DESC:FIELD OF THE INVENTION
[0001] The present invention relates to die holders. Particularly, the present invention relates to a method for repairing die holders.

BACKGROUND
[0002] Closed die forging requires use of die holders during forging operation. Parts produced with forging are usually formed to final shape in multi-stage operations. The deformation of the material such as metals and their alloys in closed die forging is done with the help of forging tools called “die”. Two dies i.e. a top die and a bottom die are typically required at every stage in the multistage operation. Depending upon the geometry of a part, pre-forming and final forging dies are designed and manufactured. The forging dies are placed into the die insert envelopes which are called 'cassette'. The cassettes and dies together are called as a die assembly. The die assembly (both upper (or top) and lower (or bottom)) is fixed to the die holders. The upper/top die holder is fixed to the ram of a forging press and the lower/bottom die holder is fixed to the anvil bolster of the press.
[0003] Forging operations require high level of precision in order to manufacture a predetermined part with an acceptable quality. Forging uses batch type production method and thereby requires frequent changing of dies and resetting of the die assembly, which results into several problems related to alignment. It is also known that the loads and stresses that the forged parts and indeed the die assembly are subjected are variable in nature and it introduces fatigue not only in the die-assembly but also in the die holders. It has been noticed that due to these issues and also due to the nature of alternating load levels, the die holders tend to develop cracks at certain critical sections. The locations on the die holder where highest stresses are produced during operation are called as “critically stressed zone”.
[0004] It is possible to repair the die-holders using conventional processes such as welding and machining. This process consists of removing the crack completely from the die holder by using the gouging process. It is found that the cracks produced in the die holders are very deep and hence, a very large amount of material has to be gouged off to completely remove the crack. The gouging process uses high density heat to locally melt the material which is then removed with the help of compressed air. Furthermore, it has been found that the heat used to melt the material locally sometimes leads to further propagation of crack which in turn increases the material removal.
[0005] After the gouging process is complete, the gouged area is filled up using welding process. For this the die holder is preheated to a temperature of around 250 ~ 350 °C. Generally, arc welding process is used for this purpose. This process is normally manual and hence, takes a very long time to complete.
[0006] Accordingly, both gouging and welding are heat intensive processes and hence, lead to distortion in the die holder.
[0007] The welding process is thereafter followed by stress relieving process which is finally followed by machining operation.
[0008] The conventional process has following drawbacks:
• The cycle time or lead time for this repair process is very large.
• After this repair process, the material structure in the “critically stressed zones” in the die holder changes from the forged – equiaxed grain structure to welded – as cast structure. Moreover, this zone will now have welding defects like porosity, blowholes etc. Due to this the strength of the material in the “critically stressed zone” is reduced drastically which leads to early failure at same location.
• The heat intensive welding and gouging process leads to problems like distortion and tensile residual stresses which has negative impact on the service life of the die holder.
• The energy consumption of this complete process is also very high. Due to the large size of the die holders, big furnaces and high energy are required to heat it during the steps such as preheating, stress relieving etc.
• The cost of this repair process is also large.
[0009] Therefore, there exists a need for a repairing process for die-holders that provides a robust and durable solution thereby extending the life of the repaired die-holders in a significant manner.
OBJECT OF THE INVENTION
[00010] Some of the objects of the present disclosure which at least one embodiment herein satisfies are as follows:
[00011] It is an object of the present invention to provide a method for repairing die holders.
[00012] It is another object of the present invention to provide a method for repairing die holders in a manner that arrests further propagation of cracks.
[00013] It is still another object of the present invention to provide a method for repairing die holders that increases the life of repaired die holders thereby minimizing waste.
[00014] It is yet another object of the present invention to provide a die holder repair method which has lower cycle time or lead time.
[00015] It is a further object of the present invention to provide a cost effective die holder repair method.
[00016] It is a further object of the present invention to provide re-usable repaired die holder.
[00017] Other objects and advantages of the present disclosure will be more apparent from the following description and accompanying drawings which are not intended to limit the scope of the present disclosure.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
[00018] Figure 1 shows a die-assembly and die-holders;
[00019] Figure 2 shows schematic of cracks on the damaged die-holder in accordance with one illustrative embodiment;
[00020] Figure 2A shows one configuration (position) of implanting stiffeners in a cracked die holder in accordance with one illustrative embodiment;
[00021] Figure 3 shows schematic diagrams of different types of stiffeners used for repairing die holders by method of the present invention; and
[00022] Figure 4 shows D-shaped stiffener in accordance with one embodiment of the present invention.

