FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
The Patent Rules, 2003
Complete Specification
(See section 10 and rule 13)
A METHOD OF MANUFACTURING A HEMISPHERICAL BOTTLE AND AN APPARATUS FOR THE SAME
Bharat Forge Limited
An Indian company registered under the Indian Companies Act, 1956.
Mundhwa, Pune Cantonment, Pune - 411036, Maharashtra, India
The following specification particularly describes the invention and the manner in which it is to be performed.

Field of invention
The present invention relates to a method of manufacturing hemispherical gas bottle made of Ti alloy which is used in satellite launch vehicle. Particularly, the present invention relates to using closed die hot forging process as an innovative manufacturing method for such bottles.
Introduction
The launch vehicles, used to launch satellites in space, generally use cryogenic rocket engines. The cryogenic rocket engines normally use oxygen and hydrogen as its fuel. These fuels are normally stored in liquid condition at cryogenic temperatures. These rocket engines also utilize liquid Helium which is used for cooling as well as pressurizing purposes. The liquid Helium is normally stored at cryogenic temperatures which can be as low as 20 K (-253 °C). The liquid Helium is stored in gas bottles, typically spherical, under high pressure. As the gas bottle has to face high storage pressures at cryogenic temperature, it has to be made from Titanium or similar alloys.
The Titanium alloys can maintain good fracture toughness even at cryogenic temperatures. The gas bottle is manufactured from an ‘α + β’ Titanium alloy when it is used at cryogenic temperatures of up to 77 K. It is manufactured from an ‘α’ Titanium alloy when it has to be used at temperatures up to 20 K.

A gas bottle is normally manufactured in two halves. Both halves, which typically have a hemispherical shape, are later joined together using electron beam welding or a similar welding process.
Each half of the gas bottle has to be manufactured using a bulk (closed die) hot forging process. This process uses a billet of the required metal or alloy. The billet is deformed sequentially in multiple steps using different dies in each step.
Conventionally, a hemispherical gas bottle is made in three forging steps of upsetting, blocker forging, and finisher forging – in that sequence. Traditionally, a forging hammer is used to apply multiple blows or strokes during each of the aforementioned three steps. However, drawbacks of using the conventional manufacturing process using a forging hammer are as follows:
1. Low productivity – Hammer forging is a very high cycle time forging. Time required to produce one part (one hemisphere) is large and may take a number of days. Thus, the productivity of this process is low.
2. Non-uniform microstructure – In hammer forging, multiple heats and booster heats are required during the aforementioned three steps to manufacture one part. Multiple heats and booster heats lead to coarsening of the microstructure as well as non-uniform microstructure in the finished part. This has significant effect on mechanical properties of the part and may render the part unsuitable for the intended purpose. Further, presence

of non-uniform microstructure leads to detection of noise during the ultrasonic testing (UT) of the part. This has dangerous consequences as the quality of the part cannot be confidently ascertained which may lead to sub-par quality parts being used. 3. Non-uniform strain penetration – The deformation in each stroke of hammer is very small. Due to this the deformation or strain does not penetrate throughout the whole part. This leads to non-uniform consolidation and microstructure.
One of the ways to overcome these drawbacks of the traditional manufacturing method is by producing the gas bottle in a forging press (or simply a press) rather than by using hammer forging. The inventors have discovered that this overcomes the aforementioned drawbacks as follows:
1. Productivity – Forging presses have higher energy and force capacity than a forging hammer. In a press, bigger deformations can be produced in the billet in each stroke thereby reducing the number of strokes required for deforming the billet to final shape of the gas bottle. Due to this the time required to produce final product will be drastically lesser than the conventional method.
2. Non-uniform microstructure – Use of slow strain rate equipment such as a hydraulic press or a screw press in combination with applying higher deformation (than conventional methods) per step helps in uniform deformation of the material throughout the part, and hence, produces more

