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ISSN : 1598-6721(Print)
ISSN : 2288-0771(Online)
The Korean Society of Manufacturing Process Engineers Vol.18 No.5 pp.9-16
DOI : https://doi.org/10.14775/ksmpe.2019.18.5.009

Study of Brittle Crack Propagation Welding for EH40 Steel Plate in Shipbuilding Steel

Kyung-Shin Choi*, Sang-Hoon Lee*, Won-Jee Chung**#, Hui-Geon Hwang**, Seok-Han Hong***, Ji-Ung Hong***
*Lloyds Register
**Changwon National University
***HYUNDAI SAMHO HEAVY INDUSTRIES CO.,LTD.
Corresponding Author : wjchung@chanwon.ac.kr Tel: +82-55-213-3624, Fax: +82-55-263-5221
11/02/2019 07/03/2019 28/03/2019

Abstract


Recent economic trends are worsening and becoming longer, and Korean shipbuilding is focused on high value added and high technology, especially for LNG carriers and large container ships. Both ship types increased in size in the 2010s but have requirements such as high strength, toughness at low temperatures and continuous weldability for preventing brittle fractures at service temperatures. In particular, as container ships become larger, the International Classification Society (IACS) has established a provision (IACS UR S33) that mandates the use of BCA (Brittle Crack Arrest) certified vessels for large container vessels contracted after 2014 to ensure safety. Therefore, studies on BCA 47Y.P are currently being undertaken, but BCA 40Y.P has not been actively studied yet. We will test BCA 40Y.P to verify why it can be applied to a large container ship and measure fatigue cracking.



조선용 EH40 강판의 용접부 취성 균열전파정지에 관한 연구

최 경신*, 이 상훈*, 정 원지**#, 황 희건**, 홍 석한***, 홍 지웅***
*로이드선급
**창원대학교
***현대삼호중공업

초록


    © The Korean Society of Manufacturing Process Engineers. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    In recent years, the number of container ships owner by shipping companies has decreased somewhat due t o a sudden recession. However, with the current revit alization of the economy of the shipping market, orde ring competition for mega container ships is increasin g again. The enlargement of container ships is advant ageous for ship owners in terms of transportation effi ciency and strategic dimensions. Since a decade ago, with the application of ultra-thick plates to the upper deck and hatch coaming parts of ships following the enlargement of 10,000-TEU or larger container ships, many studies have been conducted on 355MPa-grade steel with a thickness of 70mm or more. Furthermore, with the construction of 20,000-TEU or larger contain er ships with high-strength, ultra-thick plates, 460MPa -grade YP47 steel plate is also being researched activ ely. For large container ships, as the steel applied to ships is becoming thicker, when cracks occur in the main steel of the hull girder strength, they do not sto p but continue to progress. To address this problem, t he ClassNK added new related regulations in 2007, w hich stimulated research on the fracture safety of ultra -thick plates[1-3]. The research on crack arrest started i n 1953 when Robertson suggested the concept of crac k arrest temperature[4].

    Due to the nature of container ships, they have shar p stem and stern for fast speed, which cause insuffici ent reserve buoyancy and significant exposure in the hogging state. As a result, they are subject to high te nsion on the deck top and the deck’s large opening f or cargo loading, which affects the torsion of the enti re ship and forms a torsion box on the upper deck. Furthermore, the longitudinal members have become t hicker to compensate for the insufficient hull girder st rength. According to recent brittle crack-stopping char acteristics from the viewpoint of crack arrest for ultra -thick plates, as the steel thickness becomes larger tha n a certain value, the brittle cracks in the weld zones progress to the base metal and stop. However, for ult ra-thick plates, the brittle cracks that start from the w eld zone do not stop after progressing to the base me tal but continue to progress straight along the weld li ne, causing serious safety problems. Therefore, to pro mote safety, the International Association of Classifica tion Societies has prepared a regulation that obligates the use of BCA (Brittle Crack Arrest)-certified steel f or large container ships that have contracted since 20 14 (IACS UR S33)[5].

