A comprehensive study on the microstructure and mechanical properties of arc girth welded joints of spiral welded high strength API X70 steel pipe

4.1

Microstructure

The microstructure of the base material, the API X70 steel, is shown in Fig. 2. It is composed of polygonal ferrite and granular bainite with the presence of a small amount of martensite–austenite constituent (MA) [7].

Microstructure of steel API 5L X70 (L485), light microscope

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4.1.1

MMA welded joints

The MMA girth welded joints were made with two different combinations of filler materials. Firstly, the root passes were made with covered electrode grade Pipeliner 6P+ while the cap passes were made with covered electrode grade FOX EV 85. Secondly, the root passes were made with Pipeliner 6P+ electrode while the cap passes were made with the filler FOX EV 65 material. The results of macroscopic examination of girth welded joints are presented in Fig. 3.

Macroscopic examination of girth welded joints, a FOX EV 85+ Pipeliner 6P+ for root passes, b FOX EV 65+ Pipeliner 6P+ for root passes

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4.1.1.1

MMA welded joint made with FOX EV85 filler

In the MMA girth welded joint made with FOX EV85 filler material for cap passes and Pipeliner 6P+ for root passes, from the microstructural point of view, the two different regions can be clearly distinguished (Figs. 4, 5). The root passes, formed by two weld beads, about 5 mm thick, were separated from the cap passes of the welded joints by a clear boundary.

Microstructure of MMA girth welded joint—weld metal: a root passes made with covered electrode Pipeliner 6P+, b cap passes made with covered electrode FOX EV85, LM

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Microstructure of weld metal (FOX EV85/Pipeliner 6P+)—boundary between root and cap passes made with various filler materials, a) SEM–CBS (CBS—circular backscatter detector), b SEM–ETD (Everhart–Thornley detector)

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Figure 4 shows the difference between root and cap passes made with various filler materials. In the root passes (bottom part) ferrite prevails with a grain size of several dozen micrometres. On the borders of ferrite grains, a second structural component, occurring as small islands of a few micrometres in diameter, can be distinguished. This may be either martensitic–austenitic (M/A) component [7, 37], bainite [7, 38] or fine perlite [38, 39] (Fig. 5a, b).

Taking into account the relatively too low hardness value for this steel (177 HV10) and the presented CTPc diagram (Fig. 6), it should be assumed that the second structural component, besides ferrite, is pearlite. This is not a typical pearlite with a characteristic lamellar structure, but pearlite known as “degenerated pearlite” or “ferrite-carbide aggregate” [10].

API X70 CCT diagram showing the transformation lines identified [38]

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Microstructure in the HAZ at a distance of about 1 mm from the fusion line is shown in Fig. 7a. Near to the polygonal ferrite, ferrite of different morphology, referred to as bainitic ferrite, which is characterized by irregular grain shape and often irregular boundaries can be observed (Fig. 7b). The hardness in this area was approximately HV170—the value close to the hardness of the base material and to the hardness of the API X70 steel [40].

Microstructure of HAZ in the girth welded joint made with FOX EV85/Pipeliner 6P+—area near to the root passes 1 mm from the fusion line, a light microscopy, b SEM–ETD

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The microstructure at the middle of the girth welded joint thickness, just above the root passes, is substantially different from the microstructure in the bottom part. This microstructure is shown in Fig. 8. While the microstructure in the HAZ is similar to that in the bottom region, the weld microstructure is different, as evidenced by the use of another filler material (FOX EV 85). This microstructure is a typical bainitic structure with ferrite strips and cementite inclusions [10, 40].

Microstructure of HAZ and weld metal in the girth welded joint made with FOX EV85/Pipeliner 6P+—middle thickness of the welded joint, SEM–ETD

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The microstructure in the proximity of the fusion line, a few millimetres above, changes not only in the weld metal but also in the HAZ. The different microstructure in the HAZ at similar distances from the fusion line can be explained by the different thermal history of these areas. A typical microstructure of the area adjoining the fusion line in the middle of the welded joint is shown in Fig. 9a, b. Both micrographs represent the same area, however, the image in Fig. 9a was produced by secondary electrons (SEM–ETD technique) and the image in Fig. 9b was formed backscattered electrons (SEM–CBS technique). The bainitic structure was observed in both weld metal and HAZ, however, the HAZ was characterized by large primary austenite grains comparing to the weld metal area. Simultaneously, the boundaries of primary austenite grains were not explicitly defined. In addition, the M/A component was not observed in this area. The bainitic ferrite laths merge together and assumed complex shapes with irregular boundaries, however, the carbide precipitates within ferritic areas show the course of the original boundaries between bainitic ferrite laths. Such a structure resembles the structure observed in API X80 quenched from 950 °C and subsequently tempered at 600 °C for 1 h [41].

