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Acta Metall Sin  2015, Vol. 51 Issue (11): 1325-1332    DOI: 10.11900/0412.1961.2015.00077
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RELATIONSHIP BETWEEN THE EVOLUTION OF PHASE PARAMETERS OF GRAIN BOUNDARY M23C6 AND EMBRITTLEMENT OF HR3C SUPER-HEATER TUBES IN SERVICE
Zhifang PENG1(),Wen REN1,Chao YANG2,Fangyu CHEN3,Hongguo LIU4,Fangfang PENG5,Qingsong MEI1
1 School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072
2 Jiangsu Frontier Electric Technology Co. Ltd., Nanjing 211102
3 Research Institute of Wuhan Iron and Steel (Group) Corp., Wuhan 430080
4 Shenhua (Fujian) Energy Co. Ltd., Fuzhou 350004
5 Dongfang Boiler Group Co. Ltd., Zigong 643000
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Abstract  

The relationship of the evolution of the phase parameters (area fraction ? M 23 C 6 and equivalent width W ) of grain boundary M23C6 plates with the embrittlement of HR3C super-heater tube samples in service was studied. Based on the ASTM E112 standard charts, the total length of two dimensional austenite grain boundaries (Lgb) corresponding to each grain size number (GL) was determined in the observed area of the metallographic images and expressed as Lgb (GL). Making use of the SEM-SE images of the samples, the ? M 23 C 6 and W were determined. The relationships of W with GL and ? M 23 C 6 were established as W(GL, ? M 23 C 6 ). Combined with the result from a Charpy impact test, the function of the impact value (aKV) as the W was obtained. In addition, the grain boundary elastic modulus (Er) was measured by a nano-hardness test. The result shows that intergranular fracture occurred on all the room temperature impact test specimens taken from the super-heater tubes exposed under the operating conditions. The W was increased with the decrease of GL and the increase of Er at a constant ? M 23 C 6 , causing a corresponding decrease of aKV, and hence promoting the embrittlement of the HR3C super-heater tubes. The related mechanism for the intergranular fracture caused by the increase of the equivalent width W of grain boundary carbides (carbide coarsening) can be explained through the application of the proposed method.

Key words:  HR3C steel      embrittlement      M23C6      equivalent width      Charpy impact value     

Cite this article: 

Zhifang PENG,Wen REN,Chao YANG,Fangyu CHEN,Hongguo LIU,Fangfang PENG,Qingsong MEI. RELATIONSHIP BETWEEN THE EVOLUTION OF PHASE PARAMETERS OF GRAIN BOUNDARY M23C6 AND EMBRITTLEMENT OF HR3C SUPER-HEATER TUBES IN SERVICE. Acta Metall Sin, 2015, 51(11): 1325-1332.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00077     OR     https://www.ams.org.cn/EN/Y2015/V51/I11/1325

Sample Service condition Dimension / mm
Steam temperature / ℃ Steam pressure / MPa Service time / h Diameter Wall thickness
SH1.6 610~620 27.0~28.0 16000 63.5 11.5
SH3.2 600~610 25.0~26.5 32000 57.0 14.5
SH4.0 600~610 25.0~26.5 40000 57.0 14.5
SH5.6 520~560 24.0~25.0 56000 48.5 8.0
Table 1  Service conditions and dimensions of HR3C super-heater tubes
Sample C Si Mn P S Cr Ni Nb N Fe
SH1.6 0.068 0.385 1.18 0.015 0.0075 25.06 19.86 0.391 0.250 Bal.
SH3.2 0.055 0.425 1.19 0.017 0.0013 25.66 19.66 0.438 0.258 Bal.
SH4.0 0.071 0.397 1.17 0.016 0.0045 24.71 19.63 0.402 0.250 Bal.
SH5.6 0.063 0.420 1.19 0.026 0.0090 25.68 20.06 0.410 0.230 Bal.
ASTM 0.040~0.100 ≤0.750 ≤2.00 ≤0.030 ≤0.030 24.00~26.00 17.00~23.00 0.20~0.60 0.150~0.350 Bal.
Table 2  Chemical compositions of HR3C super-heater tube samples
Fig.1  SEM-SE images of impact fractography of HR3C super-heater tube samples

(a) SH1.6 (b) SH3.2 (c) SH4.0 (d) SH5.6

Fig.2  OM images of HR3C super-heater tube samples
Fig.3  SEM-SE images of HR3C super-heater tube samples
Fig.4  Relationships of the total length of grain boundaries (Lgb) with the austenite grain size number (GL) in the observed areas (100 times under 5000 mm2 observation area)
Fig.5  Comparison between mean values of grain size numbers GM (ASTM E112) and GL (this work) of HR3C super-heater tube samples
Fig.6  Variation of the equivalent width of grain boundary M23C6 plates (W) with their area fraction (? M 23 C 6 ) and the grain size number (GL)
Fig.7  Charpy impact value (aKV) and the equivalent width (W) of grain boundary M23C6 plates of HR3C tube samples (a) and aKV vs W (b)
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