JP7705111B2 - Fracture toughness test specimen and test method for evaluating clad steel plate bonded interface - Google Patents

Description

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

Stainless clad steel plate is a steel plate made by laminating a thin stainless steel plate to a base steel plate made of carbon steel. By using a stainless steel plate with excellent corrosion resistance as the laminating material, it is possible to ensure the same corrosion resistance at a lower cost than a thick stainless steel plate, whose entire thickness is made of stainless steel. In addition, by using carbon steel as the base steel plate of the stainless clad steel plate, it is possible to ensure the same strength as carbon steel. As it combines both corrosion resistance and strength properties in this way, there is a growing demand for it in various industrial applications, particularly in cargo tanks for chemical tankers.

On the other hand, cargo tanks are welded structures, and stress can be transmitted in the thickness direction of the clad steel plates used as the inner hull and upper deck materials due to load transmission from the surroundings and residual welding stress. When strength is required to withstand such forces and prevent delamination failure at the bonding interface between the clad steel plate and the base material, strength evaluation tests are used that target the bonding interface of the clad steel plate.

The most basic test is the shear strength test specified in JIS standard G0601 (Non-Patent Document 1). This test is very simple and easy to carry out, but many problems have been pointed out, such as the fact that the loading pattern in an actual cargo tank is not necessarily in shear mode, the evaluation value obtained is insensitive to the interface properties, and it is strongly affected by the deformation of the loading point (side of the welded material), and it is not a highly reliable evaluation method.

The above problem of the loading method of the shear strength test has been solved by, for example, the peel test described in the commentary of JIS standard G0601. In this test, if appropriate test piece dimensions are used, the interfacial peel strength against a tensile load in the plate thickness direction can be obtained. However, since shear may occur not only at the joint interface of the clad steel plate but also in the laminated material itself, it is necessary to conduct tests with various dimensions in advance to find the appropriate test piece dimensions.

The two JIS G0601 tests mentioned above are both strength tests in which defect-free material is pulled. However, cargo tanks, which are welded structures, can have small weld defects that are overlooked as passing non-destructive testing. Also, interfacial peeling may occur during welding construction. To prevent these construction defects from causing fatal damage when the tank is in use, it is useful to manage allowable defects using fracture mechanics, which is where interfacial fracture toughness comes in.

One test to evaluate the fracture toughness of the joint interface of clad steel plates is, for example, the test by Katsuta et al., in which a supplementary member is joined to each of the clad material and the base material by multi-layer welding, and then a fracture toughness test piece, such as a CT test piece, is taken and evaluated (Non-Patent Document 2).

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

Japanese Industrial Standards, JIS G0601:2012 "Test methods for clad steel" Junichi Katsuta et al., "Fracture toughness and fatigue crack propagation characteristics of bonded interface of duplex stainless clad steel plate for chemical tankers," Pressure Technology, Vol. 57, No. 1, 2019, pp. 4-12

In order to use the interface between the laminated material and the base material of stainless clad steel plate as the fracture toughness evaluation area, it is first necessary to weld a supplementary member to obtain the dimensions of the fracture toughness test piece, such as a CT test piece. When multi-layer welding is performed using a normal welding method for steel plates (e.g., shielded arc welding, gas shielded arc welding, submerged arc welding, TIG welding, etc.), a large welding residual stress distribution occurs around it, and residual stress that varies greatly depending on the thickness position of the test piece is also generated in the fracture toughness evaluation area (the interface between the laminated material and the base material), and the length of the fatigue pre-crack introduced before the fracture toughness test varies greatly depending on the thickness position of the test piece.

In addition, since the test specimen has welding residual stress as an internal force, it is difficult to accurately calculate the fracture toughness value from the external load.

Therefore, it is necessary to join the stainless clad steel plate and the supplementary member using a joining method that reduces the distribution of welding residual stress near the fracture toughness evaluation area.

