IMPEDER FOR ELECTRIC RESISTANCE TUBE WELDING

Abstract: An impeder used for an electric resistance tube welding process for welding by heating the opposite edge portions of a strip skelp continuously formed into a tubular or a spirally tubular shape comprises plural laminated metals composed of foils of a metallic magnetic material, said foils being electrically insulated from each other, disposed in an imped­ er casing as a core. The impeder improves the welding energy efficiency in the electric resistance tube welding process and is excellent in durability.

Description: Background of the Invention: Field of the Invention: This invention relates to an impeder for an electric resistance tube welding process for welding by heating opposite edge portions of a strip-skelp continuously formed into a tubular shape or a spirally tubular shape. Description of the Prior Art: The welding portion of an electric resistance tube welding process, e.g., a high frequency induction tube welding process is constituted as shown in Fig. 1 of the accompanying drawings. That is, a skelp 1 formed into a tubular shape (hereinafter, is referred to as tubular skelp) is heated at the opposite edge portions by the action of eddy current induced in the tubular skelp 1 by passing a high frequency electric current through a coil 2 and is welded under pressure by means of squeeze rolls 4. An impeder used for improving welding energy efficiency by concentrating the eddy current upon the opposite edge portions is composed of an impeder casing 5 made of an insulating material and a core 6 made of a ferromagnetic material, a mandrel pipe 7 is connected to said impeder casing 5, and the core 6 is continuously cooled by cooling water supplied from said mandrel pipe 7. The purpose of using an impeder is to improve a welding energy efficiency of an electric resistance tube welding process, the performance of the impeder is determined by the characteristics of a core, and the core must be of high magnetic permeability and high saturation flux density. That is, for improving the welding energy efficiency in an electric resistance tube welding process, it is necessary to pass a larger amount of magnetic flux through the core of the impeder. The amount of magnetic flux density () passing through a core is the product of a magnetic flux density (B) and the transverse cross-sectional area (A) of the core, that is, the amount of the magnetic flux is shown by following equation (1) ° = B x A ****** (1) In the specification the transverse cross-sectional area of the core or the laminated metal means the area of the cross section in the direction perpendicular to the longitudinal direction of the impeder. As a conventional core of an impeder, a ferrite core which consists of a magnetic oxide prepared by sintering a powdered oxide has been used. In the case.cf-a-n elec-t-r-ic-resistance tube welding process, the magnetic flux density (B) of the ferrite core is liable to saturate since the welding electric current is a large high frequency electric current and, in particular, if the diameter of the welding tube is small, the transverse cross-sectional area (A) of the ferrite core becomes small and hence the magnetic flux density of the ferrite core approaches saturation by a smaller welding electric current. In this case, as is clear from the above equation (1), the passing magnetic flux density (b) approaches saturation and the welding energy efficiency reduces. As the counterplan, core materials have been improved by ferrite core makers for increasing the saturation flux density (by') of ferrite core and the property of B51 of about 0.5 tesla has been obtained but such an improvement is not yet satisfactorily. As core materials having the saturation flux density higher than that of the ferrite core, there are such metallic magnetic materials as silicon steel, low carbon steel, Permalloy (trade name), amorphous metals, etc., and in the case of a commercial frequency (50-60 Hz), a metallic magnetic material rolled to a thickness of 0.3-0.35 mm.and heattreated has been used as an iron core for transformers, etc. In the case of an electric resistance tube welding process, since the welding electric current used in the process is a large electric current of high frequency of 10-500 KHz, the impeder is exposed under a high magnetic flux density of the high frequency and thus a large eddy current generates in the core to increase the temperature thereof. Also, the generated eddy current is in proportion to the square of the magnetic flux density and the square of the frequency, and hence the heat value of the core in an electric resistance tube welding