FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
The Patent Rules, 2003
Complete Specification
(See section 10 and rule 13)
A Forging Die And A Method Of Surface Hardening Of A Forging Die
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 hot forging die manufacturing process. Particularly, the present invention relates to a modified surface hardening process carried out to improve crack resistance and thus to increase the life of dies.
Introduction
In hot forging, the die failure occurs due to wear (adhesive and abrasive), plastic deformation and fatigue (mechanical and thermal). Among all the failure mechanisms, wear and mechanical fatigue are found to be the most common form of die failures during forging. The die wear is caused by relative motion between die surface and material being forged. The die wear is directly proportional to the normal forces working at the interface and is inversely proportional to the hardness of the surface under consideration. Thus, the wear can be reduced by increasing the surface hardness of the dies. The die cracks happen due to the cyclic impact and thermal loading which the die undergo during its operation. This leads to failure of the dies which comes under low cycle fatigue regime of fatigue failure.
State of die surface layer governs its wear and fatigue properties. So, any modification in the properties of the die surface can considerably improve or impair its wear, mechanical and fatigue behavior. The surface hardening will also have effect on the fatigue strength of the dies altering its crack resistance.

Conventionally, the hot forging dies are surface hardened in order to reduce its wear and tear during the forging production. For the surface hardening of the dies many different methods are used. The different surface hardening processes which are currently being used in the industry are Carburizing, Nitriding, Carbonitriding, Boriding, Nitrocarburising etc.
The surface hardening process increases the hardness of a small layer of the die material near its surface. This small layer of the die material is called “Case” and the depth of surface hardening is called as “Case Depth”.
As the hardness of the surface increases due to the surface hardening treatment, the toughness of this surface layer is reduced. As mentioned earlier, the failure of the dies due to cracking happens under the influence of the low cycle fatigue phenomenon. It is known that the low cycle fatigue behavior of a material depends on its surface. The reduction of the surface toughness leads to the reduction of the low cycle fatigue life/strength of the dies. In other words, the cracking tendency of the dies increases due to the surface hardening process.
Thus, the traditional method of surface hardening has the following disadvantages associated with it,
1. Harder layer formed increases the susceptibility of the dies to cracking although it increases its wear resistance.
Thus, there exists a room for advancement over the existing technology in that a forging die is manufactured with a modification in the surface hardening process.

This process would not only increase the wear resistance but also improves its crack resistance.
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 die manufacturing process.
It is another object of the present invention to provide a modified manufacturing method for hot forging die in order to increase the die life during hot forging process.
It is still another object of the present invention to provide a surface hardening process which improves the wear resistance of the die as well as increases its crack resistance.
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 illustrates the conventional process of die manufacturing;

Figure 2 illustrates the invented process of die manufacturing;
Figure 3 illustrates the effect of the surface hardening using Gaseous Ferritic Nitro
carburizing on the LCF strength of the L6 die material.
Figure 4A shows part of a typical die subjected to the method of the invention
with the crack-prone zones identified
Figure 4B shows a part of the die subjected to the method of hardening of
invention
Summary of invention
Wear and mechanical fatigue are the most common die (1) failure mechanism in hot forging. Surface Hardening processes improves die wear resistance but reduces its low cycle fatigue resistance thus increasing its susceptibility to cracking and further failure of forging dies.
The present invention discloses a modified process of die (1) manufacturing which not only improves the wear resistance but also increases its crack resistance thus increasing die life. The process consists of introduction of modified surface hardening process. In this modified process, surface hardening process/method is applied on some selected surfaces where wear phenomenon is prominent, instead of applying it to the full die (1). This process involves the identification of highly stressed / crack prone zones (1A) in the dies (1) and avoiding surface hardening in those zones (1A) using hardening inhibitor coatings, thus avoiding formation of hard layer or hardened surface in those areas (1A) and formation of hardened

layer/surface (1D) in the remaining areas. Thus the die has unhardened surface/layer (1C) in the highly stressed or crack prone areas (1A) and the remaining area has hardened surface (1D). This reduces the cracking tendency of the die without affecting its wear resistance.
Detailed description of invention
The present invention discloses a process/method of hardening of dies (1) in which a step of identification of highly stressed / crack prone zones (1A) and coating of the same with hardening inhibitor is introduced before the surface hardening operation. Figures 1 and 2 show the conventional process and the process/method of invention respectively.
Surface hardening operation increases the hardness of the surface of the dies (1). The hardened layer consists of nitrides, carbides and carbonitrides of iron and its alloying elements. The surface hardening process leads to formation of compressive residual stresses on the treated surface. Presence of hardened layer increases wear resistance of the dies but reduces its low cycle fatigue (LCF) strength. The phenomenon of LCF-strength-loss is observed in very highly stressed zones. The maximum stresses present are basically in the plastic regime of the stress-strain curve. In this regime, the resistance to fatigue failure is dependent more on the ductility/toughness of the material. The hardened case formed during the surface hardening processes is, as the name suggests, very hard

