Patent Application
20100170050
The present invention relates to a welded joint comprising a first welding member and a second welding member joined to a surface of the first welding member by welding. More particularly, the present invention relates to a welded joint suitable as a joint between a stiffener (rib) and a steel plate (deck plate) for supporting a pavement of a bridge, a steel deck using such a welded joint structure, and a process for producing the steel deck.
The steel deck includes a steel plate at its main part. Since the steel plate alone does not provide sufficient rigidity, however, a plurality of stiffeners are usually attached at regular intervals to the lower surface of the steel plate opposite the paving surface on which a pavement is placed.
In some steel decks, a plurality of plate steel members as the stiffeners, for example, are attached to the steel plate by arc welding (SAW, SMAW, GMAW, etc.), while in others, steel members having a V- or U-shaped cross section to form a closed-section structure in cooperation with the steel plate are attached as the stiffeners to the steel plate by arc welding.
As a joint between the steel plate and each stiffener of the steel deck, a welded joint is employed which is obtained by depositing a weld metal by arc welding in a single bevel groove formed at an edge of the stiffener brought into contact with the steel plate. Where the stiffener is a steel member with a V- or U-shaped cross section, single bevel grooves are formed at respective edges of the stiffener such that the single bevel grooves open outward in respective opposite directions, and the welded joint is formed at the individual edges of the stiffener brought into contact with the steel plate (see, e.g., Patent Document 1).
In the welded joint used in the steel deck, however, as the temperature of the weld metal lowers after the arc welding, the weld metal thermally shrinks, producing a tensile residual stress within the metal members. Such welding residual stress lowers the accuracy of joining between the steel plate and the stiffener, as well as the tensile strength, compressive strength and fatigue strength of these members.
In addition, where the stiffener is a steel member with a V- or U-shaped cross section, the welding operation can only be performed from outside of the stiffener. Thus, when the welding is performed while avoiding deterioration in the weld quality due to melt-through (burn-through), an unwelded region (non-penetrated region) inevitably remains at the bottom of each single bevel groove of the welded joint, namely, at the roots forming the closed-section structure.
If the welding residual stress remains in the welded joint of the steel deck for supporting a pavement of a bridge and also the welded joint includes a non-penetrated region, a crack is formed at the non-penetrated region and pierces through the steel plate or weld bead as the steel deck is repeatedly applied with bending load. As a result, a problem arises in that the fatigue strength of the steel deck suddenly lowers, possibly causing fatigue breakdown.
In conventional steel decks, moreover, in cases where a crack piercing through the steel plate or through the weld bead has been formed or is expected to be formed at the non-penetrated region, the pavement has to be removed from the bridge to repair or replace the steel deck, giving rise to the problem that the maintenance cost increases correspondingly. A solution to the problem has therefore been sought heretofore.
The present invention was created to solve the above problem, and an object thereof is to provide a welded joint which is applied, for example, to a steel deck for supporting a pavement of a bridge, is improved in fatigue strength to prevent fatigue breakdown and capable of reducing bridge maintenance costs, a steel deck using the welded joint, and a process for producing the steel deck.
A welded joint according to the present invention comprises a first welding member and a second welding member joined to a surface of the first welding member by welding, in which a single bevel groove is formed at an edge of the second welding member brought into contact with the first welding member, and a weld metal is deposited in the single bevel groove by arc welding to join the first and second welding members together. The welded joint is characterized in that the weld metal is a low transformation-temperature welding material whose martensitic transformation takes place in a predetermined low temperature range, and that a groove angle of the single bevel groove of the second welding member and welding conditions are set on the basis of data acquired so as to obtain a fixed dilution ratio of the weld metal through control of penetration rate.
Preferably, in the welded joint, the groove angle of the single bevel groove of the second welding member is set to about 45 degrees in accordance with the data acquired so as to obtain a fixed dilution ratio of the weld metal through control of the penetration rate.
