Micromechanisms of cleavage fracture at cryogenic temperatures of SA-738 Gr. B steel and 7% nickel steel

Fan, Huize (2024). Micromechanisms of cleavage fracture at cryogenic temperatures of SA-738 Gr. B steel and 7% nickel steel. University of Birmingham. Ph.D.

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Abstract

This thesis considers a detailed assessment of SA-738 Gr. B steel regarding its mechanical properties and fracture mechanisms at temperatures ranging from −196 to −40℃. Mechanical testing included hardness test, tensile test, Charpy V-notch test, and fracture toughness tests. A standard heat treatment condition (HT1) was compared with a second heat treatment (HT2) which corresponds to a simulated post-welding heat treatment in order to see the effect of post- welding heat treatment on mechanical properties and microstructure on this steel.
Tensile tests showed that yield stress, tensile strength, Lüders elongation and work hardening exponent were strongly dependent on test temperatures: as the temperature increased, yield stress, tensile strength, and Lüders elongation decreased, while work hardening increased.
The Charpy impact testing results of HT1 and HT2 exhibited closely similar ductile-to-brittle transition behaviour over the temperature range from −100 to −80℃, with a transition temperature of −91 and −87℃ estimated by tanh-fit curve respectively. The absorbed impact energy had a strong positive linear correlation with lateral expansion and ductile thumbnail extensions. There was a negative linear relationship between impact energy and percentage of cleavage area.
Fracture toughness results for HT1 and HT2 were characterised by the reference temperature T0 obtained by the Master Curve methodology. The overall application of the Master Curve methodology to estimate the reference temperature of HT1 and HT2 specimens was reliable and gave closely similar reference temperatures of −101 and −97℃ at test temperature of −120℃ respectively.
Detailed fractography of fracture toughness specimens tested at −196 and −170℃ showed that all specimens failed with transgranular cleavage, and eight out of ten cleavage cracks were inclusion-initiated. Nine out of ten specimens had the local fracture initiation sites located at positions consistent with a purely tensile stress-controlled failure criterion.
The local cleavage fracture stresses obtained from sharp-cracked specimens tested at temperatures between −196 to −80℃ were compared to those of blunt-notched specimens tested at temperatures of −196 and −170℃. The local cleavage fracture stresses appeared to be independent of test temperatures. There was a weak positive relationship between the local
fracture stress and reciprocal square root of the initiating inclusion diameter. Local cleavage fracture stresses were measured as 1678 to 2699MPa.
The study also addresses a 7% nickel steel at temperatures ranging from −196 to −60℃. Hardness, tensile, Charpy V-notch, slow blunt notch bending, fracture toughness, and fatigue crack growth tests were performed.
In tensile tests, yield stress and tensile strength decreased with increasing test temperature. The work hardening exponent of this 7% nickel steel had a weak positive correlation to test temperature, and total elongation was independent of test temperature. One tensile specimen only, tested at −196℃, showed some cleavage facets on the fracture surface.
Charpy tests exhibited ductile-to-brittle transition behaviour in the range from −170 to −140℃. The DBTT was −140℃ estimated by tanh-fit curve. Fractography of specimens tested below −140℃ showed a mixed mode of micro-void coalescence and transgranular cleavage fracture, and as the test temperature increased, the fraction of cleavage area decreased. Slow blunt notch testing of 7% nickel steel showed nonlinearity even at −196℃. Fractography was mainly transgranular cleavage, with micro-void coalescence located between cleavage facets.
In fracture toughness test of 7% nickel steel tested at −196 to −163℃, specimens failed by cleavage fracture after some amount of ductile crack growth. The fracture surfaces had a mix of transgranular cleavage facets and micro-void coalescence. KJc values of specimens tested at −196℃ were in the range of 214 to 291MPam1/2, while KJc values of specimens tested at −170 and −163℃ were in the range of 547 to 726 MPam1/2. Values of local cleavage fracture stress for 7% nickel steel were deduced to lie between 2765 to 3501 MPa.
The fatigue crack growth resistance of 7% and 9% nickel steels has also been tested at room temperature and −163℃.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):