CCT diagram Research Papers - Academia.edu (original) (raw)

The phase transformation kinetics under continuous cooling conditions for intercritical austenite in a cold rolled low carbon steel were investigated over a wide range of cooling rates (0.1–200 °C/s). The start and finish temperatures of the intercritical austenite transformation were determined by quenching dilatometry and a continuous cooling transformation (CCT) diagram was constructed. The resulting experimental CCT diagram was compared with that calculated via JMatPro software, and verified using electron microscopy and hardness tests. In general, the results reveal that the experimental CCT diagram can be helpful in the design of thermal cycles for the production of different grades of dual-phase–advanced high-strengh steels (DP-AHSS) in continuous processing lines. The results suggest that C enrichment of intercritical austenite as a result of heating in the two phases (ferrite–austenite) region and C partitioning during the formation of pro-eutectoid ferrite on cooling significantly alters the character of subsequent austenite phase transformations.

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- Microstructure, Advanced high strength steels, Dual phase steels, CCT diagram

Toothed wheels constitute essential part of the steel structural elements market. The most often hypereutectoid structural steels for carburizing are used for toothed wheels. The final producers of toothed wheels are not demanding regarding a microstructure, provided that the determined requirements will be fulfilled (e.g. metallurgical purity determined by ultrasounds). Therefore delivered forgings can
be in an annealed or quenched state. This results from the situation that the final heat treatment or heat-chemical one is being done at one of the last stages of the toothed wheel production. An essential factor allowing to develop the proper heat treatment is the knowledge of the kinetics of phase transformations of undercooled austenite and its relating to technological conditions, being at the producer disposal,
as well as to forging dimensions. Such investigations should be carried out on real melts used for forgings for toothed wheels production together with an analysis of microstructure changes on the forging cross-section. They should be based on calculation methods determining the distribution of cooling rates on its cross-section in dependence of an applied cooling medium. The mentioned above problems in relation to 18CrNiMo7-6 steel - are elucidated in this paper. The aim of the investigations was the description of the kinetics of phase transformations of undercooled austenite in this steel. The CCT diagram was constructed for the austenitizing temperature determined on the basis of phase transformations temperatures (the so-called critical points).

- by B. Pawłowski
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- Heat Treatment, Hardenability, CCT diagram

Continuous cooling transformation (CCT) diagrams and properties of four kinds of low-silicon C-Mn-Si-Al transformation-induced plasticity (TRIP) steels with different carbon contents, with or without microalloy element Ti/V, as well as a reference TRIP steel containing 1.19 wt.% Si were studied. The CCT diagrams exhibited that as the carbon equivalent (CE) increased, it caused a shift of the ferrite forming and pearlite forming temperatures to the right side and the bainite forming and martensite forming to the lower temperatures of the diagram. The microstructural evolution obtained from the dilatometry samples revealed that the highest cooling rates produced fully martensitic microstructure in all cases except the reference TRIP steel. As the cooling rate decreased, more ferrite and bainite were formed. The increase of CE caused the increase of the amount of martesite in the microstructure. Tensile test and Erichsen test of the investigated steels showed an excellent mechanical strength and ductility combination, with tensile strength between 800 and 1000 MPa, total elongation of around 20%, and a quite good formability with a dome height of about 10 mm in all cases.

- by Bruno De Cooman
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- Materials Engineering, Mechanical Engineering, Microstructure, Ductility

Microstructure formation in low carbon multiphase steels like dual phase steels is essentially governed by the thermo-mechanical schedule adopted for forming the alloy. Steps like intercritical annealing and/or isothermal holding at... more

- by Arunansu Haldar
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- Engineering, Design, Modeling, Conception