Microstructure in weld metal and HAZ in girth welded joint (FOX EV85/Pipeliner 6P+)—area in the middle of the thickness of welded joint, a SEM–ETD, b SEM–CBS

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The microstructure near the face at the fusion line is also bainitic, but in this case the bainitic ferrite strips are more evident. Also, the boundaries of primary austenite grains are better recognizable, as shown in Fig. 10a. Small amount of ferrite was often observed on the primary austenite boundaries (Fig. 10b, top). Such a ferrite is often referred to as primary ferrite [10, 42].

Microstructure of HAZ in the girth welded joint (FOX EV85/Pipeliner 6P+)—1 mm from the fusion line at the face of the weld, SEM–CBS, a lower magnification, b higher magnification

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Changes in the microstructure near the fusion line at the face of the weld are reflected in the hardness values of these areas. The hardness in the welded joint near the face of the weld was approximately 283 HV1. This value may correspond to a microstructure composed of a mixture of martensite and lower bainite. The hardness in the HAZ was 229HV1, so taking into account the hardness values presented in Ref. [7], this value corresponds likely to the bainitic microstructure.

4.1.1.2

MMA welded joint made with FOX EV65 filler

Also, in the MMA girth welded joint, made with other filler materials (FOX EV 65—cap passes, Pipeliner 6P+—root passes), the well-defined microstructural zones can be easily distinguished in the weld metal at the light microscope scale (Fig. 11).

Microstructure of weld metal in MMA girth welded joint, a root passes made with Pipeliner 6P+, b cap passes made with FOX EV65, LM

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The microstructure of the HAZ was not homogeneous (Fig. 12)—near the fusion line at the root passes a coarse-grained microstructure was observed while closer to the cap passes a fine-grained zone could be distinguished. This is a typical microstructure observed in combinations of different types of structural steels. The microstructure is a mixture of different microstructural components, depending on the thermal history of the individual areas. It should be noted, that heat input (Table 4) was two times lower than for welded joint made with FOX EV85. Therefore, the lower grain size in the HAZ was observed.

Microstructure of HAZ in the MMA girth welded joint: a near the root passes made with Pipeliner 6P+, b near the cap passes made with FOX EV65, LM

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According to Refs [10, 39], the weld microstructure may be composed of ferrite with various morphologies (polygonal, coniferous, acicular, ferrite at or around the borders of austenite grains, Widmanstätten ferrite, bainite ferrite), bainite (also with different morphology), perlite and martensite which usually occurs in the form of martensitic–austenitic islands (M/A). In the FOX EV65/Pipeliner 6P+ welded joint, the weld metal microstructure was very similar to the weld microstructure in FOX EV85/Pipeliner 6P+ welded joint. This resemblance pertains primarily to a different microstructure near the root and cap passes.

4.1.2

MAG welded joints

Two different welding technologies for the girth welded joints were applied in the presented research. The other one was the MAG process. The MAG girth welded joints with two different filler materials were produced. As first, the root passes and cap passes were made with wire LMN MoNiVa grade. Subsequently, girth welded joint was made with the wire Carbfil MnMo. Figure 13 shows the macrostructure of these welded joints. In both welded joints made by the MAG method the distinctive zones of different microstructure can be distinguished.

Macroscopic examination of MAG girth welded joints, a LMN MoNiVa, b Carbfil MnMo

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4.1.2.1

MAG welded joint made with LMN MoNiV

The microstructure of the weld metal and HAZ of the welded joint made with the LMN MoNiV filler material for root and cap passes are shown in Figs. 14 and 15.

Microstructure of weld metal in the MAG girth welded joint made with LMN MoNiVa, a root passes, b cap passes, LM

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Microstructure of HAZ in the MAG girth welded joint made with LMN MoNiVa, a root passes, b cap passes, LM

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The microstructure of the weld metal constitutes fine bainite. The hardness of the weld metal is 320 HV1, which well corresponds to the observed microstructure. In the HAZ, it is possible to distinguish a coarse-grained area, typical of all steel weld metals, which continuously alters to the fine-grained area. The microstructure near the fusion line is similar, but not uniform, throughout the joint thickness. The microstructure observed by SEM is shown in Fig. 16a (root passes) and 16b (cap passes). Near the weld face, both in the weld metal itself and adjacent HAZ, ferrite has acicular morphology (Fig. 16b), however, in the vicinity of the root passes ferrite areas take irregular shapes, while the microstructure of the weld has been preserved (Fig. 16b). A typical microstructure of the weld metal in its central part is shown in Fig. 16c. Apart from acicular ferrite, ferrite in the form of a continuous phase nucleating on the original austenite boundaries can be distinguished.

