Effect of Welding Heat Input on Microstructure and Impact Toughness of the Simulated CGHAZ in Q500qE Steel

doi:10.11900/0412.1961.2021.00175

Acta Metall Sin

2022, Vol. 58

Issue (12): 1581-1588 DOI: 10.11900/0412.1961.2021.00175

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Effect of Welding Heat Input on Microstructure and Impact Toughness of the Simulated CGHAZ in Q500qE Steel

ZHU Dongming¹, HE Jiangli²^,³, SHI Genhao²^,³, WANG Qingfeng²^,³(

)

^1.China Railway Jiujiang Bridge Engineering Co., Ltd., Jiujiang 332004, China
^2.College of Materials Science and Engineering, Yanshan University, Qinhuangdao 066004, China
^3.State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China

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ZHU Dongming, HE Jiangli, SHI Genhao, WANG Qingfeng. Effect of Welding Heat Input on Microstructure and Impact Toughness of the Simulated CGHAZ in Q500qE Steel. Acta Metall Sin, 2022, 58(12): 1581-1588.

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Abstract

As China's economy enters a stage of high-quality development, it is important to study high-performance bridge steels with high strength, high toughness, high efficiency, and easy welding. Presently, coarse-grained heat-affected zones (CGHAZs) of high-performance bridge steels are prone to coarse-grain embrittlement, which reduces its impact toughness. To improve their low-temperature toughness, the relationship between their microstructure and impact toughness has been extensively researched and discussed. However, the control connection and internal mechanism between the bainite microstructure and impact toughness have not been clarified. In this study, the simulated samples of CGHAZs in Q500qE steel with varying heat inputs, from 15 to 30 kJ/cm, were reproduced in Gleeble 3500 thermal simulation-testing machine. The effect of different heat inputs on the microstructure, impact toughness of CGHAZs, and their inherent mechanism was discussed and analyzed in-depth using OM, SEM, and EBSD. The results indicate that the microstructure of each simulated sample of CGHAZs in Q500qE steel was composed of lath-like bainite (LB) and granular-like bainite (GB). The LB increased, the GB reduced, phase-transition temperature (A_r3) declined, and the bainitic packet/block substructure refined as the welding heat input decreased. In addition, with the decreased heat inputs, the Charpy impact energy (KV₂) at -40^oC of CGHAZs is enhanced because of the refined microstructure. The impact fracture of all samples showed cleavage fracture characteristics, and the cleavage face size of the simulated samples of CGHAZs decreased due to the reduced heat inputs. Compared with prior austenite grain boundary and bainite block, the bainite packet is the most effective microstructural unit for controlling the impact toughness of CGHAZs in Q500qE steel, and its boundary effectively facilitates the prevention of further propagation of a secondary crack.

Key words: Q500qE steel CGHAZ welding thermal simulation impact toughness bainite

Received: 26 April 2021

ZTFLH:

TG401

Fund: National Natural Science Foundation of China(51671165)

About author: WANG Qingfeng, professor, Tel: 13933560072, E-mail: wqf67@ysu.edu.cn

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https://www.ams.org.cn/EN/10.11900/0412.1961.2021.00175 OR https://www.ams.org.cn/EN/Y2022/V58/I12/1581

Fig.1 Original microstructure of the Q500qE steel plate

Fig.2 Welding thermal cycles of CGHAZs with different heat inputs (CGHAZ—coarse grained heat affected zone)

Fig.3 OM images of CGHAZs with different heat inputs (LB—lath-like bainite, GB—granular-like bainite)
(a) 15 kJ/cm (b) 20 kJ/cm (c) 25 kJ/cm (d) 30 kJ/cm

Fig.4 Typical SEM images of CGHAZs with different heat inputs (PAGB—prior austenite grain boundary)
(a) 15 kJ/cm (b) 20 kJ/cm (c) 25 kJ/cm (d) 30 kJ/cm

Table 1 Size measurement results of microstructure and statistics of cleavage face size of CGHAZs with different heat inputs

Fig.5 EBSD images of CGHAZs with different heat inputs
(a) 15 kJ/cm (b) 20 kJ/cm (c) 25 kJ/cm (d) 30 kJ/cm

Fig.6 Charpy impact energies at -40^oC for CGHAZs with different heat inputs

Fig.7 Impact fracture morphologies of CGHAZs with different heat inputs
(a) 15 kJ/cm (b) 20 kJ/cm (c) 25 kJ/cm (d) 30 kJ/cm

Fig.8 Secondary crack morphologies of CGHAZs with typical heat inputs of 15 kJ/cm (a) and 25 kJ/cm (b)

Fig.9 Expansion curves and A_r3 of CGHAZs with different heat inputs (A_r3—the starting temperature for γ→α transformation)

Fig.10 Cleavage facet size as functions of PAG size, bainitic packet size, and bainitic block width

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