Refractoriness Under Load (RUL): in which furnace location is RUL the most critical property for selecting refractory bricks?

Introduction / Context:
Refractoriness Under Load (RUL) measures a refractory’s ability to resist deformation at high temperature under compressive stress. It complements simple refractoriness (PCE) by accounting for mechanical load during service. Properly prioritizing RUL is essential in zones where bricks support weight at elevated temperatures.

Given Data / Assumptions:

• Furnace zones experience different combinations of temperature and mechanical load.
• Hearths and piers bear the heaviest static loads near operating temperatures.
• Roofs/crowns are often in tension/thermal shock but with low compressive load.

Concept / Approach:
Where bricks carry substantial load at high temperature (e.g., hearth bottoms, piers supporting checkers or stacks of stock), RUL becomes the primary selection criterion to prevent creep, crushing, or plastic flow. Insulating layers, by contrast, operate at lower temperatures with minimal load, making RUL less critical than thermal conductivity and shrinkage. Roofs prioritize thermal shock resistance and spall resistance more than RUL because mechanical loads are modest.

Step-by-Step Solution:

Verification / Alternative check:
Design practices specify high-RUL bricks for soaking-pit bottoms, reheating furnace hearths, and checker supports, confirming the prioritization of RUL in load-bearing hot zones.

Why Other Options Are Wrong:

• Insulation back-up: load is low; thermal conductivity and shrinkage dominate.
• Suspended roof: low compressive load; thermal shock and spalling resistance are more critical.
• Cold stack zones: temperatures are lower; RUL is not the limiting factor.

Common Pitfalls:
Specifying materials solely by PCE without considering mechanical load; neglecting creep under sustained hot load leading to misalignment or collapse.

Final Answer:
At the hearth bottom of the furnace where stock is placed (bearing heavy load)