SUMMARY OF THE INVENTION
[00023] Accordingly, the present invention provides a method of repairing a cracked die holder; said method comprises fitting at least one stiffener of a predetermined shape in a pre-determined area of a cracked die holder and; welding, wherein said stiffener is adapted to enforce additional compressive stresses across cracked areas or critically stressed zone/s of said die holder.

[00024] In one embodiment of the present invention said method comprises the following steps:
? providing a cracked die holder;
? identifying and marking crack patterns;
? gouging the area in and around cracks with a width ranging from about 30 to about 90 mm and depth ranging from about 30 to about 90 mm;
? selecting at least one location for fitting at least one stiffener across the length of the crack based on FEA;
? machining pocket/s at pre-selected locations along the crack areas across the length of the cracks;
? providing at least one stiffener and shrink fitting said at least one stiffener in the pocket/s;
? welding the gouged area in and around the cracked areas including the spaces surrounding the pockets in which stiffeners are fitted; and
? performing a clean cut on the die holder surface including the places where stiffeners are fitted and welded.
[00025] In accordance with another aspect of the present invention there is provided a die holder comprising at least one steel stiffener having a shape selected from the group consisting of dumbbell shape, C shape with rounded curves, C shape with rectangular cuts and D shape, wherein said stiffener adapted to enforce additional compressive stresses across the cracked areas of die holder or critically stressed zone of said die holder.
DETAILED DESCRIPTION
[00026] The conventional method of die holder repair (gouging – welding – machining) requires a large amount of time, energy and material. One way in which this wastage of time, energy and material could be reduced is reducing the amount of material which is gouged out while removing the crack in die holder. But, if the material removal is reduced during gouging, there are chances that some cracks may remain in the die holder. Even if the gouged region is filled up with a weld material, a sub-surface crack may remain in the die holder. This sub-surface crack can act as a stress raiser in the critically stressed zone and can again open up to reveal crack. This problem can lead to early failure of the die holder.
[00027] One way to avoid the re-opening of the sub-surface crack could be to produce compressive residual stresses around this sub-surface crack. The present inventors found out that the conventionally used method of die repair involving gouging, welding, machining techniques do not help to induce sufficient residual compressive stress across crack prone or critically stressed regions.
[00028] Accordingly, in a first aspect of the present invention, there is provided a stiffener that can be inserted across the cracked areas for the purpose of enforcing additional compressive stresses across the cracked areas or the critically stressed zone so as to minimize the overall impact stresses generated during the forging process. The compressive stresses produced by the stiffener counteract the tensile stresses produced during the forging operation in the critically stressed zone and hence, do not allow the sub-surface cracks to re-open.
[00029] The geometry of stiffener and location of assembly of stiffener in the die holder is designed on the basis of FEM analysis. In the FEM analysis, the important factors to be considered are (i) contact area between the stiffener and slot in the die holder; (ii) strain generated in the die holder due to shrink fitting; (iii) residual stresses generated due to the shrink fitting.
[00030] Typically, the shape of the stiffener is selected from the group that includes but is not limited to dumbbell shape, C shape with rounded curves, C shape with rectangular cuts, D shaped and the like. Figure 3 shows schematic diagram of some of the shapes of stiffeners envisaged in accordance with the present invention. FEM analysis is done to understand the effect of each of these shapes on the contact area, strain generated and residual stresses generated.
[00031] In one embodiment of the present invention, the stiffeners / inserts used is made of steel materials such as AISI H13, AISI L6, and the like. In another embodiment, the stiffener is a metal stiffener, said metal stiffener is selected from the group consisting of Iron based alloys, Nickel Based alloys, Titanium based alloy and Cobalt based alloys.
[00032] In one embodiment of the present invention the length of the stiffener ranges from about 200 mm to about 400 mm.
[00033] In one embodiment, the stiffener is a D-shaped stiffener having a pair of integrated side arms, each of said side arms has an inner surface and an outer surface. The distance between the inner surfaces of respective side arms of the stiffener (Dimension D in Figure 4) is critical to achieve the efficient repairing of die holder. The optimum distance between the inner surfaces of respective side arms of the stiffener (Dimension D in Figure 4) is found to be in the range of 75 mm to 200 mm. FEM analysis showed that the D shaped stiffener’s performance is found to be better compared to other shapes tested in all three factors considered i.e. contact area, strain generated and residual stresses generated.
