It is important to carefully monitor and control the manufacturing variables of refractories to ensure that products of uniform and consistent quality are produced and shipped. I would like to remind refractory manufacturers and users about sonic testing, a quick and simple quality assurance method that has been available for decades. I learned about sonic testing in the '70s and have used it since then for a wide variety of refractory evaluation purposes.1, 2, 3
Sonic testing provides a fast, easy method of quantitatively documenting several common sense principles. Sound waves travel faster through a refractory structure that is denser and stronger, i.e., the sonic velocity (m/sec or ft/sec) is greater in a denser/stronger brick. The presence of pores, voids, cracks, laminations and non-refractory inclusions (such as bolts, wood, cigarettes, rubber, etc.) directly affect the propagation of sound and can be detected.
Additionally, if a refractory has uniform as-manufactured properties, the sonic velocity should be consistent/equivalent at any place and in any direction (e.g., through the thickness). If there is a significant difference in the sonic readings in different locations/regions, it is likely that there are internal defects and/or property differences, indicating that the quality is less than optimum.
Earlier this year, a company was experiencing inconsistent performance of its 12-in.-long SiC nozzles, which are a critical component in the company's control of molten metal flow. The nozzle manufacturer told the company that the nozzles, which are made by hand ramming, were uniform and consistent as-manufactured. But, without the need for destructive testing of these expensive pieces, manual sonic testing confirmed that was not the case. Specifically, the sonic results showed that five of the nozzles were probably acceptable (>27,000-24,300 ft/sec), while four were outside that range (down to 22,500 ft/sec).
Another recent case involved a company that experienced a significant reduction in the life of direct-bonded MgO-Cr2O3 bricks in its metal-melting operations. Various possibilities for the reduced refractory life were considered, including non-destructive sonic testing of bricks from several manufacturers. Manual sonic testing indicated that some bricks were unacceptable, with internal cracks and/or intra-brick property variations (e.g., the end of a brick, which is the hot, working face in service, is more porous and has lower strength than the middle of the brick). Sonic testing, along with confirming physical tests, revealed that brick quality was one possible factor in the lining life reduction. Better monitoring and control of the refractories has been implemented to improve the situation.
Despite the many applications and proven benefits of sonic testing of refractories, it has not been widely used. Several drawbacks have been cited, including the fact that the results are operator-sensitive. Manual sonic testing is directly affected by the contact pressure that a person applies with the two (input and receiving) transducers. However, operators with proper training and experience can obtain meaningful results. Additionally, sonic testing can be fully automated to eliminate the human factor. Automated sonic testing has been used for years to document refractory quality, e.g., 100% testing of critical components, such as slide gates for flow control of molten steel.
Some people say that the method is too slow and messy, because a coupling medium (grease, Vaseline, etc.) is needed between the test piece and the transducers. This argument is invalid, however, because the coupling requirement can be accomplished using a thin layer of dry, compressible material such as a denture adhesive on the transducers.
In addition to the sonic equipment that has been available for several decades, ongoing R&D has further enhanced the equipment and procedure.4,5 But a major need that remains is the ability to use sonic testing to non-destructively confirm the quality and properties of monolithic (cast/gunned) refractories in-situ, as-installed, and after curing and heat treatment.
1. C.E. Semler and F. Aly, Bull. Amer. Ceram. Soc.
, 64 (12) 1555-1558, 1985.
2. C.E. Semler, Proceedings, Intl. Symp. Refr., Hangzhou, China, 1988.
3. C.E. Semler, Proceedings, UNITECR '95, Kyoto, Japan, 1995.
4. T.J. Carneim, et al, Bull. Amer. Ceram. Soc., 78 (4) 88-94, 1999.
5. H. Kautz, NASA-Glenn Res. Ctr., Photonic Tech Briefs, July 2000, p. 18a.