process is very large as compared to the use at a commercial frequency. Therefore, in the case of a metallic magnetic material generating a large amount of heat by eddy current, it is impossible to keep the temperature of the core of the impeder below the Curie point in a cooling step by passing cooling water, which can be usually industrially performed. Thus, the core loses the ferromagnetism and cannot be used as the core of an impeder. This is the reason that a ferrite core generating a small amount of heat by eddy current in a high frequency region is used in an electric resistance tube welding process. Summary of the Invention: The object of this invention is to provide an impeder for electric resistance tube welding having good welding energy efficiency in electric resistance tube welding process and an excellent durability using laminated metals composed of very thin foils of metallic magnetic material such as silicon steel, low carbon steel, Permalloy, amorphous metal, etc., as a core of the impeder. That is, according to this invention, there is provided an impeder for electric resistance tube welding comprising plural laminated metals composed of very thin foils of metallic magnetic materials such as silicon steel, low carbon steel, Permalloy, amorphous metal, etc., as the core of the impeder. Brief Description of the Drawings: Fig. 1 is a perspective schematic view of the heat-welding portion in a high frequency induction tube welding process, Fig. 2 is a perspective schematic view of an embodiment of the laminated metal according to this invention, Fig. 3 is a graph showing the relation between the thickness of the metal foil of the laminated metal and a cooling temperature and the relation between the thickness of the metal foil of the laminated metal and a magnetic flux density, Fig. 4, Fig. 7, Fig. 8, Fig. 9, Fig. 12, Fig. 13, and Fig. 14 are transverse cross-sectional views of various embodiments corresponding to the apparatus shown in Fig. 1 taken on the line A-A thereof in the various cases of using the core of this invention, Fig. 5(a), Fig. 5(b), Fig. 5(c), Fig. 6(a), Fig. 6(b) and Fig. 6(c) show various embodiments of the cases of constituting the apparent laminated metal by contacting the laminated metal units with each other in various manners, Fig. 10 is a longitudinal sectional view of the apparatus of Fig. 1 taken on the line B-B thereof, and Fig. 11 (a), Fig. 11(b) and Fig. 11(c) are transverse cross-sectional views showing some embodiments of the shapes of the laminated metal according to this invention. Description of the Preferred Embodiments; Now, the constitution of this invention will be explained in detail based on the embodiments shown in the accompanying drawings. Fig. 2 shows an embodiment of a laminated metal 8 composed of very thin foils of a metallic magnetic material which are used as the core of the impeder of this invention. The laminated metal 8 is composed of very thin foils 8a (hereinafter, are referred to as metal foils) of a metallic magnetic material having a higher saturation magnetic flux density than that of a ferrite core, such as silicon steel, etc., which are adhered each other with an insulating electrically insulating and adhering the metal foils. The thickness (t) and the width (w) of the metal foil 8a are desirably less than 0.1 mm and 10 mm respectively and practically the thickness (t) and the width (w) of the metal foil 8a are 0.005-0.1 mm and 1-10 mm respectively. It is preferred that when the width (w) of the metal foil 8a is longer than 5 mm, the thickness (h) of the laminated metal 8 matches with 5-20 mm2 of the transverse cross-sectional area (S=wxh) of said laminated metal. Also, it is difficult in industrial techniques to completely insulate the metal foils 8a constituting the laminated metal 8 from each other but for the purpose of this invention, it is sufficient that the resistance between the layers or metal foils of the laminated metal 8 be higher than about 1 kQ. In addition, it is desirable that the thickness (t) of the metal foil 8a be thin and the width (w) thereof be narrow. Also, it is preferred that the resistance between the layers of the laminated metal 8 be large and the thickness (h) of the laminated metal 8 be thin in a range of reducing the workability. Furthermore, it is preferred that the laminated metal 8 be coated with varnish, etc. In addition, the ranges which are selected as the desirable ranges in this invention as described above are established on investigating and considering the easiness of cooling, that is the heat value and the magnetic characteristics. Fig. 3 shows an example of the investigations. Fig. 