Effect of pre-strain on microstructure evolution and fracture behavior of undermatching X80 pipeline steel girth weld

This work aims to study the effect of pre-strain on the fracture behavior of X80 pipeline girth weld joint, a comprehensive analysis was conducted on the microstructure and mechanical properties of the girth weld before and after pre-strain treatment. The mechanical properties were evaluated through tensile testing, Charpy impact testing, and digital image correlation (DIC) strain analysis. Furthermore, the microstructure and fracture morphology of the girth weld were observed using optical electron microscopy (OM) and scanning electron microscopy (SEM). The results show that the application of pre-strain treatment leads to dislocation accumulation at the grain boundary of X80 pipeline girth weld, resulting in stress concentration and subsequent formation of damage holes. This process disrupts the continuity of chain M-A island and initiates small cracks at the grain boundary, ultimately causing a significant decrease in impact toughness and impact work from 177 J to about 10 J.


Introduction
Pipeline transportation has been the main method of long-distance oil and gas transportation because of its advantages of convenient and efficient, economic and environmental protection, high safety factor and large transportation scale.High-strength low-alloy steels (HSLA) have been considered key materials for the petroleum industry for the construction of pipelines to move oil and gas for long distances.In recent years, the demand for oil and natural gas has gradually increased, and the transportation capacity of oil and gas pipelines development.X80 pipeline steel is a kind of high-strength low-alloy (HSLA) steel.The bainite+ferrite (B+F) dual-phase microstructure is obtained by microalloying composition design and thermal-mechanical control process (TMCP) technology, which has been widely used in the manufacture of X80 pipeline steel [1][2][3].X80 pipeline steel has good strength, toughness, weldability and corrosion resistance, and has been widely used in the west-east gas transmission project [4].X80 pipeline steel has been widely used in oil-gas pipeline transportation, the main failure cause is the fracture caused by internal cracks and welding defects in the pipeline during service.Girth-welds of long-distance transportation pipelines are characterized by nonuniformity of structure and defects of welding structure, etc.As an indispensable method for pipeline connection, the girth-welds joint was also tended to be a weak points of pipeline fracture.Due to frequent accidents caused by girth welds cracking, huge economic losses are caused [5][6][7].
Currently, the fracture behavior of pipeline steel welded joints has been widely studied.Yang et al [8] investigated the fracture toughness of the materials in welded joint of X80 pipeline steel at room temperature.The results show that the fusion zone poses a significant risk for fracture in welded joints of X80 pipeline steel, with hardening and embrittlement primarily attributed to the presence of hard brittle M-A constituents.Chen et al [9] investigated the fracture toughness of two kinds of welded joints of API X80 pipeline steel after service, and compared the critical fracture toughness of two kinds of welded joints of longitudinal submerged arc welding (LSAW) and spiral submerged arc welding (SSAW) in different regions.The results show that the fracture toughness of the base metal is the highest among the welded joints of SSAW pipes.The fracture toughness of these parts decreased, because of the coarsening of grain and the spherical precipitation of molybdenum in the welding zone and heat affected zone caused by welding heat input.Mayur P Singh et al [10] studied the dynamic fracture toughness of X80 welded joints at −20 °C under different heat input levels.Using Johnson-Cook constitutive model, a finite element model simulating Charpy impact test is established, and the experimental results are verified.Wang et al [11] studied the fracture toughness of welded joints of X80 pipeline steel at low temperature.The results show that the tough-brittle transition temperature of the weld is lower than −60 °C, and that of the heat affected zone (HAZ) is about −38 °C.In the process of service, the pipeline may be subjected to geological settlement, landslide, earthquake and other strata movement phenomena.Which will inevitably produce pipeline displacement, and thus make the girth-weld metal bear strain concentration, which will change the mechanical properties of the pipeline steel, lead to the degradation of the pipeline steel, and increase the risk of brittle fracture of the pipeline [12][13][14].Gao et al [15] studied the effect of strain aging on fracture toughness of high-strain pipeline steel welded joints.The results show that after strain aging treatment, the softening phenomenon in the heat affected zone disappears, and the fracture toughness of all region shows a decreasing trend.Chang et al [16] studied the dynamic fracture behavior of girth-welds under different load mode.And used the numerical simulation method based on Gurson-Tvergaard-Needleman (GTN) model to analyze the crack initiation and dynamic fracture behavior of welded pipes under pure bending load.The bearing capacity and fracture toughness of steel pipe under different loading conditions are analyzed.The results show that the carrying capacity of the girth weld is greatly reduced due to shearing action.Previous studies [17] clarified the strain law, deformation behavior and microstructure characteristics of undermatched X80 welded joints in practical applications.The pattern of strain concentration in the weld region varies in different tensile stages, leading to severe damage and final fracture of the weld.Guo et al [18] studied the strain aging embrittlement behavior of X80 girth weld metal under different strain aging conditions.The results show that the toughness decreases significantly after strain aging, and the increase of aging temperature or time leads to the decrease of toughness until the complete yield.The martensitic-austenitic (M-A) island (1 ∼ 2 μm) induces cleavage fracture at partial strain aging, while the submicron inclusion induces cleavage fracture at full strain aging.At present, most studies focus on the toughness and overall tensile strength of girth welds.There are few reports on the effect of pre-strain on the fracture behavior of high strength pipeline steel.As a key part of connecting pipelines, the performance of girth-weld changes cause by deformation during installation and use cannot be ignored.Therefore, it is of great significance to research the effect of pre-strain on the fracture behavior of pipeline steel girth-weld to elucidation the failure mechanism of pipeline steel during service.
In this paper, the strain rule, deformation behavior and microstructure evolution of undermatched X80 pipeline girth weld during the pre-strain has been studied.Charpy impact test was used to characterize the impact toughness of specimens with different pre-strain.Digital image correlation (DIC) was used to characterize the girth-weld strain law and deformation behavior during pre-strain.Optical microscope (OM) and scanning electron microscope (SEM) were used to characterize the microstructure evolution and fracture morphology.The effect of fracture behavior of pre-strain undermatched X80 girth weld was studied in detail.

