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FIELD OF THE INVENTION
The present invention relates to an improved method of heat-treatment of Steel to reduce welding failure. More particularly, the present invention relates to a heat-treatment process for producing welded Corten steel tubes having reduced flare failure.
BACKGROUND OF THE INVENTION
In a typical Steel plant, the High strength Corten Steel (grade IRS - M41 -1997) tubes of 3 inch diameter (Internal Diameter of 72.6 mm and wall. Thickness of 2.45 mm) are produced and used for combating sulphide corrosion in air heater assembly. These tubes are produced by side bending the Corten steel strips and joining the edges by high frequency electric resistance welding (ERW) process. After welding, the tubes are normalized at 880° C. Thereafter a flare testing is carried-out and after evolving satisfactory testing results, the tubes are supplied to the customers who manufacture air heaters. The tubes undergo flaring operation at two ends and then those are tested to conform parameters according to ASTM A450 standard.

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To perform flare testing, an opening of a cut length of the pipe is inserted in a mandrel and pressed along it; finally the flange portion is flattened against a flat base to make it 90° to the original tube. It has been frequently observed that these tubes fail along the weld seam during flaring test.
To identify the cause of such failure, a detailed metallographic study of the seam joint has been carried out to analyse the seam joint. A microscopic observation of the macro-structure reveals large ferrite-grain (equivalent to ASTM-2) on the inner and outer surface of the weld seam in the as-welded condition. A single cycle of annealing treatment is insufficient and incapable to bring about substantial changes in the grain size of metallographic structure which generally causes the weld joints to fail. To overcome the adversity in production line, it is essential that a method of heat-treatment for producing welded Corten steel is developed which reduces the flare failure, in the steel tubes. The principle of the present invention thus resides in an improvement in metallographic structure in the weld region of the Corten steel tube to reduce the flare failure.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a method of heat-treatment for producing welded Corten steel tubes having reduced flare failure, which eliminates the adversity of the prior art.

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Another object of the present invention is to propose a method of heat-treatment for producing welded Corten steel tubes having reduced flare failure by improving the metallographic structure of welded region of Corten steel tube.
A still further object of one present invention is to propose a method of heat-treatment for producing welded corten steel tubes having reduced flare failure, which decreases the rejection percentage of the produced steel tubes due to failure in flare test.
An yet further object of the present invention is to propose a method of heat-treatment for producing welded corten steel tubes having reduced flare failure, which facilitates providing welded corten steel tubes capable of being processed further without failure at the application stage in the heat exchanger.
SUMMARY OF THE INVENTION
The present invention emphasize on the introduction of a "Three -cycle heat-treatment process for achieving fine grain structure in the seam joint in ERW corten tubes. The operational process is carried-out by a plurality of steps.
Step - 1- welded Corten tubes are heat-treated in a walking-beam gas-fired furnace. The tubes are entered from one end of the furnace and taken-out from the other end over a duration of 30 to 40 minutes. The tubes are heated at a rate of 25°C to 30°C/minutes and raised to a

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maximum temperature of 800°C to 900°C, and then soaked for 4 to 8 minutes. After completion of soaking, the tubes are cooled at a rate of 25°c to 35°c/minute. The temperature at the entry and exit areas of the furnace is maintained at about 400°C to 550°C. The speed of the tube transfer within the furnace is 90cm to 100cm/min.
By implementation of the invented method of three stage heat-treatment cycle performing one after another, the coarse grain size along the seam of the welded tubes is refined to a grain size according to that provided in ASTM 8 standard, and with a homogenous grain distribution.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure la (1) Showing weld seam joint of corten steel tube.
Figure la (2) Showing welded corten steel tube fitted with mandrel at one end. (bottom end)
Figure la (3) Shows a welded corten steel tube fitted on a mandrel at one end and pressure being exerted from other end (ie from top). Deformation takes place at the bottom.
Figure la (4) Shows a complete exertion of pressure from the top, and the flange portion is flattered against a flat base to make it 90°C to the original tube.
Figure -1 (b) Shows the result of a perfect flare test.
Figure -1 (c) Shows a defect, for example, a crack developed after flare test in prior art.