3 is a graph showing the relation between the thickness of the metal foil and the cooling temperature and the reaction of the thickness of the metal foil and the magnetic flux when the frequency is 100 KHz and the amount of cooling water is 18 liters/min. in this invention. With regard to the thickness (t) of the metal foils 8a, if the thickness (t) of the metal foils 8a is increased, the amount of the magnetic flux passing through the laminated metal 8 decreases and the heat value of the laminated metal 8 increases exponentially. When the thickness (t) of the metal foil 8a is increased up to about 0.15 mm, the magnetic flux density is about twice the saturation magnetic flux density (Bs' ¯ 0.5 tesla) of a conventional ferrite core but the generation of heat in the laminated metal 8 increases too quickly to use as an impeder. On the other hand, when the thickness (t) of the metal foil 8a is 0.1 mm, the magnetic flux density is about 2.7 times the saturation flux density of a conventional ferrite core and the generation of heat in the laminated metal 8 is of an extent that the laminated metal can be sufficiently used as an impeder in an industrially applicable cooling method using water as a coolant. With regard to the disposition of the laminated metals in an impeder casing, as shown in Fig. 4, it is preferred to dispose the laminated metals 48 in such a manner that the surfaces of each laminated metal are brought into contact with cooling water as well as possible, that is, cooling water passes through the space between the inside surface of an impeder casing 45 and each laminated metal 48 as well as the spaces between the laminated metals 48. In Fig. 4, the numeral 41 stands for a tubular skelp, 43 opposite edge portions, and 49 an opening. On the other hand, as is understood from equation (1), in a magnetic field by which the magnetic flux density reaches the saturation value, the magnetic flux amount (b) passing through an impeder becomes larger as the transverse cross-sectional area (A) of a core is larger. Therefore, under a strong magnetic field, it is advantageous for welding energy efficiency to dispose a larger number of laminated metals in the impeder casing but if the laminated metals are in contact with each other, it sometimes happens that the cooling efficiency is reduced. When plural laminated metal units 58 which are adjacent each other constitute one laminated metal to all appearance (hereinafter, such a laminated metal is referred to as an apparent laminated metal) as shown in Fig. 5(a), Fig. 5(b) and Fig. 5(c) and when the length (h' or w') of the apparent laminated metal 58' is longer than 5 mm and shorter than 10 mm, it is desirable that the length (w' or h') of the other side thereof be less than 50 mm2 in the transverse cross-sectional area (S) (S' = h' x w') of the apparent laminated metal 58', while when the length (h' or w') of one side is over 10 mm, it is desirable that the length (w' or h') of the other side be less than 5 mm. This is also same when plural apparent laminated metals 58' further constitute one apparent laminated metal. In addition, when one side of an apparent laminated metal unit 68 is in contact with the side of other apparent laminated metal unit 68 to constitute an L-shape or T-shape apparent laminated metal 68' as shown in Fig. 6(a), Fig. 6(b) and Fig. 6(c), there is no limitation on the transverse crosssectional area (S") of the apparent laminated metal 68'. Now, the transverse cross-sectional area of the laminated metal is the area of the section of the laminated metal in the direction perpendicular to the longitudinal direction of the impeder casing. The transverse cross-sectional area (A") of a metal core necessary for improving the welding energy efficiency using the core (hereinafter, are referred to as "metal core") of this invention, which is the assembly of the laminated metals, over the welding energy efficiency in conventional processes may satisfy the condition shown by equation (2); A' x B ' < A" x B5, (2) S wherein A' is the transverse cross-sectional area of a ferrite core, Bs' is the saturation flux density of a ferrite core, A" is the transverse cross-sectional area of the metal core of this invention, and B is the saturation flux density of the metal core of this invention. When the magnetic flux density is in the saturation value, it is advantageous for a welding energy efficiency to increase the transverse cross-sectional area (A") of the metal core but when the magnetic flux density of the metal core does not reach the saturation flux density since the transverse cross-sectional area (A") of the metal core is sufficiently large to