DESC: FORM –2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)
EXTENSION OF DIE LIFE BY IN HOT & COLD FORGING
Applicant:
BHARAT FORGE LIMITED
Mundhawa, Pune 411036,
Maharashtra, India
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

FIELD OF INVENTION
[0001] The present invention relates to a coated die with extended die life for hot and cold forging operations. Further, the present invention also relates to a method of coating a die that reduces die wear. Still further, the present invention relates to a forging method that improves production by minimizing the change over time during forging operations.
BACKGROUND
[0002] Forging involves techniques such as hot forging and cold forging. In hot forging industry the die life is important and extensive factor that dictates the overall cost of the forging operations. High temperature and high impact load during forging process results in die wear. Particularly, the surface of the die or near surface region of the die is susceptible to the onslaught ofextreme conditions of temperature and pressure during each run. As a result, majority of the issues related to defects as well as causes of failure of forging operations are linked with the quality of the surface of the die. The die as such without any treatment can not withstand the extreme temperature and impact load conditions. Therefore, one of two options such as changing the composition of the die or modifying the surface of the die can be adopted.
[0003] Altering the composition is not a cost effective solution. Further, increasing the hardness beyond a certain limit of the die leads to cracking of die during forging process. The die life is a game changing factor in the hot & cold forging industry as the cost of the die is too high. Further, replacing a die with another during forging process is a time consuming process. Still further, because of this change over time and man power involved in it, it is considered to be a major set -back during the process and naturally it affects the production output. The other factor is rejection of product when the die is replaced with the new one as the whole process takes time to adjust with the new set up.
[0004] In view of this, the most preferred technique to increase its shelf life is surface modification of the die.Surface modification is very useful in restricting mechanical fatigue, wear, corrosion and oxidation of the die. Known surface modification techniques that are used for extending die life include nitriding, weld overlays (or hard facing) and chemical and physical vapor deposition of heat resistant ceramic materials on die surface.These known surface modification techniques however suffer from several short-comings. For example, in case of nitridation the diffusion can only be done up to maximum of 0.25mm. This wears out very soon and as a result the die has to be re-cut, polished and again subjected to nitridation. Consecutive re-cuts eventually lead to scrapping of the die. This not only increases the die change over time but it also increases the production cost.
[0005] To overcome the loss in the production cycle and improve the die life, a hitherto unknown technique is envisaged in accordance with the present invention. Accordingly, it is envisaged to provide an effective solution for surface modification of the die, which in turn allows the die to withstand extreme conditions of forging, thereby extending the life of die.
[0006] In view of the above, there still exists a need for a surface treatment method that is capable providing a durable hardened surface on the die that not extends die-life but also boosts productions and lowers productions costs.
OBJECTS
[0007] Some of the objects of the present disclosure which at least one embodiment herein satisfies are as follows:
[0008] It is an object of the present invention to provide a surface modified die that significantly reduces the change over frequency during production life cycle.
[0009] It is another object of the present invention to provide a surface modified die for forging operations that improves productivity and economy of these operations.
[0010] It is yet another object of the present invention to provide a method for coating a die for forging operations.
[0011] 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.

SUMMARY
[0012] In one aspect of the present invention, there is provided a forging die comprising a base material and a hardened surface coat comprising at least one layer of a nickel based alloy and at least one layer of a cobalt based alloy wherein the thickness of the coat is at least 2 mm. Typically, the thickness of the Ni-based alloy layers varies from 0.5 mm to 1.5mm and the thickness of the cobalt based alloy layers varies from 1.5mm to 4mm.
[0013] In another aspect, there is provided a method for coating a die. The method of coating the die of the present invention comprises following steps:
- preheating a forging die at temperature ranging from 180 to 200 deg C;
- depositing a layer of Ni-based alloy with thickness ranging from 0.5mm to 1.5mm by thermal fusion by HVOF;
- melting the Coblat based alloy and spraying said melted alloy on the Ni-based alloy layer followed by HVOF process to spread the alloy uniformly on the surface of the die; and
- cooling the die in the reduced Nitrogen atmosphere
[0014] In yet another aspect, there is provided a method for forging that employs the coated die of the present invention that is capable of manufacturing at least 10000 articles without any intermittent changeover time.
