WO2008099991A2

WO2008099991A2 - The control algorism of welding current to prevent the initial splash of automatic dc invert resistance seam welding

Info

Publication number: WO2008099991A2
Application number: PCT/KR2007/002859
Authority: WO
Inventors: Kum-Gi Park; Woo-Jeong Choi; Ho-Kyeung Kim; Moon-Sub Jeong; Sang-Myung Cho
Original assignee: Stx Shipbuilding Co., Ltd.
Priority date: 2007-02-14
Filing date: 2007-06-13
Publication date: 2008-08-21
Also published as: KR100791019B1

Description

[DESCRIPTION] [Invention Tit Ie]

THR CONTROL ALGORISM OF WELDING CURRENT TO PREVENT THE INITIAL SPLASH OF AUTOMATIC DC INVERT RESISTANCE SEAM WELDING [Technical Field]

<i- The present invention relates to a welding currenl control method that can improve work efficiency and productivity by preventing generation of initial splashes in automatic resistance SEAM welding of a thin membrane plate in the fields of ship and machine technologies. [Background Art]

<2.> The automatic resistance SEAM welding is a welding method by which nuggets are continuously formed, and used for welding requiring airt ighlness. Particularly, a cargo hold membrane of an LNG carrier is made of invar steel containing nickel of 36% and ferrous of the balance. The cargo hold membrane is formed by stacking the invar steels in triple layered structure and performing the automatic resistance SEAM welding. The minimum nugget length of the welding portion formed by the automatic resistance SEAM welding is prescribed by an original manufacturing company and ship class to prevent leakage of LNG.

<v Referring to FIG. 1. each reference numeral indicates each part of the resistance welding part. The resistance welding part is formed aI an intermediate contact surface between welding objects 4 when the welding objects are welded by upper and lower electrodes 1 and 2. The resistance welding part may include a nugget 3. a heat-sensitive part 5, a corona bond 6, penetration 7, a conceive mark 8, a middle splash and a surface splash 10.

<4> Here, the splashes 9 and 10 are melted metal of the welding object or melted electrodes splashed from the contact surfaces between welding objects 4 or between the electrodes 1 and 2 and welding object 4. The splashes include the middle splash 9 and the surface splash 10. The middle splash 9 is melted metal which breaks down the corona bond 6 between the welding objects 4 and then is splashed to the outside. The surface splash 10 is splash of the welding object 4 or the electrodes 1 and 2 melted at the contact surface between the electrodes 1 and 2 and welding object 4.

<s> FIGS. 2 and 3 show the automatic resistance SEAM welding. Welding objects 11 and 12 are overlapped in more than two layers. Then, spot uelding is continuously performed by supplying welding current to the welding objects and pressing the welding objects by rotating circular plate electrodes 14. Thus, nuggets 13 are continuously formed and the automatic resistance SEAM welding, that is, ship welding is completed.

<6^ FIG. 4 shows flow of currents 15 and 16 in resistance spot welding. Even if Lhc welding is performed under the same condition, partial current L6 flows to the previous nugget 17 when the previous nugget 17 and the present nugget 3 are too close to each other. Accordingly, current density of the present welding nugget 3 is decreased. The partial current is called as ineffective partial current. When the ineffective partial current occurs, the current density is decreased to cause the nuggets to become smaller. Here, the current density means a magnitude (A/IIIO of current flowing through a unit area of a conductor.

--7' FIG. 5 shows flow of currents 22 and 23 in the automatic resistance SEAM welding. The ineffective partial current in the automatic resistance SEAM welding is greater than that of the resistance spot welding because the nuggets 20 and 21 arc overlapped and welded. Accordingly, the welding currents 22 and 23 should be higher than that of the resistance spot welding under the same condition in order to form nuggets having a diameter that satisfies the regulation prescribed by the original manufacturing company of the LNG cargo hold and the ship class.

^s- FIG. 6 shows welding currents 25 and 27, welding voltage waveforms 26 and 28 when the current 25 of the nugget 1 is controlled to 100% the same as the current 27 of the nugget 2 at the time of forming nuggets in the automatic DC inverter resistance SEAM welding. The initial splashes were generated because the current density was high when the nugget 1 without the ineffective partial current was welded in the automatic resistance SEAM welding.

