High temperature creep behavior of friction stir welding joints for CLAM steel
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摘要: 针对核聚变堆用CLAM钢，对焊后热处理的搅拌摩擦焊接头在823 K条件和180～300 MPa应力水平下的单轴拉伸蠕变性能、断口形貌、显微组织进行了研究. 结果表明，当蠕变应力由300，260及220 MPa降低到180 MPa时，CLAM钢搅拌摩擦焊接头的蠕变寿命分别由1.5，19.2及883 h增加到6769 h以上. 临界热影响区是接头蠕变断裂的最薄弱区域，主要呈现位错控制的蠕变变形机制和穿晶韧性断裂模式. 在蠕变过程中临界热影响区组织发生回复并形成亚晶，导致位错强化作用降低；M23C6碳化物发生不同程度的粗化或周围生成Laves相，导致析出和固溶强化作用减弱；这些因素是CLAM钢FSW接头蠕变性能恶化的主要原因. 采用Monkman-Grant方程预测FSW接头在1×105 h蠕变寿命下的蠕变断裂强度估计为156 MPa，达到母材强度的88%.Abstract: The uniaxial creep tensile strength, fracture features and microstructures of friction stir welded joint with postweld heat treatment for CLAM steel have been investigated in the range of the creep applied stress from 180 MPa to 300 MPa at 823 K condition. It is found that the creep life of the FSW joints of CLAM steel increase from 1.5 h, 19.2 h and 883 h to above 6769 h respectively, when the creep stresses decrease from 300 MPa, 260 MPa and 220 MPa to 180 MPa. The inter critical heat affected zone is the weakest zone of creep rupture resistance for the FSW joint of CLAM steel, the joints mainly exhibit dislocation-controlled creep deformation mechanism and the transgranular ductile fracture mode. The microstructures of inter critical heat affected zone produce recovery and subgrain boundaries are formed in here during creep process, which result in the decrease of dislocation strengthening action; the coarser M23C6 carbides is produced or the coarser Laves phase around the M23C6 carbides is formed, which result in the reduction of precipitation and solution strengthening action, these issues are the main reasons for the deterioration of the creep performance of FSW joints. The creep fracture strength of FSW joint is estimated to be 156 MPa in the condition of 1 × 105 h creep life according to the Monkman-Grant equation, which reaches 88 % of the strength of base metal.
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Keywords:
- low activation steel /
- friction stir welding /
- creep performance /
- microstructure /
- life prediction
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图 2 焊接接头在823 K不同应力水平下应变-时间曲线和应变速率曲线
Figure 2. Strain-time curve and creep rate curve of the welded joint under different stress levels at 823 K. (a) strain-time curve; (b) creep rate curve
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图 3 CLAM钢FSW接头最小蠕变速率与应力的关系
Figure 3. Relationship between minimum creep rate and applied stress for friction stir welded joint of CLAM steel
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图 4 断裂试样
Figure 4. Rupture specimen. (a) rupture specimen at 220 MPa; (b) views of the joint at 220 MPa; (c) hardness profile of the joint at 220 MPa
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图 5 不同应力下接头热影响区的蠕变孔洞
Figure 5. Creep void of specimen under different stress levels. (a) fracture at 220 MPa; (b) fracture at 300 MPa; (c) 220 MPa; (d) 260 MPa; (e) 300 MPa; (f) quantity and area of the creep void
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图 6 不同应力下蠕变试样断口形貌
Figure 6. Morphologies of crept specimens under different stress levels. (a) macroscopic feature at 220 MPa; (b) microstructure at 220 MPa; (c) macroscopic feature at 260 MPa; (d) microstructure at 260 MPa; (e) macroscopic feature at 300 MPa; (f) microstructure at 300 MPa
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图 7 接头临界热影响区显微组织
Figure 7. Microstructure of welded joint in the inter critical heat affected zone. (a) without heat treatment; (b) with heat treatment; (c) M23C6 after heat treatment; (d) MX after heat treatment
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图 8 不同应力条件下接头临界热影响区的微观组织
Figure 8. Microstructure of ICHAZ of welded joints under different stress levels. (a) subgrain at 180 MPa; (b) subgrain at 220 MPa; (c) electron diffraction pattern of M23C6; (d) M23C6 and Laves at 180 MPa; (e) MX at 180 MPa; (f) electron diffraction pattern of Laves
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图 9 基于Monkman-Grant方程的蠕变性能曲线
Figure 9. Curve of creep rupture property based on Monkman-Grant equation
表 1 CLAM钢的化学成分(质量分数，%)
Table 1 Chemical composition of CLAM steel
C Cr Mn V W Ta Si N Fe 0.098 8.7 0.56 0.19 1.4 < 0.002 0.11 0.005 3 余量 
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表 2 焊接接头最小蠕变速率和断裂时间
Table 2 Minimum creep rate and fracture time of welded joints
应力水平σ/MPa 最小蠕变速率rmin/h−1 断裂时间t/h 180 8.09×10−6 6 769 (未断) 220 6.02×10−5 883 260 2.68×10−3 19.2 300 4.89×10−2 1.5
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