DESC:FORM – 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

HIGH STRENGTH WELDING PROCESS

Applicant: BHARAT FORGE LTD.
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
[001] The present invention relates to a welding method. Particularly, the present invention pertains to the fusion welding of metals.
[002] More particularly, the present invention relates to an improved method of welding to obtain better micro-structural as well as mechanical properties of the weld deposit. The present invention also relates to a die repairing using welding technique.

Introduction
[003] Welding typically involves localized coalescence or joining together of two metallic parts. Welding can also be used for the deposition of material which is lost due to different reasons which include but are not limited to wear, cracking, removal during repair process and the like. Welding processes can be classified into two major groups namely, Fusion Welding and Solid-state Welding.

[004] Fusion welding is a popular process compared to solid-state welding as it provides excellent properties such as flexibility, lower production cost and ability to provide strong metallic joints. There are three different regions in a fusion weld namely, the base metal, the Heat Affected Zone (HAZ) and the Fusion Zone (FZ).The FZ is in a liquid form during the welding process and becomes solid after cooling. This zone is formed by a combination of metal from the original work piece and the filler metal.

[005] The mechanical properties of the weldments are significantly affected by the microstructure of the FZ. The microstructure of the weld metal is normally columnar in nature. The columnar structure is highly anisotropic in nature which negatively impacts its properties.

[006] In the conventional method, the multi-layer weld deposit is employed in such a way that successive layers of weld metal are deposited in one direction only (Unidirectional welding). Schematic of the same is shown in Error! Reference source not found.. Schematic of the cross section of the said method is shown in Error! Reference source not found..

[007] The fusion welding process normally consists of preheating the material on which the weld deposit has to be made. After the completion of the weld deposition, the complete product has to be stress relieved in order to remove the residual stresses.

[008] The properties of the weld metal are very important as the overall performance of the product is also affected by it. Whenever, any product is repaired by using the welding process, the strength of the product depends on the strength of the weld deposit.

[009] The weld deposit has essentially a cast structure. The properties of the cast structure are not as good as forged/formed structure due to difference in the microstructure of the two.

[0010] Most of the parts which are made of metals and their alloys are normally used in forged/formed condition due to their superior properties. Whenever, welding is used to either join or repair these forged/formed structures, it can become the weak link in the structure. It is therefore recognized that improving the properties of the weld deposit is of paramount importance.

[0011] Prior art search has revealed that researchers in past have used many different ways to improve the strength of the weld deposit. Some of these methods employed by different researchers are explained below:

[0012] Chemistry modification: Some researchers have modified the chemistry of the alloys used as a welding filler material. For instance, alloying elements are added to the filler material in order to achieve the required properties. US Patent No. 3368887 discloses modification of the filler metal chemistry by addition of Tantalum which results into increase in yield strength without any side effects on the impact strength of the weld deposit prepared by electric arc welding. US Patent No. 3745294 discloses similar improvement of the mechanical properties in the submerged arc welding by controlling the chemistry of the weld material as well as that of the flux material. US Patent No. 3769491 discloses modification of chemistry of the filler material as well as the protective lime fluoride coating of the electrode to achieve better Charpy properties for non-austenitic steels.

[0013] Multiple weld material: Some researchers have used different layers of different materials in order to improve the properties of the weldments. US Patent No. 2494970 discloses improvement in the weld deposit property by using two layers of different materials. The first layer is of tougher material while the second one is that of harder material. Similarly, the inventors of US Patent 4348131 tried use of multiple layers of different welding materials having different coefficient of expansion which lead to compressive residual stresses on the surface of the weld. These compressive residual stresses give better resistance to crack formation on the surface of weld.

[0014] Modification of shielding gas: Some researchers modified the composition of the shielding gas in order to improve the properties of the weld deposits. US Patent No. 3139508 discloses improving the property of the weld deposit by modifying the composition of the shielding gas. Said patent suggests use of 40% to 98% helium, 1% to 5% oxygen and 0 to 60% argon gas.

[0015] Mechanical Treatments: Some researchers have attempted mechanical treatments to enhance the properties of the weldments. US Patent No. 6171415 B1 discloses use of ultrasonic impact technology to improve the microstructure of the deposit as well as to remove or redistribute the harmful residual stresses in the weld. Furthermore, US Patent No. 6926970 discloses use of a compression tool for selectively inducing a layer of residual compressive stress in at least a portion of the surface of the weld joint and the surface of the at least one work piece to thereby improve the material properties of the work piece.

