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Fatigue Resistance of the Welded Joint
from Weldability Investigations of 120 mm TM+ACC Thick Heavy Plate Steel, designed for Offshore App.
The fatigue resistance of the welded joint of the investigated type with a HI of 50 ± 2 kJ/cm was determined at test temperature minus 10 °С by crack tip opening displacement value δ (δCTOD) along the most brittle area of HAZ: the grain coarsened zone (GCHAZ) near the fusion line. Fig. 10 shows a characteristic graph of crack opening, which characterizes the conditions of crack development up to the moment of critical opening (loss of signal) and the type of a E36 rolled sample with a thickness of 120 mm after the end of the test. At the fracture, five main fracture zones can be distinguished according to the viscous mechanism (marked in Fig. 10b): 1) fracture initiation zone (beginning of crack development); 2) crack development zone; 3) primary zone of rupture; 4) secondary zone of rupture; 5) fracture contraction zone.
GCHAZ of the investigated steel grade is characterized by average values of the maximum applied load before fracture crack development at the level of FmCTOD = 125-130 kN and a δ CTOD average value of 1.36 mm. The results obtained testify to the high fatigue resistance of the welded joint [ 15 ].
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The microstructure and mechanical properties of welded joints of low-carbon micro-alloyed heavy plate steel of E36/S420ML quality grade with a thickness of 120 mm produced at NLMK DanSteel facility by thermomechanical treatment followed by accelerated cooling were studied.
The chemical composition, the technology for the production of heavy steel plates, as well as technological regimes and welding procedures were developed at NLMK DanSteel.
The investigated welded joints with a heat input of 15 and 50 kJ/cm are characterized by:
• Low level of carbon equivalents: Ceq ≤ 0.33%, Pcm ≤ 0.14%, CET≤ 0.21%;
• The absence of critically brittle microstructural components in the investigated heat-affected areas;
• Strength at the level of: ReH = 397-415 MPa; Rm = 503-504 MPa;
• Impact energy in the transverse direction at -40 °C in the most critical areas of the welded joint, not lower than 120 J;
• High fatigue resistance of the GCHAZ zone with a δCTOD average value of 1.36 mm;
The results of the study and testing of welded joints were used to successfully passed weldability certification tests of heavy plate rolled products of E36/S420ML quality grade with a maximum thickness of 120 mm in accordance with DNV-OS-B101-2021, ABS Rules for Materials and Welding, with acceptance and verification by maritime classification societies DNV and ABS.
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