Chrome–magnesite refractories: adding 10–30% magnesite to chromite primarily improves which service property of the original chromite brick?

Introduction / Context:
Chrome–magnesite bricks are widely used in steelmaking equipment because they combine the favorable features of chromite (Cr2O3–FeO) with magnesia (MgO). Small additions of magnesite to chromite adjust thermal expansion behavior and microstructure, changing how the brick responds to severe thermal cycling. This question targets the main benefit of such additions.

Given Data / Assumptions:

• Magnesite addition range: approximately 10–30% to chromite.
• Service conditions: frequent thermal cycling, hot spots, and chemical attack.
• Spalling refers to cracking and flaking due to thermal stress.

Concept / Approach:
Thermal spalling is driven by thermal gradients, expansion mismatches, and microcrack propagation. Magnesia addition modifies the thermal expansion coefficient and stabilizes spinel phases that help accommodate stress, thereby improving resistance to spalling. While MgO can also influence slag resistance and refractoriness, the principal and well-documented gain at these addition levels is enhanced spalling resistance in the chrome–magnesite composition range.

Step-by-Step Solution:
Identify failure mode in chromite under cycling: spalling.Recognize effect of MgO: alters expansion and microstructure to better absorb stress.Prioritize property change most sensitive to microstructural tuning: spalling resistance.Select “Spalling resistance.”

Verification / Alternative check:
Plant experience and test data show chrome–magnesite bricks outperform straight chromite in thermal-shock tests, consistent with the intended improvement from MgO addition.

Why Other Options Are Wrong:
Refractoriness: only modestly affected by the small MgO addition compared to composition extremes.Crushing strength: mainly a cold property and binder/processing dependent.Slag resistance: may improve in basic slags, but the principal cited benefit here is spalling resistance.Electrical resistivity: not the design driver for this change.

Common Pitfalls:
Equating every MgO addition with universal improvements; property changes are application-specific.Confusing chrome–magnesite (MgO-modified chromite) with magnesia–chrome (MgO-rich) bricks.

Final Answer:
Spalling resistance