Low temperature mechanical properties and fracture toughness (j-integral) of SA.738 GR.B and two nickel steels

Gao, Yunpeng (2025). Low temperature mechanical properties and fracture toughness (j-integral) of SA.738 GR.B and two nickel steels. University of Birmingham. Ph.D.

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Abstract

This thesis presents a comprehensive evaluation of SA738 Gr.B steel and 9 wt% and 7 wt% Nickel steels, focusing on their low-temperature mechanical properties and fracture mechanisms. SA738 Gr.B steel, a high-strength low-alloy steel, is widely used in pressure vessels and nuclear reactor components due to its resistance to brittle fracture. In contrast, high-nickel steels offer exceptional toughness at cryogenic temperatures, making them ideal for LNG storage tanks and other low-temperature applications. Mechanical testing included hardness testing, tensile testing, Charpy impact testing, blunt-notch fracture stress testing, and sharp-crack fracture toughness testing, conducted across temperatures ranging from -196 °C to -60 °C.
For SA738 Gr.B steel, two heat treatment conditions (HT1 and HT2) were assessed. Tensile testing revealed that both conditions exhibited splitting fracture behaviour, likely caused by the development of a high triaxial stress state within the necked region during tensile deformation. A pronounced temperature dependence of yield stress was also observed: the yield stress decreased systematically as the test temperature increased. The ductile-to-brittle transition behaviour was similar for HT1 and HT2, showing a range of -100 °C to -80 °C. Their transition temperatures were estimated as -90 °C and -86 °C, respectively, from tanh curve fitting. Strong linear correlations were observed between Charpy impact energy and fracture surface parameters, including lateral expansion, ductile thumbnail extension, and cleavage area fraction.
Fracture stress testing with blunt-notched specimens tested at -196 °C showed cleavage initiation stresses between 1803 and 2071 MPa, while fracture toughness testing with sharp-cracked specimens produced similar local fracture stress values, independent of temperature. Inclusions are found at all initiation sites and considered to be the primary reason triggering cleavage fracture in these fracture stress tests. Inclusion analysis identified calcium-, aluminium-, and titanium-bearing inclusions at crack initiation sites; however, no clear relationship between inclusion size and fracture stress was observed. The effective surface energies for cleavage fracture ranged between 5.9 and 21.1 J/m², with cleavage behaviour predominantly controlled by local tensile stress.
In Charpy impact testing, the 7 wt% Nickel steel shows a ductile-to-brittle transition region of -180 °C to -140 °C, with a ductile-to-brittle transition temperature of around -165 °C. The 9 wt% Nickel steel demonstrated superior ductility and fracture toughness, with no ductile-to-brittle transition observed down to -196 °C.
Fracture toughness testing of 9 wt% and 7 wt% Nickel steels was conducted using single-edge notched bending (SENB) specimens at -163 °C. The unloading compliance technique proved reliable for pre-crack length estimation. J-resistance curves indicated higher fracture toughness and higher local cleavage stress thresholds for the 9 wt% Nickel steel compared to the 7 wt% Nickel steel, consistent with microstructural observations, as fractography revealed a mixed mode of transgranular cleavage fracture and micro-void coalescence on the fracture surfaces of both Nickel steels.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):