Analysis of fatigue crack growth rate of welded joint after immersion corrosion
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摘要:
服役于海洋环境中的风力发电机，其塔筒上的焊接结构不可避免地要遭受洋流、风浪、工作过程中带来的疲劳载荷及海洋复杂环境引发的腐蚀，往往成为腐蚀疲劳失效中的薄弱环节，因此，研究焊接接头的腐蚀疲劳现象，探索其背后的运作机制具有十分重要的意义. 选用S355低合金高强钢作为试验材料，进行S355钢焊接接头在5%NaCl环境中的预腐蚀疲劳裂纹扩展试验. 试验结果表明，与未经预腐蚀的试件相比，预腐蚀48 h的试样疲劳裂纹扩展速率并没有明显增加，而预腐蚀336 h的试样疲劳裂纹扩展速率增加显著. 利用扫描电子显微镜对预腐蚀后的疲劳试件进行微观表征，观察到在裂纹扩展路径周围存在微裂纹和分支裂纹，表明金属发生了脆化. 在预腐蚀疲劳断面上，观察到了裂纹扩展模式的转变，表层金属呈现平面脆性特征，内部金属保留了疲劳韧性特征，微观表征的结果表明，预腐蚀脆化了表层金属，从而加快了试件整体的疲劳裂纹扩展速率.
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关键词:
- S355钢 /
- 预腐蚀疲劳裂纹扩展试验 /
- 脆化
Abstract:Wind turbines operating in marine environments inevitably face fatigue loads from ocean currents, wind, waves, as well as corrosion from the complex marine environment, which often makes the welded structures on their towers vulnerable to corrosion fatigue failure, therefore, studying the corrosion fatigue phenomena in welded joints and exploring the underlying mechanisms is of great significance. S355 high strength low alloy structural steel is selected as the test material to carry out the pre-corrosion fatigue crack growth test of S355 steel welded joint in 5%NaCl environment. The test result shows that the fatigue crack growth rate of the test specimens after 48 hours of pre-corrosion is not significantly increased, while the fatigue crack growth rate of the test specimens pre-corrosion for 336 hours is significantly increased. Scanning electron microscope is used to characterize the pre-corrosion fatigue test specimens. Micro-cracks and branching cracks are observed around the crack growth paths, indicating that the metal is embrittlement. On the pre-corrosion fatigue section, the change of crack growth mode is observed, the surface metal shows brittleness fracture and the inner metal retaines fatigue toughness fracture. The results of microstructure characterization show that the pre-corrosion embrittles the surface of metal, which result in acceleration of the fatigue crack growth rate of the whole test specimen.
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Keywords:
- S355 steel /
- pre-corrosion fatigue crack growth test /
- embrittlement
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图 2 S355钢焊接接头微观形貌
Figure 2. Micro-morphology of S355 welded joints. (a) BM; (b) WM; (c) HAZ; (d) incomplete recrystallization zone; (e) coarse-grained/fine-grained HAZ; (f) fusion zone
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图 3 试件取样形式与形状尺寸(mm)
Figure 3. Test specimens' sampling form and shape size. (a) pattern of getting specimens; (b) shape and size of specimens
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图 4 疲劳裂纹扩展试验
Figure 4. Fatigue crack growth test. (a) BM-secant; (b) HAZ-secant; (c) WM-secant; (d) BM-Smith; (e) HAZ-Smith; (f) WM-Smith
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图 5 预腐蚀的脆化机制示意图
Figure 5. Schematic diagram of embrittlement mechanism of pre-corrosion. (a) electrochemical corrosion on inclusions; (b) hydrolysis and acidification in corrosion pits; (c) surface metal embrittlement by H infiltration; (d) passivation film protects the base metal
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图 6 裂纹扩展路径微观表征
Figure 6. Microscopic characterization of crack growth path. (a) BM-0 h; (b) BM-48 h; (c) BM-336 h; (d) HAZ-0 h; (e) HAZ- 48 h; (f) HAZ-336 h; (g) WM-0 h; (h) WM-48 h; (i) WM-336 h
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图 7 疲劳试件断面微观表征
Figure 7. Microscopic characterization of fatigue specimen. (a) BM-0 h; (b) BM-48 h; (c) BM-336 h; (d) HAZ-0 h; (e) HAZ-48 h; (f) HAZ-336 h; (g) WM-0 h; (h) WM-48 h; (i) WM-336 h
表 1 S355钢化学成分(质量分数，%)
Table 1 Chemical components of S355 steel
C Si Mn P S Cr Mo Ni Cu Fe 0.09 0.13 1.53 0.007 0.001 8 0.11 0.13 0.31 0.15 余量 
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表 2 焊条LB−52NSU化学成分(质量分数，%)
Table 2 Chemical components of LB−52NSU electrode
C Si Mn P S Ni Ti Mo Fe 0.05 0.57 1.18 0.006 0.001 0.52 0.011 ＜0.01 余量
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表 3 焊接参数
Table 3 Welding parameters
焊接方法 焊接位置 电流类型 焊接电流I/A 预热温度T1/℃ 层间温度T2/℃ SMAW 平焊 DCEP 90 ~ 130 93 ~ 107 93 ~ 107
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表 4 疲劳裂纹扩展试验参数
Table 4 Parameters of fatigue crack growth test
波形 频率f/Hz 应力比R 平均应力F/N 正弦波 120 0.5 7 000
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