DESC:FORM – 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

Title

A PROCESS FOR REMOVAL OF PLASMA NITRIDED LAYER

Applicant:
BHARAT FORGE LIMITED
An Indian Company of
Mundhwa, Pune – 411036, Maharashtra, India.

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

Field of the invention
The present invention relates to process of removing a surface hardened layer.
Particularly, the present invention relates to a process of removing a hard plasma nitrided surface hardened layer present on a shaping tool.
More particularly, the present invention relates to removing a surface hardened layer present on a forging die by a chemical etching process.

Background and Introduction
Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
The forging industry uses batch type production for the automotive and other components. The size of the batch is restricted by the structural integrity of the forging die among other things.
The structural integrity of the forging die is affected by two different phenomena. The first phenomenon is cracking of the forging dies while the other is wear of the forging dies. Die crack leads to complete failure of the dies and causes problems like cold shuts and protrusions in the forged components. Die wear is a more gradual phenomenon and leads to dimensional oversize, forged part surface quality degradation and/or under filling problems in the forged components. Experience has shown that the die wear is a ‘production batch size’ limiting factor in most of the cases. Therefore, to increase the production batch size, which in turn will increase productivity and O.E.E (Overall equipment effectiveness); the wear resistance of the forging dies has to be increased.
The forging dies have to face strenuous working condition in the form of relative motion with the material being forged. The relative motion between hot metal and the die surface leads to abrasive die wear. This abrasive die wear, among other factors, is inversely proportional to the hardness of the surface under consideration. A surface hardening operation increases the hardness of the surface of the dies. Thus, the wear of the dies can be reduced by increasing the surface hardness of the dies. This results in increase in the batch size. Hence, to counter the die wear phenomenon, surface hardening of impression/cavity is performed on the forging dies.
For the surface hardening of the forging dies different methods are used. The different surface hardening processes which are currently being used in the industry are carburizing, nitriding, plasma nitriding, carbonitriding, boriding, nitrocarburizing etc.
However, this increase of the hardness of the die surfaces has an unintentional negative impact on the die repair process after the completion of certain forging runs. After run of certain batches, there can be various structural changes happened on the impression or cavity of forging dies. These includes surface cracks, deep cracks, wear of high pressurised cavities etc. To have defect free forging products, it is required to remove these structural changes from the die impression or cavity by repairing the die. It is also happened that a portion of the hardened surface layer (high pressurised cavities) is removed due to the wear phenomenon, but on many of the die surfaces the hardened layer is still left behind. Hence, in both above situations, for removing the defects in forging dies, first it required to remove the hardened layer in the affected zones and in case of recut of the dies to remove the entire hardened layer on the impression or cavity. Due to the presence of surface hardened layer, it becomes difficult to machine these die surfaces, which causes increase in machining time and cutting tools breakage. Hence there is loss of overall productivity and thereby increase in tooling cost.
Accordingly, it is requirement of the art to remove the surface hardened layer from impression or cavity of the used forging die which is typically of 100-400 µm hence the die repair work will be with less machining time, ease of machining and without tool breakage.
Accordingly, it is envisaged to remove the surface hardened layer from the used forging die using chemical process with ease and thereby improving the machining time significantly.

Object Of the Invention
Some of the objects of the present disclosure, which at least one embodiment herein satisfies are as listed herein below.
It is an object of the present invention to provide a process to remove the surface hardened layer from impression or cavity of the used forging die using chemical process.
It is another object of the present invention to provide a process to remove the surface hardened layer from impression or cavity of the used die, which is less time consuming.
It is still another object of the present invention to reduce the cost involved in repairing of dies using machining process and avoid breakage of machining tool during die repair.
It is yet another object of the present invention to provide a die repair process which involves chemical etching technique.
Other objects and advantages of the present invention will be apparent in following the description which is not intended to limit the scope of the present invention.
Summary of the invention
The present invention provides a process for removal of a surface hardened layer (nitrided layer) present on an impression or cavity of a forging die. The process comprises treating a forging die surface, from which surface hardened layer is to be removed, with an aqueous ferric chloride acid solution. The process comprises a pre-step of masking a non-treatment area of the die with a masking agent, thereby forming a masking film. The present invention also provides a method for machining a forging die. The method comprises removing a surface hardened layer present on an impression or cavity of a forging die to obtain a repaired die; and subjecting the repaired die to machining operation using a machining tool.