[0016] The aforementioned prior art has demonstrated that for improving the properties of the weld deposit, either some kind of modification in the elements of welding process (i.e. filler material, shielding gas, flux chemistry etc.) is required or some kind of additional treatments are desired. In order to follow these techniques, the welder has to either change his established practices or he has to carry out some additional work. These kinds of modifications require trials for establishment of the new process and its economic benefit. Thus, a need in the art can be identified for a method to improve the properties of the weldment without the need to change any of the basic components of the welding. The present invention addresses this need by proposing a process which can improve the properties of the weld deposit without any change in the components of welding.

OBJECTS OF THE INVENTION
[0017] Some of the objects of the present disclosure which at least one embodiment herein satisfies are as follows:

[0018] It is an object of the present invention to provide a fusion -multilayer welding method.

[0019] It is another object of the present invention to provide an improved fusion-multilayer welding method which provides enhanced properties such as tensile strength, fatigue strength, impact strength and the like.

[0020] It is still another object of the present invention to provide a fusion-multilayer welding method which provides improved micro-structural as well as mechanical properties to the weld deposit.

[0021] It is yet another object of the present invention to provide a fusion-multilayer welding method which can be used for repairing applications in various metal based industries.

[0022] It is a further object of the present invention to provide a process for repairing a forging die by fusion-multilayer welding in order to improve the die life.

[0023] 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:
[0024] Figure 1 illustrates the schematics of Shielded Metal-Arc Unidirectional welding;

[0025] Figure 2 illustrates the cross section of unidirectional layer welding;

[0026] Figure 3 illustrates the schematics of Shielded Metal-Arc cross- layer welding in accordance with one embodiment of the present invention; and

[0027] Figure 4 illustrates the Cross section of high strength welding of the present invention welding in accordance with one embodiment of the present invention.

SUMMARY OF THE INVENTION
[0028] In a first aspect of the present invention, there is provided a method for high-strength multi-layer welding, said method comprising:
- providing a part to be subjected to welding; optionally, removing defects from the part ;
- preheating the part while concurrently heating the welding electrodes;
- depositing a first set of layers and a second set of layers of a welding material in longitudinal and transverse directions on the part respectively to obtain a welded part with first and second set of layers deposited in a criss-cross manner; and
- subjecting the welded part to stress relieving and optionally machining the part.

[0029] In a second aspect of the present invention, there is provided a method of repairing a hot forging die using the method for high-strength multi-layer welding of the present invention.

DESCRIPTION OF THE INVENTION
[0030] Definitions:
[0031] In the context of the present invention the terms “longitudinal & transverse direction”; criss-cross manner; cross welding; perpendicular direction and X & Y direction are interchangeably used.

[0032] The present invention is directed to a method of multi-layer welding which results into a weld deposit having better mechanical and fatigue properties. The present invention is applicable to any of the fusion welding method which uses filler material for welding. In one embodiment the present multilayer welding method is applicable to processes which include but are not limited to Gas welding (Oxyacetylene welding), Arc welding (Shielded metal arc welding, Gas–tungsten arc welding, Plasma arc welding, Gas–Metal Arc welding, Flux-cored arc welding, Submerged arc welding) and the like.

[0033] The present invention provides a welding method in which weld material is deposited layer by layer. In one embodiment, two successive layers are deposited perpendicular to each other. To put it in other words, the first weld layer is deposited in X direction, the second weld layer is deposited in Y direction. Typically, said deposition pattern can be repeated subsequently in similar fashion (Cross-layer welding).

[0034] By changing the weld deposition pattern, the inventors found that the properties of the weld deposit are significantly improved. On micro-structural analysis, it was found that the change in the welding pattern affects the solidification of the weld deposit and hence, alters its microstructure. This modification in microstructure is such that it leads to improved properties of the weldment.

[0035] The inventors have found that the main advantage of the proposed process is that it improves all the properties of the weld deposit. i.e. there is an improvement in the tensile properties, Charpy impact as well as the fatigue properties of weldment.

[0036] In one aspect of the present invention, there is provided a method for high-strength multi-layer welding. The method of the present invention comprises the following steps:
• In the first step, a part to be subjected to welding is provided. In one embodiment, defect/s from the part are removed.
• In the next step, the part is subjected to preheating while concurrently heating the welding electrodes.
• A first set of layers and a second set of layers of a welding material are deposited in longitudinal and transverse directions on the part respectively to obtain a welded part with first and second set of layers deposited in a criss-cross manner.
• Finally, the welded part is subjected to stress relieving. In one embodiment, the obtained part is subjected to machining.

[0037] In accordance with a second aspect of the present invention there is provided a method of die repair using the high strength welding method of the present invention.

[0038] In hot forging process, a billet is heated to a high temperature (normally above re-crystallization temperature for steels) and then pressed between a set of dies in order to achieve the required shape. During production, dies are continuously under thermal and mechanical cyclic loading. Heating due to contact with high temperature billet and cooling due to lubrication provided on the die surface, lead to thermal fatigue of dies. This leads to small heat checks or serrations on the die surface. Moreover, during the production process, the dies have to face impact loads which lead to mechanical fatigue. The combination of these two phenomena may lead to big cracks and die failure.

