Mechanical and Metallurgical Characterization of HSLA X70 Welded Pipeline Steel Subjected to Successive Repairs

doi:10.1007/s40195-016-0422-1

Abstract

Abstract:

The aim of this work is to study the influence of successive weld repairs on the microstructure and the mechanical behavior of the heat-affected zone (HAZ) of an HSLA X70 steel. Detailed microstructural examination combined to grain size measurement showed that beyond the second weld repair, the microstructure of the HAZ undergoes significant change in the grain morphology and grain growth. The results of the X-ray diffraction analyzed using MAUD software indicated an increase in the crystallite size and a decrease in the dislocation density according to the number of weld repair operations. Consequently, a loss of mechanical properties, namely the yield strength and the toughness with the number of weld repairs, was recorded. Beyond the second weld repair operation, the properties of the welded joint do not fulfill the standards applied in piping industry.

Key words: HSLA X70 steel, Successive weld repair, Heat affected zone, Dislocation density

Bouzid Maamache, Mabrouk Bouabdallah, Abdelhalim Brahimi, Youcef Yahmi, Billel Cheniti, Brahim Mehdi. Mechanical and Metallurgical Characterization of HSLA X70 Welded Pipeline Steel Subjected to Successive Repairs[J]. Acta Metallurgica Sinica (English Letters), 2016, 29(6): 568-576.

Figures/Tables 19

Table 1 Chemical composition of the API 5L X70 steel pipe

Elements (%)	Ceq	C	Si	Mn	P	S	Cr	Mo	Ni	Nb	Ti	V	Cu
Base metal	0.36	0.073	0.08	1.65	0.037	<0.01	-	0.006	0.044	0.061	0.01	<0.01	0.123
Weld metal	-	0.1	0.48	0.2	0.02	0.017	0.037	0.039	0.35	-	-	-	-

Table 2 Parameters of welding process

Layers	Root	Filling	Cap
Welding process	SMAW	SMAW	SMAW
Welding position	5G	5G	5G
Current and polarity	DC (-)	DC (+)	DC (+)
Filler metal	E6010	E7010	E7010
Φ Electrode (mm)	3.25	4	4
Amp. range (A)	90-110	120-130	120-130
Volt. range (V)	25-35	25-38	25-38

Fig. 1 Sequences of repairs

Fig. 2 Schematic of tensile test specimen

Fig. 3 Schematic of V-notch position

Fig. 4 Cross section of the different weld repair conditions: a as-welded, b the first repair, c the second repair, d the third repair

Table 3 Width of the heat-affected zone in each weld repair

Number of repair	Wo	R1	R2	R3
Width of HAZ (mm)	2-4	2-5	3-5	3-5

Fig. 5 Typical weld joint microstructures: a base metal, b top HAZ, c middle HAZ, d bottom HAZ, e base metal

Fig. 6 Optical micrographs of the microstructure evolution in middle HAZ at 300 µm from FL of each repair

Fig. 7 Grain size in HAZ as a function of the repair number

Fig. 8 XRD patterns of BM and HAZ of different samples

Table 4 Microstructural parameters after Rietveld refinement of base metal and HAZ of each weld repair

	BM	Wo	R1	R2	R3
a (Å)	2.862	2.864	2.868	2.867	2.869
D (Å)	1154.089	3864.485	2058.045	3215.580	3325.952
rms	7.955 × 10^-4	4.647 × 10^-4	5.143 × 10^-4	6.127 × 10^-4	4.550 × 10^-4
ρ (m^-2)	2.638 × 10¹⁰	0.460 × 10¹⁰	0.955 × 10¹⁰	0.728 × 10¹⁰	0.570 × 10¹⁰

Fig. 9 Dislocation density and crystallite size variation with repair number

Fig. 10 Microhardness evolution in top, middle and bottom region of the welded joint for each repair

Table 5 Result of tensile tests

	Yield strength (MPa)	Tensile strength (MPa)	Strain (%)	Failure zone
Wo	515	694	22.81	Base metal
R1	513	681	18.25	Base metal
R2	485	660	15.25	HAZ
R3	436	668	14.48	HAZ

Fig. 11 Stress-strain profiles of each repair

Fig. 12 Failure zone for different samples

Fig. 13 Average values of absorbed energy for the different samples

Fig. 14 Fracture morphology of Charpy impact samples: a Wo, b R3

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