Claims:WE CLAIM

1. A process for the heat treatment of the MDN 350 material comprising:
solutionizing the MDN 350 material at 820ºC for 1 hour;
age hardening the MDN 350 material at 495-510ºC for 3-6 hours;
wherein, the chemical composition of MDN 350 material comprising: Carbon (C) : 0.03% max, Manganese (Mn) : 0.10% max, Silicon (Si) : 0.10% max, Phosphorus (P) : 0.10% max, Sulphur (S) : 0.10% max, Nickel (Ni) : 18.00%-19.00%, Cobalt (Co) : 11.50%-12.50%, Molybdenum (Mo) : 4.60%-5.20%, Titanium (Ti) : 1.30% -1.60%, Aluminium (Al) : 0.05%-0.15%, Chromium (Cr) : 0.50% max, Copper (Cu) : 0.50% max, Iron (Fe): Balance.

2. The process as claimed in claim 1, wherein the MDN 350 produced an an impact strength of around 11 Joules with 59 HRC.

3. A process for the heat treatment of the MDN 250 material comprising:
solutionizing the MDN 350 material at 820ºC for 1 hour;
age hardening the MDN 350 material at 485ºC for 3-6 hours;
wherein, the chemical composition of MDN 350 material comprising: Carbon (C): 0.03% max, Ni: 17.0%- 19.0%, Co: 7.0%-8.5%, Mn: 0.10% max, Si: 0.10% max, Al: 0.05%-0.15%, Mo: 4.6%-5.2%, S: 0.010% max, P: 0.010% max, Ti: 0.3%-0.5%, Cr: 0.50% max, Cu: 0.50% max, Fe: Balance.

4. The process as claimed in claim 3, wherein the MDN 250 produced an impact strength of around 20 Joules with 53 HRC hardness.

5. The process as claimed in claims 1-4, wherein the microstructure of the treated materials is tempered martensitic structure with uniform carbide precipitation.
, Description:FIELD OF THE INVENTION:

[001] The present invention relates generally to the manufacturing of roller and closing catch of 145kV, SF6 spring loaded switchgear assembly with an objective to improve the impact resistance properties to enhance the life of roller and closing catch assembly in an SF6 switchgear.

BACKGROUND/PRIOR ART OF THE INVENTION:

[002] 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.

[003] AISI O1 tool steel which is 'O' group steels are relatively inexpensive and their high carbon content produces adequate wear resistance for short run applications at room temperatures. This steel is currently being used for rollers in the SF6 switch gears and withstands around 800 operations when hardened to 58 HRC. The impact strength with this hardness is around 1J only. The impact strength was tested on standard 10mm x 10mm x 55mm Charpy 2mm V notch specimen (Fig.3). Lower impact strength is found to be the reason for early failure.

[004] Alternately EN 36 Case Carburized Steel grade capable of producing a high core strength combined with good resistance to shock, EN36 steel was tried. EN36 was carburized and heat treated to impart the hardness of around 50 HRC with reasonable impact strength. Roller of this material was employed in the switch gear but it got compressed in a few operations. This material was, therefore, discarded.

[005] In a prior art, Guerard, Norbert (European Patent No. EP0476496) explains the process of arranging the inserts and casting them. The process consists in casting an insert in a first mould, in arranging the insert thus cast in a second mould and in casting the part in this second mould around the insert, so as to form a mechanical link between the two castings. The wear-resisting part thus cast advantageously comprises an insert with a high resistance to wear, whilst the rest of the part is made from a more ductile material which is resistant to mechanical stresses. Application is in grinding rolls and hammer mill hog plates.

[006] Guerard, Norbert (US Patent No. 5238046A) explains the process of manufacturing bimetallic casting of rolls, wherein bimetal casting of wearing parts including a cast retaining part enclosing a plurality of inserts is undertaken. The method consists in casting an insert in a first mould, disposing the insert thus cast in a second mould and in casting the part in this second mould around the insert in such a way so as to form a mechanical bonding between the two castings. The wearing part thus cast advantageously comprises an insert having a high resistance to wear while the rest of the part is made of a more ductile material which is resistant to mechanical stresses. The primary application is in the realization of crushing wheels, termed ferrules, and crusher hammers.

[007] De Vylder, Jean et.al. (US Patent No. 5819838A) explains the method of manufacturing bimetallic grinding wheels and arranging the inserts in the periphery of the die. The method comprises the step of arranging at the periphery of a shell capable of withstanding a temperature of at least 400°C, inserts of highly wear-resistant material, preheating the shell and the inserts in an oven, removing rapidly the shell together with the inserts from the oven, placing them on a centrifugal casting machine which is set in rotation, pouring ductile cast iron and de-moulding after cooling.