uniform microstructure in the part. As compared to the conventional methods, lesser number of heats and booster heats are required when producing the part on the forging press. This further helps the uniformity of the microstructure. Uniform microstructure also results in noise being absent during the UT testing. 3. Non-uniform strain penetration – Greater deformation in each stroke helps in better penetration of strain throughout the part. This helps in the uniform consolidation of the part, and thus, rather desirably, uniform properties.
The use of forging press for the manufacturing of gas bottle has its own set of challenges. The titanium alloy is one of the most difficult to forge material. To deform this alloy, very high forces are required. Therefore, in order to overcome the aforementioned drawbacks of the conventional methods, if one were to use forging presses to carry out conventional methods deploying the steps of upsetting, blocker forging, and finisher forging, one would need to use very high capacity forging presses, which is a drawback in itself.
Thus, there exists a room for advancement over the existing technology in that an innovative method of manufacturing has to be designed to suit the manufacturing of hemispherical gas bottle through hot forging process (closed die forging process) on a forging press thus, reducing manufacturing cycle time and improving the properties of the part.

Objects of invention
Some of the objects of the present disclosure which at least one embodiment
herein satisfies are as follows:
It is an object of the present invention to provide a closed die hot forging process for hemispherical gas bottle.
It is another object of the present invention to provide manufacturing method suitable for a forging press.
It is still another object of the present invention to provide a forging process design to reduce the forces required to forge the part.
Other objects and advantages of the present disclosure will be more apparent from the following description which is not intended to limit the scope of the present disclosure.
Brief description of accompanying drawings
Figure 1 shows a hemispherical gas bottle (1)
Figure 2 shows the shaped upset preform (4). Section AA shows the cross section
of the shaped upset preform.

Figure 3 shows the central spreader preform (6). Section BB shows the cross section of the central spreader preform.
Figure 4 shows the peripheral spreader preform (11). Section CC shows the cross section of the peripheral spreader preform.
List of parts
1. Hemispherical gas bottle 7. Central Spreader hub
2. Bottle hub 8. Central Spreader Arm
3. Bottle wall 20 9. Bulbous Portion
1A. Rolled or extruded or forged 10. Heated Central Spreader
billet Preform
1B. Heated Billet 11. Peripheral spreader preform
4. Shaped upset preform 12. Peripheral spreader hub
4A. Preformed Hub 25 13. Peripheral spreader Arm
4B. Peripheral portion 14. Heated peripheral spreader
5. Heated shaped upset preform preform
6. Central Spreader Preform
Summary of invention
The present invention is directed to a method of manufacturing a hemispherical gas bottle (1) suitable for use in satellite launch technology. The process design of the invention is such that it allows the closed die hot forging of the gas bottle (1)

on a forging press rather than a forging hammer. Such ‘press forged hemispherical gas bottles’ have better strength and mechanical properties than conventionally produced ‘hammer forged hemispherical gas bottles’.
The present invention provides a closed die hot forging process which can be used for the manufacturing of gas bottles on a forging press. The manufacturing process comprises of multiple forging steps. Process steps like shaped upset and central spreader forging are used to give major deformation to the cylindrical billet used as an input to forging. These operations are followed by operations like peripheral spreader forging and finish bending and ironing.
All the forging steps are performed preferably on a low strain rate forging press like hydraulic press or screw press. The process design is developed such that it minimizes the forging load generated during each step.
Description of the invention
The present invention relates to the manufacturing of a hemispherical gas bottle (1) having a hub (2) and a wall (3). The bottle (1) is made up of Titanium alloy, used in a satellite launch vehicle, using the closed die hot forging process. The key inventive feature of this invention is the design and development of the manufacturing process allowing production of the gas bottle (1) on a forging press rather than a forging hammer while maintaining low forging loads which allows use of small and medium capacity presses.
The invented process will be now explained in detail.

