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Acta Metall Sin  2017, Vol. 53 Issue (8): 957-967    DOI: 10.11900/0412.1961.2016.00551
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Study on the Method of Improving the Toughness of CGHAZ for High Heat Input Welding Steels
Zongyuan ZOU1(), Xiaokui XU1, Yinxiao LI2,3, Chao WANG2
1 Key Laboratory of Advanced Forging & Stamping Technology and Science of Ministry of Education, Yanshan University, Qinhuangdao 066004, China
2 The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
3 Chinese 91315 People's Liberation Army Troops, Dalian 116041, China
Cite this article: 

Zongyuan ZOU, Xiaokui XU, Yinxiao LI, Chao WANG. Study on the Method of Improving the Toughness of CGHAZ for High Heat Input Welding Steels. Acta Metall Sin, 2017, 53(8): 957-967.

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Abstract  

Compared with the low heat input welding steel structures, the high strength low alloy (HSLA) steel structures after high heat input welding keep high temperature with longer time, and the cooling speed is slower, then the austenite crystal grains of coarse-grained heat affected zones (CGHAZ) grow up sharply, and coarse upper bainite (UB) and ferrite side plate (FSP) are generated easily in original austenite crystal, thus toughness of CGHAZ deteriorates seriously. At present, the approach of improving toughness of CGHAZ is to produce massive interleaved acicular ferrite (AF) in the original austenite crystal. However, with the improvement of welding capability for thick plate, welding heat input will be greater, and the hold time of high temperature will be more prolonged. In this case, AF coarsens much seriously, thus the improvement of CGHAZ toughness is limited severely. In this work, a new method for improving the toughness of CGHAZ in high heat input welding steels by studying the distribution map of HAZ impact value was proposed. This new method changes the grain boundary ferrite (GBF) and AF of the CGHAZ to polygonal ferrite (PF) of the fine-grained heat affected zones (FGHAZ) at same peak temperature, which improves the toughness of CGHAZ significantly. Comparing the microstructures and toughness of CGHAZ in Ti-V-N and Al-Ti-V-N micro alloy welding steels, the transformation condition and nucleation mechanism of PF in the CGHAZ of Al-Ti-V-N steel were analyzed. It is found that micron oxide inclusions is a key factor to inducing the nucleation of massive PF in CGHAZ, and nanoscale carbonitride is a key factor to draging and pinning the grain boundaries of austenite and ferrite. Therefore, the effective combination of above two factors guarantees the generation of a large number of PF, which improves the impact toughness greatly at low temperature.
Key words:  high heat input welding      CGHAZ      AF nucleation      PF transformation      toughness     
Received:  07 December 2016     
ZTFLH:  TG142.1  
Fund: Supported by National Natural Science Foundation of China (No.51675465)
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Zongyuan ZOU
Xiaokui XU
Yinxiao LI
Chao WANG

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https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00551     OR     https://www.ams.org.cn/EN/Y2017/V53/I8/957

Fig.1  Distribution of impact energy of welding heat affected zone in low carbon steel[23] (HAZ—heat affected zone, CGHAZ—coarse-grained heat affected zone, FGHAZ—fine-grained heat affected zone, ICHAZ—incomplete recrystallization heat affected zone, BZ—brittle zone, NAZ—not affected zone)
Steel C Si Mn S Ti Al V N O Cr+Mo+Cu+Ni Fe
A1 0.08 0.10 1.61 0.005 0.025 - 0~0.1 0.007 0.005 <0.1 Bal.
A2 0.08 0.10 1.60 0.005 0.015 0.05 0.05 0.008 0.005 Bal.
Table 1  Chemical compositions of the tested steels (mass fraction / %)
Fig.2  Two stage rolling process by using thermo-mechanical controlled process (TMCP)
Fig.3  Schematic of temperature vs time in the welding thermal simulation (t8/5—cooling time from 800 ℃ to 500 ℃, A—peak temperature, B and C—final cooling temperatures, h—plate thickness)
Steel Rp / MPa Rm / MPa A / % AKV of BM (-40 ℃) J AKV of CGHAZ (-20 ℃) / J
t8/5=138 s t8/5=198 s
A1-0 389 496 30.2 321 244 220
A1-0.05 459 565 25.5 299 244 211
A1-0.1 514 617 20.3 279 181 83
A2 456 562 24.3 310 181 240
Table 2  Basic mechanical properties and impact energy of the CGHAZ in the two groups of steels
Fig.4  Dissolution and precipitation of second phase particles in A1-0.05 (a) and A2 (b) steels
Fig.5  Curves of temperature vs time of A1-0.05 steel in the welding thermal simulation (E—heat input)
Fig.6  Relationship between the impact energy and the heat input in the CGHAZ of A1-0.05 steel
Fig.7  OM images of microstructure evolutions in the CGHAZ of A1-0.05 steel under different heat inputs (GBF—grain boundary ferrite, AF—acicular ferrite, FSP—ferrite side plate, PF—polygonal ferrite)

(a) 50 kJ/cm (b) 100 kJ/cm (c) 200 kJ/cm (d) 300 kJ/cm (e) 400 kJ/cm (f) 500 kJ/cm

Fig.8  Curves of temperature vs time of A2 steel in the welding thermal simulation
Fig.9  Relationship between the impact energy and t8/5 in the CGHAZ of A2 steel
Fig.10  OM images of microstructure evolutions in the CGHAZ of A2 steel for different times (P—pearlite)

(a) 50 s (b) 100 s (c) 200 s (d) 300 s (e) 400 s (f) 500 s

Fig.11  OM images of microstructure evolutions in the vicinity of the welded joint of A1-0.05 steel (a~d) and A2 steel (e~h)

(a, e) CGHAZ (b, f) FGHAZ at higher temperature (c, g) FGHAZ at lower temperature (d, h) ICHAZ

Fig.12  SEM images of micron sized inclusions (points 1~5) and microstructure of PF in the CGHAZ of A2 steel

(a) point 1 (b) point 2 (c) point 3 (d) points 4 and 5

Fig.13  TEM images (a~c) and EDS analysis (d) of nano precipitated particles in A2 steel

(a) point 1 (b) particles (c) point 2 (d) EDS of point 2

Fig.14  OM images of A2 steel at different cooling rates (UB—upper bainite)

(a) 0.5 ℃/s (b) 1 ℃/s (c) 1.5 ℃/s (d) 2 ℃/s (e) 3 ℃/s (f) 5 ℃/s (g) 7 ℃/s (h) 10 ℃/s (i) 20 ℃/s

Fig.15  Simulated HAZ continuous cooling transformation in A2 steel (A—austenite, B—bainite, F—ferrite, Ac3—temperature of all ferrite transformed to austenite when steel is heated, Ac1—temperature of austenite begins to form when steel is heated)
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