Stages of Quenching and Self-Tempering Cycle

The special heat treating cycle of quenching and self tempering has three stages.

First Stage

This consists of a fast cooling operation applied to the bar after it enters the cooling line. The efficiency of this cooling is very high so that it quenches to a certain depth below the surface to the martensite transformation range.

“Thermex" installation

Fig. 2.13 “Thermex" installation.

At the end of this operation, the bar has an austenite core surrounded by a layer composed of a mixture of austenite and martensite.

Phase formation in TMT Rolled Bar

Fig. 2.14. Phase formation in TMT Rolled Bar.

Microstructure of ATMT Bar

Fig. 2.15 Microstructure of ATMT Bar

It is also important to note that in some cases, the layer below the martensitic ring can be transformed partially and even completely to bainite structure depend only upon the operating parameters of process. The martensite content decreases with increasing distance from the surface.

Second Stage

The bar leaves the area of drastic cooling and is exposed to air. The heat transfer co-efficient of the environment is very small and the temperature gradient within the cross section of the bar is very large. The core reheats the quenched surface layer by conduction. As a result, the martensite formed dining the first stage is subjected to self tempering to ensures adequate ductility while maintaining a high yield strength level.

The second stage could be considered upto the stage of temperature equalization is dependent on the bar diameter and the cooling conditions which applied during the first stage.

Third Stage

This stage occurs as the bar lies on the cooling bed. During this stage the austenite core transforms to fenite and pearlite with micro-structure depending on the steel chemical composition, bar diameter, rebar entry temperature to cooling-line, cooling efficiency and duration of cooling.

The mam physical phenomena involved in the above mentioned three stages of the quenching and self tempering process are :

  • • Heat transfer from the rebar surface to the surroundings.
  • • Reheating of the surface from core by conduction.
  • • Metallurgical phenomena such as transformation of austenite to martensite, bainite, ferrite and pearlite.

Other advantages of this process lie in the fact that no expensive addition of micro-alloys is necessary on the one hand and the requirements are fulfilled even with reduced carbon contents on the other hand.

Main Equipments of TMT Process

(я) TMT quenching system.

TMT Quenching System

Fig. 2.16 TMT Quenching System.

  • (д) TMT quenching system.
  • (b) Pinch rolls, flying shears and cooling bed.
  • (c) Dedicated water system (Water Boxes & Equalization Troughs).
  • (d) Universal testing machine cum bend testing machine.

The success of high-speed operation and the ability to produce a superior finished product depends upon the design of the post finishing block equipments. Alignment of the water boxes and troughs are critical to continuous highspeed operation and for the elimination of cobbles. Wear resistant, split bore cast stainless steel nozzles and troughs are used throughout in the water boxes and equalization trough zones, to ensure the alignment is to be maintained. The nozzles and troughs are located on stainless steel wear pads mounted on water-cooled headers to negate misalignment due to thermal expansion during rolling.

Cast Stainless Steel Nozzles & Dual bore troughs

Fig. 2.17 Cast Stainless Steel Nozzles & Dual bore troughs.

The latest designed high-efficiency split bore water box nozzles provide quick and easy access for inspection and for cobble removal, contributing to maintain of a high operating efficiency. The latest design provides reduced flows for the same cooling length as original design nozzles providing reduced water consumption or increased cooling capacity over the same cooling length (Fig. 2.17).

All the water boxes in each outlet utilize a similar design, differing only in the size of the stainless steel nozzles used to accommodate the respective product size ranges.

Due to the size of the products being rolled through the straight bar outlet to brake slide and the resulting weight of the water box nozzles, the water boxes have been made traversing with two paths of water box nozzles and one by-pass roller table to cover the complete product size range. These water boxes have been split into 3 m sections to allow the total cooling length to be quickly adjusted for optimum cooling for both TMT products and plain bar products. (Fig. 2.18).

Water Box & Equalization Troughs

Fig. 2.18 Water Box & Equalization Troughs.

The water boxes are installed at various points to provide controlled temperature rolling in order to get the desired mechanical properties to different grades of steel.

The result is improved mechanical properties; including finer and improved grain size suitable for getting high yield strength with optimal sectional weight. This provides consistency of mechanical properties throughout the bar and from bar to bar.

 
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