Continuous Cooling Transformation Diagrams Research Articles

In-situ Crystallization of Highly Volatile Commercial Mold Flux Using an Isolated Observation System in the Confocal Laser Scanning Microscope

The in situ crystallization behavior of highly volatile commercial mold fluxes for medium carbon steels was investigated using the confocal laser scanning microscope (CLSM) equipped with an optimized isolated observation system. The highly volatile compounds of the mold flux were suppressed during heating allowing direct observation in the CLSM. Cooling rates of 25, 50, 100, 400, and 800 K/min were incorporated and continuous cooling transformation (CCT) diagrams of 4 different commercial mold fluxes for medium carbon steels were developed. Identification of the crystalline phase was conducted with XRD and SEM–EDS analysis. A cuspidine crystalline was observed in all samples at various cooling rates. With higher basicity, CaF2, and NaF, the crystallization of the fluxes was enhanced according to the CCT diagram. As the slag structure becomes depolymerized, the diffusion rate of the cathodic ions seems to increase.

Calculation of hardness distribution in the HAZ of micro-alloyed steel

This study describes a method for the determination of hardness based on Vickers micro-hardness of the phase constituents in the heat-affected zone (HAZ) of single-layered metal active gas (MAG) and electron beam (EB)-welded components. The finite element (FE) simulation was performed incorporating of austenite grain size (AGS)-dependent γ–α decomposition during cooling from an austenitizing temperature in the HAZ. In this simulation, a 2D symmetric thermo-metallurgical model was conducted using SYSWELD software version 2013. Two welding continuous cooling transformation (CCT) diagrams were incorporated which were experimentally determined for 1000 and 1300 °C peak temperatures using dilatometry. The implementation of CCT diagrams was performed as the function of the calculated austenite grain sizes (AGS). It was found that the phase constituents themselves had the most significant contribution to the hardness. An increase in hardness was predicted in the electron-beam-welded HAZ, whereas the MAG welding showed a reduction in hardness. The amount of reduction was depending on heat input during welding. The prediction of the hardness distribution in the HAZ was found in good accordance with experimental results.

Continuous Cooling Transformation Behavior of a Ti Addition Steel

The continuous cooling transformation (CCT) behavior of a Ti attached steel was studied through thermal simulation tests, and the influences of different cooling rates on the microstructure and transformation were investigated. The results show that the microstructure changes with the cooling rate, and the CCT curve of studied steel is plotted, which indicates that the solid-state phase transformation mainly consists of four regions. The CCT diagram made it possible to predict the microstructures of studied steel with different cooling rates.

Phase Transformation under Continuous Cooling Conditions in Medium Carbon Microalloyed Steels

Several 35CrMo4 and 38MnV7 steels with different additions of Ti and V were manufactured by electroslag remelting. The influence of the alloying and microalloying elements on phase transformation at different cooling rates was studied and the continuous cooling transformation diagrams were plotted. In order to optimize the heat treatment and improve the mechanical properties, the range of cooling rates leading to a fully bainitic microstructure (without ferrite, pearlite and especially without martensite) was determined. Bainite and martensite transformation start temperatures (Bs, Ms) were also established and compared with the values predicted by empirical equations. The important role of precipitates (especially V carbonitride particles) on final microstructure and mechanical properties was assessed.

Joint-site structure friction welding method as a tool for drive pinion light weighting in heavy-duty trucks

To satisfy the applied compressive stresses of friction welded drive pinion fabricated by using the joint-site structure (JSS) method, three different variants were followed: (A) the initial design with two joints was carried out. Two different burn-off lengths were examined for this variant. (B) The optimum burn-off length was considered for only one weld zone. (C) The weld zone was moved radially from the initial location and two different gap sizes were compared. The smallest gap size for the third variant led to the largest weld length. The lack of structural welding defects for this variant was assessed by ultrasonic testing. Hardness of the material after friction welding (FW) was correlated to the Continuous Cooling Transformation (CCT) diagram of the used materials and revealed the phase/microstructure transformation of the material. The simulated applied stresses on the optimized friction welded design of the drive pinion showed suitable results. The new drive pinion friction welded by the JSS method reduced the weight of the component by approx. 14%.