1. Supplying Input Raw Material: According to the invented process, the manufacturing process starts with a forged, extruded or rolled billet as a raw material. The billet is made up of titanium alloy. Preferably a cylindrical billet is selected.
2. Billet Heating: The billet is heated to the required temperature in a furnace. Preferably an electric furnace or gas fired furnace is used for the billet heating. Titanium alloys, in gas bottle applications are normally forged below the β transus temperature. Thus, the furnace temperature is maintained at a temperature below β transus by 20 to 60 ºC.
3. Shaped Upset Preform Forging: The heated billet is put in a shaped upset die in a forging press. In this operation, the billet is deformed in between two dies to a predefined height. These two shaped upset dies namely the shaped upset top die (SUT) and shaped upset bottom die (SUB), together termed as shaped upset dies or shaped upset preform dies, are used for this operation. The impressions or die cavity on the SUT and SUB dies are such that when they come together for shaped upset preform forging the enclosed cavity has the shape required for production of the shaped upset preform (4). The deformation in the shaped upset preform forging step can be performed in one stroke or multiple strokes. Use of one stroke or multiple stroke depends on the type of Titanium alloy being used. For some alloys, especially the α+β alloy, as the deformability of alloy is very good, the shaped upset preform forging step can be done in one stroke. For some difficult to deform alloys like α alloy, this deformation cannot be done in one stroke. In such cases, multiple strokes are required to complete the deformation. In between each stroke, the deformed part has to be again put in the furnace for booster heating. This step of booster heating helps in restoring the temperature of the preform to uniform

forging temperature. The outcome of the shaped upset preform forging is a shaped upset preform (4). Following the shaped upset operation, the shaped upset preform (4) is cooled to room temperature so that the preform may be inspected for potential crack development. The shaped upset preform (4) is shown in Figure 2. As can be seen the shaped upset preform (4) has central shape (4A) or portion, also called preformed hub (refer section AA in Figure 2), which is similar to the hub (2) portion of the finished gas bottle (1). The peripheral portion (4B) of shaped upset preform (4) is deformed more and its thickness is less than that of the preformed hub (4A). Further, the shaped upset preform dies (SUT and SUB) are designed such that the orientation of the peripheral portion (4B) of the shaped upset preform is such that the centre line passing through it is at a first included angle (ζ) with the horizontal axis. In one embodiment, the first included angle (ζ) can vary from 5º to 25º. Further, the shaped upset dies (SUT and SUB) are designed such that the material of shaped upset preform (4) becomes thicker from the joint of the preformed hub (4A) and the peripheral portion (4B) moving out towards the outer periphery. The thickening of the peripheral portion (4B) is produced by having a second included angle (θ) between the top die (SUT) surface and bottom die (SUB) surface in peripheral portion (4B) region which may vary between 5º to 25 º. Heating of Shaped upset preform: The shaped upset preform (4) after cooling to room temperature is then reheated in a furnace to produce a heated shaped upset preform (5). The parameters described in step 2 of Billet Heating are applicable here also. The furnace temperature is maintained at a temperature below β transus by 20 to 60 ºC.
Central Spreader Forging: The heated shaped upset preform (5) is subjected to a central spreader forging step to produce a central spreader

preform (6) as shown in Figure 3. Two central spreader dies namely the central spreader top die (CST) and central spreader bottom die (CSB), together termed as central spreader dies, are used for this operation. The impressions or die cavity on the CST and CSB dies are such that when they come together for central spreader preform forging the enclosed cavity has the shape required for production of the central spreader preform (6). In this step, the heated shaped upset preforms (5) material is further spread in radial direction as well as some amount of bending of the material is achieved. In this step, the heated shaped upset preform (5) is deformed partially by pressing only a central region. The said central region is inclusive of 4A and the portion of 4B of the said shaped upset preform (4) (refer section AA in Figure 2). The material near the circumference or periphery of the shaped upset preform (4) is given free space to move without any further contact with the central spreader top die (CST) used in Central Spreader Forging step. Free movement of the material of the peripheral portion (4B) is achieved by providing a special feature in the central spreader top die (CST) called “Gutter”, which is nothing but a relief provided in the central spreader top die (CST) used in central spreader forging step for free movement of the material being forged. The free flowing material (9) is the bulbous portion shown in section BB of Figure 3, which goes into the gutter of the central spreader top die (CST) used in central spreader forging step and hence, does not remain in contact with the dies. Due to this partial deformation (focused in the central region of shaped upset preform (4)), the load or forces required for this step are reduced significantly. The central portion of central spreader preform (refer Figure 3 section BB) has two parts called central spreader hub (7) and central spreader arm (8). The shape of central spreader hub (7) is similar to hub in hemispherical gas bottle. Central