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Figure -2 Shows photographic picture of a crack along the seam line in prior art.
Figure -3 Shows a metallographic structure having ferrite grains along weld seam for which failure occurs according to prior art.
Figure -4 Shows a time-treatment curve for a three stage normalizing process according to the present invention.
Figure -5 Shows photographic picture of a metallographic structure after multi stage heat-treatment of welded corten steel tube depicting fine grain structure at welded region.
Figure - 6 Shows a photographic picture of a welded corten steel tube after multi-stage heat-treatment.
DETAIL DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Figure 1. Shows a Corten steel tube (1) having a welded seam (2) line. The tube (1) is required to undergo a flare test as per specification. For flare testing, a mandrel (3) is placed at end of tube at the bottom. A specified load (P) is applied from the top of the tube (1) for flare testing. Due to increase in load, the bottom of the tube (1) starts deforming (Fig 1 (C)). After application of the full load (P), a flange portion is flattered against a flat base (4) to make it 90° to the original tube (1).
Figure - 1 (b) Shows that there is no seam (2) failure during flare test.

6 Figure 1 (c) Shows a crack (5) developed during flare testing.
Figure -2 Shows a photographic view of the crack (5) developed during flare testing. Flare failure during flare testing is due to the presence of large ferrite grains (6) (Fig - 3). The heat-treatment in a single stage of the welded corten tube constitutes performing only a single stage normalizing at 800°C to 900°c.
Figure - 4 Shows a time-treatment cycle (SI+SII+SIII) of the heat-treatment process of the present invention. The welded corten steel tube is heat treated (SI) in a walking-beam gas-fired furnace at a temperature of 400°C to 500°C (7), the tubes is heated at a rate of 25°C to 30°C/minutes (8) until it reaches to a maximum temperature at 800°C to 900°C (9) where the tube is allowed to stay for soaking (10) for 4 to 8 minutes. During this period, homogenization of grains takes place. After expiry of the soaking period, the steel tube is cooled at a rate of 25°C to 30°C/minute (11) until it reaches 480°C to 550°C (12) and the tube is taken out through the other end of the furnace. The speed of the tube movement within the furnace is 90cm to 100cm/min. Three of such heat cycles are carried out subsequently one after the other to achieve a fine grain microstructure of size prescribed in ASTM 8 standard.
Figure - 5 Shows a refined fine grain-size (13) of the welded region with uniform and homogenous distribution.
Figure - 6 Shows a photographic view of a welded corten steel tube (14) after 3 stage heat-treatment and flare testing. No crack developed during and after flare testing.

WE CLAIM
1. A method of heat-treatment for producing welded corten steel
tubes having reduce flare failure comprising:
- carrying-out a three stage (SI,SII,SIII) heat-treatment cycle of welded corten steel tube in a walking-beam gas-fired furnace, wherein each stage (SI, SII, SIII) of heat-treatment cycle comprising.
(i) starting heat-treatment temperature at a furnace temperature of 400°C to 500°C, and maintaining a heating range of 25°C to 35°C/min,
(ii) raising the furnace temperature to 880°C to 900°C in the furnace;
(iii) soaking the tube in the furnace at 880°C to 900°C for 4 to 8 minutes,
(iv) cooling the tube by reducing the furnace temperature between 880°C to 900°C and 400°C to 500°C @ 25°C to 30°c/min, and
(v) maintaining the speed of tube transfer within the furnace at 90 to 100 cm/min.
2. The method as claimed in claim 1, wherein a change in
metallographic structure of the weld joint from ASTM 2 to ASTM 8
with a homogenous distribution is achieved.

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3. The method of heat-treatment for producing welded Corten steel tubes having reduced flare failure as substantially described herein with reference to the accompanying drawing.
Dated this 27th day of SEPTEMBER 2006
The present invention emphasize on the introduction of a "Three -cycle heat-treatment process for achieving fine grain structure in the seam joint in ERW corten tubes. The operational process is carried-out by a plurality of steps.
Step - 1- welded Corten tubes are heat-treated in a walking-beam gas-fired furnace. The tubes are entered from one end of the furnace and taken-out from the other end over a duration of 30 to 40 minutes. The tubes are heated at a rate of 25°C to 30°C/minutes and raised to a

maximum temperature of 800°C to 900°C, and then soaked for 4 to 8 minutes. After completion of soaking, the tubes are cooled at a rate of 25°c to 35°c/minute. The temperature at the entry and exit areas of the furnace is maintained at about 400°C to 550°C. The speed of the tube transfer within the furnace is 90cm to 100cm/min.
By implementation of the invented method of three stage heat-treatment cycle performing one after another, the coarse grain size along the seam of the welded tubes is refined to a grain size according to that provided in ASTM 8 standard, and with a homogenous grain distribution.