Kinetics of the polymorphic phase transformation of Cu6Sn5

Cu6Sn5 is a critical intermetallic compound in soldering and three-dimensional integrated circuit packaging technology and exists in at least five different crystal structures in the solid state, with a polymorphic phase transformation from hexagonal to monoclinic structures occurring on cooling. The kinetics of polymorphic transformations in Sn-rich Cu6Sn5 and Cu-rich Cu6Sn5 is systematically investigated in this study. This includes the generation of continuous cooling transformation diagrams as well as time–temperature transformation diagrams. Techniques used include variable temperature synchrotron powder X-ray diffraction and differential scanning calorimetry. The findings have important implications for the manufacture of solder joints and their in-service performance.

The effect of coiling temperature on the microstructure and mechanical properties of a niobium–titanium microalloyed steel processed via thin slab casting

We describe here the influence of coiling temperature on the microstructure and mechanical properties, especially toughness, in a low carbon niobium microalloyed steel processed via thin slab casting. The objective is to elucidate the impact of coiling temperature on the nature and distribution of microstructural constituents (including different phases, precipitates, and dislocations) that contribute to variation in the strength–toughness relationship of these steels. In general, the microstructure primarily consisted of fine lath-type bainite and polygonal ferrite, and NbC, TiC and (Nb, Ti)C precipitates of size ~2–10nm in the matrix and at dislocations. However, the dominance of bainite and distribution of precipitates was a function of coiling temperature. The lower coiling temperature provided superior strength–toughness combination and is attributed to predominantly bainitic microstructure and uniform precipitation of NbC, TiC, and (Nb, Ti)C during the coiling process, consistent with continuous cooling transformation diagrams.

Input Data Influence on FEM Simulation of Steam Turbine Blades Materials Hot Forming

Computer simulation/optimization is widely applied to hot forming processes nowadays. A crucial point of any kind of computer simulation is the input data. There are implemented in all FEM codes some basic material database, but usually for limited materials and material states. As the input data decide about the output results quality, it is always necessary to pay attention, what are the simulation inputs. The best solution, but not always possible, is direct measurement of materials parameters for considered material and state. As this is in some cases not possible, materials data are searched in alternative ways. One possibility is survey of published data in literature. A reliability of published data is difficult to assess, as there are usually not available sufficient information about the material investigated in publications. Other possibility how to obtain material data nowadays is utilization of programs predicting material behavior on the basis of chemical composition. One of currently available software for material properties prediction is JMatPro. The program is able to calculate a broad range of properties and behavior with the use of established thermodynamics models on the basis of chemical composition and the initial state of the considered material state. There can be for example determined thermophysical and physical properties (from room temperature to the liquid state), time-temperature-transformation/ continuous-cooling transformation diagrams, stress/strain diagrams, proof and tensile stress, hardness, coarsening of γ and γ , and creep. The current paper is dealing with the simulation of hot forming of steam blade made of Ti-based alloy. As an input material data used for FEM simulation are conventionally measured data and data obtained from JMatPro. With the use of both material data sets, a sample component forming is simulated. The results obtained from both simulations are subsequently compared with the results obtained from physical forming of the sample component. Performance of both simulations is discussed here, taking into account results of physical forming results.

보론강의 경화능과 인장 특성에 미치는C, Mo, Cr의 영향

Hardenability and mechanical properties of boron-bearing steels containing C, Mo and Cr were investigated in this study. Using quench dilatometer, the steel specimens were cooled down to room temperature at different cooling rates to construct continuous cooling transformation diagrams and then the transformation products from austenite were examined. A critical cooling rate was introduced as an index to quantitatively evaluate the hardenability. The C addition to boron-bearing steels did not significantly affect hardenability compared to boron-free steels although it increases the hardenability. With the same content, the Mo addition largely increased the hardenability of boron-bearing steels than the Cr addition because it decreased both the transformation start and finish temperatures at low cooling rates. In particular, the Mo addition completely suppressed the formation of eutectoid ferrite even at the slow cooling rate of <TEX>$0.2^{\circ}C/s$</TEX>, whereas the Cr addition nearly suppressed it at the cooling rates above <TEX>$3^{\circ}C/s$</TEX>.