Where the welded joint of the present invention is used, for example, in the steel deck of a bridge, the weld metal undergoes transformation expansion in the predetermined low temperature range so as to cancel out the thermal shrinkage, and substantially no welding residual stress remains in the steel plate as the first welding member and in the stiffener as the second welding member. As a result, even if a non-penetrated region exists, the frequency of occurrence of cracking significantly lessens, so that the lowering in the steel deck assembling accuracy as well as in the tensile strength and compressive strength of the steel deck can be avoided, making it possible to enhance the fatigue strength of the steel deck.
Also, in cases where a crack piercing through the steel plate or the weld bead has been formed or is expected to be formed, the steel deck can be repaired on the site of the bridge to recover or increase the fatigue strength. Since it is unnecessary to remove the pavement from the bridge for repair or replacement of the steel deck, the maintenance cost can be greatly cut down. Further, during the repair, the dilution ratio of the weld metal can be controlled to the fixed ratio, whereby the fatigue strength can be recovered or increased with good reproducibility.
In the welded joint of the present invention, the low transformation-temperature welding material is an iron alloy containing at least components: 0.20 mass % or less of carbon, 3.0 to 13.0 mass % of chromium, and 3.0 to 12.0 mass % of nickel; and the iron alloy has a composition adjusted such that an amount of linear expansion per millimeter in a temperature range from a martensitic transformation start temperature to room temperature is equal to or greater than −3×10−3 mm.
Where the welded joint of which the weld metal is an iron alloy having a proper composition is used in the steel deck of a bridge, for example, the weld metal undergoes appropriate transformation expansion in the predetermined low temperature range so as to cancel out the thermal shrinkage, and substantially no welding residual stress remains in the steel plate as the first welding member and in the stiffener as the second welding member, thus significantly lessening the frequency of occurrence of cracking at the non-penetrated region. It is therefore possible to avoid lowering in the steel deck assembling accuracy as well as in the tensile strength and compressive strength of the steel deck, whereby the fatigue strength of the steel deck can be further improved.
Preferably, in the welded joint of the present invention, the first welding member is a high fatigue-strength steel plate which has a high fatigue strength and of which a fatigue crack propagation speed in a predetermined stress intensity factor range falls within a predetermined low speed range.
Thus, a high fatigue-strength steel plate whose composition is adjusted so as to exhibit a low fatigue crack propagation speed is selected as the first welding member of the welded joint. Accordingly, even in the case where a crack is formed at the non-penetrated region of the first welding member, propagation of the crack can be restrained, making it possible to ensure sufficient fatigue strength.
A steel deck according to the present invention comprises a steel plate having a paving surface on which a pavement of a bridge is placed, and at least one stiffener joined by welding to a lower surface of the steel plate opposite the paving surface, wherein a single bevel groove is formed at an edge of the stiffener brought into contact with the steel plate, and a weld metal is deposited in the single bevel groove by arc welding to form the steel deck. The steel deck is characterized in that the weld metal is a low transformation-temperature welding material whose martensitic transformation takes place in a predetermined low temperature range, and that a groove angle of the single bevel groove of the stiffener and welding conditions are set on the basis of data acquired so as to obtain a fixed dilution ratio of the weld metal through control of penetration rate.
Preferably, in this steel deck, the groove angle of the single bevel groove of the stiffener is set to about 45 degrees in accordance with the data acquired so as to obtain a fixed dilution ratio of the weld metal through control of the penetration rate.
With the steel deck of the present invention, the weld metal undergoes transformation expansion in the predetermined low temperature range so as to cancel out the thermal shrinkage, and substantially no welding residual stress remains in the steel plate and the stiffener. As a result, even if a non-penetrated region exists, the crack occurrence frequency significantly lessens, so that the lowering in the steel deck assembling accuracy as well as in the tensile strength and compressive strength of the steel deck can be avoided, making it possible to enhance the fatigue strength of the steel deck.
Moreover, in cases where a crack piercing through the steel plate or the weld bead is expected to be formed, the steel deck can be repaired on the site of the bridge to recover or increase the fatigue strength. Since it is unnecessary to remove the pavement from the bridge for repair or replacement of the steel deck, the maintenance cost can be greatly cut down. Further, during the repair, the dilution ratio of the weld metal can be controlled to the fixed ratio, whereby the fatigue strength of the steel deck can be recovered or increased with good reproducibility.