Material and experiment
For the experiments was taken from the actual service X80 pipeline welded joint.The base material (BM) on both sides of the welded joint was API X80 pipeline steel with thickness of 12.8 mm and 15.3 mm, marked N1 and N2, respectively.The yield strength of N1 and N2 is 585 MPa and 565 MPa, and the tensile strength is 756 MPa and 735 MPa, respectively.Pipe diameter j1060 mm, which was an unequal wall thickness girth weld.The welds joint was fabricated by GMAW (Root) +FCAW-G (Fill, Cap), semi-automatic + automatic welding process.The root welding metals was ER70S-G and the filler metal was E81T1-Ni1.The specific welding parameters are shown in table 1.The weld plate morphology and welded joint preparation are shown in figure 1.Moreover, the base metal was pre-heated at 100 ∼ 200 °C, keep the interlayer temperature at 60 ∼ 150 °C.The Chemical composition was analyzed by Shimadzu PDA-8000 (Shimadzu Corporation, Kyoto, Japan) spectral analyzer.The weld parameters of X80 pipeline studied meet API 5 L standards, and the weld joint chemical components are shown in table 2. The welded joint was cut into a block sample in the longitudinal direction of weld and the sample were milled, ground and polished.The weld joint sample was etched with 4 vol.%nitrate alcohol for 10 s, and then sample observation was performed with optical microscope.
The tensile sample was of non-standard dog-bone shape, the length direction of the specimen was parallel to the pipe, and the girth weld specimen WM was located in the middle of the tensile specimen.The geometric shape and detailed sample size were shown in figures 2(a) and (b).
Instron 5565 universal experimental machine was used for pre-tensile deformation and tensile test at room temperature.Using the DIC experimental system, including Instron 8801 (Illinois Tool Works Inc., Norwood, MA, USA) experimental machine and Vic-2D DIC system (ACQTEC, Shanghai, China).The image acquisition