    To examine research trends related to BCA, an ana lysis of specimens collected from 65mm or thicker welded steels revealed that the existence of stiffener designed to prevent brittle crack propagation in the weld zone could not prevent brittle crack propagatio n through the entire specimen[6]. Recently, studies ha ve been conducted to address the productivity degra dation problem of welding by applying electro-gas (EGW) welding instead of flux-cored arc welding (F CAW), which is frequently used to secure the struct ural safety of the hull, considering the welding proc ess as well as the effect of the thickness in the eva luation of fracture toughness.

    Several classification societies have found that the propagation path of brittle cracks that cause unstable fractures in ultra-thick plates for ships is considerabl y affected by welding residual stress as well. The e ngineering methods of domestic shipbuilding mostly adopt straight block butt joints instead of the cascade d type, which makes it more difficult to arrest brittle cracks. Consequently, the arrest of brittle crack propa gation has been designed using arrest holes, arrest we lding, and arrest inserts, and it has been reported that unstable fractures from the arrest of brittle crack prop agation can be prevented based on research on special designs[7]. Ahn et al. conducted a study to achieve sa fety against unstable fractures by inducing cracks in BCA steel with excellent brittle crack arrest capacity regardless of the welding process when brittle cracks occur in the top structure of mega container ships[8].

    In this study, a crack-tip opening displacement (CT OD) test was performed, which is a brittle fracture te st, in addition to the conventional ESSO test for a 1 5,100-TEU large container ship that is under construct ion. In addition, a welding process that combines SA W with FCAW, which has been pointed out as a cau se of welding productivity degradation, was applied to improve the low-impact strength after thermal treatme nt and prevent fractures due to the propagation of bri ttle cracks, thus achieving the safety of the welded st ructure.

    2. Application of EH40 Certified Steel and Experimental Method

    2.1 Application of steel

    For the steels in this study, YP40 Steel Plate, which is an ultra-thick steel for ships, and 80-mm-thick TMCP (Thermo Mechanical Control Process) of 390 MPa class, which is a high-quality high-strength steel for ships and offshore structures, were used. They were used at locations where deformations are caused due to major torsional stress in the upper deck and hatch coaming part of the 0.4-L section of the ship. Tables 1 and 2 show the chemical composition and mechanical characteristics of the certified steel.

    Fig. 1 graphically shows application cases by size and thickness of large container ships. As shown in this graph, application cases are very rare for 15,000-TEU or larger ships. For mega container ships, the rule requirement of the classification societies has been satisfied using YP47, but this greatly increased the weight and cost of the ship due to over-scantling in some cases.

    With the progress of research on eco-friendly ships, an optimized design is being pursued by reducing the weight and fuel of the ships. In particular, the evaluation of YP40 is urgently needed, to which the classification rule for the evaluation of brittle crack arrest characteristics has not been applied.

    2.2 Application of welding process and evaluation of unstable fracture safety

    This study developed a welding process that can achieve the structural safety of ships against unstable fractures of ultra-thick steel plates. The developed method can arrest brittle cracks by combining FCAW and submerged arc welding (SAW) when cracks are propagated from the weld zone along the weld line while solving the productivity degradation problem of welding, which is a disadvantage of conventional FCAW. Fig. 2 shows the welding process, which combines FCAW and SAW.

    Furthermore, in addition to the ESSO test, which has been generally used in brittle fracture toughness tests, the CTOD test was performed, which can measure the behaviors of the displacement of the crack surface, which is perpendicular to the original crack plane at the crack tip. Even if the weld zone of ultra-thick plates has passed nondestructive tests, such as radiographic, magnetic, and penetrant tests, they still have ultrafine weld defects, and the behaviors of structures when these defects are subjected to a load for a certain time can also be examined. The test conditions and mechanical properties of the test in the CTOD test are outlined in Table 3. Three specimens were fabricated to apply different conditions for the ratio of the mathematical function Y to verify the reproducibility in each test.