Microstructure of weld metal and HAZ of MAG girth welded joint made with LMN MoNiVa, a weld metal on the right in the root passes, SEM–CBS, b weld metal on the right in the cap passes, SEM–CBS, c weld metal on the right in the cap passes, SEM–ETD

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The microstructure of the HAZ does not show any significant changes across the thickness of the welded joint. The evolution of the HAZ microstructure in the root passes area at different distances from the fusion line (2, 4 and 7 mm) are shown in Fig. 17a–c, respectively. Micrographs clearly indicate the vanishing of acicular ferrite morphology with the increasing distance from the fusion line. The fine-grained form was found at a distance of just 7 mm from the fusion line (Fig. 17c). Simultaneously, changes in the morphology of the precipitated carbides occur in the microstructure, which are visible at higher magnifications. There are very fine spherical precipitates in some grains. In other grains, the cementite takes the form of twisted lamella that is often referred to as the degenerate perlite [10] – Fig. 17d.

Microstructure of HAZ in the MAG girth welded joint made with LMN MoNiVa filler material, area near the root passes, a 2 mm from the fusion line, SEM–CBS, b 4 mm from the fusion line, SEM–CBS, c 7 mm from the fusion line, SEM–CBS, d 6 mm from the fusion line, SEM–CBS

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4.1.2.2

MAG welded joint made with Carbofil MnMo

The microstructure of the weld metal and the HAZ in MAG girth welded joints made with Carbofil MnMo filler material are shown in Figs. 18 and 19. Compared to the MAG welded joint made with LMN MoNiVa filler material, the microstructure is characterized by a larger primary austenite grain. Moreover, on the primary boundaries the ferrite appears as a continuous phase or the Widmanstätten ferrite with plates growing into the grain interior. In the heat-affected zone, the microstructure changes continuously from the coarse-through fine-grained structure to the structure of the base material (Fig. 20a).

Microstructure of weld metal in the MAG girth welded joint made with Carbofil MnMo, a root passes, b cap passes, LM

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Microstructure of HAZ in the MAG girth welded joint made with Carbofil MnMo, a neat the root passes, b near the cap passes, LM

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Microstructure of HAZ in the MAG girth welded joint made with Carbofil MnMo, a area near to the root passes, SEM–CBS, b area near to the cap passes, SEM–CBS, c area near to the root passes 0.5 mm from the fusion line, SEM–CBS, d area near to the root passes 1.0 mm from the fusion line, SEM–CBS, e area near to the root passes 1.5 mm from the fusion line, SEM–CBS

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The microstructure of the areas adjacent to the fusion line is shown in Fig. 20a (area near the root passes) and 20b (area near the cap passes). The microstructures in the weld metal and the HAZ are similar. However, the appearance of microstructure in HAZ is rapidly changing with the distance from the fusion line. These changes are shown in Fig. 20c–e that are characteristic for areas 0.5, 1, and 1.5 mm apart from the fusion line. The difference in the ferrite morphology is apparent.

4.2

Mechanical properties

To characterise the mechanical properties of the girth welded joints produced by MMA and MAG welding processes, hardness maps, hardness distribution as well as tensile, bend and impact tests were performed. Figure 21 shows hardness maps constructed from a grid of hardness indents (ca. 2250 in total) applied on the welded joint cross sections. The applied load was 5 kg (HV5). Clearly, the largest variation in hardness occurs in the through-thickness direction. It should be noted, however, that for the MMA welded joints three different filler materials were used: Pipeliner 6P+ for root passes as well as FOX EV65 and FOX EV85 for cap passes. The results reveal that the hardness in the root is much lower than in cap passes (Fig. 21b).

Hardness map (HV5) of MMA girth welded joints: a FOX EV65+ Pipeliner 6P+, b FOX EV85 + Pipeliner 6P+

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The difference in the highest hardness values for different filler materials in the cap passes is clearly demonstrated in Fig. 21. It results from the different mechanical properties of filler materials as well as from the thermal cycles that affected the material during welding. According to the PN-EN ISO 15653:2010 standard [42] the hardness may be used to estimate the yield and tensile strengths (Re and Rm). The following equations: Re = 2.35HV10 + 62 and Rm = 3.0HV10 + 22.1, suggested for steel weld metal (in MPa) can be adopted for the calculation. The results of mechanical tests for MMA girth welded joints are presented in Table 5. Mechanical properties of the MMA filler materials are presented in Table 6. The results in Table 6 and Fig. 22 show the good agreement between presented values. The differences between calculation and measurements result from the nature of welding process and thermal history of each welding pass. It should be noted that the hardness distribution corresponds with the microstructure of particular parts of welded joints and reflects their heterogeneity.

