Fracture toughness test piece and test method for evaluating clad steel plate bonding interface

Abstract

To provide a test piece and a test method for evaluating the fracture toughness of a joint interface between a cladding material and a base material of a clad steel plate.SOLUTION: A fracture toughness test piece for evaluation of a clad steel plate joint interface of the present invention is a fracture toughness test piece that consists of a welded part of a clad steel plate and supplementary members. In the test piece, there is a supplementary member on a surface of a cladding material that is welded through one pass with the same material as the cladding material. On a back surface side thereof, there is a supplementary member on a base material that is welded through one pass with the same material as the base material. A clad joint interface has a mechanical notch and a side groove parallel to it, and a fatigue pre-crack is introduced from the bottom of the mechanical notch along the clad interface.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a test piece and a test method suitable for evaluating the fracture toughness at the bonding interface between the clad material and the base material of a clad steel plate. (hereinafter also referred to as the "bonding interface of the clad steel plate" or simply the "bonding interface")

Background Art

[0002] A stainless steel clad steel plate is a steel plate in which a stainless steel thin plate is laminated on a base steel plate made of carbon steel. By using a stainless steel plate with excellent corrosion resistance as the clad material, it is possible to ensure equivalent corrosion resistance at a lower cost than a stainless steel thick plate with the entire thickness being a stainless steel plate. In addition, by using carbon steel as the base steel plate of the stainless steel clad steel plate, it is possible to ensure the same strength as carbon steel. By having both corrosion resistance and strength characteristics in this way, the needs are increasing in various industrial applications, mainly for the cargo tanks of chemical tankers.

[0003] On the other hand, a cargo tank is a welded structure, and stress may be transmitted in the thickness direction of the clad steel plate used as the inner hull or upper deck material due to load transmission from the surroundings and welding residual stress. When strength is required so that delamination fracture does not occur at the bonding interface between the clad material and the base material against such acting forces, a strength evaluation test targeting the bonding interface of the clad steel plate is used.

[0004] The most basic test is the shear strength test defined in JIS standard G0601 (Non-Patent Document 1). This test is very simple and easy to perform, but there are many problems pointed out, such as that it is not necessarily a shear-mode load pattern in an actual cargo tank, that an evaluation value insensitive to the interface properties is obtained, and that it is strongly affected by the deformation of the loading point (side surface of the clad material), and it is not a highly reliable evaluation method.

[0005] ​The peeling test described in the commentary on JIS standard G0601, for example, solves the problem of the loading pattern in the shear strength test described above. In this test, when appropriate test piece dimensions are used, the interfacial peeling strength against the tensile load in the plate thickness direction can be obtained. However, since shear can also occur in the backing material itself rather than at the clad of the steel plate joint interface, it is necessary to conduct tests with various dimensions in advance to find appropriate test piece dimensions.

[0006] In addition, both of the above two JIS standard G0601 tests are strength tests for pulling defect-free materials. On the other hand, in a cargo tank as a welded structure, there may be small welding defects that are overlooked as non-destructive inspection passes. Also, interfacial peeling may occur during welding construction. In order for these construction defects not to cause fatal damage during tank use, allowable defect management using fracture mechanics is useful, and thus interfacial fracture toughness is required.

[0007] As a test for evaluating the fracture toughness of the bonding interface of clad steel plates, for example, like the test by Katsuta et al., after joining a reinforcing member to each of the backing material and the base material by multi-layer welding, a fracture toughness test piece typified by a CT test piece is taken and evaluated (Non-Patent Document 2).

[0008] However, in this method, the welding residual stress distribution introduced by multi-layer welding is large, and the fatigue pre-crack length introduced before the fracture toughness test varies greatly depending on the thickness position of the CT test piece. Also, since the test piece has welding residual stress as an internal force, it is difficult to accurately calculate the fracture toughness value. Also, bonding the fatigue pre-crack aimed at the interface deviates to the base material side, bonding and in many cases, the fracture toughness of the base material part rather than the interface is obtained. As a result, bonding there is a problem that the true fracture toughness value of the interface cannot be obtained.