[0015] BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Fig 1 shows the SEM images of the interface bonding between the die operative surface, Ni Alloy & Co alloy material.
[0017] Figure 2 shows characterization of Ni-based alloy in the coat.
[0018] Figure 3 shows characterization of Co-based alloy in the coat
[0019] Figure 4 shows XRD pattern for the layer of Co-based alloy
[0020] Figure 5 shows die wear pattern when the dies are surface treated by known methods which include salt bath, nitridation
[0021] Figure 6 shows die wear pattern of a coated die in accordance with the present invention before after 10,200 runs without any intermittent changeover time.
DESCRIPTION
[0022] The terms and phrases as indicated in quotation marks in this section are intended to have the meaning ascribed to them in this ‘Definitions’ section applied to them throughout this document, including in the claims, unless clearly indicated otherwise in context.
[0023] The term “Forging Applications” means all types of forging applications including hot forging, cold Forging and warm forging irrespective of the type of metal involved in the forging process.
[0024] The term “operative surface” means the surface of a die that comes in direct contact with a metallic object to be forged.
[0025] The term HVOF means “High Velocity Oxygen Fuel Coating”.
[0026] In one aspect, there is provided a die for forging operations that comprises a base material and a hardened surface coat comprising at least one layer of a nickel based alloy and at least one layer of a cobalt based alloy.
[0027] Typically, the nickel based alloy comprises Ni in an amount ranging from 56%wt to 60%wt while the amount of cobalt in the cobalt based alloy ranges from 58%wt to 62%wt. In one embodiment, the coat comprises a single layer of a nickel based alloy. In another embodiment, there may be plurality of layers of nickel based alloy. In one embodiment, the thickness of the layers of nickel based alloy may be same. In another embodiment, the thickness of each of the layer of nickel based alloy may be different. Typically, an average thickness of the nickel based alloy may range from about 0.5mm to about 1.5mm.
[0028] Typically, the cobalt based alloy comprises cobalt in amount ranging from 58 to 62. Typically, the cobalt based alloy is a low carbon Co-Cr-W based super alloy that can withstand hot forging operations. In an embodiment, the cobalt based alloy is stellite 6.
[0029] Typically, in one embodiment, the coat comprises only one layer of cobalt based alloy. In another embodiment, the coat comprises a plurality of cobalt based layers. The thickness of layers of the cobalt-based alloy may be same in one embodiment. In an alternate embodiment, the thicknesses of layers of cobalt based alloys layer of the cobalt based alloy may be different. Typically, average thickness of the cobalt based layers may vary in the range from about 1mm to about 4 mm.
[0030] Typically, HRC of the cobalt based layer in the coat ranges from about 32 to about 49, preferably, from about 38 to about 43.This increases the surface toughness. Further, ductile nature of the cobalt based alloy helps in releasing the stresses thereby improving mold release-ability.
[0031] Typically, the first layer of the nickel based alloy is directly deposited on the base material of the die on its operative surface. The inventors of the present invention have found that this ensures better adhesion of the coat to the base material. It also diminishes the possibilities of damages to coat on account of de-lamination or chipping. In one embodiment, the nickel based layers and the cobalt based layers may be deposited on the based material in an alternate manner. In one embodiment, the layers of the cobalt based alloy are deposited on the last layer of the nickel based alloy that is farthest from the base material.
[0032] Typically, the collective thickness of the coat on the base material in accordance with the present invention is varies from about 1.5 mm to about 5mm.
[0033] Typically, the base material of the die is selected from a group consisting of DAC, DAC10, DAC55, DAC45 and YXR33. In one embodiment, the die material is DAC steel. In an exemplary embodiment, the base material is DAC steel.