[0039] The cracked dies are repaired using the process of gouging, welding, and machining. The crack in the dies is removed by gouging or scarfing. This gouged area is then filled with pre-determined filler metal used for welding. Commonly, Metal Arc Welding (MAW) is used for this process. Finally, the welded die is machined to its final shape on CNC machine. Thus, welding becomes an integral part of forging dies. Hence, strength of the repaired die depends on the quality and strength of the welding.

[0040] In one embodiment, the method for repairing a hot forging die in accordance with the present invention comprises:
In the first step, a die to be subjected to repair by welding is provided. In one embodiment said die is subjected to remove defect from the die by gouging.
In the next step, the die is preheated while concurrently heating the welding electrodes.
A first set of layers and a second set of layers of a welding material are deposited in longitudinal and transverse directions on the die respectively to obtain a welded die part with first and second set of layers deposited in a criss-cross manner.
Finally, the welded die part is subjected to stress relieving. In one embodiment the welded die part is subjected to machining.

[0041] The high strength multi-layer weld deposit method of the present invention when used for die weld repair process, not only improves the mechanical properties and strength of weldments but it also improves the die life.

[0042] In one embodiment, both the first set as well as the second set of layers of welding materials comprises plurality of unidirectional deposited layers of welding material. The manner of deposition of individual uni-directionally deposited layers within the first set and the second set of layers of the welding material may vary.

[0043] In another embodiment, the uni-directionally deposited layers of welding material in the first set and the second set are deposited in an alternate manner.

[0044] The change in direction of weld deposition, done for the high strength welding method of the present invention breaks uniformity in microstructure. The change in pattern of the deposition affects the heat flow dynamics which in turn changes the metal solidification pattern thus changing its grain structure evolution. The uniformity in the microstructure is broken and the alternate layers with different direction of deposition reveal different type of grains. The above phenomena results in alternate layers of elongated grains and equiaxed grains.

[0045] In one embodiment, the welding material used for depositing the first set of layer/s and the second set of layer/s is the same. In an alternate embodiment, the welding material used for depositing the first set of layer/s and the second set of layer/s is different.

[0046] The method of the present invention is suitable for making new parts/ material or repairing the existing parts/materials in a variety of metal industries. The part material and the welding material is at least one material selected from the group consisting of Aluminium alloy, Copper alloys, Magnesium alloys, zirconium alloys, Iron based alloys, Cobalt based alloy and nickel based alloys. The part material and the welding material can be same or different based on the requirement of the product.

[0047] Typically, the method of repairing the die in accordance with the present invention employs dies made from a variety of die materials. Typically, the die materials include but are not limited to tool steels, die steels, hot work steels, super alloys and the like which can be iron based, nickel based or cobalt based alloys.

[0048] Typically, the welding material is an iron based, nickel based or cobalt based alloy. In an embodiment the welding material is MAILAM 535.

[0049] In a still further aspect, there is also provided a hot forging die repaired by a high strength welding method of the present invention.

[0050] The method of the present invention is described herein below in light of the following non-limiting examples which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure.

EXAMPLE:

[0051] In order to understand the effects of the present invention method on the properties of the weld deposit, weld deposit was prepared on die steel using a weld material which is used for the forging die repair process. This design of experiment was prepared in such a way that it was in line with the process followed for the forging die repair.

[0052] The base material considered was die steel i.e. DIN 1.2714 which is commonly used in the forging industry. The welding material used was MAILAM 535 which is also used commonly for die repair process in the forging industry. Following steps were performed during the course of this exercise:
- 2 test blocks of die steel (DIN 1.2714) were taken.
- material from these blocks was gouged to form U channels of specified size.
- U channel in 1st block was filled using the conventional method of welding and the U channel in other block (2nd ) was filled using the present method (cross welding).

[0053] Samples were prepared from these blocks and subjected to the following tests:
a. Tensile tests
b. Charpy impact tests
c. Flexural Fatigue tests
d. Micro-structural analysis

[0054] The analysis of the results of the aforesaid test shows improvement in the properties of the weld deposit performed by the present method. Tensile test results show that the yield strength, tensile strength and % elongation of cross-layer welding is 24%, 4% and 66% higher than that of conventional welding method, respectively.

[0055] An improvement of 50% in Charpy impact strength is obtained by using cross-layer welding method of the present invention. Low cycle flexural fatigue strength of welding also increases by 71.98 % with the use of cross-layer weld method of the present invention. Micro-structural analysis reveals significant refinement of the grain structure in the weld. This increase in mechanical properties can be attributed to this refined grain structure observed in the cross layer welding method of the present invention.