Furthermore, in stainless clad steel plates, the hardness of the clad steel and the base material do not generally match completely, and hardness differences occur at each location. In this case, even if a fatigue pre-crack is introduced from a mechanical notch at the bonding interface of the clad steel plate , the fatigue pre-crack will deviate toward the softer side. In many cases, the carbon steel side of the base material is softer than the stainless steel side of the clad steel plate, and the fatigue pre-crack deviates toward the carbon steel, resulting in the fracture toughness value of the carbon steel. Therefore, measures must be taken to prevent the fatigue pre-crack from deviating from the bonding interface of the clad steel plate .

The inventors investigated test pieces for evaluating the fracture toughness of the bonded interface between the base material and the laminated material of stainless clad steel plate. As a result, they found that the fracture toughness of the bonded interface of clad steel plate can be quantitatively evaluated by using the following test piece.

When welding supplementary components to stainless clad steel plates, multi-layer welding should be avoided and a single pass full penetration weld should be used, so that the residual stress distribution in the thickness direction of the fracture toughness test specimen is minimized.

In addition, when cutting out a 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.

Furthermore, in order to prevent fatigue pre-cracks from diverting from the bonding interface of the clad steel plate , an appropriate side groove is provided on the side surface of the test piece along the bonding interface of the clad steel plate .

The present invention was made based on the above findings and through further investigation, and its gist is as follows:

(1) A fracture toughness test piece for evaluating a 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 comprising a base material and a laminated material, the first supplementary member being made of a material equivalent to the base material and welded to the base material by one-pass full-penetration welding, the second supplementary member being made of a material equivalent to the laminated material and welded to the laminated material by one-pass full-penetration welding, the bonding interface of the clad steel plate being provided with a mechanical notch and a side groove parallel to the bonding interface, a fatigue pre-crack being introduced from the bottom of the mechanical notch along the bonding interface, and the thickness of the fracture toughness test piece being 10 mm or more and 15 mm or less.

(2) The fracture toughness test piece of (1), characterized in that the one-pass full-penetration welding between the clad steel plate and each of the first and second supplementary members is electron beam welding.

(3) The fracture toughness test piece of (1), characterized in that the one-pass penetration weld between the clad steel plate and each of the first and second supplementary members is laser welding.

(4) A method for evaluating the fracture toughness of a joint interface of a clad steel plate having a base material and a laminated 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 one-pass full-thrust welding; butt-welding a second supplementary member made of a material equivalent to the laminated material to the laminated material by one-pass full-thrust welding; 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 a mechanical notch is along the joint interface of the clad steel plate; machining side grooves on both side surfaces of the CT test piece parallel to the joint interface of the CT test piece; introducing a fatigue pre-crack along the joint interface from the bottom of the mechanical notch; and subjecting the CT test piece to a fatigue test.

(5) A method for evaluating the fracture toughness of the joint interface of the clad steel plate according to (4), characterized in that one pass of full penetration welding between the clad steel plate and each of the first and second supplementary members is performed by electron beam welding.

(6) A method for evaluating the fracture toughness of the joint interface of the clad steel plate according to (4), characterized in that one-pass penetration welding between the clad steel plate and each of the first and second supplementary members is performed by laser welding.

The present invention makes it possible to quantitatively evaluate the fracture toughness of the bonded interface of clad steel plates.

FIG. 1 is a diagram showing the arrangement of the test piece material (steel piece) of the present invention. FIG. 2 is a diagram showing the location of a one-pass full penetration weld in the steel piece of FIG. FIG. 3 is a diagram showing the position from which a test piece according to the present invention (CT test piece) is cut out from the welded joint of FIG. FIG. 4 is a diagram showing a test piece according to the present invention (CT test piece).

The fracture toughness test specimen and test method for evaluating the clad steel plate joint interface of the present invention will be described in detail below with reference to the drawings.

[Welded joints]
Fig. 1 is a diagram showing the arrangement of a stainless clad steel plate 1, which is the material of the test piece in the present invention and is the subject of evaluation of the fracture toughness of the joint interface, and a first supplementary member 4 and a second supplementary member 5. The stainless clad steel plate 1 is composed of a laminated material 2 and a base material 3, and the fracture toughness of the joint interface between the laminated material 2 and the base material 3 is evaluated using a fracture toughness test piece described later.

