**Abstract**

The API 5 L X80 is a high strength microalloyed steel, widely used in the gas and oil industry to fabricate pipelines. This steel presents a combination of elevated strength and toughness. In the present investigation, the microstructural features, fracture toughness and delamination occurrence of two X80 grade steel plates with different processing routes and chemical composition were studied. The first steel depicted a homogenous bainitic matrix and the second steel exhibited a banded microstructure composed of elongated ferrite grains, with macrosegregation in the mid-plane of the plates. Fracture toughness tests were conducted for both steels on 7-mm and 15-mm thick samples. The orientation distribution functions analysis revealed crystallographic intensity distribution of the austenite to ferrite transformation texture, especially the alpha-fiber (< 011 > || rolling direction) which explained the anisotropy and delamination occurrences. Both processed plates of steel presented the alpha-fiber due to hot-rolling of plates. Delaminations occurrences were further investigated and attributed to a strong {100} <011> orientation presence despite microstructure homogeneity. A schematic model was proposed, showing the source of delamination and the reason for the lowest toughness for 45° to the rolling direction.

**Keywords:** crystallographic texture, fracture toughness, mechanical strength, microstructure characterization, TMCP, X80 plates of steel, pipeline steel

### **1. Introduction**

The efficiency of pipeline transport systems, widely employed in the oil and gas industries, depends, to a significant extent, on increasing diameters and working pressures while reducing the wall thickness to lower the cost per transport unit [1, 2]. In this context, pipeline steels have been continuously developed toward increased strength, toughness, and formability, as well as maintaining low carbon composition to ensure adequate weldability [1, 2]. In order to achieve these goals, pipeline steels, such as API 5 L X80, X100, and X120, rely upon alloy design and

Thermo-Mechanically Controlled Processing (TMCP) to produce grain refinement by controlled deformation of austenite during rolling [3, 4]. In general, rolling of TMCP steel plates are carried out in two stages: first, rough rolling is performed in the temperature range of austenite recrystallization while alloying elements are in solution (normally above 1100°C); then, at lower temperatures (typically below 1000°C, sometimes in the intercritical range) finish rolling passes are executed, cold-working the matrix [4–6]. At these lower temperatures, the presence of precipitated carbides inhibit grain growth, and fine austenite grains, substructure, and dislocations assist the formation of a refined ferritic or bainitic structure, depending on the cooling conditions [4, 7]. Because of the low-temperature rolling, diffusional phenomena are limited, and the deformed microstructure carries strong crystallographic textures, which lead to anisotropy and possibly a decrease of mechanical properties [4, 5, 8]. As such, numerous investigations have been performed recently to understand texture formation in TMCP steels and its correlation with mechanical properties, especially impact toughness behavior (Charpy tests) [5, 6, 8–10].

Fracture toughness and how crack propagation occurs in steels, depend on their chemical composition [4, 5, 11–13], resultant microstructures [4, 5, 10–12, 14], inclusions [4, 5, 11, 12], grain morphology, e.g., pancaked or elongated [4, 12], crystallographic textures [4, 5, 10–14] and residual stresses produced after the TMCP process [15]. Moreover, many studies in the literature point out the crystallographic orientation as the major cause of delamination, i.e., the presence of 001 plane more specifically [5, 8, 11, 15–18]. Most of the literature presents impact toughness results from Charpy tests. Charpy tests are recommended for qualitative estimation of toughness, with samples that can be machined with low-cost, tests are conducted rapidly, and results are easily processed [19]. For accurate measurements of toughness, crack-tip opening displacement (CTOD) tests are recommended [20]. Also, the crack-tip constraint of Charpy samples changes due to the dynamic loading [21], however, a pipeline under operative conditions do not suffer impact loads, but rather a quasi-static evolution of pressure and internal forces. CTOD tests, with samples designed to guarantee a constant crack-tip triaxiality [21], which represents a crack propagation on opening mode I in quasi-static conditions, has not been well documented in the literature related to the fracture toughness measurement of pipelines.

Delamination is a brittle fracture behavior reported in TMCP steels [9, 17, 22–25], which occurs at the weakest interface, usually near the crack tip. There are two types of delamination based on its geometry: (i) crack divider and (ii) crack arrester. The divider branches the crack into a series of cracks traveling a narrower path [22, 23]. The arrester delamination does not result in crack branching, maintaining the same width of propagation. However, it reallocates the crack at a region with no plastic zone ahead of the crack, triggering the re-initiation of the crack under conditions of nearly uniaxial tension, resulting in high absorption of energy [22, 23]. There are several reports of delaminations of both types in Charpy tests in the literature [9, 17, 23–25] and few reports of delaminations during CTOD tests in the literature [8].