spreader arm (8) is the portion where the deformation happens in the central spreader forging. It can be seen from section AA of Figure 2 and section BB of Figure 3 that the central spreader arm (8) and the bulbous portion (9) are further bent in this step, whereby the centerline passing through the peripheral portion (8 and 9) forms a third included angle (γ) with the horizontal. The third included angle (γ) varies between 15º to 45 º as the forging progresses. The impression or the die cavity in the central spreader top (CST) and central spreader bottom (CSB) dies used in this operation are such that in the peripheral region the die surface makes the third included angle (γ) with the horizontal. Further a gutter is provided in the central spreader top die (CST) which helps in formation of the bulbous region (9) in the central spreader preform. This gutter is provided, from the joint of region 8 and 9 outwards, in the form of a relief where the included angle between the die surface and the horizontal becomes less than (γ) by at least 10º. As explained in previous steps, this operation can also be performed in one step or multiple steps with booster heating in between depending on the nature of the alloy. After completion of this step, the central spreader preform (6) is cooled down to room temperature so that it may be inspected for potential crack development. Heating of central spreader preform: The central spreader preform (6) is then reheated in a furnace to produce a heated central spreader preform (10). The parameters described in step 2 of Billet Heating are applicable here also, i.e. the furnace temperature is maintained at a temperature below β transus by 20 to 60 ºC.
Peripheral Spreader Forging: The heated central spreader preform (10) is subjected to peripheral spreader forging operation to produce a peripheral spreader preform (11) as shown in Figure 4. Two peripheral spreader dies

namely the peripheral spreader top die (PST) and peripheral spreader bottom die (PSB), together termed as peripheral spreader dies, are used for this operation. The impressions or die cavity on the PST and PSB dies are such that when they come together for peripheral spreader preform forging the enclosed cavity has the shape required for production of the peripheral spreader preform (11). In this step, the material is further spread radially. In this step also the material is deformed partially by pressing only the bulbous portion (9) of the central spreader preform (6). During this operation, the material near the circumference or periphery of the heated central spreader preform (10 is deformed while the area near the centre of the preform (which was deformed in the central spreader stage i.e. (7) and (8)) is given free space to move with contact with the dies (used in peripheral spreader forging) happening at the very end of this forging step. In this case the included angle (γ) of the peripheral portion (13), as shown in section CC of Figure 4, of the peripheral spreader preform (11) is maintained same as that of the peripheral portion (8) of the central spreader preform (6). Due to this partial deformation, the load or forces required for this step are reduced significantly. As explained in previous steps, this operation can also be performed in one step or multiple steps with booster heating between any two consecutive steps. After completion of this step, the peripheral spreader preform (11) is cooled down to room temperature. The peripheral spreader preform (11) has two parts as shown in section CC of Figure 4, the peripheral spreader hub (12) and peripheral spreader arm (13). The peripheral spreader arm (13) has a thickness same as required in the finished part of the wall (3) region of the hemispherical gas bottle (1).