Prior Art Documents

Non-Patent Documents

[0009]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0010] In order to use the bonding interface between the clad material and the base material of a stainless steel clad steel sheet as a fracture toughness evaluation part, first, it is necessary to weld a supplementary member for obtaining the dimensions of a fracture toughness test piece typified by a CT test piece. When performing multi-layer build-up welding by the normal welding method of steel sheets (for example, covered arc welding, gas shielded arc welding, submerged arc welding, TIG welding, etc.) in this welding, a large welding residual stress distribution is generated around it. Therefore, residual stress that varies greatly depending on the thickness position of the test piece also occurs in the fracture toughness evaluation part (the bonding interface between the clad material and the base material), and the fatigue pre-crack length introduced before the fracture toughness test varies greatly depending on the thickness position of the test piece.

[0011] Also, since the test piece has welding residual stress as an internal force, it is difficult to accurately calculate the fracture toughness value even when trying to calculate it from the external load.

[0012] Therefore, it is necessary to join the stainless steel clad steel sheet and the supplementary member by a joining method in which the distribution of welding residual stress becomes small in the vicinity of the fracture toughness evaluation part.

[0013] Furthermore, generally, the hardness of the clad material and the base material does not completely match in a stainless steel clad steel sheet, and a hardness difference occurs in each part. In this case, the clad of the steel plateEven if a fatigue pre-crack is introduced from a mechanical notch provided at the bonding interface, the propagation of the fatigue pre-crack deviates towards the softer side. In many cases, the carbon steel side of the base material is softer than the stainless steel side of the clad material, and the fatigue pre-crack deviates into the carbon steel, resulting in the fracture toughness value of the carbon steel. Therefore, it is necessary to take measures to prevent the fatigue pre-crack from deviating from the clad of the steel plate bonding interface.

Means for Solving the Problem

[0014] The inventors investigated test pieces for using the bonding interface between the clad material and the base material of a stainless steel clad plate as a fracture toughness evaluation part. As a result, it was found that the fracture toughness of the bonding interface of the clad plate can be quantitatively evaluated by using the following test pieces.

[0015] When welding the reinforcing member to the stainless steel clad plate so that the residual stress distribution in the plate thickness direction of the fracture toughness test piece becomes as small as possible, multi-layer build-up welding is avoided and single-pass penetration welding is used.

[0016] Also, when cutting out the fracture toughness test piece from the welded joint, the thickness of the test piece is reduced in order to reduce the residual stress distribution in the thickness direction of the test piece.

[0017] Furthermore, so that the fatigue pre-crack does not deviate from the clad of the steel plate bonding interface, appropriate side grooves are provided along the clad of the steel plate bonding interface on the side surface of the test piece.

[0018] The present invention has been further studied based on the above findings, and the gist thereof is as follows.

[0019] (1) A fracture toughness test piece for evaluating the bonding interface of a clad steel plate, comprising a clad steel plate, a first supplementary member, and a second supplementary member. The clad steel plate includes a base material and a clad material. The first supplementary member is made of a material equivalent to the base material and is welded to the base material by a single-pass penetration weld. The second supplementary member is made of a material equivalent to the clad material and is welded to the clad material by a single-pass penetration weld. The bonding interface of the clad steel plate is provided with a mechanical notch and a side groove parallel to the bonding interface, and a fatigue pre-crack is introduced along the bonding interface from the bottom of the mechanical notch. of the fracture toughness test piece A fracture toughness test piece for evaluating the bonding interface of a clad steel plate, characterized in that the thickness is 10 mm or more and 15 mm or less.

[0020] (2) The fracture toughness test piece according to (1), characterized in that the single-pass penetration weld between each of the clad steel plate, the first supplementary member, and the second supplementary member is electron beam welding.

[0021] (3) The fracture toughness test piece according to (1), characterized in that the single-pass penetration weld between each of the clad steel plate, the first supplementary member, and the second supplementary member is laser welding.

