Characterisation of micro-mechanisms of cleavage fracture in SA738Gr.B steel

Xu, Weichen (2024). Characterisation of micro-mechanisms of cleavage fracture in SA738Gr.B steel. University of Birmingham. Ph.D.

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Abstract

Cleavage fracture is a significant concern for structural materials, as it can lead to sudden and unexpected failures that can have serious consequences, such as the failure of bridges, buildings, or other critical infrastructure. As a fracture mode, cleavage occurs in materials that exhibit brittleness and low toughness under various conditions. The investigation of cleavage fracture micro-mechanisms in conventional steel types has been a subject of research for several decades. Long ago, scientists began locating and predicting the precise cleavage initiation site. In the past, significant emphasis was placed on cleavage nucleation induced by carbides or inclusions and cleavage propagation caused by the fracture of cleavage facets.
This study specifically focusing on the initiation phase. The result of this investigation provides some potential advances for the understanding of the cleavage fracture mechanism by not only identifying cleavage initiation sites with precision but also analysing their role in the fracture process, particularly highlighting the influence of inclusions containing calcium, aluminium, and titanium.
This thesis delves into the cleavage fracture mechanisms in SA738Gr.B steel, a type of high-strength low-alloy (HSLA) steel that is commonly used in the construction of reactor containment vessels. The investigation employed in this study involved two heat treatment processes for SA738Gr.B steel. This included heating the steel to 900°C for two hours, followed by rapid water quenching, and then tempering it at 630°C for three hours (HT1). - Additionally, following HT1, a simulated post-weld heat treatment (PWHT) was conducted, incorporating an additional tempering stage at 620°C for 15 hours, designated as HT2. This HT2 process was introduced and compared with the original HT1 to evaluate the possibility of exempting the need for PWHT. Optical microscopy and scanning electron microscopy (SEM) were employed to conduct an in-depth examination of the steel's microstructure. These techniques facilitated a detailed analysis of the size distribution and chemical composition of inclusions on both polished and the fracture surfaces.
The microstructural characterization of the steel revealed a predominantly granular bainitic structure with carbides precipitated along the grain boundaries. To understand the material's behaviour under stress, Charpy impact tests were conducted across a range of temperatures to determine the ductile to brittle transition temperatures. The transition temperatures, -91 °C for HT1 and -87 °C for HT1, were instrumental in setting the parameters for the subsequent fracture toughness and microscopic cleavage fracture stress tests on pre-cracked and notched specimens.
Tensile tests were then carried out and the material mechanical properties such as yield stress, ultimate tensile strength, and work hardening exponent were measured and calculated at various temperatures. The fracture toughness was measured following ASTM 1820-17 standards and involved using 1T compact tension specimens under the temperature -120 °C, -100 °C and -80 °C. With the obtained test result combined with fracture distance measured from the specimen surface, local cleavage fracture stresses were obtained using McMeeking’s FEM analysis results. Additionally, microscopic cleavage fracture stress tests were conducted using a four-point bending method. These tests, performed at temperatures ranging from -196 °C to -160 °C, allowed for a detailed evaluation of the local stress distribution and the cleavage fracture stress, using the Griffith-Owen finite element analysis result.
A significant challenge highlighted in this thesis is the identification of cleavage initiation sites in 55 mm thick compact tension specimens. This required a nuanced understanding of both 'global' and 'local' initiation sites. A comprehensive analysis of 26 sharp-cracked specimens provided detailed insights into the size, location, and chemical composition of the cleavage initiators. One of the critical outcomes of this research is the establishment of a correlation between macroscopic parameters, notably the J-integral, and microscopic local parameters such as fracture distance X_0. This correlation is crucial in understanding how microscopic changes in the steel's structure can influence its overall mechanical behaviour and fracture resistance.
The comprehensive results obtained from the two distinct test-piece geometries blunt-notched and sharp-cracked specimens provide robust validation for the hypothesis that cleavage fracture is governed by a tensile stress-controlled failure criterion and that the cleavage fracture stress is independent of temperature. These principles, critical for interpreting the cleavage behaviour in ferritic steels, have been widely recognized for decades. This study, however, potentially advances the field by offering explicit experimental evidence to substantiate these foundational principles. Furthermore, this research enhances the comprehension of the micro-mechanisms driving cleavage fracture in bainitic steels, especially emphasizing the role of specific inclusions in initiating these fractures.
In conclusion, this thesis contributes significantly to the field of microscopic analysis of the cleavage mechanism in steel, especially in the context of limited data availability and the challenges associated with traditional testing methodologies for characterizing cleavage initiation. The detailed analysis of fracture surfaces, the identification of cleavage initiation sites, and the establishment of relationships between various mechanical parameters not only provide valuable insights for future research but also have practical implications for the design and application of ferritic steels across various industrial sectors.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):