8. Heating of peripheral spreader preform: The peripheral spreader preform is then reheated in a furnace to produce a heated peripheral spreader preform (14). The parameters described in step 2 of Billet Heating are applicable here also. The furnace temperature is maintained at a temperature below β transus by 20 to 60 ºC
9. Finish bender forging: This heated peripheral spreader preform (14) is subjected to a bending and ironing operation called a finish bender forging. Two finish bender dies, namely the finish bender top die (FBT) and finish bender bottom die (FBB), together termed as finish bender dies, are used for this operation. The impressions or die cavity on the FBT and FBB dies are such that when they come together for finish bender forging the enclosed cavity has the shape required for production of the hemispherical gas bottle (1). In this step, the material in the region (13) of peripheral spreader preform (11) is bent and ironed in order to achieve the final shape of the hemispherical gas bottle (1). The bending and ironing operations are performed simultaneously. The hub (12) region of peripheral spreader preform (11) is also deformed in this step to obtain the final shape. As explained in previous steps, this operation can also be performed in one step or multiple step with booster heating in between. In one embodiment, the spreader preform is bent in one stroke followed with booster heating and the final ironing stroke. After completion of this step, the final forged hemispherical gas bottle (1) is cooled down to room temperature.

The hot forging is followed by dimensional inspection, DP testing to detect any cracks, shot blasting and heat treatment process which is followed by the machining process.
In one exemplary embodiment of present invention, the process of present invention is used for producing a hemispherical gas bottle having diameter of around 500 mm and height of around 350 mm, and using a forging press. It was found that the maximum force required to produce the part is reduced by around 45%. The hemispherical gas bottle was made from Titanium alloy Ti-6Al-4V. The heating temperature used during this forging was in the range of 930 to 960 ºC as its β Transus is 995 ºC. The first included angle (ζ) and second included angle (θ) used in shaped upset preform was 15º. Further the third included angle (γ) used in central and peripheral spreader was 30º.
It is evident from the foregoing discussion that the invention has a number of embodiments.
A preferred embodiment of the present invention discloses a method of manufacturing a hemispherical gas bottle from a forged, extruded or rolled billet (1A) made of Ti alloy, wherein said method comprises the following steps:
a. heating said billet (1A) in a furnace to a temperature below the β transus temperature to produce a heated billet (1B);

b. carrying out shaped upset preforming forging on said heated billet (1B) in
a shaped upset die-set comprising a top die and a bottom die to produce a
shaped upset preform (4) having a preformed hub (4A) and the peripheral
portion (4B), and wherein said peripheral portion (4B) is deformed more
than said preformed hub (4A) and wherein the thickness of said peripheral
portion (4B) is less than that of the preformed hub (4A);
c. heating said shaped upset preform (4) in said furnace to produce a heated
shaped upset preform (5) by maintaining said furnace to a temperature
below the β transus temperature;
d. subjecting said heated shaped upset preform (5) to a central spreader
forging step wherein the material of said heated shaped upset preform (5)
is partially spread in radial direction and the peripheral portion (4B) is
bent, whereby a central spreader preform (6) is produced having a central
spreader hub (7), central spreader arm (8) and the bulbous portion (9);
e. heating said central spreader preform (6) in said furnace to a temperature
below the β transus temperature to produce heated central spreader
preform (10);
f. subjecting said heated central spreader preform (10) to a peripheral
spreader forging step to produce a peripheral spreader preform (11)
having a peripheral spreader hub (12) and peripheral spreader arm (13);
g. heating said peripheral spreader preform (11) in said furnace to a
temperature below the β transus temperature to produce a heated
peripheral spreader preform (14);

h. subjecting said heated peripheral spreader preform (14) to a bending and ironing operation to produce the final hemispherical gas bottle (1).
In one embodiment of the present invention, the billet is cylindrical in shape.
In another embodiment of the present invention, the furnace temperature is maintained at a temperature below β transus by 20 to 60 ºC for each steps a, c, e and g.
In yet another embodiment of the present invention, in between each press stroke of a multi-stroke operation of each step of process, the deformed part is placed in said furnace for booster heating.
In a further embodiment of the present invention, the die used for step b (shaped upset preform forging) is such that the orientation of the peripheral portion (4B) is at a first included angle (ζ) with the horizontal axis, and wherein there is a second included angle (θ) between the top die surface and bottom die surface in said peripheral portion (4B).
In a still further embodiment of the present invention, the first included angle (ζ) and said second included angle (θ) vary between 5º to 25 º.

