100% Inspection of Continuous Casting Break Rings

The Challenge

In horizontal continuous casting of steel, the break ring is the last refractory molten steel contacts before solidification. Made from hexagonal boron nitride for its thermal shock resistance, thermal conductivity, and non-wetting properties, these components must perform flawlessly—failure causes automatic halt to casting with significant expense.

The problem originated in manufacturing. BN production involves nitriding boron compounds at high temperature, but B₂O₃ content in the final product varies widely. This binder is necessary for hot pressing but leads to erratic wear and premature failures. After a rash of break ring failures at Baltimore Specialty Steels, Armco needed to discriminate between “good” and “bad” rings before installation.

The Solution

Armco tested three NDT methods on 100mm and 155mm square break rings. Ultrasonic velocity measurements failed—the donut-shaped geometry created interfering wave fronts. The shock excitation method (GrindoSonic) succeeded: a piezoelectric transducer captures the ring’s fundamental vibration from a light tap, with raw readings above 940 typically indicating flaws.

Sorting results by production lot revealed clear patterns: lots were either all good (readings 1592–1623), all bad (1780–1812), or mixed. Mixed lots were rejected due to property inconsistency. Field data correlated directly with actual caster production logs, confirming the test’s predictive value.

Results

Comparison testing on bars cut from break rings validated shock excitation against ultrasonic velocity and resonant frequency methods (ASTM C747)—all three closely paralleled each other. The practical advantage: shock excitation works on complete rings regardless of geometry, enabling 100% field inspection.

Upper limits were written into purchase specifications. Close cooperation between Armco and the BN manufacturer allowed development of internal product specifications meeting these requirements, achieving a 100% defect-free supply of break rings.