Influence of central segregation on the welding microstructure and properties of FH40 cryogenic steel
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摘要: 利用金相（OM）、掃描電子顯微鏡（SEM）、電子背散射衍射（EBSD）以及能譜（EDS）等手段研究了FH40低溫鋼焊接接頭顯微組織演變及其對低溫沖擊韌性的影響。結果表明，FH40低溫鋼母材具有優異的綜合力學性能，其屈服強度為420 MPa，抗拉強度為518 MPa，?60 ℃夏比沖擊功為162 J，而焊接接頭熔合線位置及熱影響區的低溫韌性急劇降低至16 J。顯微組織分析表明，低溫鋼母材為細小的多邊形鐵素體+珠光體組織，在心部位置珠光體組織呈帶狀分布。焊接熱影響區的顯微組織主要為針狀鐵素體，但是心部存在明顯的馬氏體帶。針狀鐵素體硬度為229.7 HV0.05，比原來的多邊形鐵素體高約40 HV0.05，而馬氏體的硬度為313.7 HV0.05，較原來的多邊形鐵素體高約140 HV0.05。EBSD結果顯示在馬氏體帶存在較高的內應力，這是造成焊接接頭低溫韌性急劇下降的主要原因。EDS表明，中心偏析導致熱軋低溫鋼母材形成C、Mn富集的珠光體帶，這些C、Mn富集的珠光體帶在焊接熱影響作用下重新奧氏體化，并在冷卻過程中轉變成硬質相馬氏體組織。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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圖 2 實驗鋼母材SEM顯微組織. (a) 低倍鐵素體?珠光體帶狀組織; (b) 高倍退化珠光體組織形貌
Figure 2. SEM microstructure of the base metal: (a) ferrite?pearlite banding at low magnification; (b) morphology of degenerated pearlite at high magnification
圖 3 焊接接頭金相組織. (a) 接頭處全貌圖; (b) 靠近熔合線熔敷金屬區; (c) 熔合線處; (d) 距根部1/2厚度處熱影響區; (e) 距根部1/4厚度處熱影響區
Figure 3. Optical images of the welded joint: (a) full view of the joint; (b) the zone of weld metal near the fusion line; (c) the fusion line zone; (d) the heat-affected zone 1/2 thickness from the root; (e) the heat-affected zone 1/4 thickness from the root
圖 4 熱影響區EBSD結果. (a) BC圖; (b) KAM圖
Figure 4. EBSD images of the heat-affected zone: (a) band contrast (BC) map; (b) KAM map
圖 5 焊接接頭金相組織. (a) 熱影響區; (b) 母材
Figure 5. Optical images of the welded joint: (a) heat-affected zone; (b) matrix
圖 6 熱影響區能譜分析. (a) SEM圖; (b) C、Mn元素分布圖
Figure 6. Energy spectrum analysis of the heat-affected zone: (a) SEM micrographs; (b) distribution of carbon and manganese elements
圖 7 JMatPro軟件計算的實驗鋼均勻合金成分和偏析區合金成分下CCT圖
Figure 7. CCT diagram under uniform alloy composition and alloy composition in the segregation zone of the studied steel calculated by JMatPro software
F (1%) represents the start of ferrite transformation; P (1%) and P (99.9%) represent the start and finish of the pearlite transition, respectively; B (1%) and B (99.9%) represent the start and finish of the bainite transition, respectively; 1% and 99.9% represent transformation fractions of 1% and 99.9% for different transformation, respectively; A3 represents the temperature of complete austenitization at the equilibrium state; Ms represents the start of martensitic transformation.
表 1 焊接工藝參數
Table 1. Parameters of the welding process
Welding method Weld pass Interpass temperature/℃ Diameter of wire/mm Welding current/A Welding voltage/V Welding speed/ (cm?min?1) Heat input/ (kJ?cm?1) FCAW-1G 1 room temperature Φ1.2 150–155 21–22 10.7 17.7–19.1 2 and 3 ≤100 Φ1.2 170–180 23–24 10.8 22.3–24.7 4, 5, and 6 ≤120 Φ1.2 170–180 23–24 14.8 15.9–17.5 
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表 2 不同狀態試樣的沖擊性能
Table 2. Impact properties of the samples in different states
Samples Charpy impact energy at ?60 ℃/J KV2 #1 KV2 #2 KV2 #3 Average KV2 As-rolled 150 170 165 162 Weld (FL?1) 73 43 41 52 Weld (FL) 16 13 19 16 Weld (FL+1) 15 18 19 17
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表 3 熱影響區與母材不同帶狀組織的顯微硬度
Table 3. Micro-hardness of microstructures in the heat-affected zone and base metal bands
Observation area Microstructure Vickers hardness (HV0.05) Value #1 Value #2 Value #3 Value #4 Value #5 Average value Heat-affected zone Black martensite 306.5 298.1 319.9 309.6 334.4 313.7 White acicular ferrite 234.8 219.6 229.8 233.3 227.5 229.0 Matrix Black pearlite 181.9 210.0 188.2 194.3 197.4 194.4 White quasi-polygonal ferrite 162.9 173.4 154.7 175.5 170.2 167.3
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