Microstructure Evolution of Heat-Affected Zone in Submerged Arc Welding and Laser Hybrid Welding of 690 MPa High Strength Steel and its Relationship with Ductile-Brittle Transition Temperature

doi:10.1007/s40195-022-01498-0

Abstract

Abstract:

The comparative study of submerged arc welding (SAW) and laser hybrid welding (LHW) was carried out for a 690 MPa high strength steel with thickness of 20 mm. Microstructure and ductile-brittle transition temperature (DBTT) evolution in welded zone were elucidated from the aspect of crystallographic structure, particularly, digitization and visualization of 24 variants. The impact toughness of each micro zone in LHW joint is better than that of SAW, in which the DBTT of equivalent fusion line and heat-affected zone (HAZ) can reach − 70 and − 80 °C, while that of SAW is only − 50 °C. LHW technology induces narrowing of the HAZ and refining of the microstructure obtained in weld metal and HAZ. Meanwhile, the austenite grain size and transformation driving force in the coarse grained heat-affected zone (CGHAZ) are reduced and increased, respectively. It makes variant selection mechanism occurring in CGHAZ of LHW dominate by close-packed plane grouping, which promotes lath bainite formation with high density of high angle grain boundary, especially block boundary dominated by V1/V2 pair. While for SAW, the lower transformation driving force inferred from the large amount of retained austenite in CGHAZ induces Bain grouping of variants, and thus triggers the brittle crack propagating straightly in granular bainite, resulting in lower impact toughness and higher DBTT.

Key words: High strength steel, Welding, Microstructure, Variant selection, Ductile-brittle transition temperature

Xuelin Wang, Wenjuan Su, Zhenjia Xie, Xiucheng Li, Wenhao Zhou, Chengjia Shang, Qichen Wang, Jian Bai, Lianquan Wu. Microstructure Evolution of Heat-Affected Zone in Submerged Arc Welding and Laser Hybrid Welding of 690 MPa High Strength Steel and its Relationship with Ductile-Brittle Transition Temperature[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(4): 623-636.

Figures/Tables 17

Table 1 Chemical compositions of the experimental steel and welding wires (wt%)

Material	C	Si	Mn	P	S	Cr	Nb	V	Ti	Ni	Cu	Mo	B	Alt
Experimental steel	0.08	0.3	1.55	0.008	0.002	0.32	0.04	0.035	0.015	-	-	0.016	0.0016	0.04
SAW welding wire	0.08	0.05	1.53	0.013	0.001	0.018	-	-	-	0.01	0.02	-	-	-
LHW welding wire	0.07	0.8	1.45	0.015	0.014	-	-	-	-	-	0.15	-	-	-

Table 2 Welding process parameters

Welding method	Current (A)	Voltage (V)	Welding speed (cm/min)	Heat input (kJ/cm)	Welding wire	Diameter of welding wire (mm)
SAW	501	34.1	30	34.2	AWS A5.17 F7A0-EH14	4.0
LHW	280	25	100	4.2	AWS A5.18 ER70S-6	1.2

Fig. 1 Macro morphologies of the welded joints obtained by using SAW a and LHW b, and location of the measurement of mechanical properties in the test samples

Fig. 2 Impact toughness and DBTT evolution in welded zones of SAW and LHW: a-c impact toughness of WM, FL and HAZ at series temperature, d DBTT of WM, FL and HAZ

Fig. 3 SEM micrographs showing impact fracture surface morphologies of FL samples at the test temperature of − 60 °C: a-d SAW, e-h LHW

Fig. 4 Hardness evolution in welded joints

Fig. 5 Matrix microstructures of the studied steel: a OM image, b SEM image, c boundary distribution map, and d IPF

Fig. 6 Optical microstructure evolution in welded joints of SAW a-d and LHW e-h: a, e WM, b, f CGHAZ, c, g FGHAZ, and d, h ICHAZ. (AF: acicular ferrite, GBF: grain boundary ferrite, GB: granular bainite, LB: lath bainite, QPF: quasi-polygonal ferrite, BF: bainitic ferrite, M/A: matrensite/austenite constituent)

