Material Selections for Cryogenic Pressure Vessel Design (Part Three)
6. Cold Deformation: Affecting Steel Toughness
Cold deformation adversely affects the toughness of steel, leading to a reduction in low-temperature toughness due to strain aging and an elevation in the brittle transition temperature. Notch toughness becomes paramount for large pressure vessels during utilization, considering potential embrittlement from processes such as cold deformation, cold pressing, and welding deformation. Additionally, the implementation of low-temperature annealing post-cold deformation becomes a critical consideration.
7. Stress State: Occurrence of Low-Stress Brittle Fractures
The occurrence of low-stress brittle fractures is intricately tied to the stress state. Pressure vessels with cracks or notches are more susceptible to low-stress brittle fractures. The severity of fractures is influenced by factors such as the sharpness of cracks or notches and the pre-cracking size. In welded joints with cracks and residual stress, the manifestation of low-stress brittle fractures becomes more pronounced.
Design Principles for Mitigating Low-Stress Brittleness
Contemporary design specifications for cryogenic pressure vessels hinge on allowable stresses determined by tensile strength at room temperature or yield strength. This approach effectively curtails the occurrence of significant plastic deformation. To avert low-stress brittle fractures at low temperatures, the steel must possess a specific degree of toughness, necessitating specific requirements in both design and manufacturing. The determination of the required toughness level aligns predominantly with the overarching principle of preventing brittle fractures.
Insights from blasting tests on defective pressure vessels underscore the feasibility of halting cracking under full hydraulic pressure but reveal challenges under partial gas pressure. Specific design considerations are essential to prevent further cracking extension in such scenarios.
For cryogenic pressure vessels in the petrochemical industry, crack arrest principles are unsuitable due to the internal medium being gas-liquid two-phase or liquid phase with a temperature higher than its normal boiling point. Therefore, the most effective approach for preventing brittle fractures in cryogenic pressure vessels is to prevent cracking from the outset. This principle is currently applied in pressure vessel specifications globally.
Cold deformation adversely affects the toughness of steel, leading to a reduction in low-temperature toughness due to strain aging and an elevation in the brittle transition temperature. Notch toughness becomes paramount for large pressure vessels during utilization, considering potential embrittlement from processes such as cold deformation, cold pressing, and welding deformation. Additionally, the implementation of low-temperature annealing post-cold deformation becomes a critical consideration.
7. Stress State: Occurrence of Low-Stress Brittle Fractures
The occurrence of low-stress brittle fractures is intricately tied to the stress state. Pressure vessels with cracks or notches are more susceptible to low-stress brittle fractures. The severity of fractures is influenced by factors such as the sharpness of cracks or notches and the pre-cracking size. In welded joints with cracks and residual stress, the manifestation of low-stress brittle fractures becomes more pronounced.
Design Principles for Mitigating Low-Stress Brittleness
Contemporary design specifications for cryogenic pressure vessels hinge on allowable stresses determined by tensile strength at room temperature or yield strength. This approach effectively curtails the occurrence of significant plastic deformation. To avert low-stress brittle fractures at low temperatures, the steel must possess a specific degree of toughness, necessitating specific requirements in both design and manufacturing. The determination of the required toughness level aligns predominantly with the overarching principle of preventing brittle fractures.
- Managing Defects and Cracking Prevention
Allowing certain defects while actively preventing cracking is imperative. Weld zones, often characterized by poor defects and toughness, require careful measurement of properties in the base material, heat-affected zone, and weld line. The worst toughness should be able to withstand the strain induced by external loads.
- Defect Allowance with Emphasis on Base Material
Permitting defects with a reliance on the base material to prevent cracking extension and fracture is acceptable. However, the lower toughness of weld metal, weld lines, and heat-affected zones calls for cautious consideration, making this method less reliable for preventing brittle fractures.
- Controlled Cracking from Defects
Allowing cracking from defects is viable, provided all parts of the pressure vessel can prevent further extension. While seemingly secure, this approach has drawbacks, including elevated material costs and a reliance on specific structural types for absolute safety.
Insights from blasting tests on defective pressure vessels underscore the feasibility of halting cracking under full hydraulic pressure but reveal challenges under partial gas pressure. Specific design considerations are essential to prevent further cracking extension in such scenarios.
For cryogenic pressure vessels in the petrochemical industry, crack arrest principles are unsuitable due to the internal medium being gas-liquid two-phase or liquid phase with a temperature higher than its normal boiling point. Therefore, the most effective approach for preventing brittle fractures in cryogenic pressure vessels is to prevent cracking from the outset. This principle is currently applied in pressure vessel specifications globally.