Brief Description of The Accompanying Drawings
In order to provide an understanding of the embodiments of the invention, reference is made to the appended drawings, which are not necessarily drawn to scale and in which reference numerals refers to components of exemplary embodiments of the invention. The above and other features of the presently claimed invention, their nature and various advantages will become more apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings:
Figure 1 illustrates steps for removing the surface hardened layer present on a die in accordance with one embodiment of the present invention.
Description of the Invention
As used in the specification and claims, the singular forms “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “an article” may include a plurality of articles unless the context clearly dictates otherwise. Those with ordinary skill in the art will appreciate that the elements in the figures are illustrated for simplicity and clarity and are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated, relative to other elements, in order to improve the understanding of the present invention. There may be additional components described in the foregoing application that are not depicted on one of the described drawings. In the event such a component is described, but not depicted in a drawing, the absence of such a drawing should not be considered as an omission of such design from the specification.
As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting but rather to provide an understandable description of the invention.
In accordance with one aspect of the present invention there is provided a process for removal of a surface hardened layer present on the impression or cavity of a forging die, said process comprising treating a forging die surface, from which surface hardened layer is to be removed, with an aqueous ferric chloride acid solution.
Typically, the surface hardened layer is plasma nitrided layer. The depth of this plasma nitrided layer is ranging from 100 - 150 µm. The hardness of this plasma nitride layer is 68 to 70 HRC. The hardness of base material below the layer is 42 to 46 HRC. This increase in hardness due to plasma nitrided heat treatment process is the main reason behind large machining time and machining tool breakage during the die repair process.
Typically, the process comprises a pre-step of masking a non-treatment area of the die with a masking agent, thereby forming a masking film.
Typically, the masking agent is Organosol or an industrial coating comprising resin such as polyvinyl chloride (PVC) which is suspended or dispersed in an organic fluid.
The masking agent can be applied to the die using a technique which includes but is not limited to spread coating, strand coating, dip coating, spray coating and the like.
The masking agent has sufficient adherence to enable it to withstand chemical reaction with ferric chloride, but can be easily peeled off after it has served its purpose.
The process according to the present invention is illustrated in detail as follows:
Step 1: providing a used forging die or virgin forging die having surface hardened layer on its impression of cavity.
Step 2: subjecting said die to cleaning to obtain a cleaned die.
Step 3: applying a masking agent to a non-treatment area of the die to form a masking film.
Step 4: preparing an aqueous ferric chloride acid solution and transferring said solution to a tank consisting of heater and stirrer.
Step 5: heating the solution at a temperature ranging from 50 to 65oC to obtain a hot solution and continue stirring said solution in range of 120-160 rpm.
Step 6: immersing the forging die in the hot solution for a time period of 1.0 to 10 hours.
Step 7: maintaining the temperature of hot solution in the said temperature range throughout the process.
Step 8: taking out the forging die after said period.
Step 9: cleaning the forging die with waterjet.
Step 10: removing the masking film.
Typically, the chemical treated cleaned die is taken for machining operation
Preferably, the forging die is immersed in the hot solution for about 3.0 to 4.0 hours.
Typically, the concentration of the aqueous ferric chloride acid solution is 14 to 18%. Preferably, the concentration of the aqueous ferric chloride acid solution is 16.5%.
Typically, the process comprises a step of drying the masking film. The drying time depends on the type drying. The surface dry requires 20 to 30 minutes. Hard dry requires 60 to 120 minutes. Forced drying can be done at 80oC for 5 to 10 minutes.
It is found that the ferric chloride acid reacts with the forging die’s exposed surface and material layer of 200-250 µm can be easily removed. After this step, repairing of forging dies which includes machining, becomes easy.
Typically, the forging die is made of a tool steel which includes but not limited to H13, DIN 1.2714, Hiper Die Steel and Cr7VL.
The possible chemical composition for various forging die materials is provided in the following Table 1.

