柳志鹏, 谢振家, 罗登, 周文浩, 郭晖, 尚成嘉. 中心偏析对FH40低温钢焊接组织性能的影响[J]. 工程科学学报, 2023, 45(8): 1335-1341. DOI: 10.13374/j.issn2095-9389.2022.06.21.003
引用本文: 柳志鹏, 谢振家, 罗登, 周文浩, 郭晖, 尚成嘉. 中心偏析对FH40低温钢焊接组织性能的影响[J]. 工程科学学报, 2023, 45(8): 1335-1341. DOI: 10.13374/j.issn2095-9389.2022.06.21.003
LIU Zhi-peng, XIE Zhen-jia, LUO Deng, ZHOU Wen-hao, GUO Hui, SHANG Cheng-jia. Influence of central segregation on the welding microstructure and properties of FH40 cryogenic steel[J]. Chinese Journal of Engineering, 2023, 45(8): 1335-1341. DOI: 10.13374/j.issn2095-9389.2022.06.21.003
Citation: LIU Zhi-peng, XIE Zhen-jia, LUO Deng, ZHOU Wen-hao, GUO Hui, SHANG Cheng-jia. Influence of central segregation on the welding microstructure and properties of FH40 cryogenic steel[J]. Chinese Journal of Engineering, 2023, 45(8): 1335-1341. DOI: 10.13374/j.issn2095-9389.2022.06.21.003

中心偏析对FH40低温钢焊接组织性能的影响

Influence of central segregation on the welding microstructure and properties of FH40 cryogenic steel

  • 摘要: 利用金相(OM)、扫描电子显微镜(SEM)、电子背散射衍射(EBSD)以及能谱(EDS)等手段研究了FH40低温钢焊接接头显微组织演变及其对低温冲击韧性的影响。结果表明,FH40低温钢母材具有优异的综合力学性能,其屈服强度为420 MPa,抗拉强度为518 MPa,−60 ℃夏比冲击功为162 J,而焊接接头熔合线位置及热影响区的低温韧性急剧降低至16 J。显微组织分析表明,低温钢母材为细小的多边形铁素体+珠光体组织,在心部位置珠光体组织呈带状分布。焊接热影响区的显微组织主要为针状铁素体,但是心部存在明显的马氏体带。针状铁素体硬度为229.7 HV0.05,比原来的多边形铁素体高约40 HV0.05,而马氏体的硬度为313.7 HV0.05,较原来的多边形铁素体高约140 HV0.05。EBSD结果显示在马氏体带存在较高的内应力,这是造成焊接接头低温韧性急剧下降的主要原因。EDS表明,中心偏析导致热轧低温钢母材形成C、Mn富集的珠光体带,这些C、Mn富集的珠光体带在焊接热影响作用下重新奥氏体化,并在冷却过程中转变成硬质相马氏体组织。

     

    Abstract: With the development of energy extraction to offshore, deep sea, and polar fields, the service environment is becoming increasingly harsh. Hence, developing cryogenic steel with high strength, high toughness at low temperatures, and excellent welding properties has become an urgent requirement for economic development. With equipment and technology innovation, although the FH40-grade cryogenic steel base metal can be developed by grain refinement, the low-temperature impact toughness of its welded joints might be drastically reduced. Thus, the application of FH40-grade cryogenic steel has been severely restricted. To examine the evolution of the microstructure of welded joints of FH40-grade cryogenic steel and its effect on low-temperature impact toughness, the macrostructure, microstructure morphology, and composition at the welded joints were analyzed using a metallographic optical microscope and through scanning electron microscopy, electron backscatter diffraction (EBSD), and energy dispersive spectroscopy (EDS) analysis, respectively. The results indicate that the FH40 cryogenic steel base metal has excellent comprehensive mechanical properties with a yield strength of 420 MPa, tensile strength of 518 MPa, and Charpy impact energy of 162 J at −60 ℃, while the low-temperature toughness of the joint fusion line and the heat-affected zone was drastically reduced to 16 J. Results of a microstructure analysis indicate that the base metal of cryogenic steel was a fine polygonal ferrite and pearlite structure and pearlite bands occurred at the core position. The microstructure of the heat-affected zone of welding was mainly acicular ferrite, but evident martensitic bands were observed in the core. The results of the Vickers hardness test revealed that the hardness of 229.7 HV0.05 for acicular ferrite and 313.7 HV0.05 for martensite, which were approximately 40 HV0.05 and 140 HV0.05 higher than the original polygonal ferrite, respectively. An EBSD analysis indicates that the kernel average misorientation of the martensitic band was high with high internal stresses, which was the main cause of the sharp decrease in the low-temperature toughness of the welded joint. The presence of severe bias of carbon and manganese elements was confirmed through the EDS analysis of the banding in the heat-affected zone. In the rolling process, many continuous pearlite-banded structures were formed due to the severe central segregation of the base metal. In the welding process, the local hardenability increases due to the high local composition, and the martensite of hard and brittle phases was formed in the rapid cooling process, causing the increase in the local stress and hardness. Thus, the mismatch between soft and hard phases and organization led to a sharp decrease in the low-temperature toughness of the welded joint.

     

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