Analysis of Fracture Delamination Defects of Ship Plate Tensile Specimen

Abstract : Through the low-magnification and high-magnification inspection of the tensile fracture delamination cracking specimen, the cause of delamination cracking was analyzed, and the interaction between delamination cracking, steel plate thickness and mechanical properties was statistically analyzed . The results show that delamination cracking is the result of three-dimensional stress and has no significant effect on the tensile and impact properties of the knotted steel plate
Keywords : delamination cracking ; tensile fracture ; ship plate

In recent years , during the factory inspection of ship plates , delamination and cracking of tensile specimen fractures often occur . The ship inspector judges this as delamination , and the whole steel plate with delamination batch number needs to be UT flaw detected or rejected .
1 Metallographic analysis of tensile fracture delamination samples

The layered part ) presents deeply corroded dark spots and concentrated voids , indicating that the heart tissue is different from other normal tissues , and there is serious central looseness and central segregation , and no layered defects are seen . The typical tensile fracture morphology with delamination phenomenon is shown in Fig. 1 . The fracture is a pure ductile fracture , and a crack parallel to the rolling surface appears in the center of the sample, with a depth of about 3-5 mm , and plastic deformation occurs at the edges on both sides of the crack .
A low-magnification sample is made from the tensile sample after fracture ( avoiding the deformation area as much as possible ) . The length of the sample is the length direction of the tensile sample , and the width is the thickness of the plate . Heat to 65-80 °C with 50% industrial hydrochloric acid , etch for 20 min , rinse with clean water and blow dry . In the central part of the acid leaching sample ( that is, at the fracture

1-2 high magnification analysis
Transverse and longitudinal metallographic specimens are intercepted at the center of the plate thickness with tensile specimens after fracture (avoiding the deformed area as much as possible). After grinding and polishing, it was etched with 4% nitric acid ethanol solution , and the inclusions and microstructure were observed with a metallographic microscope. The content of inclusions in the steel is very low, the metallographic structure of the sample is ferrite + pearlite, the grains are uniform and fine, and there is a small amount of bainite in the center of the plate thickness. Severe banding at the center of plate thickness.
2 Statistical analysis of tensile fracture delamination samples
First of all , the statistics of the samples detected in a period of time were carried out to find the frequency and distribution of the delamination phenomenon in the tensile fracture .
According to the statistics of one year's data, the ratio of tensile fracture delamination of different grades of ship plating is shown in Figure 5. The ratio of tensile fracture delamination of A grade ship plating is only 1.3% . Delamination rarely occurs, mainly in 32 kg and 36 kg high-strength ship plates. Among the high-strength ship plates, the AH36 steel grade with a large amount of inspection is selected for further statistical analysis according to the inspection specifications. There is no tensile fracture delamination in the steel plate below 12 mm thickness, and the proportion of delamination phenomenon increases with the increase of steel plate thickness in the steel plate thicker than 12 mm .

Influence of Tensile Delamination Phenomenon on Mechanical Properties of AH36 Steel
Taking the mechanical properties of AH36 steel plate with a thickness of 16 mm as an example , the influence of tensile fracture delamination on mechanical properties was analyzed , and 48 samples with tensile delamination and 264 samples without tensile delamination were counted . It can be seen from Table 1 that the mean and standard deviation of each statistic of the stratified sample and the non-stratified sample are basically the same .

Table 1 Comparison of tensile and impact properties between samples with and without delaminationTable 1 Tensile and strike properties

sample	sample size	Statistics	R eH / MPa	R m / MPa	A / %	A kv (20 °C)/j
		average value	473	561	27 . 0	239
Stratified sample	48		20 . 5	17 . 41	2 . twenty one	39 . 2
		standard deviation
		average value	479	562	27 . 5	234
non-stratified sample	264		25 . 2	16 . 65	2 . 17	40 . 7
		standard deviation

Analysis and Discussion
(1) After the above inspection, it shows that there is no delamination in the sample before stretching . Due to the severe band structure of the steel plate after rolling, a steel plate parallel to the surface of the steel plate is formed.
The layered weak interface makes the strength along the thickness direction of the steel plate significantly lower than the longitudinal direction, and it is easy to generate cracks parallel to the rolling direction . When the tensile test is carried out , under the action of three-dimensional stress , the crack first cracks at the center of the steel plate, such as tiny flaky inclusions or cracks and other defects. With the increase of the test force , the crack tip produces a large plastic deformation . The section of the crack tip of the sample is necked , and the local deformation of the crack along the thickness direction of the sample is constrained by the surrounding materials , resulting in a larger tensile stress from the surface. The closer to the crack tip , the greater the stress from the surface. surface stress is maximum . When the out-of-plane stress along the thickness direction exceeds the tensile strength of the material's weak interface , delamination cracks are first generated near the center of the sample thickness .
( 2 ) The delamination fracture behavior of the specimen depends on the toughness and internal defects of the material , Ordinary-level ship boards rarely appear , It mainly occurs in high-strength high-strength materials with high toughness ; in addition , The proportion of delamination phenomenon is related to the thickness of the material. Thin samples below 10 mm rarely appear, and the frequency of occurrence is higher in thick samples . As the thickness of the sample material increases , The proportion of delamination increases ; the thickness of the sample affects the degree of delamination . The smaller the thickness of the sample , the smaller the size of the delamination cracks , and there are delamination cracks with larger length and width in the middle of the thickness sample . This indicates that delamination cracks are generated under the action of higher out-of-plane stress

in conclusion
( 1 ) The fracture delamination of the tensile specimen is caused by the three-dimensional stress on the defects such as segregation and banded structure inside the steel plate when the tensile specimen is deformed during the test .
( 2 ) The fracture delamination phenomenon of the tensile specimen is related to the material , Ordinary-grade ship plates occur rarely , mainly in high-strength and tough materials ; in addition , the proportion of delamination is related to the thickness of the material, and thin samples below 10 mm rarely occur. , the proportion of delamination phenomenon increases .
( 3 ) There is no significant difference in tensile properties and impact toughness between samples with tensile delamination and samples without delamination . Therefore, it is recommended to further track the UT test results of steel plates with stratified batch numbers , verify the necessity of UT inspection, and optimize the production organization .

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