Accelerated Thermal Shock Testing for Refractories

The Challenge

Standard thermal shock tests (ASTM C38, PRE/BS methods) distinguish excellent from poor materials but struggle to differentiate “very good” from “excellent”, particularly when resistance is quoted as “+20 or +30 cycles.” These methods measure failure points, not progressive damage, and sample thickness significantly affects results: a dense firebrick shows measurable resistance at 38mm but fails on the first cycle at 65mm.

Morgan Refractories needed quantitative damage tracking to compare materials and guide product development.

The Solution

The ribbon test, originating from Taylor Refractories in the 1930s, uses a 5-foot Maxon segmented burner to cycle samples (230 × 114 × 38mm) through 15 minutes heating to 1000–1040°C followed by 15 minutes cooling. The key innovation: measuring elastic modulus by transient vibration (GrindoSonic) after 1, 2, 5, and 10 cycles rather than cycling to failure.

Percentage retained modulus tracks microcrack accumulation before visible damage appears. Table 1 in the study showed excellent agreement between % retained MOE and % retained MOR across 45–90% alumina materials and magnesia-chrome refractories.

Key takeaway: As few as 5-10 thermal cycles with modulus measurement can characterize thermal shock resistance, replacing destructive cycle-to-failure tests that take days.

Results

Testing commercially available refractories demonstrated clear differentiation: silicon carbide retained highest strength while magnesia, rated +20/+30 cycles by BS 1902, showed poorest retention. Grading changes to a 66% alumina material improved shock resistance by 12%. Increasing firing temperature by 200°C almost halved retained MOE.

The method proved sensitive to thickness effects (60% alumina showed marked drops with increasing thickness) and process variations. Research by Semler at Ohio State confirmed that 5–10 cycles may be sufficient to characterize thermal shock resistance, making the combined ribbon test and modulus measurement practical for both R&D and quality control.