    2.3 Reverse bending test

    The general standards for the CTOD test are British Standard 7448 Part 1: 1991 and Part 2: 1997. Part 2 presents three test methods for preliminary fatigue crack morphologies: local compression, reverse bending, and stepwise high-R ratio. They give the maximum deviation allowance within 20% of the average value for preliminary fatigue crack morphologies. For residual stress deformations, local compression has been frequently used until recently as it is considered efficient for improving the preliminary crack morphologies of the weld zone. However, it is difficult to apply due to a rapid increase in the load resulting from the increased strength and thickness of materials[11].

    The reverse bending method of BS 7448 Part 2: 1997 applies a bending load to the entire specimen through bending work after quenching the specimen and induces local plastic deformation at the notch tip. Thus, the load size is small and the procedure is simple. The coefficient of reverse bending stress intensity is determined with Eq. (1) using the maximum bending load by generating plastic deformation and a certain tensile residual stress at the notch root. In this equation, L denotes the notch constraint of a general rectangular specimen and ωrb denotes the plastic deformation area size as a result of reverse bending.

    Krb = LR​ p 0.2 8 ω r b π
    (1)

    When the size of the applied load is determined by calculating the coefficient of reverse stress intensity, the material toughness measured by the stress intensity factor must not exceed a certain value. Furthermore, the properties related to the fracture toughness must be determined experimentally. Thus, it is difficult to apply the reference load before the test. Therefore, the load at the time when macroscopic specimen damage occurs was defined as the reference load and the applied load was determined by applying the ratio derived from it.

    3. Results and Discussion

    3.1 Fatigue pre-cracking length and original crack length

    The fatigue pre-cracking length was determined by measuring the actual fatigue crack length of one specimen in accordance with BS 7448 Part 1: 1991. It was derived from Eq. (2), and the actual crack length a0 was determined by adding the fatigue crack length and the machined notch length, as shown in Eq. (3).

    a f = a f 0 + a f 8 * 0.5 + a f 1 + a f 2 + a f 3 + a f 4 + a f 5 + a f 6 + a f 7 8
    (2)

    a 0 = M + a f
    (3)

    As shown in Fig. 3, the measurements must be performed at nine points with equal intervals positioned at 1% of the thickness from the surface between the two external points, af0 and af8. For the seven points excluding these two points, that is, between af1 and af7, the points with equal intervals must be positioned on the inside of the surface.

    Furthermore, the 0.2% resistance temperature for the fracture test can be calculated by the following Eq. (4):

    σ Y = σ Y 0 + 10 5 491 + 1.8 * T 189
    (4)

    where T is the experimental temperature, σY0 is the fatigue crack temperature, and σY is the fracture test temperature. All these temperatures mean 0.2% resistance temperatures.

    Table 4 shows the fatigue pre-cracking results of the three specimens. The frequency is 10 times per second, that is, 10 Hz; the applied load in the final step of the fatigue crack extension is 5 kN, which is the maximum applied load for appropriate plastic deformation within a range that does not damage the material features; and the stress ratio is 10:1. Furthermore, the three specimens showed similar values for the maximum fatigue stress intensity factor applied in the fatigue crack stress step. The above-mentioned strength factor appeared in between 29,000 and 30,800 cycles on average. The ratios of the strength factor to the modulus of elasticity were also similar among the three specimens.

    The average of the actual crack length was divided by the average of the length, and the differences between the maximum and minimum crack lengths are shown in Table 5. As shown in Figs. 4 and 5, none of the specimens had differences; thus, it can be seen that the reproducibility of the experiment was satisfied. Fig. 6

    3.2 Crack tip opening displacement test

    To cool down the specimens, they were placed in a mixture of liquid nitrogen, alcohol, and ice, and the experiment was performed after 40 min at the temperature of -10 ± 2℃. The temperature holding time was 30 s/mm, which was measured at around 2mm below the crack tips on both sides of the specimen. Fig. 5 shows the measuring instrument and tester used. The three-point bending test used the 1MN universal testing machine. The force and opening displacement were measured with voltageand strain-measuring instruments. The experiment setup and measuring instruments are shown in Fig. 7.

