Issue 61

E. Entezari et alii, Frattura ed Integrità Strutturale, 61 (2022) 20-45; DOI: 10.3221/IGF-ESIS.61.02

API- 5XL Steel grades

Steel manufacturing process

Microstructure

Microstructural features

Bainite-Martensite multiphase steel: * B+M+F

* Slender bainitic sheaves along with martensite laths [61]. * Smaller prior austenite grain (PAG) and fine ferrite grains [62]. * Martensite laths [61]. * High volume fraction of filmy austenite [63, 64]. ¤ polygonal and granular ferrite act as the second phase in the bainite matrix [65]. Fine ferrite acts as the second phase in the martensite matrix [66].

X70

TMCP + QT

Bainite dual-phase steel: ¤ B+F

TMCP + AcC

X80

Bainite-Martensite multiphase steel: * B+M+A

TMCP + AcC + HOP

Bainite dual-phase steel: ¤ B-F Martensite dual-phase steel: M+F Tempered Lath Martensite: TLM

X100

TMCP + AcC + HOP

X120

TMCP + AcC

Martensite laths [67].

Table 4: Classification of the new generation of high-strength pipeline steels based on microstructural control. Polygonal and granular ferrite, austenite, martensite laths, slender bainitic sheaves, smaller prior austenite grains, and fine ferrite grains are microstructural features that can be accomplished by controlling temperature parameters during TMCP [60-66]. Such microstructural features can be achieved based on the manufacturing process mentioned in Tab. 4 and positively influence the mechanical properties of pipeline steels, as shown in Fig. 1 and Tab. 5 [48, 50, 52, 57, 62- 69]. Microstructure and mechanical properties Tab. 6 presents data about the relationship between microstructure and chemical compositions of pipeline steels and their influence on tensile properties [39, 41, 48, 50, 52, 57]. As shown in Tab. 6, API X120 steel is micro-alloyed with different chemical elements, and also the amount of carbon content is reduced [41].

Figure 1: Minimum tensile properties of the new generation pipeline steels.

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