[0056] The microstructure analysis of the weld deposit has shown that change in the pattern of weld deposition significantly modifies the microstructure of the deposit. In the conventional method, the direction of material deposition is uniform. This leads to a uniform microstructure. The microstructure analysis performed on the conventionally deposited weld material confirms this as it shows uniform microstructure throughout the cross section. The inherent characteristics of melting and solidification of the filler material leads to formation of elongated grain structure.

[0057] On the other hand, the change in direction of weld deposition, done for the cross welding method, breaks this uniformity. The change in pattern of the deposition affects the heat flow dynamics which in turn changes the metal solidification pattern thus changing its grain structure evolution. The uniformity in the microstructure is broken and the alternate layers with different direction of deposition reveal different type of grains. The above phenomena results in alternate layers of elongated grains and equiaxed grains.

[0058] The presence of the equiaxed grains (alternate layers) has profound effect on the properties of the weld deposit. Grain refinement is the only mechanism by which all the properties of a material can be simultaneously improved. As discussed earlier, the Tensile, Charpy and Low Cycle Fatigue properties of the weld deposit has shown improvement. Basically the ductility of the material has improved drastically as is evident from the results.

Technical Advance
[0059] Tensile test results show that the yield strength, tensile strength and % elongation of high strength welding of the present invention is 24%, 4% and 66% higher than that of conventional welding method, respectively.
[0060] An improvement of 50% in Charpy impact strength is obtained by using high strength welding of the present invention.
[0061] Low cycle flexural fatigue strength of welding also increases by 71.98 % with use of high strength welding of the present invention.
[0062] Microstructural analysis reveals the presence of alternate layers of equiaxed and elongated grains in high strength welding of the present invention as against elongated grains in conventional welding. This increase in mechanical properties can be attributed to this mixed grain structure observed in the high strength welding of the present invention.
[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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 for high-strength multi-layer welding, said method comprising:
a. providing a part to be subjected to welding; optionally, removing defect from the part ;
b. preheating the part while concurrently heating the welding electrodes;
c. depositing a first set of layers and a second set of layers of a welding material in longitudinal and transverse directions on the part respectively to obtain a welded part with first and second set of layers deposited in a criss-cross manner; and
d. subjecting the welded part to stress relieving and optionally machining the part.

2. A method of preparing a repaired die; said method comprising the following steps:
• providing a die to be subjected to repair by welding; optionally, removing defect from the die by gouging ;
• preheating the die while concurrently heating the welding electrodes;
• depositing a first set of layers and a second set of layers of a welding material in longitudinal and transverse directions on the die respectively to obtain a welded die part with first and second set of layers deposited in a criss-cross manner; and
• subjecting the welded die part to stress relieving and optionally machining the die.

3. The method for high-strength multi-layer welding as claimed in any of the preceding claims, wherein the first set of layers and the second set of layers comprise plurality of unidirectional deposited layers of welding material.

4. The method for high-strength multi-layer welding as claimed in any of the preceding claims, wherein the depositing step comprises providing successive layers of welding material perpendicular to each other.

5. The method for high-strength multi-layer welding as claimed in any of the preceding claims, wherein the uni-directionally deposited layers of welding material in the first set and the second set are deposited in an alternate manner.

6. The method for high-strength multi-layer welding as claimed in any of the preceding claims, wherein the welding material used for depositing the first set of layers and the second set of layers is same.

7. The method for high-strength multi-layer welding as claimed in claim 1, wherein the welding material used for depositing the first set of layers and the second set of layers is different.

8. The method for high-strength multi-layer welding as claimed in claim 1, wherein the part material is selected from the group consisting of Aluminium alloy, Copper alloys, Magnesium alloys, zirconium alloys, Iron based alloys, Cobalt based alloy and nickel based alloys.

9. The method for high-strength multi-layer welding as claimed in claim 2, wherein the material of the hot forging die is selected from the group consisting of tool steels, die steels, hot work steels, superalloys or the like which may be iron based, nickel based or cobalt based alloys.

10. The method for high-strength multi-layer welding as claimed in any of the preceding claims wherein the welding material is at least one selected from the group consisting of Aluminium alloy, Copper alloys, Magnesium alloys, zirconium alloys, Iron based alloys, Cobalt based alloy and nickel based alloys.

11. The method for high-strength multi-layer welding as claimed in claim 1, wherein said welding method is selected from the group consisting of fusion welding; gas welding, and arc welding; said arc welding is selected from the group consisting of shielded metal arc welding, gas–tungsten arc welding, plasma arc welding, gas–metal arc welding, flux-cored arc welding and submerged arc welding.

Dated this 4 April 2017

PRASHANT PATANKAR
OF NOVOIP
Applicants’ Patent Agent