<9- The initial splash causes partial damage of the elect i ode and the partial damage of the electrode causes recurrent appearance defect and change of nugget diameter in the resistance welding part. Accordingly, the resistance welding part could not satisfy the nugget regulation prescribed by the original manufacturing company of the LNG cargo hold and the ship class, and productivity is greatly decreased due to repairing work by TfG welding. In addition, when the partial damage of the electrode is continued, the circular plate electrodes 18 and 19 should be more frequently touched. As a result, lifetimes and exchange periods of the circular plate electrodes 18 and 19 are reduced to affect productivity. In addition, when the welding current is decreased to prevent the initial splashes, the nugget diameters are reduced to cause defects. [Disclosure] [Technical Problem]

^;'io> An object of the present invention is to provide a welding current control method that can improve work efficiency and productivity by preventing generation of initial splashes in automatic resistance SEAM welding of a thin membrane plate.

⁾⁾> Additional advantages, objects and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. [Technical Solution] i2> According to an aspect of the present invention, there is provided a welding current control method in automatic DC inverter resistance SEAM welding, which includes: preventing initial splashes by controlling welding current for forming a first nugget to a level of 30 to 90% of welding current for forming a second nugget or normal nugget .

:π-. According to another aspect of the present invention, there is provided a welding current control method in automatic DC inverter resistance SEAM welding, which includes: preventing initial splashes by selling welding nuggets of each step respectively to nuggets 1, 2, 3, 4, ... . n-v>, n-2, n-1, and n (normal nugget) and controlling welding currents of nuggets 2. .3. ... , n-2 and n-1 respectively to Λl+(Λn-Al)/(n-l). A14 L2(An-Λl)/(n-l)] . ... , Λlf[n-3(An-Al)/(n-l)l and Al+[n-2(Aii-Al)/(n-l)] by applying a welding current step-up function to the welding current of each step, where Al is the welding current of the nugget 1 and An is the welding current of the iiuggel n.

[Advantageous Effects]

<I5> According to the welding current control method of the present invention, work efficiency and productivity are improved by preventing initial splashes in the automatic resistance SEAM welding. [Description of Drawings]

<I6> FIG. 1 is a schematic view illustrating each parts of a resistance welding part;

'i7-> FIGvS.2 and 3 are views illustrating automatic resistance SEAM welding;

<J8> FIG.4 is a view illustrating flow of currents 15 and 16 in resistance spot welding;

<I9> FIG. 5 is a view illustrating flow of currents 22 and 23 πι the automatic resistance SEAM welding;

<20> FIG. 6 is a view illustrating welding currents 25 and 27, welding voltage waveforms 26 and 28 when the current 25 of a nugget 1 is controlled to 100% the same as the current 27 of a nugget 2;

<2i> FIG. 7 is a view illustrating currents 30 and 32, welding voltage waveforms 31 and 33 when the current 30 of the nugget 1 is controlled to 50% of the current 32 of Ihe nugget 2; and

<-22_^ FIG. 8 is a view illustrating modification and application examples of the initial splash control waveform according Io the present invention. [Best Mode]

<23-> FIG.7 shows currents 30 and 32 and welding voltage waveforms 3J and 33 when the current 30 of the nugget 1 is controlled to 50% of the current 32 of the nugget 2. In the embodiment, generation of the initial splashes has been prevented by decreasing current density in welding by controlling welding current 30 of the nugget 1 without ineffective partial current in the automatic resistance SEAM welding.

<-24> Factors causing generation of the initial splash in the automatic resistance SEAM welding include welding current, pressure, current conduction time and a surface state of a welding object. Here, the initial splash may occur when only the welding current is changed without change of other condit ions.

<25> Accordingly, when each nugget is welded in the automatic resistance SEAM welding, it is possible to prevent the initial splashes by supplying current lower than the welding current for forming a nugget that satisfies the nugget diameter prescribed by the original manufacturing company of the LNG carrier and the ship class, in other words, current corresponding to 30 to 90% of the welding current if other welding conditions are not unchanged.

-26> According to the present invention, there is provided a welding cuπent control method that can prevent the initial splashes by applying a welding current step-up function to the above described principle in the automatic DC inverter resistance SEAM welding. In other words, the welding current control method prevents the initial splashes by controlling the cuπent 35 of the nugget 1 to 30 to 90% of the current 36 of the nugget 2. The 'welding πirrent step-up function' will be described below.