Fig. 7 BC maps showing the microstructure evolution in welded joints of SAW a-c and LHW d-f: a, d WM, b, e FGHAZ, and c, f ICHAZ. (RA: retained austenite)

Fig. 8 BC maps a, d, boundaries distribution maps b, e and IPF maps c, f showing CGHAZ microstructure evolution in welded joints of SAW a-c and LHW d-f

Fig. 9 Grain boundaries distribution a, and volume fraction and average size of retained austenite (RA, fcc phase) b obtained in CGHAZ of SAW and LHW joints

Fig. 10 IPF maps a,e showing prior austenite grains reconstructed in CGHAZ of SAW a-d and LHW joints e-h and the corresponding {100}bcc pole figures b, f with symbols and numbers indicating the ideal 24 variants, and {110}bcc c, g and {111}fcc d, h pole figures calibrated by theoretical variants for inner microstructure of prior austenite

Fig. 11 Gran boundaries distribution maps a, e, and CP b, f and Bain c, g group maps showing the crystallographic features in CGHAZ of SAW a-d and LHW e-h joints, and the corresponding transformation mechanisms d, h

Fig. 12 Fraction of 24 variants a and length of intervariant boundaries between V1 and other variants b in reconstructed austenite grains of CGHAZ. (Three colors are used to highlight the 24 variants that separately belong to three different Bain groups, corresponding to the Bain group maps in Fig. 9c and g

Fig. 13 In situ observation of the secondary cracks on the fractured V-notch Charpy sample of CGHAZ in SAW a-h and LHW i, j joints: a position of crack observation, b, c experimental and theoretical {100} pole figures of reconstructed austenite grains in Fig. 13d-f, d-f IPF maps of in situ reconstructed austenite grains, g-j grain boundaries distribution maps and Bain group maps of crack propagation

Fig. 14 Average length fraction of intervariant boundaries between V1 and other variants in CGHAZ a and the density of various grain boundaries b

Table 3 Size of the microstructures formed in welded zone and BM of the studied steel, and the proportion of each sub region contained in the impact sample

Welding method	Size and proportion	WM		CGHAZ		FGHAZ		ICHAZ		BM
SAW	Structure size (μm)	PCG-W	81.2 ± 11.4	PAG	42.7 ± 6.3	PAG	-	PAG-W	6.4 ± 2.5	PAG-W	6.4 ± 2.5
		AF	2.1 ± 1.3	Packet	4.7 ± 3.2	QPF	4.6 ± 2.2	QPF	2.8 ± 1.5	Packet	11.2 ± 3.8
		GBF-W	28.3 ± 13.5	Block	7.7 ± 5.4	M/A	1.8 ± 0.7	M/A	1.6 ± 0.6	Block	2.4 ± 0.5
		RA	0.8 ± 0.3	RA	0.8 ± 0.2	RA	0.4 ± 0.2	RA	0.4 ± 0.1	-	-
	CVN-WM	100%		-		-		-		-
	CVN-FL	50%		25%		25%		-		-
	CVN-HAZ	-		16.7%		16.7%		16.6%		50%
LHW	Structure size (μm)	PCG-W	68.8 ± 7.1	PAG	36.1 ± 5.6	PAG	-	PAG-W	6.4 ± 2.5	PAG-W	6.4 ± 2.5
		AF	1.5 ± 0.9	Packet	4.2 ± 2.8	QPF	4.2 ± 1.6	QPF	3.8 ± 1.4	Packet	11.2 ± 3.8
		GBF-W	18.5 ± 10.1	Block	3.4 ± 1.6	M/A	1.4 ± 0.5	M/A	1.3 ± 0.4	Block	2.4 ± 0.5
		RA	0.6 ± 0.2	RA	0.7 ± 0.1	RA	0.3 ± 0.1	RA	0.4 ± 0.2	-	-
	CVN-WM	100%		-		-		-		-
	CVN-FL	50%		16.7%		16.7%		16.6%		-
	CVN-HAZ	-		16.7%		16.7%		16.6%		50%

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