Processing Magazine

Busting myths about prestressed glass/metal windows

June 5, 2006

Myth: Hastelloy prestressed glass/metal windows using soda-lime glass provide a greater view diameter than those formed of duplex stainless steel and borosilicate glass.

Fact: The real key in determining viewing area is the thermal coefficient of expansion of the glass, not the composition of the metal ring. Since soda-lime glass has a higher coefficient of expansion than borosilicate glass it accommodates less compression and does provide a larger viewing area. But less compression means less physical strength, so a tradeoff in safety must be considered. And, borosilicate glass also offers more corrosion resistance, better thermal shock capabilities and greater physical strength.

Larger-diameter Metaglas® windows using soda lime have long been offered as an option, but the majority of Metaglas window users prefer borosilicate glass for the added margin of safety they provide.

Myth: There is no real fusion between the Duplex stainless steel and borosilicate glass in Metaglas windows.

Fact: Not true. The interface between glass and metal is literally fused. In a misdirected demonstration by a competitor, a Metaglas window was compromised by cutting a wedge out of the steel ring. Relieving the compression provided by the steel ring in this way, allowing it to expand to its theoretical diameter, overcame the fusion between the metal and glass, thus separating the two. The demonstration proves nothing and is hardly a likely event in actual service.

Myth: Duplex stainless steel is not a good material for fused glass/steel windows because its poor thermal properties lead to cracking along the interface where glass and steel fuse together.

Fact: Quite the opposite! The major reason Duplex stainless steel is the alloy of choice for Metaglas windows is its coefficient of thermal expansion. This makes it ideally suited to create a dynamically fused interface with borosilicate glass. Any tiny hairline cracks that might form at the interface are insignificant, simply a peripheral result of the powerful compressive forces that make Metaglas windows the safest windows available.

In addition, Duplex stainless steel has superior chemical and chloride stress-cracking resistance compared to 316 stainless steel; it has twice the yield strengths of conventional austenitic steels; and has more impact resistant that ferritic alloys.

Myth: The strength of fused metal/glass windows is the result of fusion between the metal and glass surfaces at the interface.

Fact: The strength of the Metaglas window actually comes from the radial compressive forces of the metal ring onto the glass disc. The fused interface that results is only a bi-product of the manufacturing of the Metaglas window. The real objective of fusing the two is to capture the strong compression characteristic of the glass while minimizing the weaker tensile characteristic.