the magnetic field formed by a high frequency electric current passing through the work coil (Fig. 1), the increase of the transverse crosssectional area (A") of the core metal does not improve the welding energy efficiency too much. In such a case, it is advantageous from the economical aspect as well as the aspect of workability to reduce the transverse crosssectional area (A") of the metal core in the range that the magnetic flux density of the metal core does not reach the saturated value. In addition, with regard to the disposition of the laminated metals constituting the metal core in the impeder casing, the laminated metals may be disposed in a manner of sufficiently performing cooling and in a number satisfying the condition of equation (2), and even when there is not particular restriction about the disposition of the laminated metals, that is, they may be randomly disposed, the welding energy efficiency is improved. However, in the case of welding a pipe having a small diameter, the transverse cross-sectional area of the impeder casing is usually small and hence although it is necessary to dispose many laminated metals in the impeder casing in the range of not reducing the cooling efficiency, the number of the laminated metals disposed in the casing is frequently reduced if the laminated metals are randomly disposed. Also, it sometimes happens that rolling of a tubular skelp or twisting.of an impeder occurs in an electric resistance welding tube and in such a case the welding energy efficiency changes as the case may be. In particular, if the transverse cross-sectional area of the laminated metal is large, the foregoing tendency takes place. In order to overcome these difficulties, laminated metals 78 are so disposed in an impeder casing 75 that the laminated metals 78 are arranged in concentric circles with regard to the impeder casing 75 as shown in Fig. 7 as the transverse cross-sectional arrangement. In other words, the laminated metals are disposed in the impeder casing 75 in such a manner that both edge corners of the surface of each laminated metal 78 facing the center of the impeder casing 75 are in contact with the edge corners of the surfaces of other adjacent laminated metals to form apparent concentric cylinders formed by the laminated metals 78. The number of the concentric cylinders formed by the laminated metals 78 in the impeder casing 75 shown in Fig. 7 is three but the number may be one or there is no restriction about the number of the apparent concentric cylinders. In addition, in the case of disposing plural apparent concentric cylinders formed by the laminated metals 78 in the impeder casing 75, if the distance between the outside diameter of the inner apparent concentric cylinder and the inside diameter of the outer apparent concentric cylinder is about 1 mm, a necessary amount of cooling water can pass through the space between the outer concentric cylinder and the inner concentric cylinder. Furthermore, as shown in Fig. 8, it is preferred to form a slight gap (a) (a is about 0.2-1 mm) between the laminated metals 88 disposed under the opening 89 formed between the opposite edge portions 83 of the tubular skelp 81. In the figure, the numeral 85 stands for an impeder casing. When the transverse cross-sectional area of the core can be increased to the magnetic field formed by a high frequency electric current passing through the work coil 2 (Fig. 1), the cooling efficiency is good and the life of the core is prolonged in the case of disposing the laminated metals 98 constituting concentric cylinders separately from each other as shown in Fig. 9. In this case, it is unnecessary to form a gap between the laminated metals 98 disposed under the opening 99 of the tubular skelp 91. Fig. 10 shows a longitudinal sectional view of the impeder of this invention shown in Fig. 1 taken on the line B-B thereof. The impeder core which is the assembly of the laminated metals 108 is placed in the impeder casing 5 with a space (b) (b = 2-5 mm) between the down stream end 108a of the impeder casing 5 and the down stream end 5a of the impeder casing 5. The laminated metals 108 are connected or fixed to supports 10 composed of a non-metallic insulator such as an epoxy resin at the opposite or up stream end 108b of the laminated metals 108 and further the support 10 is fixed to the impeder casing 5 by means of a pin 11. It is preferred that the support 10 is fixed to the laminated metals 108 with an insulating adhesive such as an epoxy resin and the support 10 can be utilized as a frame in the case of constituting the apparent concentric cylinders by the use of the laminated metals 108. In