In another embodiment of the present invention, after the central spreader arm (8) and the bulbous portion (9) are further bent in said step d, the peripheral portion (8 and 9) forms a third included angle (γ) with the horizontal.
In a still further embodiment of the present invention, the third included angle (γ) varies between 15º to 45 º.
In yet another embodiment of the present invention, in said step d (Central Spreader Forging), the material is deformed partially by pressing only a central region, i.e. a region comprising central hub (4A) and the part of peripheral portion (4B) that is near to the central hub (4A), of said shaped upset preform (4), and wherein the material of said peripheral portion (4B) of the shaped upset preform (4) is allowed to move without any further contact with the top die of Central Spreader Forging of to form a bulbous portion (9).
In a still further embodiment of the present invention, in said step f (peripheral spreader forging step), the material near the circumference or periphery of the heated central spreader preform (10) is deformed while the area near the center of the preform is allowed to move freely with contact with the dies of peripheral spreader forging happening at the very end of the stroke.
In another embodiment of the present invention, after the material near the circumference or periphery of the heated central spreader preform (10) is

deformed in said step f, the peripheral portion (13) has the same third included angle (γ) with the horizontal.
In another embodiment of the present invention, the deformation in any of the steps b, d and f is performed in one stroke in the case said alloy is of α+β type.
In a further embodiment of the present invention, the preforms of steps b, d, and f are allowed to cool down for inspecting them for potential crack development before the respective steps c, e, and g of preform heating are carried out.
In a still further embodiment of the present invention, the forging process is carried out on a forging press.
In another embodiment of the present invention, the forging press used for the forging process is preferably a hydraulic press or a screw press.
The invention also discloses an apparatus for manufacturing a hemispherical gas bottle from a forged, extruded or rolled billet (1A) made of Ti alloy. The apparatus comprises forging press, in which are placed a set of top and bottom shaped upset preform dies to carry out shaped upset preform forging, a set of top and bottom central spreader forging dies to carry out the central spreader forging, a set of top and bottom peripheral spreader forging dies to carry out peripheral spreader forging, a set of top and bottom finish bender forging dies to carry out finish bender forging.

In an embodiment of the apparatus of the present invention, the impressions or die cavity on the SUT and SUB dies are such that when they come together for shaped upset preform forging, the enclosed die cavity has the shape required for production of the shaped upset preform (4).
In another embodiment of the apparatus of the present invention, the impressions or die cavity on the SUT and SUB dies are such that when they come together for shaped upset preform forging the orientation of the enclosed die cavity corresponding to the peripheral portion (4B) of the shaped upset preform produced using these dies is such that the centre line passing through the die cavity corresponding to the peripheral portion (4B) is at a first included angle (ζ) with the horizontal axis, said first included angle (ζ) being between 5º and 25º.
In yet another embodiment of the apparatus, the impressions or die cavity on the SUT and SUB dies are such that a second included angle (θ) between the top die (SUT) surface and bottom die (SUB) surface in the enclosed die cavity corresponding to the peripheral portion (4B) region is between 5º and 25 º.
In a further embodiment of the apparatus of the invention, the impressions or die cavities of the CST and CSB dies is such that once the central spreader arm (8) and the bulbous portion (9) are further bent in the central spreader forging step, the centerline passing through the enclosed die cavity corresponding to the

peripheral portion (8 and 9) forms a third included angle (γ) with the horizontal, said third included angle (γ) being between 15º and 45 º.
In a still further embodiment of the apparatus, a gutter is provided in the form of a relief channel in the central spreader top die (CST) from the die surfaces corresponding to the joint of said central spreader arm (8) and said bulbous portion (9) outwards, such that the included angle between the die surface and the horizontal becomes less than said third included angle (γ) by at least 10º.
In yet further embodiment of the apparatus, the impressions or die cavity on said PST and PSB dies are such that when they come together for peripheral spreader preform forging, the enclosed cavity has the shape required for production of the peripheral spreader preform (11).
In yet another embodiment of the apparatus, the impression or the die cavity of said peripheral spreader dies is such that said included angle (γ) of the enclosed die cavity corresponding to the peripheral portion (13) of the peripheral spreader preform (11) is maintained same as that of the peripheral portion (8) of the central spreader preform (6) with horizontal.
In one more embodiment of the apparatus, the impressions or die cavity on the FBT and FBB dies are such that when they come together for finish bender