[0022] (4) A method for evaluating the fracture toughness of the bonding interface of a clad steel plate having a base material and a clad material, comprising the steps of butt-welding a first supplementary member made of a material equivalent to the base material to the base material by a single-pass penetration weld, butt-welding a second supplementary member made of a material equivalent to the clad material to the clad material by a single-pass penetration weld, cutting out a CT test piece from the clad steel plate to which the first supplementary member and the second supplementary member are welded such that the center line of the mechanical notch is along the interface of the clad steel plate, bonding processing a side groove parallel to the bonding interface of the CT test piece, introducing a fatigue pre-crack along the bonding interface from the bottom of the mechanical notch, and subjecting the CT test piece to a fatigue test. A method for evaluating the fracture toughness of the bonding interface of a clad steel plate, characterized by comprising the above steps. on both side surfaces of the CT test piece A method for evaluating the fracture toughness of the bonding interface of a clad steel plate, characterized by comprising the above steps.

[0023] (5) The method for evaluating the fracture toughness of the bonding interface of the clad steel plate according to (4) above, characterized in that a single-pass through-welding between the clad steel plate, the first reinforcing member, and the second reinforcing member is performed by electron beam welding.

[0024] (6) The method for evaluating the fracture toughness of the bonding interface of the clad steel plate according to (4) above, characterized in that a single-pass through-welding between the clad steel plate, the first reinforcing member, and the second reinforcing member is performed by laser welding.

Advantages of the Invention

[0025] According to the present invention, the fracture toughness of the bonding interface of the clad steel plate can be quantitatively evaluated.

Brief Description of the Drawings

[0026]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0027] Hereinafter, with reference to the drawings, the fracture toughness test piece and test method for evaluating the bonding interface of the clad steel plate of the present invention will be described in detail.

[0028] [Welded Joint] FIG. 1 is a view showing the arrangement of the stainless steel clad steel plate 1, which is the material of the test piece in the present invention and is the object of evaluating the fracture toughness of the bonding interface, the first reinforcing member 4, and the second reinforcing member 5. The stainless steel clad steel plate 1 is composed of a cladding material 2 and a base material 3, and the fracture toughness of the bonding interface between the cladding material 2 and the base material 3 is evaluated using the fracture toughness test piece described below.

[0029] On the base material 3 side of this stainless clad steel plate 1, a first supplementary member 4 made of a material equivalent to the base material 3 and having substantially the same area (length L × width W) as the surface of the base material 3 is butted, and further, on the clad material 2 side of this stainless clad steel plate 1, a second supplementary member 5 made of a material equivalent to the clad material 2 and having substantially the same area (length L × width W) as the surface of the base material 3 is butted. One-pass welding is performed so as to penetrate each butted portion (width W) from the side surface, and a welded joint provided with a one-pass penetration weld portion 6 as shown in FIG. 2 is obtained.

[0030] Here, the equivalent material is, for example, defined within a roughly range according to the material used for the base material 3 and the clad material 2, which is specified by the same material standard in industrial standards.

[0031] The welding when welding the supplementary member to the stainless clad steel plate is made a one-pass penetration weld in order to minimize the residual stress distribution in the plate thickness direction of the fracture toughness test piece. Therefore, it is necessary to avoid multi-layer build-up welding. For one-pass penetration welding, for example, electron beam welding or laser welding can be used.

[0032] [Fracture toughness test piece] Next, a CT test piece is taken as a fracture toughness test piece for evaluating the clad steel plate bonding interface from the cutting position of the CT test piece shown in FIG. 3 of the obtained welded joint. At this time, it is cut out so that the center line of the mechanical notch is along the clad bonding of the steel plate interface. The thickness of the CT test piece is preferably 10 mm or more and 15 mm or less.

[0033] The reason for setting the thickness to preferably 10 mm or more and 15 mm or less is to reduce the residual stress distribution in the thickness direction of the test piece. However, an overly thin test piece becomes a plane stress state and a shear-type fracture mode, and fracture toughness cannot be obtained. Therefore, it is necessary to set an appropriate test piece thickness so that the opening-type fracture mode becomes dominant, and the above thickness is preferable.

