Characteristics and Evolution of the Spot Segregations and Banded Defects in High Strength Corrosion Resistant Tube Steel
Bo LI1,Zhonghua ZHANG2,Huasong LIU1,Ming LUO2,Peng LAN1,Haiyan TANG1,Jiaquan ZHANG1()
1. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China 2. Tube & Pipe Department, Baosteel Research Institute, Baoshan Iron & Steel Co. , Ltd. , Shanghai 201900, China
Cite this article:
Bo LI,Zhonghua ZHANG,Huasong LIU,Ming LUO,Peng LAN,Haiyan TANG,Jiaquan ZHANG. Characteristics and Evolution of the Spot Segregations and Banded Defects in High Strength Corrosion Resistant Tube Steel. Acta Metall Sin, 2019, 55(6): 762-772.
C110 casing tube is one of the high strength corrosion resistant steel products for deep well oil exploration. Due to the co-existence of acidic media such as H2S and the high pressure, there are frequently sulfide stress corrosion cracking (SSC) failures produced in the tubes, which are supposed to be closely connected with their banded segregation defects. The relationship between the as-cast spot segregation and the following as-rolled banded defects, together with the impacts of quenching and tempering (QT) treatment have been revealed. The banded defects in high strength corrosion resistant oil tube have been studied experimentally from its very beginning of as-cast state. With aids of OM, SEM, EDS and EPMA observation and analysis, the various spot like segregations in round casting were revealed along with their following banded structure in both as-rolled and QT tubes. The mechanism and appearance of the segregation induced banded defects were investigated comparatively of the both tubes. It is pointed out that there are normally two kinds of spot like segregations in steel castings, speckle type and porosity type, respectively. There are not only severe positive segregations of solutes, such as C, Cr, Mo and Mn etc., in the macro-etched spot like areas, a finer dendritic sub-structure has also been observed in the speckle type spot segregation zones. It has been found that the width of the banded defects in the as-rolled tubes is closely related to the types of segregations, and the severe banded defects, which are difficult to remove by heat treatment, are recognized to originate directly from the spot like segregations. Solute segregations are found in the microstructure of banded defects of the both as-rolled and QT tubes but with different existences. A kind of pearlite plus bainite banded structure is present in the former tube, while the banded defect of latter is composed of concentrated granular carbides, which explains the difference of their hardness behavior.
Fig.3 Macro-etching and typical dendritic morphology of the round casting (1/4 cross section, A—equiaxed crystal, B—crossed dendrite, C—columnar crystal, dash circles show spot segregations)
Fig.4 As-cast dendritic morphologies of the round casting (From near surface No.1 to center No.9 shown in Fig.1)
Fig.5 Variation of the surface inward secondary dendrite spacing of the round casting (CET—columnar to equiaxed transition)
Fig.6 Dendritic morphology and EPMA analyses of map scanning (a) and line scanning (b) in the spot segregation zones
Fig.7 Banded structures of hot rolled (HR) tube (a) and high magnified image of it shown in Fig.7a (c), and quenching and tempering (QT) tube (b) (The lefts are the inner wall of the tubes and the rights are the center of their wall thickness)
Fig.8 Frequency distribution of the banded defects in the as-rolled and QT tubes
Fig.9 Low (a) and high (b) magnified SEM images of banded structure in hot rolled tube
Fig.10 Morphology and EPMA analyses of map scanning (a) and line scanning (b) on the banded structure of hot rolled tube
Fig.11 OM (a) and SEM (b) images of banded structure in QT tube
Fig.12 SEM images of the substrate (a) and banded (b) areas of QT tube
Position
C
Fe
1
1.63
98.37
2
1.71
98.29
3
1.90
98.10
4
1.82
98.18
5
1.23
98.77
6
1.37
98.63
7
1.16
98.84
Table 1 EDS analyses of the tube substrate area in Fig.12a
Fig.13 EPMA analyses of map scanning (a) and line scanning (b) on banded structure of QT tube
Fig.14 Hardnesses of the hot rolled (a) and QT (b) tubes
Position
C
Ti
Cr
Mn
Fe
Nb
Mo
1
2.34
6.73
85.44
5.49
2
2.59
0.83
1.68
94.90
3
2.11
1.38
96.51
4
2.46
97.54
5
1.39
96.53
2.09
6
1.11
98.89
Table 2 EDS analyses of tube banded area in Fig.12b
Fig.15 Morphologies of as-cast spot segregations of speckle type (a) and porosity type (b)
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