Zong et al. studied the influence of crystallographic orientation upon impact toughness through Charpy impact tests on an API X100 steel [17]. The influence of microstructure on the toughness results was excluded by using the same steel plate. The influence of crystallography orientation was assessed by milling out samples from the different orientation, 0, 30, 45, 60 and 90° tilted from rolling direction. The best condition was found at 0° and then at 90° [17], where the fracture orientation factor, a factor used to characterize the anisotropy of the fracture strength based on <100>, presented lower values compared to 45° to RD, the direction in which maximum fracture orientation factor was obtained. Bakshi et al. [5] studied

*Effect of Textures and Microstructures on the Occurrence of Delamination… DOI: http://dx.doi.org/10.5772/intechopen.88001*

the influence of the TMCP, microstructure and crystallography on an X70 also varying the orientation of the machined samples for Charpy impact tests. As many cited works, Bakshi et al. [5] also reported that the presence of 001 plane induces delamination.

Pyshmintsev et al. [26] suggested that the clustering of (001) cleavage plane parallel to the crack plane does not lead to delamination and splitting phenomena, but that microstructure plays the major role. It was reported that prior austenite grain boundary with cube rotation texture lying parallel to crack plane propagation causes severe splitting [26]. Another study [27], reported that the anisotropy in Charpy tests results could be correlated to factors as the spatial grain distribution, grain shape, and the distribution of the phases and microconstituents, mainly the hardening ones. Kimura et al. reported that grain size was the main key to control yield strength and delamination. By applying a TMCP, ultra-fined grain was obtained, increasing the yield strength and triggering delaminations of crack-arrest type, increasing toughness as well [23].

In this work, we aim to determine the causes of the occurrence of delamination occurring during and after fracture toughness tests. The present investigation was conducted using crystallographic textures, microstructures analysis by light optical microscopy (LOM), scanning electron microscopy (SEM) coupled with energy-dispersive x-ray spectroscopy (EDS), electron back-scattered diffraction (EBSD), X-ray diffraction (XRD), and mechanical assessment with tensile tests and fracture toughness test, specifically crack-tip opening displacement (CTOD) tests, in two steel plates of X80 fabricated by TMCP. In addition, a complete fractographic analysis was conducted. The first steel was air-cooled after the last rolling pass in the intercritical region, referred in the text as the AIR-steel. It presented a banded microstructure and segregation in the mid-thickness of the plate, with elongated manganese sulfide particles and pearlite colonies. The second steel, hereafter designated as ACC-steel, underwent accelerated-cooling after finish rolling and exhibited a bainitic matrix with equiaxed bainite packages.

#### **2. Materials and methods**

Two plates of X80 grade TMCP steels, produced elsewhere using different cooling conditions, air-cooling (AIR-steel) and accelerated-cooling (ACC-steel) after finish rolling in the intercritical range, were used in the present study. The chemical composition of both steel plates is shown in **Table 1**, which is in good agreement with the ISO 3183 standard [28] requirements.

The fracture toughness assessment was conducted using CTOD tests, with experimental testing according to the ASTM 1820-13 standard [29]. Rectangular


#### **Table 1.**

*Chemical composition of the ACC-steel and AIR steel (wt.%).*

(Bx2B) single edge bending notched samples with different thicknesses (B) of 7 and 15 mm were assessed. First, the 7-mm-thick samples were tested at 25°C to analyze the effect of the crack propagation direction in fracture toughness; therefore notches were located through the transverse (L-T) and longitudinal (T-L) direction (more details about crack orientation the ASTM E1823 standard [30]). After that, the direction of best fracture toughness result of the 7-mm-thick samples in each steel was chosen to conduct tests in thicker plates (15 mm), to increase the constraint of the crack-tip and assess the effect of temperature. 15-mm-thick samples were assessed at 0, −20 and −40°C, using the L-T direction in the AIR-steel and T-L direction in the ACC-steel. Notice that ACC-steel CTOD results were previously reported by Avila et al. [31]. Side grooves were machined on 15-mm-thick samples after pre-cracking to increase triaxiality state at the crack-tip, with straight cracktip fronts during the CTOD tests.

Tensile tests were conducted in cylindrical samples with a diameter of 6 mm in the reduced area, following ASTM E8 standard [32]. Tensile samples were machined in rolling and transverse directions. The tensile tests were also conducted at temperatures 25, 0, −20 and −40°C.

X-ray Diffraction (XRD) was conducted to assess crystallographic texture along the normal and transverse directions (ND-TD). CuKα radiation with wavelength λ = 1.54059 Å, a continuous scanning speed of 0.14°/min, 0.02° per step and 2θ range of 40 to 100° were used during the X-ray measuring. Experimental pole figures were determined by varying azimuthal angle phi (ϕ) 0 to 360° in 3° steps and tilting angle chi (χ) from 0 to 87° in 3° steps. Orientation distribution functions (ODFs) were obtained from independent measurements of the (110), (200) and (211) planes.

