Influence of hydrogen on the microstructure and fracture toughness of friction stir welded plates of API 5L X80 pipeline steel

Highlights

• Hydrogen embrittlement of an FSW welded joint of a pipeline steel is studied.

• The role of hydrogen on the microstructure and fracture toughness.

• Samples were hydrogenated at room temperature during 10 h in a solution.

• The embrittlement affected much more the stir and hard zone, than the base metal.

• H-charged samples with fine bainite and ferrite present high toughness.

Abstract

In this work, the influence of hydrogen on the microstructure and fracture toughness of API 5L X80 high strength pipeline steel welded by friction stir welding was assessed. Samples were hydrogenated at room temperature for a duration of 10 h in a solution of 0.1 M H₂SO₄ + 10 mg L⁻¹ As₂O₃, with an intensity current of 20 mA cm⁻². Fracture toughness tests were performed at 0 °C in single-edged notched bending samples, using the Critical Crack Tip Opening Displacement (CTOD) parameter. Notches were positioned in different regions within the joint, such as the stir zone, hard zone, and base material. Hydrogen induces internal stress between bainite packets and ferrite plates within bainite packets. Besides, hydrogen acted as a reducer of the strain capacity of the three zones. The base metal had a moderate capacity to resist stable crack growth, displaying a ductile fracture mechanism. While the hard zone showed a brittle behavior with CTOD values below the acceptance limits for pipeline design (0.1–0.2 mm). The fracture toughness of the stir zone is higher than that of the base metal. Nevertheless, the stir zone displayed higher data dispersion due to its high inhomogeneity. Hence, it can also show a brittle behavior with critical CTOD values.

Introduction

Friction Stir Welding (FSW) is a solid-state joining process, which eliminates the melting and solidification problems associated with the use of the fusion welding process on high strength steels. Therefore, several efforts have been made during the past years for the successful implementation of FSW on these steels, which are used in energy, nuclear, petrochemical, and building industries [1], [2], [3], [4], [5], [6], [7], [8].

FSW does not significantly increase the hydrogen content in API 5 L X80 high strength pipeline steel during dry and underwater welding [1], [9]. This statement suggests that FSW reduces the risk of hydrogen embrittlement in comparison to fusion welding processes during welding, which has been associated with the absence of hydrogen sources such as filler metal and process temperatures below the melting point.

Nevertheless, the validation of FSW also requires assuring the mechanical properties under hydrogen effects. As it is well known, hydrogen input could also take place during service conditions (cathodic charging or corrosive environments) [10]. After the hydrogen diffusion, atoms can get positioned at the lattice and crystal imperfections or traps, such as inclusions, voids, grain boundaries, and dislocations [11]. This hydrogen interaction between welded joints and some environment lead to embrittlement and reduces the mechanical properties of steels [12], [13].

The influence of hydrogen on the microstructure and mechanical behavior of high strength pipeline steels [14], [15], [16], [17], [18], [19], [20] and their welded joints [21], [22], [23], [24] have been widely studied.

For pipeline steels, the influence of the hydrogen in the yield strength and tensile strength cannot be deemed significant [14], [15], [25], [26]. Nevertheless, the ductility (fracture elongation) [14], [27], [28] and fracture toughness decrease significantly in hydrogen environments [9], [12], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [29]. In addition, the loss of ductility increases when the strength level of the steel is high [14], [27]. Hydrogen induces the fracture mechanism transition from ductile to brittle, from microvoids coalescence to transgranular cleavage or intergranular fracture, respectively [17], [28], [30], [31]. The detrimental effect of hydrogen is associated with the reduction of strain capacity of the matrix [32], which is usually a mixed ferrite-perlite or ferrite-bainite microstructure [21].

A similar trend was observed in welded joints of pipeline steels. For API 5L X70 and X52 pipeline steels, Chatzidouros et al. [21] showed that fracture toughness of base metal (BM) and heat-affected zone metal (HAZ) decreased after hydrogen cathodic charging. Lee et al. [22] showed that electrochemical hydrogen charging reduced the impact toughness of BM and coarse-grained HAZ at room temperature and −40 °C in API 5L X70 welded by shielded metal arc welding. An et al. [24] showed that hydrogen gas decreased the ductility (elongation and area reduction) and fracture toughness of BM and weld metal (spiral submerged arc welding), and had little influence on the yield strength and tensile strength of API 5L X80 steel. Also, they showed that hydrogen increased the fatigue crack growth rate of the BM and weld metal.

As fusion welding processes result in a wide variety of microstructures, the influence of hydrogen in mechanical behavior is attributed to several factors. On the one hand, failure is affected by local hydrogen concentration, which depends on the binding energy traps. The low binding energy traps, grain boundaries, and dislocations are considered as the main sources for hydrogen embrittlement [22]. On the other hand, some microstructures are proven more susceptible to hydrogen embrittlement such as coarse microstructures of welding joints [22], inclusions, and hard constituents of bainite and martensite/austenite (M/A) [16], [21], [24], [33].

Few studies have reported the hydrogen effects on fracture toughness on FSW welded joints of pipeline steels [9], [29]. The hydrogen decreased the ductility (elongation at fracture) of both the BM and stir zone (SZ) [9] and accelerated fatigue crack growth rate of the BM, SZ, and HAZ [29]. Sun and Fujii [9], showed that the SZ showed higher resistance to hydrogen embrittlement than the BM. By contrary, Ronevich et al. [29], showed that the fatigue crack growth rate was slightly highest in SZ.

Although the FSW welded joint contains a hard zone (HZ), which is considered as a local brittle zone [4], [34], there is no information reporting its behavior under hydrogen effects. This is important since the hydrogen would have a strong influence on this zone in comparison to the BM, restricting the commercial pipeline applications.

The current work examines the influence of hydrogen on fracture toughness of welded joints of API 5L X80 pipeline steel (ISO 3183 × 80 M). Chemical composition (0.04 %wt. C, 0.32 %wt. Si, 1.56 %wt. Mn, 0.06 %wt. Cr), microstructure (stir and hard zone with granular bainite, acicular ferrite, and bainite packets with irregular and straight ferrite plates) and mechanical properties (yield strength of 593 ± 21 MPa, ultimate strength of 658 ± 34 MPA and elongation of 17 ± 1%) were reported in previous work [2]. Fracture toughness was measured at 0 °C using CTOD with notches placed in different regions within the joint. The results suggest that FSW has limited applicability under hydrogen effects since CTOD values of the hard zone are below the acceptance limits, 0.1–0.2 mm, for pipeline design or defect acceptation [4].