and hence, brittle in nature. The brittle case leads to early initiation as well as propagation of cracks.
The fatigue failure basically consists of three phenomena. These are crack initiation, stable crack propagation and then final failure. LCF strength is a measure of crack propagation zone. If a crack length reaches a value above which rate of its propagation becomes unsustainably fast then it is called critical crack length. Hence, critical crack length governs its rate of propagation and thus fatigue life. Lower the critical crack length, higher will be the rate of propagation and thus lower will be fatigue life.
During surface hardening, the chemistry of the surface layer is modified due to introduction (diffusion) of hardening elements like carbon, nitrogen etc. Typically, this modification in chemistry takes place up to a limited depth (0.2 to 0.6 mm). Below this depth, exists the base metal, where the chemistry of the material is unchanged. Increase in the brittleness of the material reduces the critical crack length for the material. The presence of case (developed due to Surface Hardening process) reduces the critical crack length to a value lower than case depth (hardened layer) and thus leads to early failure. Thus, the tougher base metal does not get a chance to resist the crack propagation.
The present invention proposes a modification in die manufacturing process. Modified process is given as follows:

1. Machining of the hot forging die: When a new forging die (1) used in a hot forging process is being made, the raw material or die steel block is machined using various machining processes to manufacture the die. During the die (1) repair cycle when a die has already been used for forging production (called a “run die”) is being repaired, the run die is repaired by using a process which may comprise of different processes like gouging, welding, machining, grinding etc. Thus, the finished die thus manufactured is called a “Machined Die”.
2. Identification of the highly stressed / Crack prone zones: In this step the zones where the die is highly stressed during its operation are identified and marked. These zones (1A) can be identified based on forging simulation process (in case of new product development) or by visual observation in case of a run die (highly stressed zones are the regions where the dies crack). The marking of the identified areas can be done using chalk or paint. All area other than the crack prone zones is the normal die surface (1B).
3. Application of hardening inhibitors: In this step, highly stressed / crack
prone zones are coated with hardening inhibitor. The hardening inhibitor is
normally available in the form of a paste. This paste is applied on the
identified surfaces (see part 1A of figure 4 – coated areas) with the help of
a brush or any other suitable means. The normal die surface (1B) remains
uncoated. After application, the coating is allowed to dry.

4. Surface hardening of the dies: Next, the die surface is hardened using any of the hardening processes like Carburizing, Nitriding, Carbonitriding, Boriding, Nitrocarburising etc. During this process the uncoated surface becomes the hardened surface (1D) while the coated surface (1A) remains as unhardened surface (1C).
Figure 2 describes the modified process. This has the following advantages over conventional process:
1. Increase in crack resistance and thus, increase in die life without affecting its wear resistance.
2. Increased number of parts produced per production run
3. Reduction in the die repair work required after each production run.
The process of the invention is also applicable to cold forging dies if they are surface hardened.
The dies made using the method disclosed herein is different and physically distinguishable from dies made using conventional method. The distinction is in the crack prone region (where surface masking is done during the surface hardening process), where there is an unhardened layer/surface in dies made using the process of the invention, whereas in the adjoining region (where masking has not been provided), a surface hardened layer/surface is obtained. The presence of the hardened surface/layer may be ascertained by hardness testing (using portable hardness tester) on the surface at that location which will show lower hardness

due to absence of surface hardened layer. This could also be ascertained using destructive testing where the dies will have to be cut and micro structure will have to be studied from cut samples in the transverse direction. Another way could be to take a hardness traverse from surface towards core on the cut sample in the transverse direction. This will also show absence of hardened surface layer.
Example:
As an example to illustrate the findings of the invention, a hot forging die material, AISI L6 or DIN 1.2714 was considered for trial. LCF strength of materials in as received condition and after Gaseous Ferritic Nitrocarburizing (GFN) process was determined using Servo-Hydraulic Machine of INSTRON make with 250 kN capacity. Figure 3 shows the LCF test results. From the results it can be inferred that LCF strength after GFN process reduces approximately by 34% as compared to base material. GFN process makes the dies more prone to cracking and thus reduces die life. This led to the modification in the existing GFN process. Highly stressed zones / areas more prone to crack, of the dies were identified and nitrocarburising inhibitor (antinitriding compound) was applied before GFN process. Production trial was taken with the modified process and a reduction of approximately 40% in die cracking was observed. As an example, by using conventional method, 80 die blocks (out of 100) cracked during forging production runs. But, when 100 die blocks produced by invented method were