For metallographic purposes, samples were ground from 100-grit up to 1200 grit SiC paper. Polishing was performed with diamond pastes of 3 and 1 μm and final polishing was performed in a silica suspension with 0.06 μm particle size.

Microstructural characterization was carried out on all three planes: rolling, transverse, and normal, where the rolling plane is perpendicular to the rolling direction (RD), likewise the transverse plane to TD and normal plane to ND. LOM, SEM coupled with an EDS and EBSD detectors, and Vickers hardness measurements were performed. The EBSD measurements were conducted on non-etched samples and two different magnifications were used. Areas of 1500x1300 μm2 with a step size of 2.5 μm and 75 × 65 μm2 with step sizes of 0.1 μm were used. Misorientation above 15° was used to considered grain boundaries. Then the effective grain size was determined using the area method. Grain size measured by the linear intercept technique, was carried out on LOM images. Samples were etched with 2% Nital.

### **3. Results**

#### **3.1 Microstructure**

**Figure 1a**–**c** shows the microstructure of both steel plates. Microstructural misorientation cubes in **Figure 1d**, **e** present the microstructure morphology and distribution near the mid-thickness. The fine secondary phases and constituents (SP) presence between ferrite grains in steels depends on the alloying elements and their effect on the transformation kinetics during cooling [33]. The air-cooling after the finishing rolling pass provides enough time at elevated temperature, enabling diffusion and resulting in a variety of incomplete transformations and microconstituents classified into martensite-austenite (M-A), degenerated pearlite (DP), bainite and martensite [33–35]. The AIR-steel exhibited, in **Figure 1a**, **b**, a banded

#### *Effect of Textures and Microstructures on the Occurrence of Delamination… DOI: http://dx.doi.org/10.5772/intechopen.88001*

microstructure composed by polygonal ferrite, some quasi-polygonal ferrite, pancake grains evidencing a finishing rolling pass at an intercritical temperature, with the formation of banding of fine phases and constituents. A macrosegregation band in the mid-plane induced by solidification is visible on the transverse plane.

The ACC-steel presented a homogenous microstructure, as shown in **Figure 1c**, **d**, over the transverse and rolling planes composed by granular bainite, with more equiaxed shape, some elongated grains, and SP. Grain size measurements using EBSD data and conventional optical procedure showed similar values, around 1.2 μm, as shown in **Table 2**. However, based on the standard deviation, the grain sizes can be considered similar in both steel plates.

The ACC-steel presented a chemical composition with lower alloying elements content than AIR-steel, resulting in less MA and microconstituents dispersed on a bainitic matrix. The ACC-steel depicted a fine ferrite and bainite matrix with dispersed SP. In addition, the accelerated-cooling suppressed diffusion and favored the formation of bainite products, as packets of bainite and granular bainite [33, 36]. According to Bhadeshia et al. [37], bainite formation takes place first by the growth of one single crystal and formation of clusters, known as packet sheaves, by the cooperative growth of other crystals, with low misorientation angles between the sub-units. As ACC steel had a bainitic microstructure, it presented a higher content

#### **Figure 1.**

*SEM micrographs of the rolling plane of the plates: (a, b) AIR-steel presenting segregation and secondary phases (SP), (c) ACC-steel showing a homogeneous microstructure. Misorientation angle distribution cubes of (d) AIR-steel and (e) ACC-steel. Grain boundary misorientation >10°.*


#### **Table 2.**

*Grain size values of the studied steels.*

of low angle misorientation, between 5 and 15°, than the AIR-steel. The ACC steel presented similar morphology on both planes, rolling and transverse, as shown in **Figure 1e** since the accelerated-cooling resulted in a larger ferrite//bainite nucleation rate. Furthermore, the microstructure did not present elongated ferrite, evidencing a finishing rolling pass at a full austenitization temperature.

SP were observed in both plates of steel distributed around the ferritic matrix, as shown in **Figure 2a**, **b**. However, the AIR-steel presented a higher number of constituents within the light contrast bands, such as SP bands depicted in **Figure 2a** and zoomed-in in **Figure 2b**. The SP was composed of elongated and massive shapes of DP and M-A, and manganese sulfide (MnS), as detailed in **Figure 3**.

EDS analysis conducted at the mid-thickness on a specific region confirms the presence of MnS in the AIR-steel. These elongated MnS particles also depict silicon and titanium presence, as shown in **Figure 4**. ACC-steel did not show segregation in the mid-thickness.