Preferably, in the steel deck of the present invention, the stiffener forms a closed-section structure in cooperation with the lower surface of the steel plate, and single bevel grooves are formed at respective edges of the stiffener brought into contact with the steel plate to form the closed-section structure such that the single bevel grooves open outward in respective opposite directions. More specifically, the stiffener preferably comprises a shaped steel with a U-shaped cross section.
With this steel deck, even though the welding operation can only be performed from outside of the stiffener with a V- or U-shaped cross section and thus a non-penetrated region is liable to be formed at the bottom of each single bevel groove, the weld metal undergoes transformation expansion in the predetermined low temperature range so as to cancel out the thermal shrinkage, and substantially no welding residual stress remains in the steel plate as well as in the stiffener. Consequently, sufficient rigidity of the steel deck is secured and also the crack occurrence frequency significantly lessens, so that the fatigue strength of the steel deck can be enhanced. Further, shaped steels with a U-shaped cross section are easily available, making it possible not only to save the labor of obtaining stiffeners but to improve the fatigue strength of the steel deck while at the same time securing sufficient rigidity.
Preferably, in the steel deck of the present invention, the low transformation-temperature welding material is an iron alloy containing at least components: 0.20 mass % or less of carbon, 3.0 to 13.0 mass % of chromium, and 3.0 to 12.0 mass % of nickel; and the iron alloy has a composition adjusted such that an amount of linear expansion per millimeter in a temperature range from a martensitic transformation start temperature to room temperature is equal to or greater than −3×10−3 mm.
With the steel deck using, as the weld metal, the iron alloy having a proper composition, the weld metal undergoes appropriate transformation expansion in the predetermined low temperature range so as to cancel out the thermal shrinkage, and substantially no welding residual stress remains in the steel plate as well as in the stiffener, thus significantly lessening the frequency of occurrence of cracking at the non-penetrated region. It is therefore possible to avoid lowering in the steel deck assembling accuracy as well as in the tensile strength and compressive strength of the steel deck, whereby the fatigue strength of the steel deck can be further improved. Since the dilution ratio of the weld metal can be controlled to the fixed ratio, moreover, the fatigue strength of the steel deck can be recovered or increased with good reproducibility.
Preferably, in the steel deck of the present invention, the steel plate is a high fatigue-strength steel plate which has a high fatigue strength and of which a fatigue crack propagation speed in a predetermined stress intensity factor range falls within a predetermined low speed range.
Thus, in this steel deck, a high fatigue-strength steel plate whose composition is adjusted so as to exhibit a low fatigue crack propagation speed is selected as the steel plate. Accordingly, even in the case where a crack is formed at the non-penetrated region of the steel plate, propagation of the crack can be restrained, making it possible to ensure sufficient fatigue strength.
Also, according to the present invention, there is provided a process for producing the steel deck, which comprises: a first step of selecting, as the weld metal, a low transformation-temperature welding material whose martensitic transformation takes place in the predetermined low temperature range; a second step of forming a single bevel groove at the edge of the stiffener brought into contact with the steel plate; and a third step of performing welding under the welding conditions to deposit the weld metal in the single bevel groove.
Thus, in the steel deck production process of the present invention, a low transformation-temperature welding material whose martensitic transformation takes place in the predetermined low temperature range is selected as the weld metal in the first step, then, a single bevel groove is formed at the edge of the stiffener in the second step, and in the third step, welding is performed under the welding conditions to deposit the low transformation-temperature welding material, selected as the weld metal, in the single bevel groove. Accordingly, the weld metal undergoes transformation expansion in the predetermined low temperature range so as to cancel out the thermal shrinkage, and substantially no welding residual stress remains in the steel plate as well as in the stiffener, thus significantly lessening the frequency of occurrence of cracking at a non-penetrated region which is liable to be formed at the bottom of the single bevel groove. It is therefore possible to provide a steel deck with high fatigue strength. Further, since the dilution ratio of the weld metal can be controlled to the fixed ratio, the fatigue strength of the steel deck can be recovered or increased with good reproducibility.