Microstructure of the girth-weld
The macroscopic morphology of the of X80 pipeline girth weld is illustrated in figure 5.As shown in figure 5, the girth-welded joint comprises weld metal (WM), heat affected zone (HAZ), and base metal (BM).Due to welding thermal cycling, the heat affected zone consists of fusion zone (FZ), coarse-grained heat affected zone (CGHAZ), fine-grained heat affected zone (FGHAZ), and inter critical heat affected zone (ICHAZ) [22].
Figure 6 shows metallographic microstructure of different areas of girth-welds of X80 pipeline steel.As shown in figure 6(a), the cap weld composed of coarse granular bainite (GB) and lath bainite (LB), containing a bit of martensite-austenite (M-A) island and acicular ferrite (AF).The process is multi-layer and multi-pass welding.As a welded metal, the content of granular bainite (GB) in the cover pass is higher than that in the fill pass.M-A island is a hard and brittle phase, which will reduce the toughness of girth welds.The size and distribution of M-A island both affect the toughness of girth welds [23].evenly distributed.Figure 6(c) shows the supplementary A of the three-pass weld.The microstructure mainly consists of granular bainite (GB), a small amount of polygonal ferrite (PF) and M-A distribution.Figure 6(d) shows the third supplementary region B. The microstructure is mainly granular bainite (GB), M-A distribution and polygonal ferrite (PF).Figure 6(e) shows the second layer weld, and it can be seen that the grain size is large, which may be due to the growth of the grain after the third thermal cycle.The typical bainite structure is presented, in which lath bainite (LB), granular bainite (GB) are evenly distributed, and chain M-A island are evenly distributed at the grain boundaries.Figure 6(f) shows the first layer weld, in which the quasi-  polygonal ferrite (QF) is evenly distributed and contains lath bainite (LB) granular bainite (GB) structures.The microstructure of the bottom welding layer is very different from that of other filled layers.As shown in figure 6(g), the root welding layer is mainly composed of small equiaxed crystals, including polygonal ferrite (PF), quasi-polygonal ferrite (QF), pearlite (P) and a certain amount of granular bainite (GB), and the carbides (C) on the matrix are evenly distributed.The microstructure of all girth weld layer is similar, and different fill pass are affected by the welding thermal cycle, resulting in differences microstructure, and the grain size and microstructure distribution are non-homogeneity.When the heat generated by the latter phase makes the peak heat treatment temperature of the former phase between Ac1 and Ac3, it is conducive to the formation and growth of M-A island.

Strain distribution and microstructure evolution of girth-weld joint
In order to investigate the strain law of welded joints at different strain value, the strain distribution of girth welds during tensile process was studied by using digital image correlation technique (DIC).The tensile process can be divided into four stages, namely (1) elastic stage, (2) yield stage, (3) uniform deformation stage, and (4) fracture stage.In the elastic stage, the strain distribution is uniform and there is almost no deformation concentration.Figure 7 shows the DIC strain distribution cloud diagram of the girth weld under tensile stress.The stress-strain curve of the sample was shown in figure 7(a).The strain was calculated using the data measured by the displacement sensor, and its strain was defined as the macroscopic strain, represented by e .
M When e M = 2%, the base metal (BM), heat affected zone (HAZ) and girth-weld zone in the elastic stage, and the total strain of the sample is very small.The axial strain distribution of the sample is shown in figure 7(b).It can be seen that the strain distribution is not homogeneous when the pre-strain of 2% is applied, and there are strain concentration micro-regions in the N1-HAZ region for L1-L3, and strong strain concentration occurs in the weld center and the heat affected zone.When e M = 4%, loading belongs to the end of the linear growth stage, and the strain distribution gradually becomes regular.The strain distribution is greatest near the heat affected zone.The axial strain distribution is shown in figure 7(c).With the gradual progress of stretching, the strain concentration of the sample is concentrated in the weld and the base material, but the strain concentration degree in the weld is smaller than that in the base material, and there is still a weak strain.At 2% and 4%, the tensile curve of the sample shows that the whole is in the elastic stage, as shown in figure 7(a).As shown in figures 7(b) and (c), when the total deformation reaches 2%, the strain variable at the center of the weld is about 0.15%.When the total deformation reaches 4%, the strain at the center of the weld is about 2%.The evolution to these strain distributions indicates the non-homogeneity of the welded joint structure, resulting in different mechanical properties.
The girth-weld joint strain distribution of each layer is different during the tensile process, the girth-weld joint microstructure evolution is also different in each layer.As shown in figure 8, three different filler layer structures of the girth weld were selected for scanning electron microscopy (SEM) observation.Figures 8(a), (d),  and g show the microstructure of the girth weld at 0% pre-strain, which is similar and consists of lath bainite (LB), granular bainite (GB), polygonal ferrite (PF), and M-A island.It has been confirmed in some literatures [24,25] that the effect of small pre-strain on weld microstructure is not significant.Nevertheless, dislocation slip and lattice rotation lead plastic deformation of girth welds in tensile process [26].After 2% pre-strain treatment, some small hole is generated in the girth weld matrix at the grain boundary, and the chain M-A continuity along the grain boundary is interrupted, as shown in figures 8(b), (e), (h).With the increase of the stress value to 4%, the number of small holes in the matrix increase and the lath bainite widens gradually.Conventional welding processes may produce important microstructural changes, which negatively impact the mechanical behavior of the pipeline material.The reduction of ductility is by the effective grain size.Particularly under high heat input conditions, which can promote the expansion of the coarse grain region and the increase of texture strength [27].