[0034] The forging operations involve large temperature variations during various method steps such as hammering and cooling down. The present inventors carried out extensive experiments using different coating materials and it has been observed that compatibility of the coating agents with the die material is one of the factors that decide the success of efficient coating that is suitable for forging operations. Thermal coefficients and thermal conductivities of respective materials provide an indication as to the compatibility of these materials.
[0035] The process of the present invention requires a balance of both hardness as well as toughness. The Co-Cr-W based super alloy with low to medium carbon, particularly in combination with Ni alloy employed in the present invention offers a combination of high strength as well as high ductility. Further, it also has higher conductivity. It is found to provide resistance to fracture, failure and damage.
[0036] Typically, the thermal coefficient (X 10-6/°C) of the base material of the die of the present invention at 700°C ranges from about 8 to about 20, preferably from about 12 to about 15. Typically, the thermal conductivity(W/m•K[cal/cm•s•°C]) of the die material at 700°C ranges from about 22 to about 35, preferably from about 26 to about 30.
[0037] Increasing the thickness of the coat, particularly on a die suitable for forging operations always poses many challenges. The cobalt based alloy as employed in the coat possesses characteristics that enable it to maintain sufficient ductility at temperatures encountered during typical forging operations. This attribute ensures better stability of the coat for a longer duration of time. This in turn translates to an extended died life and minimal change over frequency.
[0038] The coat on the operative surface area of the base material of the die improves mechanical wear resistance of the die during forging operations under varying temperature and pressure. Further, the coat also protects the die from cavitation, corrosion, erosion, abrasion, and galling.
[0039] In accordance with a further aspect of the present invention, there is provided a method of coating operative surface of a die. It comprises the following steps:
- preheating a forging die at temperature ranging from 180 to 200 deg C;
- depositing a layer of Ni-based alloy with thickness ranging from 0.5mm to 1.5mm by thermal fusion by HVOF;
- melting the Coblat based alloy and spraying said melted alloy on the Ni-based alloy layer followed by HVOF process to spread the alloy uniformly on the surface of the die; and
- cooling the die in the reduced Nitrogen atmosphere.
[0040] Typically, the cooling is done by slow cooling rate process ranging between ramp rate from 0.5 degree C/min, preferably between 6 to 10 hours for attaining maximum stability in the Nitrogen atmosphere.
[0041] In one of the embodiments of the present invention the coating comprises a fluidized spray coating in the presence of a combination of carrier and combustion gases. In one embodiment, oxygen is used as a combustion gas and acetylene is used as a carrier gas.
[0042] The process of the present invention does not affect or disturb the structure as well as properties of the die. The process of the present invention provides a uniform coating with well-defined compact atomized microstructure with metallurgical bonding between the base material steel & the interface material Ni- base alloy and the coating alloy (Co- Cr- W) material as shown in the SEM image (Fig 1), where the interface is not separated, which further leads to improvement of die life.
[0043] In still another aspect, there is provided in accordance with the present invention a hot forging process that employs the die of the present invention. Typically, the die in accordance with the present invention brings down the die change over from 3 to 2, preferably from 3 to 1 and most preferably it obviates the need for die change over during the production cycle. This in turn boots the productivity of the forging operations by at least 200%; preferably by at least 300%. Further, the forging process of the present invention also brings down the operative costs for forging operations by minimizing the need for maintenance and manpower.
[0044] In one embodiment of the present invention, the forging process of the present invention allows continuous production for up to 9000 cycles, preferably up to 10000 cycles and most preferably up to 11000 cycles using a single die same die without any changeover or repair or maintenance interruption in between. Further, the Co-Cr based super alloy, due to its ductility has the property to shrink and expand according to the temperature, pressure and load applied on the die surface.
[0045] In another aspect, there is provided in accordance with the present invention a die for hot forging applications that increases the productivity of hot forging process at least 3 times, preferably at least 4 times and most preferably by at least 5 times. Typically, the die material is a type of steel that is compatible with Co-Cr-W based super alloy characterized by HRC ranging between 32 and 49, preferably, between 38 and 43.

[0046] Technical advance and economic significance
• The die of the present invention has enormous potential application in extrusion, hot and cold forging industry for extending the shelf life of the die.