[00034] In Figure 4 of accompanying drawings the stiffener is illustrated, wherein length “L” is the total length of the stiffener and length “D” is the distance between inner surfaces of the stiffener’s side arms.
[00035] In another aspect, there is provided a die holder comprising at least one steel stiffener having a shape selected from the group consisting of dumbbell shape, C shape with rounded curves, C shape with rectangular cuts and D shape, wherein said stiffener adapted to enforce additional compressive stresses across the cracked areas of die holder or critically stressed zone of said die holder. In one embodiment, the stiffener in the die holder is a D-shaped stiffener having a pair of integrated side arms, each of said side arms has an inner surface and an outer surface, said stiffener characterized in that the distance between the inner surfaces of respective side arms (D) of the stiffener is in the range of 75 mm to 200 mm.
[00036] In yet another aspect, there is provided a method for repairing die holders that employs stiffeners as described herein above.
[00037] In one embodiment, the method of repairing the cracked die holder comprises fitting at least one stiffener in the pre-determined area of cracked die holder (critically stressed zone) followed by welding.
[00038] In one illustrative embodiment, the method of the present invention comprises the following steps:
[00039] Initially, a cracked die holder is provided/obtained (refer Figure 2). Thereafter, crack patterns are identified and marked. In the next step, the area in and around the cracks is gouged with a width ranging from about 30 to about 90 mm and depth ranging from about 30 to about 90 mm. Post gouging operation, at least one location for fitting at least one stiffener across the length of the crack based on FEA is selected. One illustration of positioning of the stiffeners across the crack is shown in Figure 2A.
[00040] Pocket/s at pre-selected locations along the crack areas across the length of the cracks are machined. Then at least one stiffener is provided and shrink fitted in the pocket/s. The stiffener is a metal stiffener, said metal stiffener is selected from the group consisting of Iron based alloys, Nickel Based alloys, Titanium based alloy and Cobalt based alloys.
[00041] In the next step, the gouged area is welded in and around the cracked areas including the spaces surrounding the pockets in which stiffeners are fitted. Finally, a clean cut is performed on the die holder surface including the places where stiffeners are fitted and welded.
[00042] In one embodiment, shrink fitting of said at least one stiffener inside the die holder involves heating said stiffener/s in order to fit said stiffener/s in the machined pockets inside the die holder and allowing said stiffener/s to cool naturally. The thermal expansion of the stiffeners is converted to shrink fitment after stiffeners regain machined dimensions.
[00043] Typically, the shapes of the stiffeners along the length of the cracks are selected on the basis of FE analysis to have maximum induced compressive residual stress zone. Further, the spacing between the stiffeners is maintained such that the compressive residual stress fields produced by the neighbouring stiffeners intersect with each other.
[00044] In an embodiment, the dimensions of the pockets machined in the die holder surface and the dimensions of the stiffeners are such that the stiffeners can be shrink-fitted inside the pockets. The depth of the pockets is such that after the shrink fitting of the stiffener/s in the pocket, a predetermined pocket height near the die holder surface is left free/vacant in which a weld material can be filled during welding. It is illustrated in section AA of Figure 2A.
[00045] In accordance with another aspect of the present invention there is provided a die holder comprising at least one stiffener of pre-determined size and shape, prepared by the method of the present invention.
TECHNICAL ADVANCE AND ECONOMIC SIGNIFICANCE:
[00046] The method of the present invention arrests further propagation of cracks in the repaired die holder.
[00047] The method of the present invention increases the life of repaired die holders thereby minimizing waste.
[00048] The present invention provides an economic die holder repair method which has lower cycle time or lead time.
[00049] 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.
[00050] 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.
[00051] 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.
[00052] 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.
[00053] 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 limitation.
,CLAIMS:We claim:
1. A method of repairing a cracked die holder; said method comprises fitting at least one stiffener of a predetermined shape in a pre-determined area of a cracked die holder; and welding, wherein said stiffener is adapted to enforce additional compressive stresses across cracked areas or critically stressed zone(s) of said die holder.

