Issue 78

A. A. Frantsuzov et alii, Fracture and Structural Integrity, 78 (2026) 474-488; DOI: 10.3221/IGF-ESIS.78.27

and pattern indexing accuracy. The quality of EBSD patterns can be compromised by the high dislocation density and residual strain fields typical of quenched and tempered martensitic structures, resulting in lower indexing rates and increased uncertainty in orientation determination. As is well known, plastic deformation not only increases the dislocation density that may impede EBSD indexation, but also introduces residual stresses of Types II and III that have a pronounced effect on the mechanical properties, including fracture resistance [11,12]. The abovementioned technical constraints necessitate a systematic comparison of PAGS analysis methodologies to quantify their consistency, identify systematic biases, and define the operational limits of each technique when applied to fine-grained advanced high-strength steels. Such a correlation study is particularly urgent for the development of the hypothesized link between refined PAGS and enhanced sulfide SSC resistance producing statistically robust microstructural metrics. Without a clear understanding of how different characterization methods converge or diverge in measuring ultra-fine PAGS, it remains difficult to establish reliable structure-property relationships or to validate computational models predicting SSC performance based on microstructural inputs. Therefore, bridging the gap between traditional chemical etching and advanced crystallographic reconstruction through direct experimental comparison is essential for standardizing PAGS measurement protocols and ensuring that microstructural optimization strategies use accurate, comparable data. The present study aims to evaluate the applicability of EBSD-based prior austenite grain reconstruction for the determination of PAGS in high-strength C110 OCTG steel. The reconstruction approach is compared with conventional metallographic techniques, including direct measurement of chemically revealed grains, the linear intercept method, and comparison with standard grain size charts. The main objective is to determine whether reconstructed prior austenite grains provide results consistent with traditional measurements and whether the additional statistical information obtained from EBSD, particularly pertaining to the grain size distribution, can improve microstructural characterization of fine-grained casing steels used in sour-service environments. he material used in this work was C110 grade pipeline steel made according to API Spec 5CT [13] after plant heat treatment. This type of pipes is used for casings in oil and gas drilling and production processes, where it should withstand high mechanical loads and is exposed to sour environment enriched with H 2 S. Combined, these factors could lead to sulfide stress cracking, if the material is not specifically designed for such use. The chemical composition and mechanical properties of the material according to the specification are shown in Table 1 and Table 2, respectively. The microstructure of C110 grade steel is typically presented as tempered martensite with carbides located mostly along grain boundaries. The microstructural morphology is hierarchically based on PAG and is dictated by its size and shape. PAGS should be taken into account in order to manipulate the resulting microstructure and therefore control the mechanical properties. T M ATERIALS AND METHODS

C

Mn

Mo

Cr

Ni

P

S

≤ 0.35 ≤ 0.005 Table 1: Chemical composition (mass fraction, %) of steel according to API Spec 5CT specification [13]. σ y , MPa σ t , MPa Hardness 758 – 828 ≥ 793 ≤ 29 HRC Table 2: Mechanical properties of the material according to API Spec 5CT specification [13]. ≤ 1.20 0.25 – 1.00 0.40 – 1.50 ≤ 0.99 ≤ 0.020

A longitudinal cross-section of a C110 grade pipe was cut and mechanically ground and polished until mirror-surface finish was achieved. For EBSD mapping, the surface was then polished using colloidal silica (0,03 µm) for 30 minutes to remove the residual stresses associated with the surface preparation. EBSD maps were acquired using JEOL 500IT LV scanning electron microscope equipped with Bruker eFlash EBSD detector. The working distance was set to 19 mm, and the detector was placed approximately at 16 mm from the focal point. Kikuchi patterns for each point of the acquired map were accumulated for 20 ms for two frames, which were then averaged. The resulting data was extracted and processed using MTEX [9] software for MATLAB. Using the Kurdjumov-Sachs orientation relationship as an initial estimate, the actual parent-to-child orientation relationship was first optimized for the experimental EBSD dataset using the iterative approach proposed by Nyyssönen. This optimization minimizes the misfit between measured child-to-child misorientations and those predicted by the orientation relationship, after which a variant

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