Continuous Cooling Transformation, Microstructure and Mechanical Properties of High-Strength Low-Alloy Steels Containing B and Cu

This study investigated the continuous cooling transformation, microstructure, and mechanical properties of highstrength low-alloy steels containing B and Cu. Continuous cooling transformation diagrams under non-deformed and deformed conditions were constructed by means of dilatometry, metallographic methods, and hardness data. Based on the continuous cooling transformation behaviors, six kinds of steel specimens with different B and Cu contents were fabricated by a thermomechanical control process comprising controlled rolling and accelerated cooling. Then, tensile and Charpy impact tests were conducted to examine the correlation of the microstructure with mechanical properties. Deformation in the austenite region promoted the formation of quasi-polygonal ferrite and granular bainite with a significant increase in transformation start temperatures. The mechanical test results indicate that the B-added steel specimens had higher strength and lower upper-shelf energy than the B-free steel specimens without deterioration in low-temperature toughness because their microstructures were mostly composed of lower bainite and lath martensite with a small amount of degenerate upper bainite. On the other hand, the increase of Cu content from 0.5 wt.% to 1.5 wt.% noticeably increased yield and tensile strengths by 100 MPa without loss of ductility, which may be attributed to the enhanced solid solution hardening and precipitation hardening resulting from veryfine Cu precipitates formed during accelerated cooling.

Phase Transformation Behaviors of Nb-V-Ti Microalloyed Pipeline Steel X70

The CCT (continuous cooling transformation) diagrams of the Nb-V-Ti without Mo containing microalloyed pipeline steel X70 were investigated. The microstructures observed in continuous cooled specimens are composed of P (pearlite), PF (polygonal ferrite), QF (quasi-polygonal ferrite), and GF (granular bainite ferrite). At low cooling rates between 0.1°C/s and 1°C/s, the microstructure of the steel consisted of banded ferrite and pearlite but higher cooling rates suppressed its formation.

Microstructure and mechanical properties of spheroidized D6AC steel

The influence of spheroidization on the microstructure and mechanical properties of D6AC steel was investigated by means of dilatometry, micrograph analysis, and tensile testing. The continuous cooling transformation diagram was presented, and the critical cooling rate to obtain a fully martensitic microstructure was determined. Undissolved Mo carbide particles in the martensite matrix were observed after the solid solution treatment at 860°C. The kinetics of the spheroidized cementite particles accelerated due to the carbide particles that were undissolved during the solid solution treatment. As the holding time for the spheroidization was increased, the total elongation increased, while the tensile strength and hardness were reduced according to a proportional relationship.

Evolution of Phases and Mechanical Properties of Thermomechanically Processed Ultra High Strength Steels

The present study concerns the development of two low carbon microalloyed ultra high strength steels on a pilot scale. This recent endeavour has been made towards the reduction of weight by achieving high strength to weight ratio together with improved weldability for the various prospective high performance defence applications such as explosive ammunition, gun barrel, missile skins, light-weight military bridges etc. These steels were thermomechanically processed and finished at different finish rolling temperatures followed by water quenching. Variation in microstructure and mechanically properties at different finished rolling temperatures was studied. The experimentally determined continuous cooling transformation diagrams have revealed that adequate hardenability is achievable in these steels usually at a cooling rate >5 °C/s. Lath martensite along with the microalloy (Ti, Nb) CN precipitate particles are the characteristic microstructural feature of the investigated steels. The high strength value obtained in the present steels is due to the accumulated contribution of fine grained pan-caked austenite, highly dislocated lath martensite along with the presence of tiny precipitates of microalloy carbide/carbonitride and Cu rich precipitates. The good combination of strength (1,364–1,538 MPa) and ductility (11–16 %) has been achieved for the selected range of finish rolling temperature. The Charpy impact toughness values (30–80 J) reveal approximately consistent fall with the lowering of testing temperature.

