transformation kinetics in a precipitation-strengthened automotive 0.2C–1.5Mn–0.5Si steel with Nb and Ti microadditions. The analysis of the precipitation processes was performed while taking into account equilibrium calculations and phase transitions resulting from calculated time–temperature–transformation (TTT) and continuous cooling transformation (CCT) diagrams. The austenite decomposition was monitored based on thermodynamic calculations of the volume fraction evolution of individual phases as a function of temperature. The calculations were compared to real CCT and DCCT (deformation continuous cooling transformation) diagrams produced using dilatometric tests. The research included the identification of the microstructure of the nondeformed and thermomechanically processed supercooled austenite products formed at various cooling rates. The complex interactions between the precipitation process, hot deformation, and cooling schedules are linked.

Microstructure modelling of laser metal powder directed energy deposition of alloy 718

A multi-component and multi-phase-field modelling approach, combined with transformation kinetics modelling, was used to model microstructure evolution during laser metal powder directed energy deposition of Alloy 718 and subsequent heat treatments. Experimental temperature measurements were utilised to predict microstructural evolution during successive addition of layers. Segregation of alloying elements as well as formation of Laves and δ phase was specifically modelled. The predicted elemental concentrations were then used in transformation kinetics to estimate changes in Continuous Cooling Transformation (CCT) and Time Temperature Transformation (TTT) diagrams for Alloy 718. Modelling results showed good agreement with experimentally observed phase evolution within the microstructure. The results indicate that the approach can be a valuable tool, both for improving process understanding and for process development including subsequent heat treatment.

Effect of chemical composition and microstructure on the fatigue crack growth resistance of pearlitic steels for railroad application

This work characterized the microstructure and evaluated the mechanical behavior of two pearlitic steels used in Brazilian railroads, a C-Mn-Si steel and a V-microalloyed steel. The microstructures were observed by light optical, scanning electron and atomic force microscopy. Prior austenite grain size, pearlite colony size and pearlite interlamelar spacing were measured. Continuous cooling transformation diagrams for both steels were obtained. The mechanical behavior was evaluated by tensile tests, hardness tests, fracture toughness tests (linear elastic fracture mechanics - KIC), and fatigue crack propagation tests (da/dN × ΔK). The fatigue tests were performed with different R-ratios (R = 0.1 and 0.7) applying two recent empirical models proposed by Rubaie et al. (2008) and Jones et al. (2012) to predict the material behavior. Different loading histories including overloads in function of the crack size were also applied. The fracture surfaces of all tested specimens were analyzed by scanning electron and atomic force microscopy. The presence of vanadium in steel can certainly provide a more refined microstructure, changing the mechanical properties. However, an appropriate range of simple chemical composition and an adequate thermomechanical processing can provide a steel in accordance to standard specifications for railroad application without the need of microalloying element additions that could make the final product more expensive. In the specific case of fatigue, both steels presented a dependence to fatigue R-ratio. The two empirical models were satisfactory aiming to fit the experimental data, although the equation of Jones et al. is simpler to be manipulated. The studied steels also had a dependence to fatigue overloads that can represent a positive effect related to crack growth retardation with increasing fatigue overloads, but also a negative effect, because increasing the crack size for which the overload is applied and increasing the number of overloads, the intensity of retardation effect decreases. These fatigue results are important to predict the actual behavior of steels used in the railway sector and to perform a proper maintenance control, avoiding a premature failure and a consequent catastrophic accident.

