Welding method

Abstract

A welding method wherein the root gap aperture displacement is measured before welding begins, and a welding material having a Mn/S ratio and a Mn/Si ratio compatible with the measured root gap aperture displacement is selected from gas-shielded arc welding materials. Gas-shielded arc welding is then performed using the selected welding material.

Claims

1. A method for performing welding under fluctuating stress, the method comprising: measuring a first root opening width variation for a predetermined period of time before start of welding; setting a maximum value of the measured first root opening width variation as an allowable value; selecting, from among gas shielded arc welding consumables, a welding consumable having Mn/S and Mn/Si ratios configured such that the selected welding consumable is adequately crack resistant when the measured first root opening width variation is equal to the allowable value; measuring a second root opening width variation while welding is performed; if the second root opening width variation exceeds the allowable value, inspecting, after welding, a quality of only a welded joint formed at a time when the second root opening width variation was measured as exceeding the allowable value; and if the quality of the welded joint inspected differs relative to a predetermined value, repairing the welded joint.

2. The method of claim 1, wherein if none of the gas shielded arc welding consumables have the Mn/S and Mn/Si ratios configured to be adequately crack resistant when the measured first root opening width variation is equal to the allowable value, the welding consumable is selected with a reduced root opening.

3. The method of claim 1, wherein: the selected welding consumable is a first welding consumable for use in a first layer; and a second welding consumable having lower Mn/S and Mn/Si ratios than the first welding consumable is used to perform welding to form a second and subsequent layers.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 illustrates chemical compositions of target electrodes for a trans-varestraint test conducted by the applicants of this application.

(2) FIG. 2 illustrates the relationship between each of the ratio of manganese (Mn) to sulfur (S) in an electrode and the ratio of Mn to silicon (Si) therein and the crack resistance, which was shown by the trans-varestraint test conducted by the applicants of this application.

(3) FIGS. 3(a) and 3(b) illustrate the state of a test for examining the applicability of in-service gas shielded arc welding. The test was conducted by the inventors of this application.

(4) FIG. 4 illustrates chemical compositions of welding consumables for use in the welding test illustrated in FIGS. 3(a) and 3(b).

(5) FIG. 5 illustrates the results of conducting the welding test illustrated in FIGS. 3(a) and 3(b) using the welding consumables illustrated in FIG. 4.

(6) FIG. 6 is a flow chart of a welding method according to an embodiment.

(7) FIG. 7 illustrates how a vehicle runs on a steel plate deck girder bridge.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

(8) The applicants of this application examined the resistance of each of a plurality of electrodes (4 mm) to a crack formed by imposing strain on the electrode using a trans-varestraint test. The electrodes have such various chemical compositions (unit: weight percent (wt. %)) as illustrated in FIG. 1. The examination showed that the Mn/Si and Mn/S ratios in each electrode significantly affect the crack resistance.

(9) Specifically, it was recognized that as illustrated in FIG. 2, with increasing Mn/S and Mn/Si ratios in an electrode, the crack resistance under fluctuating load increases.

(10) In view of the foregoing, to examine the applicability of in-service gas shielded arc welding, the inventors of this application used welding consumables having different Mn/S ratios and different Mn/Si ratios (specifically, gas shielded arc welding consumables, such as wires, used to perform a single welding operation over a longer distance than when electrodes, such as shielded metal-arc electrodes, are used) to conduct a welding test while varying the root opening (G) and the root opening width variation (), i.e., the amount by which the width of the root opening varies.

(11) Specifically, as illustrated in FIG. 3(a), substrates 1 and 2 each having a thickness T faced each other with the root opening G therebetween, a backing member 3 was provided at the bottom of a groove defined between bevels of the substrates 1 and 2 as illustrated in FIG. 3(b), and gas shielded arc welding was performed using carbonic acid gas to form a welded joint 4, thereby bonding the substrates 1 and 2 together.

(12) A welding consumable A (Mn/S ratio=160, Mn/Si ratio=2.98) and a welding consumable B (Mn/S ratio=130, Mn/Si ratio=26) each having a corresponding one of the chemical compositions illustrated in FIG. 4 was used as welding consumables.