A first supplementary member 4 made of the same material as the base material 3 and having approximately the same area (length L x width W) as the surface of the base material 3 is butted against the base material 3 side of the stainless clad steel plate 1, and a second supplementary member 5 made of the same material as the base material 2 and having approximately the same area (length L x width W) as the surface of the base material 3 is butted against the mating material 2 side of the stainless clad steel plate 1. One pass welding is performed from the side to penetrate each butt joint (width W), resulting in a welded joint with a one-pass full-penetration weld 6 as shown in Figure 2.

Here, equivalent materials are those that are stipulated in the same material standard, for example, in industrial standards, and the approximate range can be determined according to the materials used for the base material 3 and the laminated material 2.

When welding supplementary components to stainless clad steel plate, the welding is performed with a single pass full penetration welding in order to minimize the residual stress distribution in the plate thickness direction of the fracture toughness test specimen. For this reason, multi-layer welding must be avoided. For example, electron beam welding or laser welding can be used for the single pass full penetration welding.

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

The thickness is preferably 10 mm or more and 15 mm or less in order to reduce the residual stress distribution in the thickness direction of the test piece. However, an excessively thin test piece will be in a plane stress state, resulting in a shear-type fracture mode, and no fracture toughness can be obtained. Therefore, it is necessary to select an appropriate test piece thickness so that the opening-type fracture mode becomes dominant, and the above thickness is preferable.

As shown in FIG. 4, the CT test piece has a pin hole 10 for inserting the pin of a test device, and further, side grooves 8 are machined on both sides of the CT test piece. Furthermore, a fatigue pre-crack 9 is introduced from the bottom of the machine notch along the bonding interface of the clad steel plate .

The dimensions of the side groove are, for example, an angle of 45 degrees, a radius of curvature at the bottom of 0.25 mm, and a depth of 1 mm. The site groove is provided to prevent the fatigue pre-crack from diverging from the bonding interface of the clad steel plate . The side groove also serves to prevent the shear type fracture mode (shear crack) from occurring on the side of the test piece.

The details of the conditions for introducing fatigue pre-cracks are not limited, but conditions that comply with ISO standard 15653, for example, are preferable.

The CT test specimen obtained in this manner is subjected to a fatigue crack propagation test, and the fracture characteristics from the tip of the fatigue pre-crack (critical CTOD) are measured, thereby making it possible to evaluate the fracture toughness of the bonding interface of the stainless clad steel plate 1. The conditions of the fatigue crack propagation test are not limited, and may be set appropriately depending on the properties required of the clad steel plate.

The present invention will be described in more detail below with reference to examples. The present invention is not limited to these examples.

The fracture toughness of the joint interface was evaluated for a stainless clad steel plate with a thickness H2 of 3 mm and a base material of NK ship classification standard KA32 with a thickness H3 of 21 mm. KA32 with a height H4 of 30 mm was prepared as the first supplementary member to be attached to the base material, and SUS316L with a height H5 of 35 mm was prepared as the second supplementary member to be attached to the combined material. Each was butt-welded to the stainless clad steel plate to produce an electron beam welded joint with a length L of 170 mm and a width W of 15 mm for each weld.

Electron beam welding was performed downward at a vacuum 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 first supplementary member and a second supplementary portion were joined by multi-layer welding using shielded metal arc welding.

A CT test piece was taken from approximately the center of the welded joint as a fracture toughness test piece. At this time, the center line of the machine notch was cut out along the joint interface. The thickness of the CT test piece was set to 10 mm. Furthermore, side grooves with an angle of 45 degrees, a bottom curvature radius of 0.25 mm, and a depth of 1 mm were machined on both sides of the CT test piece. For comparison, a specimen without side grooves was also made.

The CT test specimen was placed in a hydraulic servo type fatigue testing machine, and a fatigue pre-crack approximately 1.5 mm long was introduced from the bottom of the mechanical notch.

