Structure–Property–Fracture Mechanism Correlation in Heat-Affected Zone of X100 Ferrite−Bainite Pipeline Steel
Structure–Property–Fracture Mechanism Correlation in Heat-Affected Zone of X100 Ferrite−Bainite Pipeline Steel
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DOI：
10.1007/s40553-014-0036-3
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发表时间：
2015-01
期刊：
影响因子：
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通讯作者：
Xueda Li;Xiaoping Ma;S. Subramanian;R. Misra;C. Shang
中科院分区：
文献类型：
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作者：
Xueda Li;Xiaoping Ma;S. Subramanian;R. Misra;C. Shang
焊接接头的组织性能主要取决于焊接热影响区（HAZ）的显微组织特征。在这方面，X100铁素体-贝氏体管线钢的热影响区进行了研究，分离的热影响区的亚温再热粗晶（ICCG）热影响区的含和不含区域。这两个区域分别进行了夏比冲击韧性的评价，其特征在于电子背散射衍射（EBSD）。当冲击试样的缺口中含有ICCGHAZ时，获得了~50 J的低韧性，而当缺口中不含ICCGHAZ时，获得了~180 J的高韧性。在冲击测试样品中，在不存在ICCGHAZ的情况下，断裂表面约为60%脆性，在存在ICCGHAZ的情况下，断裂表面约为95%脆性（不包括剪切唇）。其根本原因是ICCGHAZ的显微组织由粒状贝氏体和上贝氏体组成，沿晶界有项链型马氏体-奥氏体（M-A）成分沿着。项链型M-A组元的存在显著增加了解理微裂纹形核的敏感性。ICCGHAZ既是整个断裂的起始点，也是脆性断裂扩展阶段解理面的起始点。EBSD对CGHAZ和ICCGHAZ下二次裂纹的研究表明，由于ICCGHAZ中存在项链型M-A组元，CGHAZ的断裂机制由形核控制转变为扩展控制。这两种断裂机制都导致了含ICCGHAZ的试样韧性差。
Structural performance of a weld joint primarily depends on the microstructural characteristics of heat-affected zone (HAZ). In this regard, the HAZ in X100 ferrite−bainite pipeline steel was studied by separating the HAZ into intercritically reheated coarse-grained (ICCG) HAZ containing and non-containing regions. These two regions were individually evaluated for Charpy impact toughness and characterized by electron back-scattered diffraction (EBSD). Low toughness of ~50 J was obtained when the notch of impact specimen encountered ICCGHAZ and high toughness of ~180 J when the notch did not contain ICCGHAZ. Fracture surface was ~60 pct brittle in the absence of ICCGHAZ, and 95 pct brittle (excluding shear lip) in the presence of ICCGHAZ in the impact tested samples. The underlying reason is the microstructure of ICCGHAZ consisted of granular bainite and upper bainite with necklace-type martensite−austenite (M−A) constituent along grain boundaries. The presence of necklace-type M-A constituent notably increases the susceptibility of cleavage microcrack nucleation. ICCGHAZ was found to be both the initiation site of the whole fracture and cleavage facet initiation site during brittle fracture propagation stage. Furthermore, the study of secondary microcracks beneath CGHAZ and ICCGHAZ through EBSD suggested that the fracture mechanism changes from nucleation-controlled in CGHAZ to propagation-controlled in ICCGHAZ because of the presence of necklace-type M-A constituent in ICCGHAZ. Both fracture mechanisms contribute to the poor toughness of the sample contained ICCGHAZ.