FIELD OF INVENTION
The present invention relates to method of manufacturing of a Bi-metallic tube having excellent corrosion resistance at room and elevated temperature. More specifically, the present invention relates to Bi-metallic tube that can be used in Boilers, Heat exchangers, Pipelines of Chemical and Food processing industries.
BACKGROUND OF INVENTION
Alloy steels comprise a wide variety of alloying elements which are not present in plain carbon steel. Steels containing more than 3.99% chromium are classified as Stainless steels and Tool steels depending on the other alloying elements.
Stainless steels are high-alloy steels and have superior corrosion resistance than other plain carbon and low alloy steels. These steel are also resistant to oxidation at room and high temperatures. It is mainly because of formation of chromium oxide passive layer on the surface of the steel. The Chromium content in these Stainless steels can vary from 4 to 30 per cent depending on the applications. Stainless steels can be broadly divided into three groups based on their crystalline structure at room temperature: Austenitic, Ferritic, and Martensitic stainless steels. Each ASTM Material specifications (ASTM A 213, ASTM A 249, ASTM A 358, A-312) for stainless steel tubes generally cover a variety of grades or types that indicate a specific material composition. Stainless steels are used in Chemical plants, Refineries, Paper mills, and Food processing industries. Stainless steels are also used in equipment that are usually used for carrying acids, brines, sulphur water, seawater or halogen salts. Other applications of Stainless steel pipes and tubing are: Aviation, Electronics, Automotive, Cryogenic, Marine, Air conditioning and heating, Medical, Architectural and Textiles.

The corrosion resistance property of the conventional tubes is usually enhanced by increasing chromium content. Galvanized tubes are also used by applying a protective zinc coating to steel in order to prevent rusting. The corrosive atmosphere in a tube could be prevalent either on inside or outside of the tube. According to ASME standards, a minimum thickness of minimum 1.57 mm is required for corrosion resistance in addition to the thickness required
for structural strength.
However, use of Stainless steels, Alloy steel and Coated steel increase the overall expense of the equipment. Various efforts have been put forward by the researchers on how to make use of plain steel in such applications. The use of coated mild steel has not been that effective because of highly corrosive atmosphere in above mentioned applications. Bimetallic tubes can be used in such cases where only one side of the tube is exposed to corrosive atmosphere.
Chinese patent CN102182874 discloses method of bi-metallic tube with cold deformation for wear resistance application. However, the bi-metallic tube thus produced cannot be used at high temperature due to its loss of bonding achieved during cold deformation. Most of the chemical processes and transportation of chemical takes place at high temperature and hence, this bi-metallic tube cannot be used where operating temperature of the process is high. WO2009014001 also disclosed similar innovation on bi-metallic tube for oil transportation equipment. This also fails where operating temperature is relatively high. US patent 4125924 indicates that the alternate method of achieving a composite metal pipe or tube is formed by cold sink drawing a telescopic combination of work hardened pipe inside a soft pipe. Another

US patent 4332073 disclosed that an outer pipe is diametrically expanded by heating and an inner pipe is diametrically contracted by cooling and then coolant is pressurized to expand the inner pipe and the pressure is removed after specified expansion is taking place. However, there is no Metallurgical (or Diffusion) bond between the tubes. US patent 4556240 disclosed a process of producing double wall tube by thermo mechanical process. Commercially available processes like hydroforming and explosive forming are often expensive and since there is no diffusion bonding at the interface, the composite tube will not act as a monolithic tube and the interface can become source of failures like leakage at the interface, interface as weak region etc. Thus the existing composite applications are limited.
OBJECTIVES OF THE INVENTION
The object of this invention is to devise a method of producing a bimetallic tube using cold
drawing process.
Another object of the invention is to manufacture a bimetallic tube where tubes are bonded
together via metallurgical (Diffusion) bonding between inner and outer tube, i.e. at the
interface.
ADVANTAGES OF THE INVENSION ACHIEVED
If the metallurgical (Diffusion) bond could be achieved by cold drawing process followed by heat treatment then it will become as a monolithic tube would find wide range of applications at lower cost compared to the equivalent thickness of the tube which is of complete Stainless
steel.