(13) A rolled steel SM490A having a thickness T of 16 mm was used as each of the substrates 1 and 2. The root opening G was selected from two different values, i.e., 4 mm and 10 mm, and when each value was selected as the root opening G, the root opening width variation () (before welding) was selected from among three different values, i.e., 0.2 mm, 0.3 mm, and 0.4 mm. Thus, a welding test was conducted. The substrates 1 and 2 were displaced using servomechanisms retaining portions of the substrates 1 and 2 opposite to the bevels, and the root opening width variation () was measured with a displacement gauge, thereby allowing the root opening width variation to be equal to the above-described selected value. In this welding test, the frequency of variation in width of the root opening was set at 3.7 Hz, which is close to the frequency of vibrations induced by a running vehicle.

(14) FIG. 5 illustrates results of the welding test in which the above-described welding consumables were used. Here, the results were obtained by examining whether a discontinuity, such as a crack or a porosity, is caused in a welded joint through an ultrasonic test (UT) and a radiograph test (RT). A crack in an end portion of the welded joint can be prevented by the provision of an end tab, and thus, was not considered as a discontinuity.

(15) As illustrated in FIG. 5, in a case where the welding consumable A was used, when the root opening G was 4 mm, which is a typical value, a discontinuity was not caused even in a situation where the root opening width variation was 0.4 mm, which is relatively large. Also when the root opening G was set at 10 mm in consideration of an adequate margin for operation, and the root opening width variation was 0.2 mm, substrates were able to be welded using the welding consumable A without any discontinuity.

(16) In contrast, in a case where the welding consumable B was used, when the root opening G was 4 mm, any discontinuity was not caused in a situation where the root opening width variation was 0.2 mm. However, when the root opening G was 10 mm, a discontinuity was caused in a situation where the root opening width variation was 0.2 mm.

(17) Based on the above-described results of the welding test, the inventors of this application found that if the root opening width variation before the start of welding is measured, and a welding consumable having a Mn/S ratio and a Mn/Si ratio accommodating the measurement result is selected from among shielded metal-arc welding consumables each used to perform a single welding operation over a longer distance than when a shielded metal-arc electrode is used, gas shielded arc welding can be performed under fluctuating stress. The inventors of this application also found that if the root opening width variation is measured also during an actual welding operation at the site of welding, and the measured root opening width variation exceeds an allowable value assumed when a welding consumable was selected, the quality of a welded joint formed at the time when the measured root opening width variation exceeds the allowable value is inspected (through visual inspection, an ultrasonic test (UT), a radiograph test (RT), or any other test) to identify a deteriorated portion of the welded joint, and the welded joint can be, therefore, easily inspected and repaired (if necessary). Furthermore, the inventors of this application found that if, to select a welding consumable, not only the measured root opening width variation before the start of welding but also the root opening itself is taken into consideration, a more appropriate welding consumable can be selected.

(18) A welding method according to an embodiment of the present invention will now be described with reference to the drawings. The welding method according to this embodiment is an in-service gas shielded arc welding method (i.e., under fluctuating stress), and the gas shielded arc welding method is based on the findings described in the above-described section Assumptions of the Present Invention.

(19) FIG. 6 is a flow chart of the welding method according to this embodiment.

(20) First, in step S1, for example, the steel grade, shape, and size of a target joint for welding are examined based on, for example, a construction plan.

(21) Next, in step S2, to form the joint examined in step S1, the root opening width variation (before welding) under service conditions is measured.

(22) Next, in step S3, a welding consumable having Mn/S and Mn/Si ratios that can accommodate the root opening width variation measured in step S2 (a gas shielded arc welding consumable, such as a wire, used to perform a single welding operation over a longer distance than when an electrode, such as a shielded metal-arc electrode, is used) is selected. Specifically, a maximum root opening width variation (an allowable root opening width variation) is determined based on the measurement result of the root opening width variation , and a welding consumable having Mn/S and Mn/Si ratios that allow the welding consumable to be adequately resistant to a crack also when the root opening width variation is equal to the allowable root opening width variation is selected.

(23) For example, in a case where the root opening G is determined in, for example, a construction plan to be equal to or less than 4 mm, if the allowable root opening width variation is 0.2 mm, either of the above-described welding consumables A and B can be used, whereas if the allowable root opening width variation is 0.4 mm, the above-described welding consumable A can be used. Alternatively, if the allowable root opening width variation exceeds 0.4 mm, a welding consumable that is adequately resistant to a crack in a situation where the root opening width variation is equal to the allowable root opening width variation, i.e., a welding consumable having higher Mn/S and Mn/Si ratios than those of the above-described welding consumable A, needs to be selected.