Physical Simulation of Weldability of Weldox 1300 Steel

In the present study, the investigation of weldability of new ultra-high strength - Weldox 1300 steel has been presented. The thermal simulated samples were used to investigate the effect of welding cooling time t8/5on the microstructure and mechanical properties of the heat affected zone (HAZ). In the frame of these investigation the microstructure was studied by the light (LM) and transmission electron microscopies (TEM). It has been shown that the microstructure of the Weldox 1300 steel is composed of tempered martensite, and inside the laths the minor precipitations mainly V(CN) and molybdenum carbide Mo2C were observed. Mechanical properties of parent material were analysed by the tensile, impact and hardness tests. In details the influence of cooling time in the range of 2,5 - 300 s. on hardness, impact toughness and microstructure of simulated HAZ was studied by using welding thermal simulation test. The results show that the impact toughness and hardness decrease with the increase of t8/5under the condition of a single thermal cycle in simulated HAZ. The continuous cooling transformation diagrams (CCT-W for welding conditions) of Weldox 1300 steel for welding purposes was also elaborated. The steel Weldox 1300 for cooling time in the range of 2,5 - 4 s showed martensite microstructure, for time from 4 s to 60 s mixture of martensite and bainite, and for longer cooling time mixture of ferrite, bainite and martensite. The results indicated that the weldability of Weldox 1300 steel is limited and to avoid the cold cracking the preheating procedure or medium net linear heat input should be used.

Phase transformations in WB36 heat resistant steel under continuous cooling conditions

Aiming to optimise the thermal treatment technology of WB36 heat resistant steel, the characteristics of phase transformations during continuous cooling at 5–2000°C min−1 were investigated by means of microstructure observation and dilatometric measurements. The continuous cooling transformation diagram of WB36 heat resistant steel was obtained, as well as the fractions of different transformation products obtained under various cooling conditions. The results showed that the transformation products in continuously cooled WB36 heat resistant steel involve polygonal ferrite, pearlite, granular bainite and lath martensite, with increasing cooling rates from 5 to 2000°C min−1. Owing to the Mo addition to the steel, the super cooled austenite in WB36 heat resistant steel was stabilised significantly, and granular bainite can be obtained in a wide range of cooling rate. However, the substructures of granular bainite vary significantly with the increase in cooling rate, including matrix morphology and the distribution of M/A islands in the matrix.

Continuous cooling transformation behaviors of CLAM steel

The continuous cooling transformation (CCT) behaviors of CLAM (China Low Activation Martensitic) steel were studied, the CCT diagram was constructed, and the influence of cooling rates on the microstructures was also investigated. The microstructures were investigated using optical microscopy (OM) and microhardness tests were also carried out. The results showed that CLAM steel possessed high hardenability and there were ferrite and martensite transformation regions only. The maximum cooling rate to form ferrite microstructure was found to be 10–12K/min. In order to obtain fully ferrite microstructure, the cooling rate should be lower than 1K/min. The CCT diagram also gave relevant parameters such as the transformation temperatures, i.e., Ac1, Ac3, Ms and Mf were 1124K, 1193K, 705K and 593K, respectively. The diagram made it possible to predict the microstructures and properties of CLAM steel with different cooling rates.