forging, the enclosed cavity has the shape required for production of the hemispherical gas bottle (1).
The benefits of this invention are as follows:
1. The new process design allows production of the hemispherical gas bottle on a forging press. Due to this the hemispherical gas bottle can be produced with better productivity, better mechanical and metallurgical properties as well as better UT response.
2. Uniform and refined grain structure is obtained in the gas bottle which assures uniform properties throughout the part. This also improves the UT response of the product.
3. The new process reduces the force generated during the forging process significantly and thus, allows use of lower capacity press for production. This reduces the overall manufacturing cost of the product.

We claim:
1. A method of manufacturing a hemispherical gas bottle from a forged, extruded or rolled billet (1A) made of Ti alloy, characterised in that said method comprises the following steps:
a. heating said billet (1A) in a furnace to a temperature below the β transus
temperature to produce a heated billet (1B);
b. carrying out shaped upset preforming forging on said heated billet (1B) in
a shaped upset die-set comprising a top die and a bottom die to produce a
shaped upset preform (4) having a preformed hub (4A) and the peripheral
portion (4B), and wherein said peripheral portion (4B) is deformed more
than said preformed hub (4A) and wherein the thickness of said peripheral
portion (4B) is less than that of the preformed hub (4A);
c. heating said shaped upset preform (4) in said furnace to produce a heated
shaped upset preform (5) by maintaining said furnace to a temperature
below the β transus temperature;
d. subjecting said heated shaped upset preform (5) to a central spreader
forging step wherein the material of said heated shaped upset preform is
spread in radial direction and the peripheral portion (4B) is bent, whereby
a central spreader preform (6) is produced having a central spreader hub
(7) ,central spreader arm (8) and the bulbous portion (9);
e. heating said central spreader preform (6) in said furnace to a temperature
below the β transus temperature to produce a heated central spreader
preform (10);

f. subjecting said heated central spreader preform (10) to a peripheral
spreader forging step to produce a peripheral spreader preform (11)
having a peripheral spreader hub (12) and peripheral spreader arm (13);
g. heating said peripheral spreader preform in said furnace to a temperature
below the β transus temperature to produce a heated peripheral spreader
preform (14);
h. subjecting said heated peripheral spreader preform (14) to a bending and ironing operation to produce the final hemispherical gas bottle.
2. The method as claimed in claim 1, wherein said billet is cylindrical in shape.
3. The method as claimed in claims 1 and 2, wherein the furnace temperature is
maintained at a temperature below β transus by 20 to 60 ºC for any of steps a, c, e and g.
4. The method as claimed in claims 1 to 3, wherein in between each stroke of a
multi-stroke process, the deformed part is placed in said furnace for booster heating.
5. The method as claimed in claims 1 to 4, wherein the die used for step b is such
that the orientation of the peripheral portion (4B) is at a first included angle (ζ) with the horizontal axis, and wherein there is a second included angle (θ) between the top die surface and bottom die surface in said peripheral portion (4B).
6. The method as claimed in claims 1 to 5, wherein said first included angle (ζ)
and said second included angle (θ) vary between 5º to 25 º.