addition, the pin 11 may be made of a metal if the up stream end 108b of the laminated metals 108 is sufficiently separated from the pin 11. Also, in Fig. 10, the laminated metals 108 are constituted in a body without joints in the lengthwise direction of the impeder casing 5 but the laminated metals 108 may be constituted by connecting plural laminated metal units in the lengthwise direction of the impeder casing 5. Many advantages of this invention will be explained. By laminating metal foils each having very thin thickness and a narrow width so that the metal foils are electrically insulated well from each other to form a laminated metal having a small cross sectional area and using these laminated metals as a core of an impeder, an impeder generating less heat by eddy current and having good cooling efficiency is obtained. Thus, the temperature of the core of the impeder can be sufficiently kept below the Curie point in a cooling step by passing cooling water, which can be usually performed industrially. As the results thereof, an impeder having very excellent welding energy efficiency can be obtained. Also, by concentrically disposing the apparent cylinders composed of the laminated metal as shown in Fig. 7 and Fig. 8, the metal foils constituting the laminated metals are radially distributed from the center of the impeder casing and hence even when rolling of a tubular skelp or twisting of an impeder occurs in the tube welding process, a stably high welding energy efficiency is always obtained. Furthermore, by forming a slight space or gap between the laminated metals 88 disposed under the opening 89 of the tubular skelp 81 as shown in, for example, Fig. 8, the magnetic flux formed by the high frequency electric current passing through the work coil can pass through the laminated metals 88 disposed at the inner portion in the impeder casing without being magnetically shielded by the laminated metals 88 disposed at the outer portion in the impeder casing. Still further, by supporting the up stream ends 108b of the laminated metals 108 and forming a space (b) between the down stream ends 108a of the laminated metals and the inside wall of the down stream end 5a of the impeder casing 5 as shown in Fig. 10, the hydraulic pressure of cooling water supplied from a mandrel pipe into the electric resistance welding tube does not act as a compressive force to the laminated metal 108. Therefore, the deformation and stripping of the laminated metal 108 can be prevented, thereby the life of the core of the impeder is greatly improved. Moreover, at performing electric resistance tube welding, the intensity of magnetic field differs in the direction of the longitudinal axis of the tube. That is, since the intensity of the magnetic field under the work coil is strongest, it sometimes happens that the insulating adhesive of the laminated metal located under the work coil is deteriorated and extraordinarily heated. In such a case, if the impeder core is constituted by connecting the laminated metal units in the lengthwise direction of the impeder casing according to an embodiment of this invention, the deteriorated laminated metal unit only may be replaced with a new laminated metal unit and hence the invention is economical. In addition, the transverse cross-sectional shape of the laminated metal is not limited to the shape illustrated in Fig. 2 but it may be an arc shape or a parallelogramic shape as shown by numeral 118 in Fig. 11 and in such cases, the thickness (h), width (w), and the transverse cross-sectional area (S) of the laminated metals are same as those discussed with regard to the embodiment shown in Fig. 2. Also, in the embodiments illustrated in Fig. 7, Fig. 8, and Fig. 9, the laminated metals are uniformly disposed on a concentric circle or concentric circles but if a sufficient transverse cross-sectional area of the laminated metals is obtained for the magnetic field formed by a work coil, the laminated metals 128 may be disposed on a part of the concentric circle or circles in an impeder casing 125 as shown in Fig. 12. In addition, the numeral 121 in the figure stands for a tubular.skelp. Furthermore, when the cross sectional area of an impeder casing is small, the laminated metals may be disposed as shown in Fig. 13 and Fig. 14, wherein numerals 131 and 141 stand for a tubular skelp, 135 and 145 an impeder casing, and 138 and 148 laminated metals. An example of performing a high frequency induction tube welding process using the impeder of this invention is shown in Table 1 together with a comparative example of performing the same tube welding process using a conventional impeder. Table 1 