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Acta Metall Sin  2016, Vol. 52 Issue (9): 1025-1035    DOI: 10.11900/0412.1961.2015.00610
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INFLUENCE OF NECKLACE-TYPE M-A CONSTITU-ENT ON IMPACT TOUGHNESS AND FRACTUREMECHANISM IN THE HEAT AFFECTED ZONE OF X100 PIPELINE STEEL
Xueda LI1(),Chengjia SHANG2,Changchai HAN3,Yuran FAN4,Jianbo SUN1
1 College of Mechanical and Electronic Engineering, China University of Petroleum, Qingdao 266580, China
2 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
3 Petroleum China West East Gas Pipeline Company, Shanghai 200120, China
4 China Petroleum Pipeline Research Institute, Langfang 065000, China
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Abstract  

After decades of development, mechanical properties of pipeline steels have a good combination of strength and toughness. But after welding, in the heat affected zone (HAZ), microstructure of the base plate was erased by the welding thermal cycle. Several subzones with different microstructures were formed in the HAZ due to different thermal histories they went through. Toughness of the HAZ varies due to the heterogeneous microstructure. In this work, toughness of the HAZ of X100 pipeline steel was examined with two notch locations. Low toughness of 51 J was obtained when the notch encountered intercritically reheated coarsen-grained (ICCG) HAZ and high toughness of 183 J when the notch did not contain ICCGHAZ. Meanwhile, different sub-zones in the HAZ were simulated using Gleeble thermal simulation machine. Simulated coarsen-grained (CG) HAZ, fine-grained (FG) HAZ and intercritically reheated (IC) HAZ with uniform microstructure had good toughness of 244, 164 and 196 J, respectively. In contrast, toughness of simulated ICCGHAZ was only 32 J. Therefore, ICCGHAZ consisting of coarse granular/upper bainite and necklace-type martensite-austenite (M-A) constituent along grain boundaries was proved to be the primary reason for low toughness. Instrumented Charpy impact test results showed that ICCGHAZ could notably embrittle the sample and lower the crack initiation energy. Characterization on the fracture surfaces of the as-fractured Charpy impact specimens showed that ICCGHAZ was found to be the crack initiation site of the whole fracture, and M-A constituent in the ICCGHAZ was characterized as cleavage facet initiation. Fracture mechanisms in the CGHAZ and ICCGHAZ were separately investigated using EBSD. The results showed that necklace-type M-A constituent in the ICCGHAZ notably increased the frequency of cleavage microcracks nucleation. Fracture mechanism changed from nucleation controlled in the CGHAZ to propagation controlled in the ICCGHAZ due to the existence of necklace-type M-A constituent. Therefore, the formation of necklace-type M-A constituent in the ICCGHAZ could not only cause notable drop of toughness in the HAZ, but also change the fracture behavior/mechanism. Hence, research on how to control the distribution status of M-A constituent in the ICCGHAZ is the key to improve the toughness of a weld joint.

Key words:  pipeline steel      heat affected zone (HAZ)      necklace-type M-A constituent      Charpy impact toughness      fracture mechanism     
Received:  27 November 2015     
Fund: Supported by National Basic Research Program of China (No.2010CB630801), China Postdoctoral Science Foundation (No.2015M582159) and Qingdao Postdoctoral Application Research Foundation (No.2015240)

Cite this article: 

Xueda LI,Chengjia SHANG,Changchai HAN,Yuran FAN,Jianbo SUN. INFLUENCE OF NECKLACE-TYPE M-A CONSTITU-ENT ON IMPACT TOUGHNESS AND FRACTUREMECHANISM IN THE HEAT AFFECTED ZONE OF X100 PIPELINE STEEL. Acta Metall Sin, 2016, 52(9): 1025-1035.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00610     OR     https://www.ams.org.cn/EN/Y2016/V52/I9/1025

Fig.1  Schematic of weld geometry and Charpy impact specimen orientation (a), macrograph of longitudinal submerged arc welding (LSAW) weld joint (b), notch position and EBSD map position (c), schematic showing how EBSD samples were sectioned (d), and EBSD sample mounted in epoxy (e) (HAZ—heat affected zone, IC—intercritically reheated, FG—fine-grained, CG—coarsen-grained, ICCG—intercritically reheated CG, UACG—unaltered CG, UAFG—unaltered FG )
Fig.2  Schematic of thermal simulation curves (CGHAZ, FGHAZ, ICHAZ were simulated by curves (a)~(c), respectively; ICCGHAZ simulated by curves (a)+(d))
Fig.3  Microstructures of X100 base plate (a), CGHAZ (b), FGHAZ (c), ICHAZ (d), ICCGHAZ (e) and weld metal (f) (a, f—EBSD band slope maps; b~e—OM images; M-A—martensite-austenite)
Fig.4  Detailed characterization of the ICCGHAZ microstructure(a, b) SEM images(c) EBSD all-Euler map (Inset show the enlarged image of square)(d) TEM image of M-A constituent
Sample Position Charpy impact energy Average
Real weld joint Base plate 255 228 254 268 251
Notch I 211 174 229 119 183
Notch II 38 51 64 50 51
Weld metal 87 82 78 80 82
Simulated CGHAZ 268 259 206 243 244
sample FGHAZ 163 154 179 159 164
ICHAZ 178 212 169 224 196
ICCGHAZ 28 30 36 33 32
Table 1  Charpy impact energies of X100 base plate, HAZ, weld metal and simulated sub-zones of the HAZ
Fig.5  OM images of simulated CGHAZ (a), FGHAZ (b), ICHAZ (c) and ICCGHAZ (d)
Fig.6  OM images of the fracture surfaces of sample with notch I (a) and II (b), and instrumented Charpy impact load-absorbed energy curves (c) (E1—crack initiation energy, E2—crack propagation energy, E3—energy absorbed during brittle fracture, E4—post brittle fracture energy, Pm—maximum impact load, Pf—brittle fracture start load, Pa—brittle fracture arrested load)
Fig.7  SEM images of fracture surface of sample I (a, b) and II (c, d)
(a) ductile fracture (b) cleavage fracture (c) cleavage fracture in the CGHAZ (d) cleavage fracture initiated from M-A constituent in the ICCGHAZ (Inset shows the enlarged SEM image of M-A particles locating at the center of cleavage facet)
Fig.8  SEM images of crack initiation site of the whole fracture with different magnifications (a~c)
Fig.9  Secondary microcracks underneath the CGHAZ fracture surface
(a) in situ OM image of EBSD scanning area (b) OM image of polished EBSD specimen surface before scanning (c) EBSD all-Euler map (Yellow lines represent >45° grain boundaries, green lines represent 15°~45° grain boundaries)
Fig.10  Secondary microcracks underneath the ICCGHAZ fracture surface
(a, b) OM images of EBSD scanning area (c) OM image of polished EBSD specimen surface before scanning (d) EBSD all-Euler map (Yellow lines represent >45° grain boundaries, green lines represent 15°~45° grain boundaries)
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