• The process of the present invention also provides improved resistance to thermal fatigue, high temperature, oxidation and thermal shock.
• The present invention provides a cost effective solution for frequent die replacement issue arises during hot forging application.
• The coated die of the present invention helps to increase the production which is at least 3fold more. i.e. ~ 75% increase in the production without replacement of the die can be achieved.
• The changeover of die time is reduced to Zero from 3 times.
• The production target invariably achieved in a shift compared with existing practices of 3 shifts.
• Only the surface coating have to be replaced and not the die, therefore the base material of the die remains intact through the die-life.
[0047] The following examples are given by way of illustration only and therefore should not be constructed to limit the scope of the present invention.
[0048] EXAMPLE
[0049] An upset die for of following die material was used for coating it with the coating material of the present invention.
[0050] Composition of the die material:
[0051] Die Material: DIN& H13 Chemical Composition
Table No 1.
Standard C Si Mn Cr Mo V P S

DIN 0.32~0.42 0.80~1.20 0.20~0.50 4.75~5.50 1.10~1.75 0.80~1.20 =0.030 =0.030
[0052] Thermal coefficient of the die material and its thermal conductivity was taken into consideration while selecting the coating material. These parameters were taken into consideration in determining the composition of Ni-based alloys material and cobalt based material.
Table 2
PARAMETERS Temperature DAC DAC10 DAC55 DAC45 YXR33

Thermal Coefficient
X 10-6/°C
100°C
11.7
10.7
11.6
10.5
11.6

700°C 14.0 13.2 13.7 13.6 13.2
Thermal Conductivity
W/m•K[cal/cm•s•°C] 20°C 30.5
[0.073] 32.2
[0.077] 34.5
[0.082] 26.4
[0.063] 27.2
[0.065]
700°C 28.0
[0.067] 28.5
[0.068] 28.0
[0.067] 27.6
[0.066] 29.7
[0.071]
[0053] Composition of the Coating Material :
[0054] Ni-based alloy material for depositing a first layer on the operative surface of the die had the following composition as shown in Table 3.
Element Wt% At %
Ni 56.20% 47.01
Cr 18.82% 17.77
O 7.29 22.38
Fe 1.5 5.44
Mo 9.26 5.1
Si 0.5 1.75
Nb 4
[0055] The Ni-based alloy was characterized. The characterization of Ni-based alloys is shown in Figure 2.
[0056] The Co-based alloy material used for depositing layers of cobalt based material had the following composition as shown in Table 4.
Element %wt At%
Co 58.30 56.01
Cr 29.82 17.77
O 7.29 22.38
Fe 2.5 5.44
Si 0.5 1.75
Ni 2.6 47
[0057] The characterization data for the cobalt based alloy is provided in Figure 3.
[0058] Coating Process:
[0059] A die with above mentioned die material was heated to 170 oC. A layer of Ni-based alloy of the above mentioned composition was deposited on the operative surface of die using thermal fusion through thermal fusion using HVOF process with following set parameters as shown in Table 5.
Oxygen Pressure (KPa) Fuel Pressure (KPa) Air Pressure(KPa) Powder Feed Rate m3/h Spray Rate Kg/h Spray Distance (m)
1030 600 715 0.78 6.25 0.27
[0060] The thickness of the Ni-based alloy as deposited was 1.5mm. This was followed by deposition of Cobalt based alloy: Stellite 6. The total thickness of the coat after depositing about 2.5 mm thick layers of the cobalt based alloy was found to be 4mm.
[0061] Characterization and Testing of the Coated Die:
[0062] By introducing the hard facing of low-carbon steel die with a sandwich layer of Ni-base alloy and Stellite-6. The deposition parameter window was investigated; it was possible to form deposits on the die surface and the coatings of Stellite-6 exhibited a crack-free structure, low porosity and excellent bond strength with the substrate. The coating grain size was found to be submicron (200–400 nm), resulting in superior wear-test performance in comparison to conventional nitridation. The layer of the cobalt based alloy was characterized by XRD. This is shown in Figure 4.