Table 1: Chemical composition
Forging Die Material
Elements Din 1.2714 H13 Hiper Die Steel Cr7VL
C 0.5 0.32-0.45 0.35 0.42
Mn 0.65-0.9 0.2-0.5 0.5 0.40
Si 0.4-1.1 0.8-1.2 =0.35 0.50
S =0.004 0.03 =0.003 -
P =0.015 0.03 =0.025 -
Cr 1.0-1.2 4.75-5.5 2.70 6.50
Ni 1.5-1.8 0.3 - -
Mo 0.45-0.55 1.1-1.75 1 1.30
V 0.07-0.12 0.8-1.2 0.2 0.80
Cu - 0.25 - -
Fe Balance Balance Balance Balance

In accordance with another aspect of the present invention there is provided a process for repairing a used or virgin forging die; said process comprises removal of a hard plasma nitrided layer present on said impression of cavity of the forging die by the process as described herein above; and subjecting said die to machining.
In accordance with still another aspect of the present invention there is a method for machining a forging die, said method comprises
a) removing a surface hardened layer present on an impression or cavity of a forging die by the process as described herein above to obtain a repaired die; and
b) subjecting the repaired die to machining operation using a machining tool.

Present Invention is now illustrated with the help of non-limiting embodiments which can be combined with each other in any manner.
Embodiment 1: The process for removal of a surface hardened layer present on an impression or cavity of a forging die, said process comprising treating a forging die surface, from which surface hardened layer is to be removed, with an aqueous ferric chloride acid solution.
Embodiment 2: The process according to embodiment 1, wherein the surface hardened layer is a plasma nitrided layer.
Embodiment 3: The process according to embodiment 2, wherein the depth of the plasma nitrided layer is ranging from 100 - 150 µm.
Embodiment 4: The process according to embodiment 1, wherein the process comprises a pre-step of masking a non-treatment area of the die with a masking agent, thereby forming a masking film.
Embodiment 5: The process according to embodiment 4, wherein the masking agent is Organosol or an industrial coating comprising polyvinyl chloride.
Embodiment 6: The process according to embodiment 4, wherein the masking agent is applied to the die using a technique selected from spread coating, strand coating, dip coating, and spray coating.
Embodiment 7: The process according to embodiment 4, wherein the process comprises a step of drying the masking film.
Embodiment 8: The process according to embodiment 7, wherein the drying comprises either surface dry which is performed in 20 to 30 minutes or hard dry which is performed in 60 to 120 minutes.
Embodiment 9: The process according to embodiment 1, wherein the process comprises the following steps:
? providing a used forging die or virgin forging die having surface hardened layer on its impression of cavity;
? subjecting said die to clean with waterjet to obtain a cleaned die;
? applying a masking agent to a non-treatment area of the die to form a masking film;
? preparing an aqueous ferric chloride acid solution and transferring said solution to a tank consisting of a heater and stirrer;
? heating the solution at a temperature ranging from 50 to 65oC to obtain a hot solution and continue stirring of said solution in range of 120-160 rpm.
? immersing the forging die in the hot solution for a time period of 1.0 to 10 hours;
? maintaining the temperature of the hot solution in the temperature ranging from 50 to 65oC;
? taking out the forging die;
? cleaning the forging die with waterjet; and
? removing the masking film.
Embodiment 10: The process according to embodiment 9, wherein the forging die is immersed in the hot solution for about 3.0 to 4.0 hours.
Embodiment 11: The process according to embodiment 9, wherein the concentration of the aqueous ferric chloride acid solution is 14 to 18%.
Embodiment 12: A method for machining a forging die, said method comprises removing a surface hardened layer present on an impression or cavity of a forging die by the process according to embodiments 1 to 11 to obtain a repaired die; and subjecting the repaired die to machining operation using a machining tool.

The invention is now illustrated with the help of non-limiting examples. The examples are merely for illustration purpose and should not be construed to limit the scope of the invention.

