Microstructure and texture of the crystalline phase of steel for high-frequency welded pipelines X65 (2023)

Chemistry and physics of materials

Tom 302,

July 1, 2023

, 127758

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https://doi.org/10.1016/j.matchemphys.2023.127758access rights and content


The microstructure and crystal texture of high-frequency electrowelded (HF-ERW) X65 pipeline steel and their correlation with impact strength were investigated. Base metal X65 after HF-ERW produces a typical hourglass weld with four distinct zones including the thermomechanically affected zone (TMAZ), fine grainheat affected zone(FGHAZ), gross heat affected zone (CGHAZ) and connection line (weld zone). Base metal X65 exhibits different texture components (113)[110] and (112)[110] due to thermomechanically controlled processing. Due to partial recrystallization during welding, TMAZ presents a random texture of cubic (001) [110] elements of lower strength. In FGHAZ and CGHAZ near the glue line, the matrix metal texture is converted to Rotated Gauss(110)[110], Gauss(110)[001] and Rotated Cube(001)[110], thanks toausteniticand shear deformation in the austenitic temperature range. A high density of {100} and {110} cleavage planes was observed in CGHAZ, which are often found as fracture sites in case of impactendurance test.

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Microstructure and texture of the crystalline phase of steel for high-frequency welded pipelines X65 (3)
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Pipelines are the primary means of transporting oil and gas products over long distances. Due to the optimal combination of strength and toughness, the most suitable material for these pipes is microalloyed steel, certified and classified by the American Petroleum Institute (API) [1], [2], [3], [4]. Microalloyed steels are carbon-manganese steels that contain small amounts of microalloying elements, including niobium, titanium and vanadium [1,5]. Thanks to the addition of microalloying elements and controlled thermomechanical processing, the desired strength and impact strength are achieved by grain crushing, dislocation strengthening, solid solutions and precipitation hardening [1]. However, these carefully controlled microstructural features are subject to drastic changes during the welding process, resulting in poor mechanical properties [1,5]. High-frequency resistance welding (hereafter HF-ERW) is sometimes used for longitudinal welding of smaller pipes (less than 24 inches in diameter); not used for field installation, i.e. in HF-ERW hot rolled steel strip is cold formed into pipe. A high-frequency alternating current is used to heat the strip edge, which is then compressed without adding additional material to form the weld [6,7].

One of the challenges with welded HF-ERW pipeline steels is poor impact strength at low temperatures. It has been widely reported that the impact strength of the rear bonding layer (seam zone) of HF-ERW is lower than that of the pipe body [8], [9], [10]. Therefore, HF-ERW pipeline steel welds are usually post-weld heat treated (PWHT) to optimize the post-weld microstructure [ [11] , [12] , [13] , [14] , [15] , [16] However, the ductility is often unsatisfactory even after heat treatment after welding [15,16]. Karaji et al. [8] studied the effect of different PWHT cycles on the Charpy V impact strength (CVN) of HF-ERW X60 pipeline steel. Their results showed significant differences in the value of absorbed energy. In another attempt, Sisi et al. [9] reported similar changes in impact toughness values ​​in a peak temperature and time test during normalization after welding of high frequency induction welded steel X52. However, the mechanism of the change in the impact strength value is not clear. Angie Dan et al. [10] reported that the hardness change was due to a locally formed crystal structure near the weld zone. Their research showed that X52 microalloy steel, whose main components are brass, S and copper, has strong post-weld texture and post-weld normalization. Similarly, Yan et al. [15] studied the poor ductility of high-frequency welded X65 steel pipes after PWHT. Their results showed that the presence of non-metallic inclusions as welding defects, large grain size and crystalline structure are responsible for reduced impact strength. In another study by Zhang et al. [16] reported that the impact strength values ​​of high-strength low-alloy steels after high-frequency welding are affected by the combination of microstructure and crystal texture. Moreover, it has been reported that the texture components formed during the HF-ERW process are retained even after PWHT [10,15,16]. However, the crystallographic texture evolution mechanism during welding and the weld texture behavior after PWHT have not been elucidated. During HF-ERW, the structure of the base metal is locally changed by a combination of heating, deformation and transformation (narrowing of dimensions due to skin and proximity effects in high-frequency heating) [7]. Therefore, the texture mechanism of HF-ERW is complex and little information is available to researchers.

Published literature on HF-ERW welded joints of steel pipelines includes studies of microstructure and texture after postweld heat treatment [8,9,11,12,15]. It is worth noting that few studies have focused on welding conditions. This may be due to the fact that in practice HF-ERW pipeline steels are subjected to post-weld heat treatment before commissioning [4]. However, welds can affect the evolution of microstructure and texture during subsequent post-weld heat treatment; therefore, the impact strength at low temperatures is high. On the other hand, the texture evolution mechanism during PWHT must be affected by the weld texture, which has not been fully addressed anywhere [[15], [16], [17].

Therefore, the aim of this work is to characterize the as-welded HF-ERW X65 pipeline steel to help understand the microstructure and texture of HF-ERW directly after thermal and deformation cycling. This understanding will provide insight into PWHT process parameters to improve useful microstructures and textures.

partial extract

Materials and methods

API X65 grade steel was used as the base metal in this study. Nominal compositions are given in Table 1. HF-ERW X65 steel pipe is obtained from commercial pipe factories that undergo a special in-house welding process. Thermomechanically treated X65 grade steel strip is cold formed into a cylindrical shape by roll forming. Use of high-frequency alternating current (welding) to heat adjacent strip edges above the melting point (~1550°C)

organization of base metals

As shown in Figures 2a and b, the microstructure of the X65 base metal has profiled ferrite and a small amount of minor components. During cooling, austenite forms an inhomogeneous ferrite phase. As a result, the remaining austenite becomes enriched in carbon and other alloying elements and transforms into pearlite, bainite and martensito-austenite (M-A) islands. Their formation will depend on the history and percentage of thermomechanically controlled machining (TMCP).

microstructural evolution

With HF-ERW, the microstructural composition of the X65 base metal changed dramatically. The heat and deformation applied during HF-ERW produced a typical hourglass-shaped weld with four distinct regions as shown in Figure 6. This unique weld profile is due to the skin effect and the proximity of the high-frequency alternating current supplied during welding [23 ] . Due to the nature of the high frequency current, the heat will be concentrated only on the surface, resulting in shrinkage

in the end

In this study, the microstructure and texture evolution of X65 pipeline steel after high frequency resistance welding (HF-ERW) was investigated. In conclusion, the following conclusions can be drawn.

  • As a result of thermomechanically controlled machining, the X65 base metal exhibits deformed heterogeneous ferrite grains with visible (113)[110] and (112)[110] texture components.

  • In the thermomechanical interaction zone (TMAZ) there are mostly subgrains with grain boundaries at a small angle

CrediT Author Contribution Statement

Kopati Ravikiran:Study planning and research, methodology, evaluation, experimentation, data collection, preparation, writing of the original draft, preparation of the revised manuscript.Leijun Li:Supervision, conceptualization, fundraising, research coordination, interpretation of results, manuscript proofreading.Written by Greg Leinhoff:Study planning and research, methodology, evaluation, periodic coordination, data curation, manuscript correction/revision.Nitin Kumar

Conflict of interest statement

The authors confirm that they have no known financial interests or personal connections that could influence the work described in this article.

Thank you

The author thanks for the support fromMISCELLANEOUSThanks to Dr. Jing Su, EVRAZ NA, for help with texture data interpretation and Dr. Alejandro Hintze Cesar, EVRAZ NA, for valuable advice on Accelerate.


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