[0034] As shown in Fig. 4, the CT test piece is provided with a pin hole 10 for inserting the pin of the test device. Further, side grooves 8 are machined on both side surfaces of the CT test piece. Further, from the bottom of the mechanical notch to the clad bonding of the steel plate A fatigue pre-crack 9 along the interface is introduced.

[0035] The dimensions of the side groove are, for example, an angle of 45 degrees, a bottom radius of curvature of 0.25 mm, and a depth of 1 mm. The reason for providing the site groove is to prevent the fatigue pre-crack from deviating from the clad of the steel plate Bonding interface. The side groove also serves to prevent a shear-type failure mode (shear lip) on the side surface of the test piece.

[0036] Although the details of the conditions for introducing the fatigue pre-crack are not limited, for example, conditions conforming to ISO standard 15653 are preferable.

[0037] The CT test piece thus obtained is subjected to a fatigue crack growth test, and by measuring the fracture characteristics (limiting CTOD) from the tip of the fatigue pre-crack, the fracture toughness of the bonding interface of the stainless steel clad steel plate 1 can be evaluated. The conditions of the fatigue crack growth test are not limited and may be appropriately set according to the characteristics required for the clad steel plate.

Example

[0038] Hereinafter, the present invention will be described more specifically by way of examples. The present invention is not limited to these examples.

[0039] The fracture toughness of the bonding interface of a stainless steel clad steel plate in which the filler material is SUS316L with a thickness H2 of 3 mm and the base material is NK class KA32 with a thickness H3 of 21 mm was evaluated. KA32 with a height H4 of 30 mm was prepared as the first supplementary member attached to the base material, and SUS316L with a height H5 of 35 mm was prepared as the second supplementary member attached to the filler material. They were butt-welded to the stainless steel clad steel plate respectively, and electron beam weld joints with a length L of 170 mm and a width W of 15 mm were fabricated at each welded part.

[0040] The electron beam welding was carried out downward at a vacuum degree of 0.1 torr, a voltage of 60 kV, a current of 80 Am, an object distance of approximately 300 mm, and a speed of 90 mm / min. For comparison, a multi-layer build-up by covered arc welding joining the first supplementary member and the second supplementary part was also used.

[0041] From approximately the center of the weld joint, a CT specimen was taken as a fracture toughness test specimen. At this time, the center line of the mechanical notch was bonding cut out along the interface. Also, the thickness of the CT specimen was set to 10 mm. Furthermore, on both side surfaces of the CT specimen, side grooves with an angle of 45 degrees, a bottom curvature radius of 0.25 mm, and a depth of 1 mm were machined. For comparison, specimens without side grooves were also fabricated.

[0042] The CT specimen was installed in a hydraulic servo type fatigue testing machine, and a fatigue pre-crack with a length of approximately 1.5 mm was introduced from the bottom of the mechanical notch.

[0043] The CT specimen was cooled to 0 °C, and the pins inserted into the pin holes were pulled at a displacement rate of 0.01 mm / second to obtain the fracture characteristics (limiting CTOD) from the tip of the fatigue pre-crack. Furthermore, after the test, the fracture surface and the crack cross-section were observed to confirm the evaluation site of the fracture toughness.

[0044] A list of the examples is shown in Table 1.

[0045] In No. 1 of the present invention examples, since the introduced fatigue pre-crack length was almost the same anywhere in the specimen thickness position, it was judged that a uniform stress field could be brought about. Also, the tip position of the propagated fatigue pre-crack was approximately along the of the steel plate clad of the steel plate bonding interface, and it became a pre-crack suitable for evaluating the fracture toughness of the clad

[0046] On the other hand, in Comparative Example No. 2, a large residual stress distribution occurred at the test piece thickness position due to multi-layer fillet welding, and the difference in the introduced pre-fatigue crack length varied greatly depending on the position of the test piece thickness. Therefore, the No. 2 test piece was determined to be invalid as a test piece for evaluating the fracture toughness of the bonding interface.