According to another aspect of the present invention, there is provided a process for producing the steel deck, in which the stiffener forms a closed-section structure in cooperation with the lower surface of the steel plate and which comprises: a first step of selecting, as the weld metal, a low transformation-temperature welding material whose martensitic transformation takes place in the predetermined low temperature range; a second step of forming single bevel grooves at respective edges of the stiffener brought into contact with the steel plate to form the closed-section structure such that the single bevel grooves open outward in respective opposite directions; and a third step of performing welding under the welding conditions to deposit the weld metal in the individual single bevel grooves.
In this steel deck production process, a low transformation-temperature welding material whose martensitic transformation takes place in the predetermined low temperature range is selected as the weld metal in the first step, then in the second step, single bevel grooves are formed at the respective edges of the stiffener brought into contact with the steel plate to form the closed-section structure such that the single bevel grooves open outward in the respective opposite directions, and in the third step, welding is performed under the welding conditions to deposit the weld metal in the individual single bevel grooves. With this process, even though the welding operation can only be performed from outside of the stiffener with a V- or U-shaped cross section and thus a non-penetrated region is liable to be formed at the bottom of each single bevel groove, the weld metal undergoes transformation expansion in the predetermined low temperature range so as to cancel out the thermal shrinkage, and substantially no welding residual stress remains in the steel plate as well as in the stiffener. Consequently, sufficient rigidity of the steel deck is secured and also the crack occurrence frequency significantly decreases, so that the fatigue strength of the steel deck can be enhanced. Further, since the dilution ratio of the weld metal can be controlled to the fixed ratio, the fatigue strength of the steel deck can be recovered or increased with good reproducibility.
In the steel deck production processes of the present invention, preferably, in the first step, an iron alloy is selected as the low transformation-temperature welding material, the iron alloy containing at least components: 0.20 mass % or less of carbon, 3.0 to 13.0 mass % of chromium, and 3.0 to 12.0 mass % of nickel; and having a composition adjusted such that an amount of linear expansion per millimeter in a temperature range from a martensitic transformation start temperature to room temperature is equal to or greater than −3×10−3 mm.
With the steel deck production processes, the iron alloy having a proper composition is selected as the weld metal. Accordingly, the weld metal undergoes appropriate transformation expansion in the predetermined low temperature range so as to cancel out the thermal shrinkage, and substantially no welding residual stress remains in the steel plate as well as in the stiffener, thus significantly lessening the frequency of occurrence of cracking at the non-penetrated region. It is therefore possible to produce a steel deck with remarkably high fatigue strength.
Preferably, in the steel deck production processes of the present invention, the first step further includes selecting, as the steel plate, a high fatigue-strength steel plate which has a high fatigue strength and of which a fatigue crack propagation speed in a predetermined stress intensity factor range falls within a predetermined low speed range.
Thus, in the steel deck production processes, a high fatigue-strength steel plate whose composition is adjusted so as to exhibit a low fatigue crack propagation speed is selected as the steel plate. Accordingly, even in the case where a crack is formed at the non-penetrated region of the steel plate, propagation of the crack can be restrained. It is therefore possible to produce a steel deck with sufficiently high fatigue strength.
In the steel deck production processes of the present invention, preferably, in the third step, the weld metal is deposited in each of the single bevel grooves by single pass welding.
Thus, with the steel deck production processes, the welding is carried out by single pass welding, and not multi-pass (multilayer) welding, and it is also unnecessary to take care not to cause melt-through. Accordingly, the man-hour of the welding work can be reduced and also the efficiency of the welding operation can be improved.
Embodiments of the present invention will be described below with reference to the accompanying drawings.
In the following description, a welded joint according to an embodiment of the present invention is applied to a steel deck of a box girder for a bridge, by way of example.
As shown in
Each steel deck 3 of the box girder 1 includes, as shown in
Thus, the steel plate 10 and the stiffener 20 are welded together to constitute a welded joint by depositing a weld metal by arc welding in a single bevel groove 22 formed at each edge 21 of the stiffener 20. The single bevel grooves 22, 22 formed at the respective edges 21, 21 of the stiffener 20 open outward in respective opposite directions.