[008] Guerard, Norbert (US Patent No. 5839677A) explains the process of manufacturing tracks for mills including a series of inner wearing segments and a series of outer wearing segments with a high resistance to wear. The upper surface of the inner wearing segments and the outer wearing segments provide two concentric circular runway paths moving beneath milling rollers in order to mill material tipped out onto the track. The inner and outer wearing segments are fixed removably to a sole with an outer edge and an inner edge defining housing for the wearing segments.

[009] Horiuchi, Kaoru (US Patent No. 4099988) explains the process of producing a composite material by placing a plurality of blocks of cast iron having high wear-resisting properties on the bottom of a mould, pouring into the mould a molten impact-resisting cast steel, allowing the molten steel to solidify and subjecting the solidified mould to a heat treatment. The composite material thus produced can exhibit both excellent wear and impact-resisting properties.

[0010] In view of the above, there is requirement to devise an apparatus which can address the aforesaid shortcoming(s). Hence, the present invention has been introduced.

[0011] The information disclosed in this background of the disclosure section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

OBJECTS OF THE INVENTION:

[0012] It is therefore the object of the present subject matter to overcome the aforementioned and other drawbacks in prior method/product/apparatus.

[0013] The present invention is based on the object of providing a scheme for preparing roller and catch assembly of an improved materials to enhance the life of roller and closing catch assembly in an SF6 switchgear.

[0014] Said object is achieved according to the invention by the features of claim 1. Further, particularly advantageous embodiments of the invention are disclosed in the sub-claims.

[0015] Another objective of the present invention is to provide an upgraded material (MDN 350 & MDN 250) for roller and catch assembly for preventing frequent failure of SF6 switchgear assembly.

[0016] Still another objective is to study the chemical composition of the prospective materials to be used for the Roller & Catch assembly along with the heat treatment process, hardness and impact strength.

[0017] Yet another objective is to freeze MDN 350 Alloy for Roller and MDN 250 Alloy for Closing Catch for SF6 switchgear assembly, which yields the best hardness and impact properties and suitable for the above applications.

[0018] These and other objects and advantages of the present invention will be apparent to those skilled in the art after a consideration of the following detailed description taken in conjunction with the accompanying drawings in which a preferred form of the present invention is illustrated.

SUMMARY OF THE INVENTION:

[0019] One or more drawbacks of conventional systems and process are overcome, and additional advantages are provided through the apparatus/composition and a method as claimed in the present disclosure. Additional features and advantages are realized through the technicalities of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered to be part of the claimed disclosure.

[0020] To achieve the forgoing objectives, and in accordance with the purpose of the present invention, disclosure is made about the suitable material for Roller and Closing Catch for SF6 switchgear assembly, of chemical composition for roller application: C : 0.03% max, Mn : 0.10% max, Si : 0.10% max, P : 0.10% max, S : 0.10% max, Ni : 18.00%-19.00%, Co : 11.50%-12.50%, Mo : 4.60%-5.20%, Ti : 1.30% -1.60%, Al : 0.05%-0.15%, Cr : 0.50% max, Cu : 0.50% max, Fe: Balance. For Closing Catch application, the chemical composition is C: 0.03% max, Ni: 17.0%- 19.0%, Co: 7.0%-8.5%, Mn: 0.10% max, Si: 0.10% max, Al: 0.05%-0.15%, Mo: 4.6%-5.2%, S: 0.010% max, P: 0.010% max, Ti: 0.3%-0.5%, Cr: 0.50% max, Cu: 0.50% max, Fe: Balance.

[0021] Heat treatment carried out on both the prospective materials, roller material was solutionized at 820oC for 1 hour and age hardened at 495-510oC for 3 to 6 hours. In case of closing catch application, the material was solutionized at 820oC for 1 hour and age hardened at 485oC for 3 to 6 hours. After final heat treatment process, the microstructure of both the materials are found to be tempered martensitic in nature with uniform distribution of carbides precipitates.

[0022] MDN 350 for Roller application produced an impact strength of around 11 Joules with 59 HRC hardness after aging for 8h at 510oC. Considering its hardness and impact strength and the nature of application in the switchgear, it is found to be the best among the shortlisted materials for roller application.

[0023] MDN 250 for closing catch application had an impact strength of around 20 Joules with 53 HRC hardness when aged for 8h at 485oC. Considering its hardness and impact strength and the nature of application in the switchgear, it is found to be suitable for closing catch application.

[0024] The developed materials for roller and closing catch applications for SF6 switchgears have been successfully tested and completed 2000 operations in the switchgear assembly. Hence, benefits of the invention include improved life of the component.

[0025] The present invention is achieved by the features of principal claim and subsequent claims.

[0026] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

[0027] It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined to form a further embodiment of the disclosure.

[0028] The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:

[0029] It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. In the figures, a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods or structure in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:

[0030] Figure 1 shows the perspective view of the roller pin of SF6 switchgear.