<27 A welding engineer may set welding condition including the number of steps between the nuggets 1 and n (normal nugget). Here, the welding nuggets of each step are respectively a nugget 1, 2, 3, 4, ... , n-3, n-2, n-l and n (normal nugget). Here, if the welding current of each step, for example, the welding current of the nugget 1 is 2,00OA and the welding current of the nugget n is 4000A, welding currents of the nuggets 2, 3, ... , n-2, n-l and n are respectively 2000A+U000A-2000A)/(n-l), 2000A+12(4⁽X)OA 2000A)/(n-l) I , ... 2000A+[n-3(4000A-2000A)/(n-l)J, 2000A+[n-2(4000A-2000A)/(n-l⁾| and 2000Af[n-l(4000A-2000A)/(u-l)J.

--28> Thus, the 'welding current step-up function' means that the welding condition is gradually stepped up until the welding condition reaches the normal nugget welding condition.

<2^<)> In this time, in the automatic resistance SEAM welding, minimum and maximum welding currents generating no initial splash are respectively about 30% and 90% of the welding current generating the nugget n (normal nugget). The condition may be different according to thickness of the welding objects 11 and 12. <^ The welding current generating no initial splash is set to 30 to 90% of the welding current generating the nugget n (normal nugget) as described above because of the reason as below.

'^ If the number of steps is set to 2 by the welding current step-up function, the previous nugget of the nugget n is the nugget 1. In this lime, if the welding current of the nugget 1 is set to be lower than 30% of the welding current of the nugget n (normal nugget), the nugget 1 is not generated. Accordingly, the ineffective partial current causing decrease of current density does not exist in welding the nugget 2. Λs a result, the current density is increased to cause generation of initial splashes, [f the welding current for forming the nugget 1 is set to be higher than 90% of the welding current of the nugget n (normal nugget), the current density is high at the time of welding the nugget 1, thereby causing the initial splashes.

• ^--> Referring to FIG. 8, a waveform diagram (^L) is a magnified view of a waveform diagram (^~i). FIG. 8 shows an example that initial 'splashes are prevented by controlling welding current 37 to 41 of the corresponding nugget to be higher by a predetermined level than welding current of the previous nugget for each step until the welding current reaches welding current 12 for forming the normal nugget. For example, the total number of nuggets 1 to n may be set to 100 by using the welding current step-up function, and current value of each nugget may be increased step by step from 0.01 to 1 times of the current of the normal nugget during welding. In this time, the initial splashes are not generated in welding each nugget. [Industrial Applicability]

O3-> As described above, the initial splashes can be effectively prevented by controlling the welding currents variously in the automatic resistance SEAM welding to improve work efficiency and productivity. The welding current control method can be widely used in the fields of ship and machine technologies to improve practical and economical value.

Claims

[CLAIMS]

[Claim 1]

<35> A welding current control method in automatic DC inverter resistance SEAM welding, comprising: preventing initial splashes by controlling welding current for forming a first nugget to a level of 30 to 90% of welding current for forming a second nugget or normal nugget.

[Claim 2]

<-36> A welding current control method in automatic DC inverter resistance SEAM welding, comprising: preventing initial splashes by setting welding nuggets of each step respectively to nuggets 1, 2, 8, 4, ... , n-3, n-l\ n-1, and n (normal nugget) and controlling welding currents of nuggets 2, .'->, ... , n-2 and n-1 respectively to Al+(Λn-Al)/(n-l), Al+L2(Λn-Λl)/(n-J)l , ... , Al+[n-3(An-Al)/(n-l)l and Al+[n-2(An-Al)/(n-l)] by applying a welding current step-up function to the welding current of each step, where Al is the welding current of the nugget 1 and An is the welding current of the nugget n.

[ABSTRACT]

«-- Disclosed is a welding current control method that can improve work efficiency and productivity by preventing generation of initial splashes in automatic resistance SEAM welding of a thin membrane plate. The welding current control method prevents initial splashes by controlling welding current for forming a first nugget to a level of 30 to 90% of welding current for forming a second nugget or normal nugget. Or, the welding current control method prevents the initial splashes by increasing welding current of the corresponding nugget by a predetermined level than welding cuπ cnt of the previous nugget for each step until the welding current reaches welding current of a normal nugget spaced by a predetermined amount from the first nugget by applying a welding current step-up function.