Issue 78

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

K EYWORDS . Steel, Casings pipes, Prior austenite, Microstructure, Stress corrosion cracking.

I NTRODUCTION

H

igh-strength casing pipeline steels, commonly referred to as Oil Country Tubular Goods (OCTG), are critical components in the oil and gas industry, serving both extraction and transportation functions under increasingly aggressive environmental conditions. A primary challenge in these applications is the exposure to sour service environments containing hydrogen sulfide (H 2 S), which requires exceptional resistance to sulfide stress cracking (SSC). Sulfide stress cracking in steels proceeds by the formation of metal sulfides (MeS) and the release of atomic hydrogen that may diffuse into bulk metal, causing embrittlement and promoting fracture. The susceptibility of martensitic steels to SSC is intricately linked to their microstructural hierarchy, with the prior austenite grain size (PAGS) emerging as a pivotal parameter governing material performance [1–4]. The significance of PAGS lies in its direct influence on the resulting martensitic microstructure formed during heat treatment; specifically, it dictates the dimensions of martensite packets and blocks, which serve as the fundamental structural units controlling crack propagation and hydrogen diffusion pathways [5,6]. Recent investigations [7] have elucidated that the refinement of PAGS through multiple quenching cycles induces profound microstructural transformations beyond just grain size reduction. Concurrently, this thermal processing regime significantly increases grain boundary density and dislocation accumulation within the matrix. Although high-grade OCTG pipes typically undergo tempering that eliminates untempered lath martensite, the tempered microstructure retains the geometric imprint of the original martensitic architecture. Consequently, the refined packet and block structures derived from smaller PAG (Figure 1) persist in the final product, creating a more tortuous path for crack propagation and potentially enhancing SSC resistance by impeding hydrogen-assisted fracture mechanisms.

Figure 1: Schematically illustrated lath martensite formation in PAG.

It is worth noting that additional quenching cycles, while often observed in industrial manufacturing settings, are frequently implemented as corrective measures rather than deliberate microstructural engineering strategies. In many production scenarios, these secondary heat treatments are employed to rectify deviations in mechanical properties when initial processing fails to meet specified grade requirements, rather than being explicitly designed to optimize PAGS. Despite the substantial economic costs associated with these redundant thermal cycles, even inadvertent refinement of PAGs may result in unintended metallurgical benefits. The resulting increase in boundary density and the modification of martensitic variant selection could positively influence the material’s resilience against SSC, suggesting that strategic control of PAGS through purposeful multi-cycle quenching could be leveraged as a viable approach to enhance the service reliability of OCTG steels in sour environments.

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