Fabrication rule of thumb: for plain carbon steel vessels, butt-welded joints are typically used when the shell plate thickness is less than or equal to approximately how many centimeters?

Introduction / Context:
Pressure vessel construction uses different joint types depending on plate thickness, fabrication practicality, and code requirements. For thin to moderate shell plates in plain carbon steel, butt-welded joints are common and economical, providing good strength and inspectability. As thickness increases, bevel geometry, weld volume, preheat, and distortion control become more demanding.

Given Data / Assumptions:

• Plain carbon steel shell plates for typical process equipment.
• Interest in a practical threshold where butt-welded joints are commonly applied.
• Rule-of-thumb guidance rather than a mandatory code limit (actual projects must follow code and WPS).

Concept / Approach:
Thin plate sections are routinely joined by full-penetration butt welds with manageable heat input and distortion. Around a centimeter of thickness represents a convenient upper bound for “easy” butt-weld fabrication on many shop floors before special procedures and increased weld volume raise cost and complexity. The referenced choice reflects an indicative threshold used in many instructional contexts.

Step-by-Step Solution:

Verification / Alternative check:
Shop practices and educational problem data often use ~12 mm as a practical benchmark; exact limits vary by code, service, and fabricator capability.

Why Other Options Are Wrong:

• 0.5 cm is overly restrictive; butt welds are common above this.
• 3.8 cm and 6.8 cm enter heavy-plate territory requiring substantial weld prep and may favor different joint details or segmental construction.

Common Pitfalls:
Confusing rules of thumb with code requirements; ignoring impact toughness, PWHT, and NDE that also influence joint selection.

Final Answer:
1.2