[0031] Figure 2 shows the perspective view of the catch of SF6 switchgear.

[0032] Figure 3 shows the impact specimen drawing.

[0033] Figure 4 shows the Microstructure of as received maraging steel MDN 250 (200X Magnification).

[0034] Figure 5 shows the Microstructure of as received maraging steel MDN 250 (500X Magnification).

[0035] Figure 6 shows the Microstructure of Maraging Steel MDN 250 aged at 485oC for 6 hours (200X Magnification).

[0036] Figure 7 shows the Microstructure of Maraging Steel MDN 250 aged at 485oC for 6 hours (500X Magnification).

[0037] Figure 8 shows the Microstructure of Maraging Steel MDN 250 aged at 485oC for 8 hours (200X Magnification).

[0038] Figure 9 shows the Microstructure of Maraging Steel MDN 250 aged at 485oC for 8 hours (500X Magnification).

[0039] Figure 10 shows the Microstructure of as received Maraging Steel MDN 350 (500X Magnification).

[0040] Figure 11 shows the Microstructure of Maraging Steel MDN 350 aged at 510oC for 4 hours (200X Magnification).

[0041] Figure 12 shows the Microstructure of Maraging Steel MDN 350 aged at 510oC for 6 hours (200X Magnification).

[0042] Figure 13 shows the Microstructure of Maraging Steel MDN 350 aged at 510oC for 6 hours (500X Magnification).

[0043] Figure 14 shows the Microstructure of Maraging Steel MDN 350 aged at 510oC for 8 hours (200X Magnification).

[0044] Figure 15 shows the Microstructure of Maraging Steel MDN 350 aged at 510oC for 8 hours (500X Magnification).

[0045] The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S) OF THE PRESENT INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

[0046] While the embodiments of the disclosure are subject to various modifications and alternative forms, specific embodiment thereof have been shown by way of example in the figures and will be described below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the scope of the disclosure.

[0047] The present invention makes a disclosure regarding the technology pertinent to the development of the material for manufacturing of roller and closing catch of 145kV, SF6 spring loaded switchgear assembly to improve the impact resistance properties to enhance the life of roller and closing catch assembly.

[0048] The Present invention relates to the specific chemical composition for material and heat treatment process to develop a material with desired hardness and impact strength.

MATERIALS AND HEAT TREATMENT

Method of Manufacturing Roller Pins

[0049] AISI O1 tool steel which is 'O' group steels are relatively inexpensive and their high carbon content produces adequate wear resistance for short run applications at room temperatures. This steel is currently being used for rollers in the SF6 switch gears and withstands around 800 operations when hardened to 58 HRC. The impact strength with this hardness is around 1J only. The impact strength was tested on standard 10mm x 10mm x 55mm Charpy 2mm V notch specimen (Fig.3). Lower impact strength is found to be the reason for early failure. Alternately EN 36 Case Carburized Steel grade capable of producing a high core strength combined with good resistance to shock, EN36 steel was tried. EN36 was carburized and heat treated to impart the hardness of around 50 HRC with reasonable impact strength. Roller of this material was employed in the switch gear but it got compressed in a few operations. This material was, therefore, discarded. Later, H11 steel has good resistance to heat softening because of its medium chromium content supplemented by the addition of carbide forming elements such as molybdenum, tungsten or vanadium. The low carbon and relatively low total alloy content promote toughness at the normal working hardness of 40 to 55 HRC.

[0050] When hardened to 45 HRC, it gave an impact strength of around 12 J. However, considering the low hardness, it was given a different heat treatment and hardened to around 58 HRC, then the impact strength dropped to 2 J only. Therefore, H11 was also not found suitable for SF6 switch gear rollers. With the failure of the above steels, maraging steel grades MDN 350 and MDN 250 are selected for roller applications for SF6 switchgears, where MDN 350 grade turn out to be the best suited material for roller applications.

Table 1: Chemical composition of different alloys for Roller Applications

Element, wt%
Grade C Si Mn Ni Co Cr Mo V W Al Ti Cu S P Fe
Existing Material AISI O1 0.90 --- 1.00 0.50 0.50 Bal
Suggested Material EN 36 0.10-0.15 0.10-0.35 0.40-0.70 3.0-3.5 --- 0.6-1.0 --- --- --- Bal
H11 0.30-0.40 0.80-1.20 0.25-0.50 --- --- 4.75-5.25 1.20-1.60 0.20-0.40 --- Bal
MDN 250 0.03
max 0.10 max 0.10 max 17.0-19.0 7.0-8.5 0.50 max 4.6-5.2 0.05-0.15 0.3-0.5 0.50 max 0.010 0.010 Bal
MDN 350 0.03
max 0.10 max 0.10 max 18.0-19.0 11.50-12.50 0.50 max 4.6-5.2 0.05-0.15 1.3-1.6 0.50 max 0.010 0.010 Bal

