Microstructure and Mechanical Properties of X80 Pipeline Steel Joints by Friction Stir Welding Under Various Cooling Conditions

doi:10.1007/s40195-019-00940-0

Abstract

Abstract:

X80 pipeline steel plates were friction stir welded (FSW) under air, water, liquid CO₂ + water, and liquid CO₂ cooling conditions, producing defect-free welds. The microstructural evolution and mechanical properties of these FSW joints were studied. Coarse granular bainite was observed in the nugget zone (NZ) under air cooling, and lath bainite and lath martensite increased significantly as the cooling medium temperature reduced. In particular, under the liquid CO₂ cooling condition, a dual phase structure of lath martensite and fine ferrite appeared in the NZ. Compared to the case under air cooling, a strong shear texture was identified in the NZs under other rapid cooling conditions, because the partial deformation at elevated temperature was retained through higher cooling rates. Under liquid CO₂ cooling, the highest transverse tensile strength and elongation of the joint reached 92% and 82% of those of the basal metal (BM), respectively, due to the weak tempering softening. A maximum impact energy of up to 93% of that of the BM was obtained in the NZ under liquid CO₂ cooling, which was attributed to the operation of the dual phase of lath martensite and fine ferrite.

Key words: Friction, stir, welding, Pipeline, steel, Microstructure, Mechanical, properties, Cooling

G. M.Xie, R. H. Duan, P. Xue, Z. Y. Ma, H. L. Liu, Z. A. Luo. Microstructure and Mechanical Properties of X80 Pipeline Steel Joints by Friction Stir Welding Under Various Cooling Conditions[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(1): 88-102.

Figures/Tables 20

Fig. 1 FSW process under liquid CO2 cooling a, temperature measurement system b

Table 1 Chemical composition and mechanical properties of X80 pipeline steel

Composition (wt%)	Fe	C	Si	Mn	P	S	Nb	V
	Balance	0.038	0.2329	1.611	0.0098	0.017	0.0535	0.0029
Mechanical property	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)	Impact energy (J/cm²)
	589	693	34	211.6

Fig. 2 Schematic diagram of microstructure and property test sample locations a, Charpy V-notch specimen dimension b, tensile specimen dimension c

Fig. 3 Macrographs of FSW joints under various cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling

Fig. 4 Microstructures of the welded joint at air cooling: a BM, b OHAZ, c MHAZ, d IHAZ, e NZ

Fig. 5 OM images of the OHAZs at various cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling

Fig. 6 OM images of the IHAZs under different cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling

Fig. 7 OM images of the NZs under different cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling

Fig. 8 TEM images in the NZs at various cooling conditions: a air cooling, b water cooling, c water + liquid CO2 cooling, d liquid CO2 cooling

Fig. 9 Misorientation distribution maps in the NZs at various cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling

Fig. 10 Grain boundaries distribution maps in the NZs under different cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling

Fig. 11 Fraction of grain boundaries in the NZs at various cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling

Fig. 12 Average grain sizes of prior austenite in the NZs at various cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling

Fig. 13 ODF maps in the NZs under different cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling, e ideal shear texture of BCC lattice metals

Fig. 14 Hardness profiles of the joints under different cooling conditions

Fig. 15 Tensile properties of BM and joints under different conditions: a BM, b GMAW joint, c air cooling, d water cooling, e liquid CO2 + water cooling, f liquid CO2 cooling

Fig. 16 Fracture tensile samples of the joint under different cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling

Fig. 17 Impact properties of BM and joints under different conditions: a BM, b GMAW joint, c air cooling, d water cooling, e liquid CO2 + water cooling, f liquid CO2 cooling

Fig. 18 SEM images of impact fractural surface in the NZ under different cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling

Fig. 19 SEM images of impact crack propagation paths under different cooling conditions: a air cooling, b water cooling, c liquid CO2 + water cooling, d liquid CO2 cooling

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