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

A list of examples is shown in Table 1.

In the present invention example No. 1, the length of the introduced fatigue pre-crack was almost the same anywhere in the thickness of the test piece, so it was judged that a uniform stress field was created. In addition, the tip position of the propagated fatigue pre-crack was roughly along the bonding interface of the clad steel plate , so it became a pre-crack suitable for evaluating the fracture toughness of the bonding interface of the clad steel plate . Therefore, the test piece No. 1 was judged to be effective as a test piece for evaluating the fracture toughness of the bonding interface.

On the other hand, in the comparative example, No. 2, a large residual stress distribution occurred at the thickness of the test piece due to the multi-layer welding, and the difference in the length of the introduced fatigue pre-cracks varied greatly depending on the position in the thickness of the test piece. Therefore, the No. 2 test piece was judged to be invalid as a test piece for evaluating the fracture toughness of the joint interface.

Furthermore, in the comparative example No. 3, since no side groove was introduced, the introduced fatigue pre-crack gradually deviated from the bonding interface of the clad steel plate toward the base material as the fatigue crack propagation test progressed, and the final position of the tip of the fatigue pre-crack was about 0.5 mm away from the bonding interface of the clad steel plate toward the base material. Therefore, the obtained fracture toughness was that of the base material, and the test piece was judged to be invalid as a test piece for evaluating the fracture toughness of the bonding interface.

Although the embodiments of the present invention have been described above, the above-mentioned embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-mentioned embodiments, and it is possible to implement the above-mentioned embodiments by appropriately modifying them as long as they do not deviate from the spirit of the present invention.

The test specimen and test method of the present invention can be used to quantitatively evaluate the fracture toughness of the bonding interface of clad steel plates, and can be used to develop clad steel plates with excellent interfacial fracture toughness, as well as to manage interface defects and prevent fracture in welded structures that use clad steel plates, resulting in extremely significant industrial benefits.

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

Claims (6)

A fracture toughness test piece for evaluating a bonding interface of a clad steel plate,
The present invention provides 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 one pass of penetration welding;
The second supplemental member is made of the same material as the mating material and is welded to the mating material by one pass of penetration welding;
The clad steel plate has a mechanical notch and a side groove parallel to the joint interface,
A fatigue pre-crack is introduced from the bottom of the mechanical notch along the bonding interface;
A fracture toughness test piece for evaluating a clad steel plate bonding interface, characterized in that the thickness of the fracture toughness test piece is 10 mm or more and 15 mm or less. The fracture toughness test piece according to claim 1, characterized in that the one-pass penetration welds between the clad steel plate and each of the first and second supplementary members are electron beam welds. The fracture toughness test piece according to claim 1, characterized in that the one-pass penetration welds between the clad steel plate and each of the first and second supplementary members are laser welds. A method for evaluating fracture toughness of a joint interface of a clad steel plate having a base material and a laminated material, comprising:
a step of butt-welding a first supplementary member made of a material equivalent to the base material to the base material by a single pass full penetration weld;
butt-welding a second supplemental member made of the same material to the composite material by a single pass full penetration weld to the composite material;
A step of 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 a center line of a mechanical notch is along a joint interface of the clad steel plate;
A step of machining side grooves on both side surfaces of the CT test piece parallel to the bonding interface of the CT test piece ;
introducing a fatigue pre-crack along the bonded interface from the bottom of the mechanical notch;
A method for evaluating the fracture toughness of a bonding interface of a clad steel plate, comprising a step of subjecting the CT test piece to a fatigue test. The method for evaluating the fracture toughness of the joint interface of a clad steel plate according to claim 4, characterized in that one pass of full penetration welding between the clad steel plate and each of the first and second supplementary members is performed by electron beam welding. The method for evaluating the fracture toughness of the joint interface of a clad steel plate according to claim 4, characterized in that one-pass penetration welding between the clad steel plate and each of the first and second supplementary members is performed by laser welding.

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