[0047] Furthermore, in Comparative Example No. 3, since the side groove was not introduced, the introduced pre-fatigue crack of the steel plate departed gradually from the clad of the steel plate bonding interface toward the base material side as the fatigue crack growth test progressed, and the position of the tip of the final pre-fatigue crack was about 0.5 mm away from the clad

[0048] [Table 1]

[0049] As described above, the embodiments of the present invention have been explained. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiments, and it is possible to appropriately modify and implement the above-described embodiments without departing from the gist thereof.

Industrial Applicability

[0050] By using the test piece and test method of the present invention, the fracture toughness of the bonding interface of the clad steel plate can be quantitatively evaluated, and it can be utilized for the development of clad steel plates with excellent interfacial fracture toughness, the interfacial defect management of welded structures using clad steel plates, and fracture prevention. Therefore, the industrial effect is extremely remarkable.

Explanation of Reference Numerals

[0051] 1: Stainless clad steel plate 2: Lining material 3: Base material 4: First supplementary member (material equivalent to the base material) 5: Second supplementary member (equivalent material to mating material) L: Length W: Width H2: Thickness of mating material H3: Thickness of base material H4: Height of the first supplementary member 4 H5: Height of the second supplementary member 5 6: Welding line 7: Cutting position of CT test piece 8: Side groove 9: Fatigue pre-crack 10: Pin hole

Claims

1. A fracture toughness test piece for evaluating the bonding interface of a clad steel plate, comprising: a clad steel plate, a first reinforcing member, and a second reinforcing member; the clad steel plate includes a base material and a cladding material; the first reinforcing member is made of a material equivalent to the base material and is welded to the base material by one-pass through welding; the second reinforcing member is made of a material equivalent to the cladding material and is welded to the cladding material by one-pass through welding; the bonding interface of the clad steel plate is provided with a mechanical notch and a side groove parallel to the bonding interface; a fatigue pre-crack is introduced along the bonding interface from the bottom of the mechanical notch; the thickness of the fracture toughness test piece is 10 mm or more and 15 mm or less. A fracture toughness test piece for evaluating the bonding interface of a clad steel plate, characterized by the above.

2. The fracture toughness test piece according to claim 1, wherein the one-pass through welding between the clad steel plate and each of the first reinforcing member and the second reinforcing member is electron beam welding.

3. The fracture toughness test piece according to claim 1, wherein the one-pass through welding between the clad steel plate and each of the first reinforcing member and the second reinforcing member is laser welding.

4. A method for evaluating the fracture toughness of the bonding interface of a clad steel plate having a base material and a cladding material, comprising: a step of butt-welding a first reinforcing member made of a material equivalent to the base material to the base material by one-pass through welding; a step of butt-welding a second reinforcing member made of a material equivalent to the cladding material to the cladding material by one-pass through welding; a step of cutting out a CT test piece from the clad steel plate to which the first reinforcing member and the second reinforcing member are welded, such that the center line of the mechanical notch is along the bonding interface of the clad steel plate; a step of processing side grooves on both side surfaces of the CT test piece parallel to the bonding interface of the CT test piece; a step of introducing a fatigue pre-crack along the bonding interface from the bottom of the mechanical notch; a step of subjecting the CT test piece to a fatigue test. A method for evaluating the fracture toughness of the bonding interface of a clad steel plate, characterized by comprising the above steps.

5. The method for evaluating the fracture toughness of the bonding interface of a clad steel plate according to claim 4, wherein the one-pass through welding between the clad steel plate and each of the first reinforcing member and the second reinforcing member is performed by electron beam welding.

6. The method for evaluating the fracture toughness of the bonding interface of the clad steel plate according to claim 4, wherein one-pass through welding between the clad steel plate and each of the first reinforcing member and the second reinforcing member is performed by laser welding.

Images