As the weld metal, a low transformation-temperature welding material whose martensitic transformation takes place in a predetermined low temperature range is used, and as shown in the enlarged view of
In this embodiment, a high fatigue-strength steel plate with a predetermined thickness t1 (e.g., 12 mm) is used as the steel plate 10. The high fatigue-strength steel plate has its metal structure adjusted such that, as shown in the graph of
In addition, as seen from the S-N diagram of
On the other hand, the stiffener 20 with a U-shaped cross section is made of a shaped steel obtained by cutting and bending a flat steel plate with a predetermined thickness t2 (e.g., 6 to 8 mm) such that the shaped steel has a suitable width with respect to the width of the steel plate 10. The aforementioned high fatigue-strength steel plate may be used for the stiffener 20.
Also, the low transformation-temperature welding material used as the weld metal for arc welding is an iron alloy of which the component composition, heat treating conditions and the like are adjusted such that the martensitic transformation start temperature Ms at which the martensitic transformation starts for transformation expansion, indicated by the solid line in the graph of
Specifically, the low transformation-temperature welding material is an iron alloy containing at least the following components: 0.20 mass % or less of C (carbon), 3.0 to 13.0 mass % of Cr (chromium) and 3.0 to 12.0 mass % of Ni (nickel), and the composition thereof is adjusted so that the amount of linear expansion per millimeter in the temperature range from the martensitic transformation start temperature to 30° C. (room temperature) may be equal to or greater than −3×10−3 mm. To prevent weld cracking, the C (carbon) content is preferably lower than or equal to 0.12 mass %, and it is also preferable that the iron alloy contain traces of Si (silicon), Mn (manganese), Mo (molybdenum), Nb (niobium), etc.
Te process of producing the steel deck 3 will now be described.
First, the aforementioned high fatigue-strength steel plate with the predetermined thickness t1 is selected as the steel plate 10, then an ordinary shaped steel having the predetermined thickness t2 and having a U-shaped cross section is selected as the stiffener 20, and the above low transformation-temperature welding material is selected as the weld metal for arc welding. At this time, the high fatigue-strength steel plate may be used as the stiffener 20 (first step).
Subsequently, the single bevel grooves 22, 22 are formed at the respective edges 21, 21 of the of stiffener 20, which are brought into contact with the steel plate 10 to form a closed-section structure, in such a manner that the single bevel grooves open outward of the closed-section structure, namely, in respective opposite directions (second step).
Specifically, in accordance with the data acquired so as to obtain a fixed dilution ratio of the weld metal through control of the penetration rate, the groove angle θ of each single bevel groove 22 of the stiffener 20 is set to 45 degrees, as shown in
Then, using the low transformation-temperature welding material selected as the weld metal in the first step, the edges 21, 21 of the stiffener 20 and the steel plate 10 are joined together by arc welding. Specifically, as shown in
The arc welding conditions are set on the basis of the data acquired so as to obtain a fixed dilution ratio of the weld metal through control of the penetration rate, with respect to the specifications of the steel plate 10 and stiffener 20. Preferably, the welding current is set to 200 to 300 A, the voltage to 30 to 35 V, the welding speed to 30 to 40 cm/minute, and the torch angle to 40 to 50 degrees. More desirably, the welding current is set to 280 A, the voltage to 32 V, the welding speed to 35 cm/minute, and the torch angle to 45 degrees.
The following explains the operation and advantages of the steel deck of the present invention, produced in the manner described above.
The weld metal deposit 30 formed by filling the weld metal in each of the single bevel grooves 22, 22 by arc welding thermally shrinks as it is cooled, as shown in
With such welding residual stress remaining in the steel plate 10 and the stiffener 20, if the steel deck 3 is repeatedly applied with bending load as vehicles run on the steel deck 3, for example, the steel plate 10 is liable to crack mostly from the non-penetrated region.
