PSI - Issue 22

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

123

6

steel is similar, the chemical composition would has significant effect on the aging sensitivity[4-6]. When the C, N content is equal, the aging sensitivity of the pipeline steel would be reduced by increasing the content of V, Nb and Ti, among which, the effect of Nb, Ti content on aging sensitivity is more significant. the main reason should be attributed to these micro- alloying elements would form alloy precipitates with interstitial solid solute C, N, so as to reducing the segregation of interstitial solid solute C, N near dislocation. But the study on aging sensitivity coefficients of X90 pipeline steel shows that the effect of microstructures on aging sensitivity is more significant than chemical composition[7]. Microstructures analysis on the X90 pipeline steel shows that a small amount of polygonal ferrite precipitation appear in the microstructures of sample C, these ferrite precipitation has dissection effect on the original austenite grain, so as to make the bainite lath become short and fine. At the same time, the shear process and the volume expansion during the transformation from austenite to bainite will induce high density dislocation generated on the ferrite around bainite, which effectively reduces the segregation path of C, N atoms to dislocation during the aging process[8,9], as a result, the aging sensitivity of the pipeline steel would exacerbate. In addition, the C content of polygonal ferrite, bainite and MA is quite different between each other[10], this large concentration gradient is also conducive to promoting the diffusion of C atoms during the aging process, thus further exacerbate the aging sensitivity. This may be the main reason for the high aging sensitivity of sample C. Aging pipeline steel under conditions of 250 ℃ , 1h also shows a similar result, as shown in Figure 3.

Fig. 3. Aging sensitivity coefficients of X90 pipeline steel under aging conditions of 250 ℃ , 1h

6. Conclusions The impact energy of X90 pipeline steel would decrease significantly after the introduction of 200 ℃， 5min aging at -10 ℃ , While the impact energy would increase slightly with the increase of aging temperature and aging time. The tensile strength of X90 pipeline steel would decrease or increase slightly after the introduction of aging. While the yield strength and yield ratio would increase significantly after the introduction of aging, and the rising range of yield strength and yield ratio would decrease with he increase of aging temperature and aging time. The study on aging sensitivity coefficients of X90 pipeline steel shows that the effect of microstructures on aging sensitivity is more significant than chemical composition. X90 pipeline steel with dual-phase microstructure would