Generally, basic steels do not have the sufficient combination of properties for a direct use for such types of applications. The compassionate evolved will support in scheming isothermal heat treatment cycles during production of such steels. During steel processing as well as fabrication through welding, the phase transformation has always been a significant research topic for steel users and metallurgists.
Isothermal heat treatment is subsequently possible after casting process in industry on nominal cost. Better properties offer to users to reduce the overall cost for structures by reducing the thickness of steel plates. Presently, most of researchers focus on isothermal transformation treatments for medium and high grade carbon steel [ 2 , 3 , 4 , 5 ] but they lack such information in wide range of isothermal temperature on micro-alloyed steel.
In the present study, effort has been made to design isothermal transformation ITT phase diagram for micro-alloyed steel.
Microstructural evolution is characterized by optical microscopy. The present micro-alloyed steel are provided by the Steel Authority of India Ltd.
Samples were taken from the transverse direction of a 18 mm thick hot rolled plate. Resistance heating was provided by K-type Chromel-Alumel thermocouples to the samples by a machine working based on conductive heat transfer model. A Linear Variable Displacement Transducer, LVDT dilatometer quartz based was fitted above the sample where the thermocouple was welded at mid position to acquire diametric strain changes during thermal cycles.
External water quenching was arranged on both side of the hollow region of sample to freeze the microstructure and achieve the desire cooling rates as per ASTM isothermal treatments. Desired cooling rate in programmed and actual thermal cycles was achieved in less than 1 s between the solutionizing temperature and the isothermal temperatures.
The samples were kept at these isothermal temperatures until completion of phase transformation estimated through dilation strain changes.
Subsequently, the sample was water quenched to room temperature by external water quenching arrangements. Isothermal simulated samples were partitioned cross-sectionally from the position of thermocouples for microstructural examination. Optical micrographs of base material and simulated IT samples were characterized by using optical microscope Leica DMI M equipped with digital imaging facility.
The grain size of ferrite was measured by linear intercept method using Image J software. Average grain size of 10 readings is reported in the present work. The optical micrographs for base metal and isothermally simulated steel samples are shown in Figures 2 and 3. The base metal micrograph consists of pro-eutectoid ferrite and pearlite as shown by arrow in Figure 2a. The pearlite structure is found to be in banded form in the direction of rolling [ 6 ].
The average grain size of ferrite is found to be decrease with decreasing isothermal temperature as shown in Figure 4. Rest bright phase fraction reveals as the pro-eutectoid ferrite. The austenite grain boundaries are completely illuminated through ferrite allotriomorphs and retained austenite has reconstructed into pearlite as can see in Figure 2b.
The diffusion of carbon in austenite ahead of plate tip gives maximum lengthening rate due to the capillarity effects. This structure is in agreement with other micro-alloyed steels [ 9 , 10 ]. As the isothermal temperature decreases, the rest amount of enriched carbon islands is fully dissolved in soft pearlite phase region as shown in Figure 2d. The ferrite grain size is found to be decreasing gradually with a decreasing isothermal temperature Figure 4.
Carbon is extracted from pearlite and it transforms into residual austenite or cementite or carbides along the austenite grain boundary.
This formation of ferrite laths and separation by residual austenite or cementite or carbides is known as upper bainite [ 5 , 11 , 12 ]. The fraction of residual austenite, etc. Upper and lower bainite structures have formed and grown by displacive mechanism [ 13 ]. Bainite formed as an aggregate of small plates of ferrite sub-units. These ferrite plates or laths grow in the form of clusters, which are known as sheaves.
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Your browser is not current. Microsoft Internet Explorer 6. About: Isothermal transformation diagram. Daneben werden aber auch andere Methoden wie Dynamische Differenzkalorimetrie oder elektrische Widerstandsmessung angewendet. Prinzipiell unterscheidet man das isotherme und das kontinuierliche ZTU-Diagramm.
Le curve TTT riportano l'inizio, la fine e talvolta anche l'avanzamento percentuale delle transizioni di fase che avvengono in un acciaio sottoraffreddato di specifica composizione durante una fase isoterma di sufficiente durata, partendo dallo stesso acciaio completamente austenitizzato al di sopra dei suoi.
Oltre alle curve di trasformazione, i diagrammi TTT riportano normalmente anche una o due di equilibrio determinate precisamente dalla sola composizione dell'acciaio e, a volte, dati relativi alla percentuale di volume trasformato in corrispondenza delle curve di fine trasformazione o alla durezza dell'acciaio ottenuto a fine processo. They are generated from percentage transformation-vs time measurements, and are useful for understanding the transformations of an alloy steel at elevated temperatures.
An isothermal transformation diagram is only valid for one specific composition of material, and only if the temperature is held constant during the transformation, and strictly with rapid cooling to that temperature. Though usually used to represent transformation kinetics for steels, they also can be used to describe the kinetics of crystallization in ceramic or other materials.
Time-temperature-precipitation diagrams and time-temperature-embrittlement diagrams have also been used to represent kinetic changes in steels.
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