7. The method as claimed in claims 1 to 6, wherein after the central spreader arm (8) and the bulbous portion (9) are further bent in said step d, the peripheral portion (8 and 9) forms a third included angle (γ) with the horizontal.
8. The method as claimed in claims 1 to 7, wherein said third included angle (γ) vary between 15º to 45 º.
9. The method as claimed in claims 1 to 8, wherein in said step d, the material is deformed partially by pressing only a central region, i.e. a region comprising central hub (4A) and the part of peripheral portion (4B) that is near to the central hub (4A), of said shaped upset preform, and wherein the material of said peripheral portion (4B) of the shaped upset preform (4) is allowed to move without any further contact with the top die to form a bulbous portion (9).
10. The method as claimed in claims 1 to 9, wherein in said step f, the material near the circumference or periphery of the heated central spreader preform is deformed while the area near the centre of the preform is allowed to move freely with contact with the dies happening at the end of the stroke.
11. The method as claimed in claims 1 to 10, wherein the deformation in any of the steps b, d and f is performed in one stroke in the case said alloy is of α+β type.
12. The method as claimed in claims 1 to 11, wherein the preforms of steps b, d, and f are allowed to cool down for inspecting them for potential crack

development before the respective steps c, e, and g of preform heating are carried out.
13. The method as claimed in claims 1 to 12, wherein said method is carried out on a forging press, preferably the forging press is either a screw press or a hydraulic press.
14. An apparatus for manufacturing a hemispherical gas bottle from a forged, extruded or rolled billet (1A) made of Ti alloy, said apparatus comprising a forging press, in which are placed a set of top and bottom shaped upset preform dies to carry out shaped upset preform forging, a set of top and bottom central spreader forging dies to carry out the central spreader forging, a set of top and bottom peripheral spreader forging dies to carry out peripheral spreader forging, a set of top and bottom finish bender forging dies to carry out finish bender forging.
15. The apparatus as claimed in claim 14, wherein the impressions or die cavity on the SUT and SUB dies are such that when they come together for shaped upset preform forging the enclosed die cavity has the shape required for production of the shaped upset preform (4).
16. The apparatus as claimed in claims 14 and 15, wherein the impressions or die cavity on the SUT and SUB dies are such that when they come together for shaped upset preform forging the orientation of the enclosed die cavity corresponding to the peripheral portion (4B) of the shaped upset preform produced using these dies is such that the centre line passing through the die cavity corresponding to the peripheral portion (4B) is at a first included angle

(ζ) with the horizontal axis, said first included angle (ζ) being between 5º and 25º.
17. The apparatus as claimed in claims 14 to 16, wherein the impressions or die cavity on the SUT and SUB dies are such that a second included angle (θ) between the top die (SUT) surface and bottom die (SUB) surface in the enclosed die cavity corresponding to the peripheral portion (4B) region is between 5º and 25 º.
18. The apparatus as claimed in claims 14 to 17, wherein the impressions or die cavities of the CST and CSB dies is such that once the central spreader arm (8) and the bulbous portion (9) are further bent in the central spreader forging step, the centerline passing through the enclosed die cavity corresponding to the peripheral portion (8 and 9) forms a third included angle (γ) with the horizontal, said third included angle (γ) being between 15º and 45 º.
19. The apparatus as claimed in claim 18, wherein a gutter is provided in the form of a relief channel in the central spreader top die (CST) from the die surfaces corresponding to the joint of said central spreader arm (8) and said bulbous portion (9) outwards, such that the included angle between the die surface and the horizontal becomes less than said third included angle (γ) by at least 10º.
20. The apparatus as claimed in claim 14, wherein the impressions or die cavity on said PST and PSB dies are such that when they come together for peripheral spreader preform forging, the enclosed cavity has the shape required for production of the peripheral spreader preform (11).

21. The apparatus as claimed in claim 20, wherein the impression or the die cavity of said peripheral spreader dies is such that said included angle (γ) of the enclosed die cavity corresponding to the peripheral portion (13) of the peripheral spreader preform (11) is maintained same as that of the peripheral portion (8) of the central spreader preform (6) with horizontal.
22. The apparatus as claimed in claims 14 to 21, wherein impressions or die cavity on the FBT and FBB dies are such that when they come together for finish bender forging, the enclosed cavity has the shape required for production of the hemispherical gas bottle (1).