(24) When the root opening G can be optionally determined, a welding consumable is selected also in consideration of the root opening G. For example, when the allowable root opening width variation is 0.2 mm, and the root opening G is set at 4 mm, either of the above-described welding consumables A and B can be used, whereas when the allowable root opening width variation is similarly 0.2 mm, and the root opening G is set at 10 mm, the above-described welding consumable A can be used.

(25) Next, in step S4, the welding consumable selected in step S3 is used to weld the target joint for welding by gas shielded arc welding.

(26) As described above, according to this embodiment, the Mn/S ratio and the Mn/Si ratio of the welding consumable are determined based on the measurement result of the root opening width variation before the start of welding. This enables selection of a welding consumable that is adequately resistant to a crack under assumed fluctuating stress. This selection enables in-service gas shielded arc welding, and a strong welded joint that is continuous over a long distance can be, therefore, efficiently formed.

(27) In this embodiment, if the root opening width variation is measured during the time period during which gas shielded arc welding is performed in step S4, and the measured root opening width variation exceeds the allowable root opening width variation assumed to select the welding consumable, the quality of a welded joint formed at the time when the measured root opening width variation exceeds the allowable root opening width variation may be inspected. Thus, also when the root opening width variation exceeds the allowable root opening width variation during a welding operation at the site of welding to deteriorate the quality of a welded joint, a deteriorated portion of the welded joint can be easily identified and repaired.

(28) In this embodiment, for example, when the measurement result of the root opening width variation in step S2 shows that the allowable root opening width variation exceeds 0.4 mm, and selectable welding consumables include only a consumable that is resistant to a crack in a situation where the allowable root opening width variation is equal to or less than 0.4 mm, the following procedure may be performed. Specifically, for example, the provision of a constraining plate or traffic regulation reduces the root opening width variation , and thereafter, the root opening width variation is again measured in step S2. Subsequently, for example, the constraint and how traffic is regulated have been successively tightened until the allowable root opening width variation is equal to or less than 0.4 mm. Here, when the root opening G is variable, the root opening G may be reduced.

(29) In this embodiment, for example, the type of the target joint for welding and the type and thickness of steel forming the joint are not specifically limited. For example, steel having a strength substantially equivalent to or less than that of SM490A, such as SM400A, SM400B, SM490B, SM490YA, or SM490YB in Japanese industrial standards (JIS) G 3106, may be used for butt welding. The steel has a thickness of equal to or less than 16 mm.

(30) In this embodiment, the Mn/S and Mn/Si ratios of the welding consumable for use in gas shielded arc welding are not specifically limited. However, with increasing Mn/S and Mn/Si ratios, the resistance of the welding consumable to a crack under strain increases, whereas with increasing amount of Mn contained in the welding consumable, the cost required for the consumable increases, and the viscosity of a droplet during welding increases, resulting in difficulty in, for example, ensuring the uniformity of the welded joint. Thus, the use of a welding consumable having a low Mn/S ratio and a low Mn/Si ratio that can accommodate the measurement result of the root opening width variation before the start of welding is advantageous in, for example, cost and ease of operation. Applicants' studies on the electrodes having the chemical compositions illustrated in FIG. 1 show that the amount of each of C, Si, P, and S in the welding consumable is preferably as small as possible.

(31) In this embodiment, the type of a shielding gas for use in gas shielded arc welding is also not specifically limited. However, with increasing content of an inert gas (e.g., Ar) in the shielding gas, the amount of spatters, for example, decreases, whereas with increasing content of the inert gas, the cost required for the shielding gas increases.

(32) Incidentally, when multilayer welding is performed in a in-service welding operation, a first layer is most likely to be cracked, whereas when a second layer and layers following the second layer are to be formed, the root opening width variation significantly decreases. Thus, when multilayer welding is performed, gas shielded arc welding of this embodiment may be performed to form all layers. Alternatively, gas shielded arc welding of this embodiment may be performed only to form a first layer. In other words, a welding consumable having lower Mn/S and Mn/Si ratios than those of the first layer may be used to form the second layer and the layers following the second layer.

(33) A target to which the welding method of this embodiment is applied is not limited to a bridge demanding an in-service welding operation. If the welding method of this embodiment is applied also to, for example, other steel structures undergoing fluctuating stress arising from an external environment, such as a steel tower or an offshore structure, the welding method of this embodiment is highly effective.

(34) The present invention is useful as a gas shielded arc welding method for use in butt welding of, for example, steel plates.