Crystallography of Reverted Austenite in the Intercritically Reheated Coarse-Grained Heat-Affected Zone of High Strength Pipeline Steel

doi:10.11900/0412.1961.2020.00287

Acta Metall Sin

2021, Vol. 57

Issue (8): 967-976 DOI: 10.11900/0412.1961.2020.00287

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Crystallography of Reverted Austenite in the Intercritically Reheated Coarse-Grained Heat-Affected Zone of High Strength Pipeline Steel

LI Xueda(

), LI Chunyu, CAO Ning, LIN Xueqiang, SUN Jianbo

School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao 266580, China

Cite this article:

LI Xueda, LI Chunyu, CAO Ning, LIN Xueqiang, SUN Jianbo. Crystallography of Reverted Austenite in the Intercritically Reheated Coarse-Grained Heat-Affected Zone of High Strength Pipeline Steel. Acta Metall Sin, 2021, 57(8): 967-976.

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Abstract

Low carbon microalloyed high-strength pipeline steels processed by the thermomechanical controlled process have a good strength-toughness combination. However, after welding, the microstructure and mechanical properties of the heat-affected zone (HAZ) become deteriorated. Previous studies show that martensite-austenite (M-A) constituent formed in the HAZ is a key factor that lowers the toughness, especially necklace-type M-A constituent formed in the intercritically reheated coarse-grained HAZ (ICCGHAZ). However, the phase transformation mechanism of necklace-type M-A constituent in the ICCGHAZ is unclear. In this study, the crystallography of reverted austenite (γ_r) during the reversion phase transformation upon heating the ICCGHAZ of a high-strength pipeline steel was studied using Gleeble thermal simulation and electron backscatter diffraction (EBSD) technique. Two thermal cycles with peak temperatures of 1300^oC and 760^oC/800^oC/840^oC were conducted to simulate the phase transformation process in the ICCGHAZ. The samples were directly quenched to room temperature after been reheated to the second peak temperature, and the reversion behavior, distribution, and crystallography of γ_r were studied. The results showed that the volume fraction of γ_r formed at 760, 800, and 840^oC was 4.1%, 8.9%, and 25.2%, respectively. γ_r preferred to nucleate along prior austenite grain boundaries (PAGB), and posterior the block boundaries within the prior austenite grains. γ_r preferred to grow to blocky type along the PAGB, and the acicular type γ_r between bainite laths was suppressed. The crystallographic study showed that the formation of γ_r at PAGB was not free nucleation. However, γ_r was formed based on the crystallographic orientation of PAGB alongside prior austenite grain complying with Kurdjuov-Sachs (K-S) relationship, while having non K-S relationship with the prior austenite grain on the other side. After nucleation at PAGB, and at a low second peak temperature (760^oC), γ_r transformed to prior austenite grain with non K-S relationship, and the γ_r formed in blocky and necklace-type along the PAGB. With an increase in the second peak temperature (800-840^oC), γ_r transformed to the prior austenite grains on both sides. The analysis showed that the reversion behavior and crystallography of γ_r during the second pass reheating have a big impact on the phase transformation upon cooling and the corresponding microstructure and mechanical properties.

Key words: high strength pipeline steel intercritically reheated coarse-grained heat affected zone (ICCGHAZ) reverted austenite crystallography reversion phase transformation

Received: 31 July 2020

ZTFLH:

TG407

Fund: National Natural Science Foundation of China(51801233)

About author: LI Xueda, associate professor, Tel: 15621003950, E-mail: lixuedachina@163.com

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	Xueda LI
	Chunyu LI
	Ning CAO
	Xueqiang LIN
	Jianbo SUN

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https://www.ams.org.cn/EN/10.11900/0412.1961.2020.00287 OR https://www.ams.org.cn/EN/Y2021/V57/I8/967

Fig.1 Schematic of thermal simulation curve

Fig.2 SEM images with low (a) and high (b) magnifi-cations of the simulated CGHAZ microstructure of X100 pipeline steel (CGHAZ—coarse-grained heat affected zone, M-A—martensite-austenite, GB—granular bainite, AF—acicular ferrite, LB—lath bainite)

Fig.3 EBSD characterizations of the CGHAZ (Grain boundary definition: black line 3° < θ < 15°, green line 15° ≤ θ ≤45°, yellow line θ > 45°; Symbols of △, ▽, ◇, and ? in Figs.3c and d represent four different crystallographic packets, and the symbols were colored as red, green, and black to represent three different Bain groups; the same below)

Fig.4 OM images of Q760 (a), Q800 (b), Q840 (c) samples etched by LePera reagent

Fig.5 SEM images and EBSD characterizations of Q760 (a₁-a₃), Q800 (b₁-b₃), Q840 (c₁-c₃) samples

Fig.6 Orientation relationships between γ_r and γ_p in Q760 sample (Red and blue arrows in Figs.6d-f point out the locations of γ_r1 and γ_r2 in the pole figures, respectively; the same below)

Fig.7 Orientation relationships between γ_r and γ_p in Q800 sample

Fig.8 Orientation relationships between γ_r and γ_p in Q840 sample

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