In the case of the steel deck 3 of the present invention, by contrast, the component composition and linear expansion of the weld metal are adjusted as stated above by using a low transformation-temperature welding material as the weld metal. Accordingly, as seen from
Thus, even in the case where the weld metal deposit 30 is formed by single pass (single layer) welding, instead of multi-pass (multilayer) welding, the weld metal exhibits a significantly large elongation at 30° C. (room temperature), compared with ordinary weld metals, with the result that substantially no welding residual stress remains in the steel plate 10 as well as in the stiffener 20. It is therefore possible to minimize the frequency of occurrence of cracking at the non-penetrated region of the steel plate 10 while at the same time greatly reducing the man-hour of the welding work and also improving the efficiency of the welding operation.
In the steel deck 3 of the embodiment, therefore, the residual stress of the steel plate 10 and the stiffener 20 can be reduced, thereby avoiding lowering in the assembling accuracy, tensile strength and compressive strength. Also, cracking can be suppressed to thereby enhance the fatigue strength of the steel deck.
Even if melt-through occurs while a low transformation-temperature welding material is used as the weld metal, cracking can be suppressed because a non-penetrated region does not exist. Consequently, the residual stress of the steel plate 10 and the stiffener 20 can be reduced while at the same time the fatigue strength is improved, whereby deterioration in the weld quality can be minimized. It is therefore possible to carry out the single pass welding without taking care not to cause melt-through.
Particularly, in the above embodiment, a shaped steel with a U-shaped cross section, which is easily available, is used as the stiffener 20. Thus, by using a readily available shaped steel, it is possible to enhance the fatigue strength while at the same time ensuring sufficiently high rigidity.
Further, in the above embodiment, a high fatigue-strength steel plate is used as the steel plate 10. Accordingly, even if the steel plate 10 begins to crack from the non-penetrated region, propagation of such a crack can be satisfactorily restrained because the fatigue crack propagation speed is slow, making it possible to further enhance the fatigue strength and also to reliably prevent fatigue breakdown.
In addition, in cases where a crack piercing through the steel plate or through the weld bead has been formed or is expected to be formed in the welded joint of the embodiment, the steel deck 3 can be repaired on the site of the bridge to recover or increase the fatigue strength, and it is unnecessary to remove the pavement from the bridge to repair or replace the steel deck 3, whereby the maintenance cost can be drastically cut down. During the repair, moreover, the dilution ratio of the weld metal can be controlled to the fixed ratio, making it possible to recover or enhance the fatigue strength with good reproducibility.
For evaluation purposes, a welded joint of Comparative Example 1 using an ordinary welding material (Type 1) as the weld metal for joining the steel plate 10 and the stiffener 20 together by arc welding, a welded joint of Comparative Example 2 using another ordinary welding material (Type 2) as the weld metal, and the welded joint of the steel deck 3 according to the embodiment, in which the low transformation-temperature welding material having the aforementioned composition was used as the weld metal, were individual subjected to a fatigue test using cyclic stress loading. The fatigue strengths of the individual welded joints, measured by the fatigue testing, are plotted in the S-N diagram of
The horizontal axis of the S-N diagram indicates the number of times the stress was cyclically applied in the fatigue testing, and the vertical axis indicates a stress range Δσ showing the difference between maximum and minimum stresses applied to the individual specimens during the fatigue testing.
As illustrated in the S-N diagram of
In the foregoing embodiment, a shaped steel with a U-shaped cross section is used as the stiffener 20 for the steel deck 3, in order to constitute the closed-section structure in cooperation with the steel plate 10. Alternatively, a shaped steel with a V-shaped cross section or an I-section steel may be used as the stiffener 20 for the steel deck 3.
Also, in the above embodiment, a high fatigue-strength steel plate is used as the steel plate 10, but satisfactory effects can also be achieved with an ordinary steel used as the steel plate 10.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-156454 | Jun 2007 | JP | national |
This is a U.S. National Phase Application under 35 USC §371 of International Application PCT/JP2008/060565 filed on Jun. 9, 2008. This application claims the priority of Japanese Patent Application No. 2007-156454 filed Jun. 13, 2007, the entire content of which is hereby incorporated by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/060565 | 6/9/2008 | WO | 00 | 12/14/2009 |