studied, only 48 die blocks (out of 100) cracked – i.e. 40% reduction in cracked dies.
It is evident from the foregoing discussion that the invention has the following embodiments:
1. A forging die (1) for hot forging or cold forging processes characterised in that said die, has an unhardened surface (1C) in its crack prone region, whereas in the remaining areas, it has a hardened surface (1D).
2. The forging die as disclosed in embodiment 1 wherein said unhardened surface (1C) is obtained by coating the crack-prone region (1A) of the die with a hardening inhibitor coating before said die is subjected to a surface hardening operation.
3. A method of surface hardening of a forging die (1), characterised in that said method comprises a step of coating the crack-prone region (1A) of the die with a hardening inhibitor coating before said die is subjected to a surface hardening operation.
4. The method of surface hardening as disclosed in embodiment 3, wherein said method comprises the steps of:
a. in the case, the die (1) is a new die, machining it; in the case of the die is a run die, repairing it using methods such as gouging, welding, machining, grinding;

b. identifying and marking the zones (1A) where, in the case of the run
die, the die is highly stressed during its operation, and in the case of a
new die, the zones where the die is prone to high stresses or cracks;
c. coating the highly stressed or cracked zones (1A), or zones prone to
high stresses or cracking with hardening inhibitor;
d. surface hardening the dies.
5. The method of surface hardening as disclosed in embodiment 4, wherein in said step b, in case of new die development, said identifying is based on forging simulation process, and in the case of a run die, it is done by visual observation.
6. The method of surface hardening as disclosed in embodiment 5, wherein the marking of the identified areas is done using chalk or paint.
7. The method of surface hardening as disclosed in embodiment 4, wherein said hardening inhibitor is in the form of a paste.
8. The method of surface hardening as disclosed in embodiment 7, wherein said paste is applied on the identified surface using a brush, and whereafter, said paste is allowed to dry.
9. The method of surface hardening as disclosed in embodiment 4, wherein said surface hardening step is carried out using any of the following techniques: carburizing, nitriding, carbonitriding, boriding, and nitrocarburising.

While the above description contains much specificity, this should not be construed as limitation in the scope of the invention, but rather as exemplification of the preferred embodiments thereof. Many other variations are possible. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.

We claim:
1. A forging die (1) for hot forging or cold forging processes characterised in that said die, has an unhardened surface (1C) in its crack prone region, whereas in the remaining areas, it has a hardened surface (1D).
2. The forging die as claimed in claim 1 wherein said unhardened surface (1C) is obtained by coating the crack-prone region (1A) of the die with a hardening inhibitor coating before said die is subjected to a surface hardening operation.
3. A method of surface hardening of a forging die (1), characterised in that said method comprises a step of coating the crack-prone region (1A) of the die with a hardening inhibitor coating before said die is subjected to a surface hardening operation.
4. The method of surface hardening as claimed in claim 3, wherein said method comprises the steps of:
e. in the case, the die (1) is a new die, machining it; in the case of the die
is a run die, repairing it using methods such as gouging, welding,
machining, grinding;
f. identifying and marking the zones (1A) where, in the case of the run
die, the die is highly stressed during its operation, and in the case of a
new die, the zones where the die is prone to high stresses or cracks;
g. coating the highly stressed or cracked zones (1A), or zones prone to
high stresses or cracking with hardening inhibitor;

h. surface hardening the dies.
5. The method of surface hardening as claimed in claim 4, wherein in said step b, in case of new die development, said identifying is based on forging simulation process, and in the case of a run die, it is done by visual observation.
6. The method of surface hardening as claimed in claim 5, wherein the marking of the identified areas is done using chalk or paint.
7. The method of surface hardening as claimed in claim 4, wherein said hardening inhibitor is in the form of a paste.
8. The method of surface hardening as claimed in claim 7, wherein said paste is applied on the identified surface using a brush, and whereafter, said paste is allowed to dry.
9. The method of surface hardening as claimed in claim 4, wherein said surface hardening step is carried out using any of the following techniques: carburizing, nitriding, carbonitriding, boriding, and nitrocarburising.

A Forging Die And A Method Of Surface Hardening Of A Forging Die Abstract:
The present invention relates to a hot forging die manufacturing process. Particularly, the present invention relates to a modified surface hardening process carried out to improve crack resistance and thus to increase the life of dies. The present invention discloses a modified process of die (1) manufacturing which consists of introduction of modified surface hardening process wherein surface hardening process/method is applied on some selected surfaces where wear phenomenon is prominent, instead of applying it to the full die (1). Highly stressed / crack prone zones (1A) in the dies are identified (1) and surface hardening in those zones (1A) is avoided using hardening inhibitor coatings. The treated dies are thus devoid of hard layer or hardened surface in those areas (1A) and have hardened layer/surface (1D) in the remaining areas. A die has unhardened surface/layer (1C) in the highly stressed or crack prone areas (1A) and the remaining area has hardened surface (1D). This reduces the cracking tendency of the die without affecting its wear resistance.