2. The method as claimed in claim 1, wherein said method comprises the following steps:
? providing a cracked die holder;
? identifying and marking crack patterns;
? gouging the area in and around cracks with a width ranging from about 30 to about 90 mm and depth ranging from about 30 to about 90 mm;
? selecting at least one location for fitting at least one stiffener across the length of the crack based on FEA;
? machining pocket/s at pre-selected locations along the crack areas across the length of the cracks;
? providing at least one stiffener and shrink fitting said at least one stiffener in the pocket/s;
? welding the gouged area in and around the cracked areas including the spaces surrounding the pockets in which stiffeners are fitted; and
? performing a clean cut on the die holder surface including the places where stiffeners are fitted and welded.

3. The method as claimed in claim 1, wherein the shrink fitting of said at least one stiffener comprises heating said stiffener/s in order to fit said stiffener/s in the machined pockets inside the die holder and allowing said stiffener/s to cool, wherein thermal expansion of the stiffeners is converted to shrink fitment after stiffeners regain machined dimensions.

4. The method as claimed in claim 1, wherein the shape of the stiffener along the length of the cracks is selected on the basis of FEA to have maximum induced compressive residual stress zone in the critically stressed zone(s).

5. The method as claimed in claim 1, wherein the shape of the stiffener is selected from the group consisting of dumbbell shape, C shape with rounded curves, C shape with rectangular cuts and D shape.

6. The method as claimed in claim 1, wherein the stiffener is a metal stiffener, said metal stiffener is selected from the group consisting of Iron based alloys, Nickel Based alloys, Titanium based alloys, Cobalt based alloys.

7. The method as claimed in claim 1, wherein said stiffener is a D-shaped stiffener having a pair of integrated side arms, each of said side arms has an inner surface and an outer surface,
said stiffener characterized in that the distance between the inner surfaces of respective side arms of the stiffener (D) is in the range of 75 mm to 200 mm.
8. A die holder comprising at least one steel stiffener having a shape selected from the group consisting of dumbbell shape, C shape with rounded curves, C shape with rectangular cuts and D shape, wherein said stiffener adapted to enforce additional compressive stresses across the cracked areas of die holder or critically stressed zone of said die holder.
9. The die holder as claimed in claim 8, wherein said stiffener is a D-shaped stiffener having a pair of integrated side arms, each of said side arms has an inner surface and an outer surface,
said stiffener characterized in that the distance between the inner surfaces of respective side arms of the stiffener (D) is in the range of 75 mm to 200 mm.

Dated this September 21, 2018
CHIRAG TANNA
OF NOVOIP
Applicant’s Patent Agent