SUMMARY OF THE INVENTION
In this invention, the diffusion bonding between Stainless steel and Carbon steel tubes is achieved by cold drawing process followed by heat treatment. The assembly after tight fitted to each other by cold drawing process is heated treated. When the composite pipes are heated to high temperature, due to difference between coefficients of thermal expansion there will be thermal stress at the interface. This stress will help in formation of diffusion bonding at the interface. Due to high temperature and thermal stresses, diffusion of alloying elements will take place across the interface. Thermal stresses are developed at the interface will aggravate the diffusion bonding. Experimentally it is found that diffusion of Cr towards Plain carbon steel and diffusion of Carbon towards Stainless steel and hence diffusion bonding at the interface.
DETAILED DESCRIPTION OF The Invention with BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
More precisely, the invention relates to the method of making a bimetallic Tube. The bimetallic tube comprises of two tubes - a first tube of smaller diameter and a second tube of large diameter. The diameters of the first tube can be decided based upon the end application requirements. In an embodiment of the invention, the smaller diameter tube is made of corrosion resistance High alloy steel such as Stainless steel etc. and large diameter tube is made of low alloy steel such as mild steel. This embodiment of the invention is particularly useful for the applications where inside environment of the bimetallic tube is exposed to corrosive environment. In another embodiment of the invention, the larger diameter tube is

made of corrosion resistance High alloy steel such as Stainless steel etc. and smaller diameter tube is made of low alloy steel such as mild steel. This embodiment of the invention is particularly useful for the applications where outside environment of the bimetallic tube is exposed to corrosive environment. The process of manufacturing the bimetallic tube comprises the following steps:
1) Inserting tube of smaller diameter inside the tube of larger diametre
2) Cold deforming at room temperature and subjecting to 8 tol2 % tensile elongation to impart mechanical bonding
3) Subjecting to heat treatment at Hydrogen/Nitrogen/Argon atmosphere. Thus fitted tube
assembly is heat treated at 900 to 1200 °C, preferably in the range of 850 to 950 °C for a
period of 1 to 2 hr at a reducing atmosphere H2 + N2.
In another embodiment of the invention, the low alloy steel tube comprises a coating layer of Zinc or aluminum (as coating) on the surface contacting the high alloyed steel such as stainless steel. The coating layer enhances bonding between Stainless steel and carbon steel at high temperature in the presence of Hydrogen.
EXPERIMENTAL:
Tube Samples of stainless steel and carbon steel were collected. The chemical composition of
tubes was given in the Table 1 and 2.


Initially tubes were pickled with nitric acid to remove oxide layers for 10 Mins. Stainless tube is inserted in plain carbon steel and then cold drawn by tensile elongation of 5 %. After tensile elongation the composite tube is heat treated at 900 °C in H2+N2 atmosphere to avoid oxidation of tubes.
For diffusion bonding to take place the constituent materials has to be brought at close proximity. This is achieved by the invention with tensile elongation by cold drawing and subsequent heat treatment. The tensile elongation of outer tube while keeping inside the stainless steel tube brought the surfaces at 30 microns distance. Further heat treatment at 900' C closes this interface and creates an interface pressure of 80 MPa. The figure 6 and table 3 below show that after heat treatment chromium and manganese gets diffused at the interface of the two tubes. The chromium gets diffused to more than 1 %.

WE CLAIM 1. A method of manufacturing a cold formed, diffusion bonded bimetallic tube, the method
comprising:
Inserting first tube of smaller diameter inside a second tube of larger diameter;
Cold deforming at room temperature and subjecting to 8 tol2 % tensile elongation to
impart mechanical bonding; and
Heating the cold deformed bimetallic tube to a temperature in the range of 900 to 1200
°C for diffusion bonding in an inert atmosphere.
2. The method as claimed in claim 1, wherein the cold formed elongated metallic tube is heat treated for 1 to 4 hours at reducing atmosphere.
3. The method as claimed in claim 1, wherein the first tube is made of high alloy steel and the second tube is made of low alloy steel in case corrosion resistance is required inside the bimetallic tube.
4. The method as claimed in claim 3, wherein the high alloy steel is stainless steel and the low alloy steel is mild steel.
5. The method as claimed in claim 1, wherein the first tube is made of low alloy steel
and the second tube is made of high alloy steel in case corrosion resistance is required
inside the bimetallic tube in case corrosion resistance is required outside the
bimetallic tube.

6. The method as claimed in claim 5, wherein the high alloy steel is stainless steel and the low alloy steel is mild steel.
7. The method as claimed in claim 1, wherein alloying element diffusion occurs across the interface of two tubes from stainless less steel to low alloying steel.
8. The method as claimed in claim 1, wherein at least 1 weight % of chromium metal diffusion occurs across the interface of the first tube and the second tube.