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Results of hardness tests for MMA girth welded joints, a filler metal FOX EV65 and Pipeliner 6P+, b filler material FOX EV85 and Pipeliner 6P+

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It may be observed that there is a better agreement between experimental hardness values, and values calculated from Rm, compared to values calculated from Re. This is logical since it is known that hardness is more closely related to Rm than to Re according to the original works by Tabor [44]. Less scatter is observed in HV-Rm diagrams, compared to HV-Re diagrams.

It should be noted, that higher impact energy for MMA girth welded joints made with FOX EV65 filler can be observed (Table 2). It can be caused by the following reason: the heat input during MMA welding with FOX EV65 is much lower than for FOX EV85 (see Table 4), thus the cooling time t8/5 is lower and finally the impact energy can be higher [45,46,47]. Simultaneously hardness (Fig. 22) is a little bit lower at HAZ and much lower in the weld metal.

The same grade of filler material was applied for the MAG welded joints. Therefore, only one thermal cycle of welding processes influenced the hardness distribution. The hardness maps for MAG joints are shown in Fig. 23. The mechanical properties of MAG joints are presented in Table 7. Moreover, the mechanical properties of weld metals and calculated hardness values according to the PN-EN ISO 15653:2010 standard are presented in Table 8. The calculation of hardness strongly matches the experimental data (Fig. 24). However, the real welding process associated with annealing phenomena makes the experimental hardness values lower than calculated.

Hardness map (HV0.05) of MAG girth welded joints: a Carbofil MnMo, b LMN MoNiVa

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Results of hardness tests for MAG girth welded joints, a LMN MoNiVa, b Carbofil MnMo

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The results of the hardness distribution of MMA and MAG girth welded joints confirmed the microstructural evaluation presented in the previous section. The thermal cycles that high strength low alloy and carbon API X70 steel experience during welding inevitably affect microstructure and mechanical properties of the MMA and MAG welded joints. It is generally agreed that the adopted welding filler materials influence on mechanical properties of weld metals. For MMA welding with FOX EV65 as well as FOX EV85 the microstructure of weld metal is similar and is composed of ferrite with various morphologies, bainite, perlite and martensite which usually occurs in the form of martensitic–austenitic islands (M/A). However, the chemical composition of weld metals [21, 23] influences on hardness and strength of welded joints. For FOX EV85 higher strength and hardness is observed (Table 5, Fig. 22). In root pass the same filler material was applied, thus the hardness is almost the same, but lower than in cap passes. In the root passes (bottom part) ferrite prevails with a grain size of several dozen micrometres. On the borders of ferrite grains, a second structural component, occurring as small islands of a few micrometres in diameter, can be distinguished. This is perlite. Therefore, the hardness is lower than cap passes about 180 HV5.

For MAG welded joints, similar as for MMA welded joints the chemical composition of filler materials [26, 28] decided about microstructure as well as mechanical properties of weld metals. The microstructure of the weld metal fabricated with LMN MoNiVa constitutes fine bainite. The hardness of the weld metal is 320 HV1, which well corresponds to the observed microstructure. On the other side, for Carbfil MnMo MAG welded joint the microstructure is characterized by a larger primary austenite grain. Moreover, on the primary boundaries the ferrite appears as a continuous phase or the Widmanstätten ferrite with plates growing into the grain interior. In this case, the hardness of the weld metal is lower than 260 HV1 and the strength of welded joint is lower than for LMN MoNiVa filler material.

The difference in microstructure and hardness distribution correspond also to impact energy (see Table 4). The even match material Carbfil MnMo provides higher toughness than overmatch LMN MoNiVa filler material. Moreover, the heat input (Table 4) for MAG welded joint with LMN MoNiVa is a little bit higher than for Carbfil MnMo filler material. Therefore, the impact energy is lower, but hardness is higher (Fig. 24).

It can be noted, that based on the achieved results the proper selection of welding technology and filler material determine the final mechanical properties of girth welded joints. In case of demand of the highest strength of welded joints the overmatch material should be applied (MMA FOX EV85 and MAG LMN MoNiV). In the other side if the toughness is the most important factor for engineers the FOX EV65 filler material has to be used.