MCDM towards knowledge incorporation in ANN models for phase transformation in continuous cooling of steel

PurposeThe purpose of this paper is to incorporate prior knowledge in the artificial neural network (ANN) model for the prediction of continuous cooling transformation (CCT) diagram of steel, so that the model predictions become valid from materials engineering point of view.Design/methodology/approachGenetic algorithm (GA) is used in different ways for incorporating system knowledge during training the ANN. In case of training, the ANN in multi-objective optimization mode, with prediction error minimization as one objective and the system knowledge incorporation as the other, the generated Pareto solutions are different ANN models with better performance in at least one objective. To choose a single model for the prediction of steel transformation, different multi-criteria decision-making (MCDM) concepts are employed. To avoid the problem of choosing a single model from the non-dominated Pareto solutions, the training scheme also converted into a single objective optimization problem.FindingsThe prediction results of the models trained in multi and single objective optimization schemes are compared. It is seen that though conversion of the problem to a single objective optimization problem reduces the complexity, the models trained using multi-objective optimization are found to be better for predicting metallurgically justifiable result.Originality/valueANN is being used extensively in the complex materials systems like steel. Several works have been done to develop ANN models for the prediction of CCT diagram. But the present work proposes some methods to overcome the inherent problem of data-driven model, and make the prediction viable from the system knowledge.

The Effects of Alloying Elements on the Continuous Cooling Transformation Behavior of 2¼Cr-1Mo Steels

The continuous cooling transformation (CCT) behavior of eight 2¼Cr-1Mo steels from a statistically designed matrix was determined. These steels contained two levels of carbon (0.07 and 0.16 wt.%), manganese (0.35 and 0.85%), chromium (1.5 and 2.8%), and molybdenum (0.3 and 1.25%). Each steel was tested in a quenching dilatometer at five to six cooling rates between 725 and 1.2 °C/min. For each CCT sample, the change in length, microstructure, and macrohardness were determined. The ferrite content was also measured for samples cooled near the ferrite nose. Pearson correlation and multiple regression analyses were performed for various CCT diagram parameters. The correlation analysis showed that carbon and chromium contents significantly affected the critical temperatures and the bainite and martensite transformation temperatures. Increasing carbon content significantly increased the hardness for the bainite and martensite range of cooling rates, but hardness at slower cooling rates was unaffected by alloying elements. Regression equations were obtained for the critical temperatures and the ferrite nose cooling rate.

A New Neural Networks Model for Calculating the Continuous Cooling Transformation Diagrams

A New Neural Networks Model for Calculating the Continuous Cooling Transformation Diagrams

Laser Welding Dissimilar High-Strength Steel Alloys with Complex Geometries

Laser welding of dissimilar high-strength steels was performed in this study for two different geometries, flat and circular samples with material thicknesses of 5 and 8 mm. The material combinations were a low carbon to a medium or high carbon steel. Three different welding systems were employed: a Nd:YAG, a CO2 and a fiber laser. The process stability was evaluated for all the experiments. The resulting full penetration welds were inspected for their surface quality at the top and bottom of the specimens. Cross sections were taken to investigate the resulting microstructures and the metallurgical defects of the welds, such as cracks and pores. Significant hardening occurred in the weld region and the highest hardness values occurred in the Heat Affected Zone (HAZ) of the high carbon steel. The occurrence of weld defects depends strongly on the component geometry. The resulting microstructures within the weld were also predicted using neural network-simulated Continuous Cooling Transformation (CCT) diagrams and predicted the occurrence of a mixture of microstructures, such as bainite, martensite and pearlite, depending on the material chemistry. The thermal fields were measured with thermocouples and revealed the strong influence of component geometry on the cooling rate which in term defines the microstructures forming in the weld and the occurring hardness.

Fabrication of a Cu36Zr48Al8Ag8 bulk metallic glass sheet by atmosphere-controlled vertical twin roll strip casting

ABSTRACTLarge-scale fabrication of Cu36Zr48Al8Ag8 amorphous alloy sheets having with lengths above 250 mm was performed using atmosphere-controlled twin roll strip casting. By constructing a continuous cooling transformation diagram, the critical cooling rate was determined to be 212.6 K/s. The size and morphology of the crystalline phases within the amorphous matrix, which was Al-rich and Ag-depleted, were confirmed by scanning electron microscopy. The presence of a small volume fraction fine dendritic crystalline phase was necessary for the improved plasticity of the amorphous strip under tensile deformation mode.