Example 1:
A used forging die X and Y were subjected to chemical etching using ferric chloride acid solution. These used forging dies were cleaned and the areas of dies which are not to be subjected for treating was masked using Organosol to form a masking film. An aqueous ferric chloride acid solution was prepared by mixing 16.5 grams of ferric chloride in 100 ml of demineralised water and said solution was transferred two separate tanks consisting of heater and stirrer. The solution in each tank was heated at a temperature of 62oC to obtain a hot solution. These dies were immersed in the respective tank of hot solution for a time period of 4.0 hours. During the process, the temperature of solution is maintained at 62oC. After that the dies were taken out from tank and cleaned with waterjet. After cleaning, the masking film was peeled off.
The obtained dies were then subjected to machining process.
The results for comparative machining time for the die which is subjected to chemical etching process and the same die which not subjected to etching process are provided below in Table 2:
Table 2: Machining time
Die Machining time for Chemically etched die Machining time for non-etched die Saved Time Time for chemical etching process Total Saved Time Improvement
X 22.3 hrs. 35 hrs. 12.7 hrs. 5 hrs. 7.7 hrs. 22%
Y 26 hrs. 39.5 hrs. 13.5 hrs. 5 hrs. 8.5 hrs. 22%

Technical Advance:
? The present chemical etching process with the well-established combination of parameters like concentration of ferric chloride, temperature of ferric chloride during the process and immersion time of forging dies in the hot aqueous solution of ferric chloride is resulted in complete removal of the plasma nitride layer in short time.
? It is a cost-effective process.
? No tool breakage is observed during machining which saves tooling cost.
? Localized area can be exposed to chemical etching by applying Masking agent.
? Machining time reduces drastically.

The above descriptions are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the scope of the present invention.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
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.
While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other modifications in the nature of the disclosure or the preferred embodiments 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 limitation.
,CLAIMS:We Claim:

1. A process for removal of a surface hardened layer present on an impression or cavity of a forging die, said process comprising treating a forging die surface, from which surface hardened layer is to be removed, with an aqueous ferric chloride acid solution.

2. The process as claimed in claim 1, wherein the surface hardened layer is a plasma nitrided layer.

3. The process as claimed in claim 2, wherein the depth of the plasma nitrided layer is ranging from 100 - 150 µm.

4. The process as claimed in claim 1, wherein the process comprises a pre-step of masking a non-treatment area of the die with a masking agent, thereby forming a masking film.

5. The process as claimed in claim 4, wherein the masking agent is Organosol or an industrial coating comprising polyvinyl chloride.

6. The process as claimed in claim 4, wherein the masking agent is applied to the die using a technique selected from spread coating, strand coating, dip coating, and spray coating.
7. The process as claimed in claim 4, wherein the process comprises a step of drying the masking film.

8. The process as claimed in claim 7, wherein the drying comprises either surface dry which is performed in 20 to 30 minutes or hard dry which is performed in 60 to 120 minutes.

9. The process as claimed in claim 1, wherein the process comprises the following steps:
a) providing a used forging die or virgin forging die having surface hardened layer on its impression of cavity;
b) subjecting said die to clean with waterjet to obtain a cleaned die;
c) applying a masking agent to a non-treatment area of the die to form a masking film;
d) preparing an aqueous ferric chloride acid solution and transferring said solution to a tank consisting of a heater and stirrer;
e) heating the solution at a temperature ranging from 50 to 65oC to obtain a hot solution and continue stirring of said solution in range of 120-160 rpm.
f) immersing the forging die in the hot solution for a time period of 1.0 to 10 hours;
g) maintaining the temperature of the hot solution in the temperature ranging from 50 to 65oC;
h) taking out the forging die;
i) cleaning the forging die with waterjet; and
j) removing the masking film.

10. The process as claimed in claim 9, wherein the forging die is immersed in the hot solution for about 3.0 to 4.0 hours.

11. The process as claimed in claim 9, wherein the concentration of the aqueous ferric chloride acid solution is 14 to 18%.

12. A method for machining a forging die, said method comprises
a) removing a surface hardened layer present on an impression or cavity of a forging die by the process as claimed in claims 1 to 11 to obtain a repaired die; and
b) subjecting the repaired die to machining operation using a machining tool.

Dated this 29th Day of March 2023.
FOR BHARAT FORGE LIMITED,

MANISHA JADHAV
OF NOVOIP
APPLICANT’S PATENT AGENT
(IN/PA-3556)