PSI - Issue 22

Li Yan-hua et al. / Procedia Structural Integrity 22 (2019) 118–124 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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dislocation after short heating process at low temperature so as to stimulate the formation of Cottrell atmosphere which result in dislocation locking. As more and more solute atoms gathered near dislocation, the grain boundary of pipeline steel with high dislocation density would become more brittle, as a result the yield strength and yield ratio of pipeline steel would increase finally. The test results shows that under 200 ℃ , 5min aging condition, the sensitivity coefficient of C sample of X90 pipeline steel is maximum, while the sensitivity coefficient of C sample of X90 pipeline steel is minimum, the main reason for different aging sensitivity coefficient of X90 pipeline steel manufactured by different manufacturers should be mainly attributed to different microstructures and chemical compositions of the X90 pipeline steel ， as shown in Table 8 and Figure 2.

Table 8. The chemical composition and microstructure characteristics of X90 pipeline steel Sample information Chemical composition Microstructure characteristics

(C+N)=0.06, (V+Nb+Ti)=0.13, (V+Nb+Ti)-(C+N)=0.07.

A

Granular bainite

(C+N)=0.05, (V+Nb+Ti)= 0.13, (V+Nb+Ti)-(C+N)=0.08 (C+N) =0.05, (V+Nb+Ti) =0.11, (V+Nb+Ti)-(C+N)=0.06

B

Granular bainite and MA

Granular bainite and MA and small amount of PF on inner and outer surfaces, the remaining are granular bainite and MA

C

Fig. 2. The microstructure characteristics of X90 pipeline steel

Study on the early results of X80 artificial strain aging show that when the microstructures of the X80 pipeline