Effects of tungsten on continuous cooling transformation characteristics of microalloyed steels

Continuous cooling transformation (CCT) characteristics of microalloyed steels with different tungsten (W) contents (0, 0.1 and 1wt.%) were investigated to obtain the necessary information for heat treatment of these steels. The effects of W addition on the sizes of prior austenite grains and precipitates were analysed. CCT diagrams were obtained by varying the cooling rates from 0.1 to 120°C/s. Transformation characteristics were determined by using dilatometer test, microscopic observation and hardness measurement. The results showed that W had a positive effect on the refinement of prior austenite grains and precipitates. The CCT diagrams exhibited that the ranges of transformation products were shifted to the right side of the diagram when the W content increased. CCT diagram for steel with 0.1% W was similar in shape to that without W. The addition of 1% W induced two separated transformation ranges in the cooling rate range of 0.1 to 1°C/s in the diagram. Both the austenitisation starting and finishing temperatures were raised as W was added. W addition induced decreased critical cooling rates for phase transformations and obtaining complete ferrite+pearlite microstructures. The martensite transformation temperature was decreased after W addition. The addition of W caused increased hardness, and the hardness obeyed an exponential type relationship with cooling rate.

Numerical Modelling of Thermal and Structural Strain in Laser Welding Process / Modelowanie Numeryczne Odkształceń Cieplnych I Strukturalnych W Procesie Spawania Techniką Laserową

This work concerns numerical modelling of thermal and structural strain, resulting in heating and cooling of laser butt-welded joints. Numerical analysis of strain is carried out in Abaqus FEA. Through the use of additional author’s subroutines, the structural strain caused by phase transformations during heating and cooling of welded elements is taken into account in the analysis. V.I. Machnienko models as well as Continuous Heating Transformation (CHT) and Continuous Cooling Transformation (CCT) diagrams for S355 steel are implemented into UEXPAN subroutine in order to determine the kinetics of phase transformation in the solid state. The model takes into account thermomechanical properties of the base material varying with temperature. The paper presents results of numerical simulation of temperature field, predicted structural composition, thermal and structural strain in laser butt-welded joints.

Effect of Thermo‐Mechanical Process on Phase Transformation Behaviors of V–Ti–N Microalloyed Steel

AbstractThermo‐mechanical simulation tests were performed on V–Ti–N microalloyed steel under three hot working conditions by using Gleeble‐3800 thermo‐mechanical simulator to study the effects of hot deformation and post‐deformation holding process on the continuous cooling transformation behaviors of overcooled austenite. The continuous cooling transformation diagrams (CCT diagrams) were determined by thermal dilation method and metallographic method. The effects of the hot deformation, post‐deformation holding, and cooling rate on the microstructure evolution were analyzed. The results show that deformation promotes ferrite and pearlite transformation. In addition, deformation leads to an increase in bainite start temperature, which becomes more markedly with the increase in cooling rate. The post‐deformation holding process is much favorable to promote carbonitride precipitation of the microalloying elements, which contributes to ferrite nucleation and smaller austenite grains. As a result, an increase in ferrite quantity and a decrease in ferrite grain size can be observed. And further more, the post‐deformation holding process reduces the effect of hot deformation on the bainite start temperature.

Crystallization Behavior of the CaO-Al2O3-MgO System Studied with a Confocal Laser Scanning Microscope

The crystallization behavior of a calcium-aluminate system with various MgO content from 2.5 to 7.5 wt pct and CaO/Al2O3 ratios between 0.8 and 1.2 has been examined using a confocal laser scanning microscope (CLSM). CCT (continuous cooling transformation) and time temperature transformation (TTT) diagrams were constructed to identify the primary crystal phase of slag at different compositions and at cooling rates between 25 and 800 K/minutes. In the slag at a CaO/Al2O3 ratio of 1.0, crystallization temperature increased during isothermal and continuous cooling with higher MgO content, and the shortest incubation time was observed at 5 wt pct MgO. When MgO content was fixed to be 5 wt pct, crystallization temperature increased with lower CaO/Al2O3 ratio. According to the slag composition, cooling rates and temperature, the primary phase could be CA, or C5A3, or C3A, or C3MA2, or MgO, and the crystal morphology changes from dendrites to faceted crystals to columnar crystals in this composition range.