FIELD OF INVENTION
The present invention relates to a process of elimination of normalising and tempering process of Heat treatment by establishing a higher ratio of radius (R) of bend to mean Diameter (D) of bent tubes for coils In a high pressure boiler. More particularly, the invention relates to changing the tube bending radius for super heater and Re-heater coils.
BACKGROUND OF THE INVENTION
The Tubes after edge preparation are bent in a bending machine to a specific bending radius as per drawings to form a loop. Multiple loops are joined together by butt welding to form a circuit. There may be many different bending radius in one single circuit based on the design requirement. The R/D ratio of a bend is the ratio of radius (R) of the bend to mean diameter (D) of a tube. As per the Table 1, the SA213T91 material is subjected to heat treatment process based on the R/D ratio. For the bends with design temperature less than 540°C, when the R/D ratio less than 2.5, the post forming heat treatment has to be done at 745±15°C and when the R/D ratio is exceeding 2.5, no heat treatment is required. For the bends with design temperature greater than 540°C and less than or equal to 600 °C, when the R/D ratio is less than 2 Normalising have to be done at 1040°C followed by Tempering at 745±15°C, when the R/D ratio is greater than or equal to 2 and less than 10, post forming heat treatment have to be done at 745±15°C and when the R/D is greater than 10, no heat treatment is required. For the bends with design temperature greater than 600°C, when the R/D

ratio is less than 2.5 Normalising have to be done at 1040°C followed by Tempering at 745±150C,when the R/D ratio is greater than or equal to 2.5 and less than 10, post forming heat treatment have to be done at 745±15°C and when the R/D is greater than 10, no heat treatment is required. A bending former mounted on a bending machine is used for bending and it is unique for a specific bending radius.The tubes are usually subjected to cold bending. In some special cases where proper bending cannot be achieved for close R/D ratio, hot bending is performed. After post forming heat treatment the original properties of material before bending will be regained.


The Normalising and Tempering of T91 bends involves high furnace operation cost, higher cycle time, distortion of T91 bends due to higher furnace temperature. This distortion has to be corrected by cold or hot means to fit into the assembly fixture. This distortion correction involves lot of time and energy. Referring to figure 1, the tube shown is 47.63mm outside diameter with thickness of 5.6mm high alloy steel (T91) and the radius of bend used is R-90. The R/D ratio for this case is 1.88. So as per Table 1, for this bend the Normalising and Tempering have to be done. The Figure 4 shows the manufacturing process chart of super heater and re heater coil before improvement. The tubes will be subjected to bending. Since the R/D ratio for this case is less than 2.5, the bend tubes are subjected to Normalising at 1040°C followed by Tempering at 745 ± 15 °C with 30 minutes soaking time. This Normalising and Tempering have to be done in an encompassed condition i.e. the entire component to be normalised and tempered should be kept inside the furnace. Then the Normalised and Tempered tube portion will be Butt welded with the other portion of the circuits. Then the assembly will be formed and stress relieving of the entire assembly will be done at 745 ± 159C with 30 minutes soaking time. It is understood from the Table 1 that if the R/D ratio of T91 bends are kept more than 2.5 the entire Normalising and tempering process can be avoided, thereby cycle time reduction, cost saving, and distortions can be avoided.
Hence there is a need to design Super heater and Re-heater coils with higher R/D ratio by the way of experiments.

OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a process of elimination of normalising and tempering process of Heat treatment by establishing a higher ratio of radius (R) of bend to mean Diameter (D) of bent tubes for coils in a high pressure boiler, which is capable of establishing increased bend radius experimentally to eliminate Normalising and tempering process.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Shows the existing bend with bending radius.
Figure 2: Shows the modified bend with bending radius.
Figure 3: Shows the total circuit with all dimensional details.
Figure 4: Shows the flow process chart before modification.
Figure 5: Shows the flow process chart after modification eliminating heat treatment cycles.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Accordingly, experiments were conducted to increase the bend radius from R-90 mm to higher value thereby the R/D ratio will increase. The tube outside diameter is a constant parameter and cannot be increased. Referring to Figure 2 many

experiments were conducted to increase the radius_and it is found that with the increase in radius the length `L´ between the tube center increases. The radius of bend was finally fixed to R-175 mm. With this radius the R/D ratio of the bend become 3.67 which is higher than 2.5. In Figure 3 the increase in the dimension `L´ was compensated by decreasing the end limb length `ELI´ (04) and `EL2´ (03). With this improvement the overall height of the coil `H´ was kept same, thereby not altering the overall arrangements of the coils.
Referring to Rgure 3, the radius of bend was finally fixed to R1-175 mm . With this radius the R/D ratio of the bend become 3.67 which is higher than 2.5. In Figure 3 the increase in the dimension `L1´ between the tube centres was compensated by decreasing the end limb length `ELI´ (04) and `EL2´ (03). With this improvement the overall height of the coil `H´ was kept same, thereby not altering the overall arrangements of the coils.
The Figure 5 shows the manufacturing process chart of super heater and re heater coil after improvement. The tubes will be subjected to bending. Since in this case the bending radius and R/D ratio is revised to higher value, Normalising (N) and Tempering (T) is not required. The bend tubes are then subjected to Butt welding. After formation of the assembly they are subjected to stress relieving at 745 ± 15 °C with 30 minutes soaking time. With this improvement the heat treatment cycle time of 187 cycles (1122 production shifts) is saved.


RH=Re-Heater, SH=Super Heater, D=Outside Diameter in mm, R= Radius of Bend in mm
The total number of normalising and tempering cycle saved as per the above table is 187 cycles. This savings is for one typical 500MW tower type boiler.

WE CLAIM
1. A process of elimination of normalising and tempering process of Heat treatment
by establishing a higher ratio of radius (R) of bend to mean Diameter (D) of bent tubes
for coils in a high pressure boiler, the said process comprising;
establishing an increased bend radius of 175 mm from existing radius of 90 mm by conducting experiments to increase R/D ratio to more than 2.5 where `D´ is mean diameter of tube and is a constant parameter; wherein, an increase in length `L´ is compensated by decreasing end limb length EL 1 (04) and EL 2 (03) maintaining the overall height (H) of the coil same, so that overall arrangements of the coils are not altered, when achieved R/D ratio being more than 2.5, the entire Normalising and Tempering process are avoided.
2. The process as claimed In claim 1, wherein the established R/D ratio is 3,67.