STUDY ON MICROSTRUCTURE AND DYNAMIC FRACTURE BEHAVIOR OF Q460 STEEL WELDING JOINTS

doi:10.11900/0412.1961.2015.00617

Acta Metall Sin

2016, Vol. 52

Issue (7): 787-796 DOI: 10.11900/0412.1961.2015.00617

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STUDY ON MICROSTRUCTURE AND DYNAMIC FRACTURE BEHAVIOR OF Q460 STEEL WELDING JOINTS

Xiangli FENG^1,²,Lei WANG¹(

),Yang LIU¹

1 Key Lab for Anisotropy and Texture of Materials, Ministry of Education, School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
2 China Metallurgical Group Corporation, Beijing 100088, China

Cite this article:

Xiangli FENG,Lei WANG,Yang LIU. STUDY ON MICROSTRUCTURE AND DYNAMIC FRACTURE BEHAVIOR OF Q460 STEEL WELDING JOINTS. Acta Metall Sin, 2016, 52(7): 787-796.

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Abstract

Effects of welding heat input on the microstructure and dynamic fracture toughness (J_Id) of the CO₂ shielded arc welded joints of Q460 high strength low alloy steel were investigated. The mechanism of effects on the dynamic facture behavior of the welded joint was also discussed. The results showed that there existed the allotriomorphic ferrite at the columnar interface in the fusion zone of welded joint under the condition of low heat input. The morphological characteristics of columnar crystal in the fusion zone gradually decreased and the allotriomorphic ferrite disappeared as the heat input increased. The fusion zone was mainly composed of acicular ferrite, and its average size increased with increasing heat input. The welded joint exhibited the optimal dynamic fracture toughness under the condition of medium heat input while it showed the lowest value under low heat input within the temperature range of -70 ℃ to room temperature. When the temperature decreased from room temperature to -70 ℃, the dynamic fracture mechanism of Q460 welded joint changed from ductile fracture to brittle cleavage fracture. Under the condition of low heat input, the allotriomorphic ferrite characterized by the planar growth at the columnar interface in the fusion zone of welded joint can lead to the rapid intergranular crack propagation at low temperature. The fine acicular ferrite in the fusion zone of the welding joint obtained at medium heat input which can hinder the crack propagation during the dynamic fracture at low temperature to the greatest extent is the reason why the welded joint exhibits high dynamic fracture toughness.

Key words: low alloy high strength steel dynamic fracture toughness impact absorted energy fracture behavior mirostructure

Received: 01 December 2015

Fund: Supported by National Natural Science Foundation of China (Nos.51371044 and 51571052) and Fundamental Research Funds for the Central Universities of China (No.L1502027)

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https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00617 OR https://www.ams.org.cn/EN/Y2016/V52/I7/787

Fig.1 Schematics of samplling positions (a) and geometry dimensions (b) of specimens for dynamic fracture of the CO₂ shielded arc welding joints of Q460 steel (BM—base metal, FZ—fusion zone)

Fig.2 Microstructure of Q460 steel as the base metal for welding joints

Fig.3 Microstructures of overall views (a, d, g), FZs (b, e, h) and HAZs (c, f, i) in the CO₂ shielded arc welded joints of Q460 steel under the welding heat inputs of 12.6 kJ/cm (a~c), 16.4 kJ/cm (d~f) and 19.5 kJ/cm (g~i) (HAZ—heat affected zone)

Fig.4 Microhardness distributions of CO₂ shielded arc welding joints of Q460 steel under welding heat inputs of 12.6 kJ/cm (a), 16.4 kJ/cm (b) and 19.5 kJ/cm (c)

Fig.5 Impact absorted energy (A_K) (a) and dynamic fracture toughness (J_Id) (b) of the CO₂ shielded arc welded joints of Q460 steel under different welding heat inputs

Fig.6 Macromorphologies of the fracture surface of dynamic fracture toughness specimens of the CO₂ shielded arc welded joints of Q460 steel under welding heat imputs of 12.6 kJ/mol (a, d, g, j), 16.4 kJ/mol (b, e, h, k) and 19.5 kJ/mol (c, f, i, l) at RT (a~c), -20 ℃ (d~f), -50 ℃ (g~i) and -70 ℃ (j~l) (RT—room temperature)

Fig.7 SEM images of the fracture surface of dynamic fracture toughness specimens of the CO₂ shielded arc welded joints of Q460 steel under welding heat imputs of 12.6 kJ/mol (a, d, g, j), 16.4 kJ/mol (b, e, h, k) and 19.5 kJ/mol (c, f, i, l) at RT (a~c), -20 ℃ (d~f), -50 ℃ (g~i) and -70 ℃ (j~l)

Fig.8 SEM image of the fracture surface of dynamic fracture toughness specimens (at -50 ℃) near the initial cracking point of the CO₂ shielded arc welded joints of Q460 steel under low welding heat input

Fig.9 SEM images of the fracture surface of dynamic fracture toughness specimens near the initial cracking point of the CO₂ shielded arc welding joints of Q460 steel under medium 16.4 kJ/cm (a, c, e, g) and high 19.5 kJ/cm (b, d, f, h) welding heat inputs at RT (a, b), -20 ℃ (c, d), -50 ℃ (e, f) and -70 ℃ (g, h)

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