Effect of long term high temperature aging on CGHAZ microstructure of T23 water wall welded joint
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摘要:
通过高温时效方法分析了焊后未热处理T23水冷壁管焊接接头粗晶热影响区(coarse grained heat affected zone，CGHAZ)在服役过程中形成再热裂纹的微观机理，揭示了工程中未热处理T23水冷壁接头在机组启机后，短期运行容易发生开裂泄漏的内在原因. 采用材料表征手段对未时效和高温时效处理后的水冷壁焊接接头CGHAZ硬度、微观组织、析出物物相等进行系统分析. 结果表明，在530 ℃时效100 h后，CGHAZ硬度出现由晶内弥散强化导致的二次硬化现象，随着时效(运行)时间增加，CGHAZ硬度逐渐降低，但时效1000 h后，CGHAZ硬度仍有319 HV高于标准要求；在600 ℃温度下，随着时效时间的增加，CGHAZ硬度随之降低，组织回复、再结晶、马氏体板条宽化、位错密度降低、C元素及合金元素从基体析出等因素导致的CGHAZ硬度降低作用高于MX碳化物在晶内弥散析出导致的硬度升高，M23C6型碳(氮)化物在晶界、亚晶界逐渐析出和长大.
Abstract:The micro-mechanism of reheat crack formation in coarse grain heat affected zone (CGHAZ) of welded joint of T23 water wall tube without heat treatment after welding was analyzed by high temperature aging method.It reveals the internal reason that T23 water wall joint without heat treatment is easy to crack and leak in short-term operation after unit startup.The hardness, microstructure and precipitates of welded joints of water wall after unaged and high temperature aging treatment were systematically analyzed by means of material characterization.The results show that after aging at 530 ℃ for 100 h, the hardness of CGHAZ appears secondary hardening caused by intragranular dispersion strengthening. With the increase of aging (running) time, the hardness of CGHAZ gradually decreases, but after aging for 1000 h, the hardness of CGHAZ is still 319 HV, which is higher than the standard requirement.After aging at 600 ℃, the hardness of CGHAZ decreases with the increase of aging time. The hardness of CGHAZ decreased due to the recovery of microstructure,recrystallization, broadening of martensite lath, reduction of dislocation density, and precipitation of C and alloy elements from the matrix, which is higher than the hardness increased due to dispersion and precipitation of MX carbide in the grain. M23C6 carbide gradually precipitates and grows at grain boundaries and subgrain boundaries.
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Keywords:
- high temperature aging /
- T23 steel /
- water cooling wall /
- CGHAZ /
- reheat crack
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图 3 不同时效时间试样CGHAZ显微硬度
Figure 3. Microhardness of CGHAZ samples with different aging time. (a) aging temperature is 530 ℃; (b) aging temperature is 600 ℃
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图 4 CGHAZ金相组织形貌
Figure 4. Metallographic structure and morphology of CGHAZ. (a) S1-Y; (b) S2-Y; (c) S3-Y; (d) S4-Y; (e) S5-Y; (f) S6-Y; (g) S7-Y
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图 5 CGHAZ扫描电镜形貌
Figure 5. SEM morphology of CGHAZ. (a) S1-Y; (b) S2-Y; (c) S3-Y; (d) S4-Y; (e) S5-Y; (f) S6-Y; (g) S7-Y
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图 6 焊接接头CGHAZ的TEM形貌
Figure 6. TEM morphology of CGHAZ. (a) S1-Y; (b) S2-Y; (c) S3-Y; (d) S4-Y; (e) S5-Y; (f) S6-Y; (g) S7-Y
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图 7 大析出物能谱分析结果
Figure 7. Energy spectrum analysis results of large precipitates. (a) Energy spectrum of large-size precipitates in Fig. 6(d); (b) Energy spectrum of large-size precipitates in Fig. 6(f)
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图 8 小析出物能谱分析结果
Figure 8. Energy spectrum analysis results of small precipitates. (a) Energy spectrum of small-sized precipitates in Fig. 6(e). (b) Energy spectrum of small-sized precipitates in Fig. 6(g)
表 1 焊接工艺参数
Table 1 Welding parameters
预热温度
T1/℃层间温度
T2/℃焊接电流
I/A电弧电压
U/V150 ℃ 200 ~ 300 100 ~130 10 ~14 
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