This paper considers the issues related to plotting of continuous cooling transformation (CCT) phase diagram based on the dilatometric test results. The numerical data processing algorithm for uniformly analysis of the dilatometric curves is developed and implemented. Computing of the phase transformation ranges and coefficient of thermal expansion (CTE) were made in Microsoft Excel. Numerical algorithm contains determination of the critical points as the deviation dots from linear approximation function of the monotonous segment of experimental curve which is extrapolated to the start (or finish) of the phase transformation. Method of CCT diagram plotting based on the quantitative analysis of CTE during austenite continuous cooling is proposed. CTE quantitative evaluation of austenite and its decomposition products were accomplished by the analysis of linear approximation function of monotonous segment of dilatometric curve. Dependence of the CTE values on the range of cooling rates were used to define the phase transformation ranges on the CCT diagram. Obtained CTE dependence clearly defines critical cooling rate (ССR) as an intersection point of functions, describing austenite CTE changes with cooling rates and austenite-ferrite mixture. Point of intersection was found by solving the system of functions equations. Confirmation and clarification of results, based on the numerical analysis of the dilatometric curves, could be done as usual, using metallographic analysis and microhardness testing. The application of the developed numerical algorithm for dilatometric curves provides opportunity to unify dilatometric test analysis and to improve the accuracy of the CCT diagram plotting.

- by Anton Tsvetkov
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- CCT diagram

ABSTRACTThis article outlines the use of quenching dilatometry in phase transformation kinetics research in steels under continuous cooling conditions. For this purpose, the phase transformation behavior of a hot-rolled heat treatable steel was investigated over the cooling rate range of 0.1 to 200 °C/s. The start and finish points of the austenite transformation were identified from the dilatometric curves and then the continuous cooling transformation (CCT) diagrams were constructed. The experimental CCT diagrams were verified by microstructural characterization using scanning electron microscopy (SEM) and Vickers micro-hardness. In general, results revealed that the quenching dilatometry technique is a powerful tool for the characterization and study of solid-solid phase transformations in steels. For cooling rates between 200 and 25 °C/s the final microstructure consists on plate-like martensite with the highest hardness values. By contrast, a mixture of phases of ferrite, baini...

- by Alfredo Flores
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- Microstructure, Multidisciplinary, Advanced high strength steels, Metals

Continuous cooling transformation (CCT) characteristics of microalloyed steels with different tungsten (W) contents (0, 0.1 and 1. wt.%) 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.

- by Chong Soo Lee
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- Microstructure, Transformation, Materials Design, CCT diagram

Present study aims to predict the effect of Ti, B, Cu and Ni on continuous cooling transformation diagrams of low carbon (0.04-0.05 wt%) steels by artificial neural network model. The predicted results are validated with dilatometric studies. Comparison of the phase fields in different continuous cooling transformation diagrams demonstrated that in Ni containing 1.5 wt% Cu-added Ti-B microalloyed steel it is possible to achieve dual phase (ferrite-martensite) microstructure in directly air-cooled condition (i.e., at cooling rate close to 1°C/s) by suppressing pearlite formation. Addition of Cu has remarkably improved the hardness of the dilatometric samples.

- by Arunansu Haldar
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- Engineering, Design, Modeling, Conception

Alternative materials to HY-80, quenched and tempered structural alloy steel, like the microalloyed steels HSLA-80 and ULCB, allow the suppression of the quenching and tempering heat treatment and offer better weldability due to their extra-low C content. In the HSLA-80 steel copper precipitation is one of the main hardening mechanisms available, while in the ULCB steel the contribution of the bainitic transformation plus solid solution hardening is vital. The aim of this work was to determine the continuous cooling austenite transformation (CCT) diagrams of both steels and its performance during age hardening. It was verified that both alloys developed a bainitic microstructure with low C content, commonly designed by the literature as "granular" bainite. The hardenability of the ULCB steel was greater than the HSLA-80 due to the presence of Nb, B and Mo in the first alloy. The age hardening behavior of these alloys was slightly different between each other. The HSLA-80 steel developed maximum hardness during a 600 • C age hardening, while for the ULCB steel this occurred at a 500 or 600 • C age hardening and took less time. Both steels showed a significant hardness decrease during the 700 • C age hardening, that was probably due to overaging and tempering effects.

- by Paulo Roberto Mei
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- Materials Engineering, Mechanical Engineering, Heat Treatment, Copper

- by Antonio Gorni
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- Materials Engineering, Mechanical Engineering, Heat Treatment, Copper