Materials for pressure vessels – identify the one seldom used Among the listed steels, which is seldom used for pressure vessel construction due to quality and through-thickness property concerns?

Introduction / Context:
Pressure vessels operate under significant internal pressure and cyclic loads. Material selection must ensure adequate toughness, ductility, weldability, and cleanliness. Steelmaking practice (rimmed, killed, semi-killed) strongly affects internal soundness and suitability for thick, welded pressure parts.

Given Data / Assumptions:

• Typical codes (e.g., ASME) favor steels with predictable composition and deoxidation.
• Rimmed steel is low in deoxidation and prone to compositional segregation.
• Killed and semi-killed steels are more homogeneous and weld-friendly.

Concept / Approach:
Rimmed steel solidifies with gas evolution, producing rim zones of relatively pure iron but interior segregation and blowholes. This lack of through-thickness quality makes it undesirable for pressure-containing weldments. Killed steels are fully deoxidized (e.g., Al-killed), yielding uniform properties and fewer inclusions—preferred for pressure service. Semi-killed steels sit in between but are still used in certain vessel applications. Plain “mild steel” is a composition descriptor (e.g., low-carbon) that may be killed or semi-killed per specification and is commonly used when produced by suitable practice.

Step-by-Step Solution:

Verification / Alternative check:
Standards discourage rimmed steel for welded pressure parts; specifications call for killed steels with controlled impact properties (e.g., fine-grain practice).

Why Other Options Are Wrong:

• Mild steel: Common when produced as killed/semi-killed to code.
• Killed steel: Preferred for homogeneity and toughness.
• Semi-killed steel: Acceptable in some services; not “seldom used”.

Common Pitfalls:
Assuming “mild steel” means rimmed steel; in practice, purchase specifications ensure deoxidation suitable for vessels.

Final Answer:
Rimmed steel