Impeder Core --------------------------------------------------------------------------------------------------------- Example of the invention Comparative example Specimen of weldes tube Dimensin of welded tube 34.0 #mm x 4.2t mm do Kind of steel 0.28% C steel do --------------------------------------------------------------------------------------------------------- Welding apparatus High frequency induction welding epparatus Frequehcy f = 180 KHz do --------------------------------------------------------------------------------------------------------- 3% silicon steel Fe-B-Si smorphous alloy Mn-Zn series ferrite Material Thickness of metal Thickness of metal foil: 0.05 mm foil: 0.02 mm width of metal foil: width of metal foil: 2.5 mm 2.5 mm --------------------------------------------------------------------------------------------------------- Dimensions Thickness of lamipated Thickness of laminated Outer diameter: 18 #mm shape metal: 2.5 mm metal: 2.5 mm Inside diameter: 9 #mm Length of laminated Length of laminated Length: 400 mm Core arrangement metal: 400 mm metal: 400 mm Only one core in impeder casing Number of laminated Number of laminated metal: 24ncentric metal: 24 Double concentric Double concentric structure structure ---------------------------------------------------------------------------------------------------------- Resistance batween 10 k# 10 k# layers Saturation flux 2.0 tesla 1.6 tesla 0.5 tesla dinsity ----------------------------------------------------------------------------------------------------------- Welding speed 5.0 m/min. do do ----------------------------------------------------------------------------------------------------------- Required electric power 514 KW 630 KW 750 KW ------------------------------------------------------------------------------------------------------------ In addition, the welding energy coefficients in the case of using the impeder of this invention and the case of using the conventional impeder in the tube welding process may be compared by the required electric power (the product of the plate voltage and the plate current of the oscillator) As is clear from the results shown in Table 1, by using the impeder of this invention, the welding energy efficiency can be greatly improved and more than 30% of the electric power can be saved. On the other hand, a conventional impeder using a ferrite core causes the deterioration in ferromagnetism during use and must be replaced, while the impeder of this invention exhibits such excellent effects that the durability of about 1.3 times that of a conventional impeder, etc., is obtained.

Claims: What is claimed is: Claim 1. An impeder for electric resistance tube welding comprising plural laminated metals composed of foils of a metallic magnetic material, said foils being electrically insulated from each other, disposed in an impeder casing as a core. Claim 2. The impeder for electric resistance tube welding as claimed in claim 1 wherein the total sum of the transverse cross-sectional areas of the laminated metals satisfy the following equation; A' x B ' < A" x B n S S wherein A' : the transverse cross-sectional area of a ferrite core, B ' : the saturation flux density of a ferrite S core, A" : the total sum of the transverse cross sectional areas of laminated metals, and B " : the saturation flux density of laminated S metal. Claim 3. The impeder for electric resistance tube welding as claimed in claim 1 or 2 wherein the laminated metals are disposed in the impeder casing in such a manner that the laminated metals are disposed on wholly or partially a concentric circle or concentric circles with regard to the impeder casing in the transverse crosssectional views of the impeder casing and the laminated metals. Claim 4. The impeder for electric resistance tube welding as claimed in claim 1 wherein a gap is formed between the laminated metals disposed under an opening of the tubular skelp among the laminated metal disposed in the impeder casing. Claim 5. The impeder for electric resistance tube welding as claimed in claim 1 wherein the laminated metals are fixed to a support at the end of the upper stream thereof. Claim 6. The impeder for electric resistance tube welding as claimed in claim 1 wherein the laminated metal units are connected as a core in the longitudinal direction of the impeder casing.

Inventors: ICHIHARA, HIROHISA, ING., KUMAI, KO, ING., NOUGATA, HIROSHI, ING., SATO, TAKASHI, ING., TAKECHI, HIROSHI, ING., YAMAMOTO, TAKAAKI, ING., YOSHIZAWA, MITSUO, ING.
Applicants: NIPPON KINZOKU CO., LTD., NIPPON STEEL CORPORATION
Application year: 1981
Main classification: B23K13
IPC codes: B23K13