Charpy impact test
As shown in figures 6 and 8, the differences in microstructure between the areas of the welded joint necessarily leads to non-uniform mechanical properties.The samples with 0%, 2% and 4% pre-strain were subjected to impact test at −10 °C, and the results were shown in table 3. The pre-strain will lead to the decrease of impact toughness and the embrittlement of weld.This is caused by the microstructure evolution during the predeformation process.The stress concentration caused by dislocation accumulation during the tensile process, leads to the hole formation at the grain boundary, resulting in a significant decrease impact toughness with the increase of stress variables.As shown in figure 9, the impact toughness of the pre-strain sample has a large toughness scatter phenomenon, and the impact toughness decreases with the increase of strain variable.

Fracture morphologies
The macroscopic morphology of fracture in different pre-strain value as shown in figure 10.Figures 10(a), (b) and c were 0%, 2% and 4% samples with pre-strain value respectively.As shown in figure 10(a), the fracture of 0% samples is uneven and irregular in shape, with obvious fiber region, radiating region and shear lip.The fracture separation characteristics are significantly affected by the pre-strain.With the increase of strain value, the area of the cleavage surface increases.The macro morphology of the fracture changes from the surface uneven shape to the smooth fracture morphology, showing a typical brittle fracture characteristic.
Figure 11 shows the micro-morphology of the fracture with different pre-deformation amounts.It is found by comparison that after pre-deformation treatment, the crack initiation region of the girth weld is different from that of the undeformed specimen.At 0% pre-strain, the fracture mode is ductile fracture, and there are  more dimples in the fracture morphology with small size and uniform In addition, there are smaller honeycomb-shaped equiaxial dimples around the large dimples, which are conducive to improving the toughness of the girth weld.After the pre-deformation treatment, the fracture mode changed from ductile fracture to brittle fracture, and the fracture morphology showed a typical cleavage fracture morphology, with a large area of smooth and flat cleavage surface.The fracture morphology of the samples after 2% and 4% prestrain showed river pattern morphology and tear ridge, and the crack in notch front dimple size was smaller, as shown in figures 11(c)-(f).

Effect of pre-strain on microstructure characteristics
The girth welds different filling layers of are affected by the welding thermal cycle, which leads to the nonuniformity of grain size and microstructure distribution.When the peak heat treatment temperature generated by the latter process is between Ac1 and Ac3, it is conducive to the formation and growth of M-A island subjected to the previous process.During the tensile process, strain concentration occurred in the heat affected zone (HAZ), as shown in figure 7. The grain recovery and recrystallization caused by thermal cycling will reduce the dislocation density, inside the ferrite to form a softened ferrite with lower hardness, and cause the C atom in the matrix diffuse to the grain boundary, lead to M-A island or carbides at the grain boundary, as shown in figure 8.
In pre-strain process, the dislocation moves continuously under the tensile stress, and the hardness of M-A or carbide is different from the bainite ferrite matrix, so the dislocation movement is prevented and accumulated at the cross section of the matrix and hard phase.A large number of accumulated dislocations will generate  dislocation entanglement near the matrix and the interface, and the long-term stress will hinder the dislocation movement and make the dislocation movement difficult.When the dislocation motion accumulates, it will form damage holes in the cross section.The microscopic holes introduced by the pre-strain will destroy the geometrical continuity of the material, and then cause the local stress concentration to promote the formation and growth of cracks.