    BS 7448: Part 1: 1991 shows a correlation graph between the force and notch opening displacement. The calculation formula for the bending specimen is shown in Eq. (5).

    K Q = F Q S B W 1.5 * f a 0 W
    (5)

    The above equation can be converted to Eq. (6) using the stress intensity factor (KC) and the correct factor (Y (ξ)) .

    δ c = K C 2 * 1 υ 2 2 σ Y E + 0.4 W a 0 * V p 0.4 W a 0 + a 0 + Z
    (6)

    The stress intensity factor (KC) can be calculated by Eq. (7) using the maximum force pmax, strain coefficient (Y (ξ)) , span, thickness, and width, and the correction factor can be expressed as Eq. (8):

    K C = P max * S * Y ξ B * W 1.5
    (7)

    Y ξ = 3 ξ 0.5 [ 1.99 ξ ( 1 ξ ) ( 2.15 3.98 ξ + 2.7 ξ 2 ) ] 2 ( 1 + 2 ξ ) ( 1 ξ ) 1.5
    (8)

    where δc is the CTOD (mm), E=Young's modulus is 206 GPa, υ is Poisson's ratio (0.3), a0 is the original crack length (mm), the span is 4W(mm), Z is the knife edge thickness (mm), and, Y (ξ) is the correction factor (N).

    Figs. 810 show the graphs for the correlation between the clip gauge opening displacement and force. Fig. 9

    The initial elastic zone is directly set by a person. In the graph, the y axis represents the force (kN) and the x axis represents the clip gauge opening displacement (mm), which means the (VP) of the notch opening elasticity. As shown in 77-SAW-1, the line that starts from the (0, 0) point is an elastic section. The gauge opening displacement section (VP) is 3.41 mm when the maximum load is 294.4 kN, and the opening displacement is 1.03 mm. This means that the material has more of its innate characteristic than the maximum 0.015 in the CTOD acceptance value required by API-PR 2Z.

    Table 6 outlines the opening displacement results of the three specimens.

    4. Conclusion

    This study applied a welding process combining FCAW and SAW and satisfies the classification rules of EH40 BCA (Brittle Crack Arrest) steel for large container ships. The experiment results confirm the improved productivity of welding that can arrest the propagation of brittle cracks and ensure the hull structural safety of large container ships against brittle fatigue fractures.

    Figure

    KSMPE-18-5-9_F1.gif
    Growth in containership size
    KSMPE-18-5-9_F2.gif
    Welding sequence of FCAW+SAW
    KSMPE-18-5-9_F3.gif
    Location of measurement point for specimen
    KSMPE-18-5-9_F4.gif
    Fracture surface of 77t-SAW-1
    KSMPE-18-5-9_F5.gif
    Fracture surface of 77t-SAW-2
    KSMPE-18-5-9_F6.gif
    Fracture surface of 77t-SAW-3
    KSMPE-18-5-9_F7.gif
    Testing machine and measuring Instrument for CTOD test
    KSMPE-18-5-9_F8.gif
    Force and clip gauge opening of 77t-SAW-1
    KSMPE-18-5-9_F9.gif
    Force and clip gauge opening of 77t-SAW-2
    KSMPE-18-5-9_F10.gif
    Force and clip gauge opening of 77t-SAW-3

    Table

    Chemical composition of EH40 steel of used
    Mechanical properties of base meter for EH40 steel of used
    Test condition and mechanical properties
    Test results of fatigue pre-cracking parameters
    Test results of original crack length
    Test results of CTOD

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