Maraging Steel, MDN 250

[0051] Maraging steels are known to have very high room temperature strength, hardness and good impact strength. Maraging steels are iron base alloys containing 18% Ni 8% Co 5%Mo with small additions of Al and Ti and 0.03%C. These steels are strengthened by precipitation of intermetallic compounds at temperatures of about 485oC. During age hardening at 485oC, there is hardly any dimensional change in the component. Thus fairly intricate shapes can be machined in the soft condition and subsequently hardened with least distortion. Fracture toughness is considerably better than conventional high strength steels. There is virtually no risk of decarburization, distortion or cracking. These steels are manufactured and used for space applications and reliable quality of steels is locally available from MIDHANI. Heat treatment is also simpler as low temperature aging at 480-500oC, may produce around 52 - 54 HRC hardness and 20 J impact strength in MDN 250.

[0052] The hardness of as received MDN 250 is 30 HRC. To determine the hardness and impact strength and optimize for the roller application, it has been aged at 485oC for 4, 6 and 8 hours. The hardness and impact strength in all the three conditions are 52 HRC & 20 J (4 hours aging), 53 HRC & 18 J (6 hours aging) and 53 - 54 HRC & 23 J (8 hours aging), respectively. MDN 250 was also aged at 510oC for 4 hours. The hardness and impact strength in this condition are 51 HRC and 26 J, respectively. These mechanical properties are listed in Table.2 and microstructures are shown in Figs.4 to 9.
Heat Treatment
Solutionizing : 820oC, 1h
Aging : 485oC, 3 to 6 hours

Table.2 Mechanical Properties of MDN250
Aging Temp, oC Aging Time, h Hardness, HRC Impact Strength, J
485 4 52 20
485 6 53 - 54 18
485 8 53 - 54 23
510 4 51 26

Maraging Steel, MDN 350

[0053] If heat treated with care, MDN 350 imparts around 60 HRC hardness and 10 J impact strength.

[0054] The hardness and impact strength of as received MDN350 are 38 HRC and 146 J, respectively. MDN 350 was aged at 510oC for 4, 6 and 8 hours. The hardness and impact strength in three conditions are 58 HRC & 8 J, 59 HRC & 10 J and 59 HRC & 11 J, respectively as given in Table.3. The microstructures are shown in Figs.10 to 15. MDN 350 rollers were put in the switchgear assembly and have already successfully completed 2000 operations.

Heat Treatment
Solutionizing : 820oC, 1h
Aging : 495-510oC, 3 to 6 hours
Table.3 Mechanical Properties of MDN 350
Aging Temp, oC Aging Time, h Hardness, HRC Impact Strength, J
As received 38 146
510 4 58 8
510 6 59 10
510 8 59 11

Method of Manufacturing Closing Catch

[0055] The closing catch is constantly subjected to high dynamic loads. During closing operation, the closing catch is supposed to rest gently on the roller pin and not exert any impact load. However, on certain occasions, the closing catch may experience high impact loads due to inadequate damping resulting in higher speed of closing.

[0056] After introduction of MDN 350 rollers, the original closing catches made of Mn-Cr steel started failing. During failure they created dents on the roller. After replacing with the new Mn-Cr closing catch, MDN 350 roller also failed due to presence of dents on roller surface before the intended numbers of operations. Keeping in view the required no. of 2000 operations, it was decided to manufacture the catches also out of improved material which shall be able to impart higher hardness and tensile strength and better impact properties. Out of the available materials, MDN 250 was selected for this purpose. Five catches of MDN 250 were machined and aged at 485oC for 8 hours. The hardness of 53 HRC was obtained. The heat treated catches with improved profile and rollers had been handed over to Switchgear Laboratory for testing in the switchgear assembly. The catches and rollers have already completed 2000 operations in the SF6 switchgear assembly.

[0057] The advantage of present Invention is improved life of roller and catch assembly and better maintenance strategy.

[0058] The other advantage of present Invention is that, it reduces frequency of replacing damaged rollers and catch resulting in huge amount of savings on account of reduced outages due to reduction in maintenance of the switchgear.

[0059] One more advantage of present Invention is that, it is a very cost effective method.

[0060] The material above has been given as an example without restricting scope of the invention to the same. Thus, other materials readily apparent to a person skilled in the art are within scope of the invention.

[0061] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases, it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.

[0062] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all groups used in the appended claims.

[0063] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particulars claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogues to “at least one of A, B and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B”.

[0064] The above description does not provide specific details of manufacture or design of the various components. Those of skill in the art are familiar with such details, and unless departures from those techniques are set out, techniques, known, related art or later developed designs and materials should be employed. Those in the art are capable of choosing suitable manufacturing and design details.

[0065] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.

[0066] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.

[0067] While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.