[0063] The SEM images of the coated die are shown in Figure 1. The coating was found out to be uniform and without any pores.
[0064] Forging Process using coated die :
[0065] The coated die of the present invention was used for forging operations at temperature of about 1100 oC and impact load 2500 ton press for forging. It was observed that the coated die could manufacture 10,200 articles without a single intermittent changeover break. The die wear pattern of the coated die before and after forging operations is shown in Figure 6.
[0066] In contrast, when the same uncoated die surface treated with nitridation was used for forging to manufacture 100 articles, the die had to be re-cut 19 times. Provided herein below in Table 6 is a comparative data that shows comparative performance of the coated die of the present invention.
Existing Die Life 2000
Die Life After coating by the method of the present invention 10200
Up Set Die change Time min (For Die surface treated with Nitridation) 45
Per day number of up set up (For Die surface treated with Nitridation) 3
Total Down Time for up set die change 135
Cycle Time Sec 7.5
[0067] It was found out that the coated die of the present invention could be re-used by changing the coating. This completed obviated the need to re-cut the die which in turn saved costs involved in die tooling and machining.
[0068] Following Table 7 provides a comparison of the coats applied by known techniques and the coat applied by the method of the present invention.
Coating Method Die material Thickness
of Coat Die temperature during coating Die changeover time
Method of the Present Invention DAC 2-5mm 180 to 200 deg c Zero
Plasma Nitriding DAC 0.2 to 0.4 mm 150 to 200 deg c Max 2500 shots
Salt Bath DAC 0.2 to 0.4 mm 150 to 200 deg c 1500 to 1800 shots
Gas Nitriding DAC 0.2 to 0.4 mm 180 to 200 deg c Max 2500 shots

[0069] The inventors also compared the die wear associated with the known methods. This is shown in Figure 5. It was observed that the extent of die wear in case of known methods of surface treatment as shown in the above Table was substantially higher in comparison to the die wear in case of the coated die of the present invention even after 10,200 runs. The die wear pattern for the coated die of the present invention after and before the forging operations is shown in Figure 6.
[0070] Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
[0071] The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
[0072] While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitations. ,CLAIMS:We claim:
1. A forging die comprising a base material and a hardened surface coat comprising at least one layer of a nickel based alloy and at least one layer of a cobalt based alloy wherein the thickness of the coat is at least 2 mm.
2. A forging die as claimed in claim 1, wherein the thickness of nickel based alloy ranges from 0.5 to 1.5mm.
3. A forging die as claimed in claim 1, wherein the thickness of the layer of cobalt based alloy ranges from is about 1.5 to 4mm.
4. A forging die as claimed in claim 1, wherein the grain size of the layer of cobalt based alloy is characterized by grain size ranging from 200nm to 400nm.
5. A forging die as claimed in claim 1, wherein the thickness of the coat ranges from about 2mm to about 5mm.
6. A forging die as claimed in claim 1, wherein the base material is at least one selected from the group consisting of DAC, DAC10, DAC55, DAC45, H11 and H 13.
7. A forging die as claimed in claim 1, wherein the die material is DAC and the cobalt based alloy composition is stellite 6.
8. A method of coating a forging die that is essentially devoid of nitridation; said method comprising the following steps:
- preheating a forging die at temperature ranging from 180 to 200 deg C;
- depositing a layer of Ni-based alloy with thickness ranging from 0.5mm to 1.5mm by thermal fusion by HVOF;
- melting the Coblat based alloy and spraying said melted alloy on the Ni-based alloy layer followed by HVOF process to spread the alloy uniformly on the surface of the die; and
- cooling the die in the reduced Nitrogen atmosphere.
9. A method of coating a forging die as claimed in claim 10, wherein the method step of spraying is fluidized spray coating in presence of oxygen as combustion gas and acetylene as a carrier gas.
10. A method of manufacturing a forged article by using the forging die as claimed in claim 1, wherein the die is capable of manufacturing at least 10,000 articles without any intermittent die change over time.