Effect of pre-strain on fracture behavior
Different pre-strain value affected the performance of the sample.After 2% and 4% pre-strain, the sample loses toughness, the fracture separation characteristics are obviously affected by the pre-strain, and the impact toughness of the sample gradually decreases with the increase of the pre-strain value.The pre-strain leads to the change of the internal microstructure of the material, and cracks are easily to initiation and growth at the microscopic holes (figure 7), resulting in a decrease the impact toughness.Meanwhile, the pre-strain changes the stress state inside the material, resulting in stress concentration, stress gradient and residual stress.These stress states change will affect the deformation behavior and fracture mechanism of the material, and thus affect the impact toughness.The pre-strain also causes the change of crack propagation path.Under the condition of no pre-strain, there is no tiny hole in the matrix, which is not easy to initiate crack.In the case of pre-strain, cracks initiation and growth in tiny holes, and dislocation accumulation occurs in the matrix, hindering dislocation slip, making the flow stress greater than the interatomic binding force, resulting in the fracture of the atomic surface.So that the fracture morphology changes from the uneven dimple shape to flat and smooth cleavage fracture surface.After 2% and 4% pre-strain, the fracture morphology of the sample shows river pattern and tear ridge, and the crack notch front dimple size was smaller.

Conclusion
(1) The strain distribution is not uniform during the tensile process.When the strain value reaches at 2%, the strain is mainly concentrated in the weld center and near the heat affected zone.With the increase of strain, the girth weld is uniformly deformed, and the strain concentration is transferred from the girth weld to the base material.
(2) The microstructure of girth weld is mainly composed of granular bainite (GB), lath bainite (LB), polygonal ferrite (PF), acicular ferrite (AF) and M-A island.A few of carbide are uniformly distributed in the matrix, and M-A is uniformly distributed in chain shape at the grain boundaries.After the pre-strain, the continuous failure of chain M-A at the grain boundary, results in stress concentration, the lath bainite widens gradually, and micro-cracks initiation in the matrix, resulting in a significant reduction of impact energy.
(3) By the study of the fracture morphology, the results that 2% strain value is enough to make the girth weld produce tough-brittle transition.After the pre-strain treatment, there are great differences in the initiation region of the girth weld impact samples.The fracture morphology of the samples after 2% and 4% prestrain is typical cleavage fracture morphology, with a large area of smooth and flat cleavage surface, and the fracture morphology presents river pattern morphology and tear ridge.

Figure 1 .
Figure 1.The X80 pipeline steel (a) Girth weld; (b) The size of groove shape.

Figure 2 .
Figure 2. Diagram of sample (a) Sampling direction; (b) The size of tensile specimen.
device consists of two high-resolution 2448 × 2048 pixel CCD cameras, as show in figure3.Two cameras are angled at about 30°in front of the sample, and two LEDs are used for lighting.The measurement accuracy of DIC is closely related to the quality of speckle images.Speckle affects the grayscale distribution of images and the selection of the subset size in DIC analysis[19,20].Before DIC test, the pattern size of matte spray paint is 3-5 pixels to ensure the best recognition effect[21].The nominal strain of the parallel section is set to 2% and 4%, the loading speed is 2 mm min −1 , and the extensometer with a measuring range of 25 mm is used for measurement.DIC was used to record the strain during tensile deformation and analyze the strain response characteristics of girth weld.According to the national standard GB/T.229'Charpy pendulum impact test Method for Metal Materials', processed 10 * 10 * 55 mm impact specimens.Charpy impact test was carried out on 0%, 2% and 4% pre-strain sample at −10 °C by Pendulum impact testing machine (ZBC2302-B) to study the influence of different prestrain value on the impact toughness of girth welds.The sampling position and opening direction of the impact sample are sampled according to Q/SY GDJ 0110-2007 'Technical Specification for Welding of Second-line Pipeline Engineering Lines of West-East Gas Transmission', and the sample size are shown in figure4.
Figure 6(b)  shows the weld of the third layer.The M-A island is distributed in chain along the grain boundary, and the carbide (C) inside the grain is

Figure 3 .
Figure 3. Diagram of DIC Scheme (a) Zone and mark point locations; (b) DIC system schematic diagram.

Figure 4 .
Figure 4.The size of Charpy impact specimen.

Figure 7 .
Figure 7. Stress and Strain curve (a) Tensile curve of test specimen; (b) Axial strain distribution diagram of specimen at e M = 2%; (c) Axial strain distribution diagram of specimen at e M = 4%.

Table 1 .
Welding specifications and constant parameters.