Duplex stainless steel centrifuge brittle failure

So some people say brittle is anything below 0.2 percent, which is the yield point of a metal. Some people, and this is in the pipeline business, will say the yield point is defined at 0.5 percent, which is two and a half times as great. Some people will say you need to have five percent ductility to be ductile. Let's just say anything above one percent is ductile and means the thing has stretched. I didn't bring the sample, but I passed a sample around last time, my brittle centrifuge, and if you look very carefully, just a little bit of stretch before the thing snapped. It's a continuous spectrum of materials. Something very brittle like glass, depending on the flaw size, could fracture anywhere along that elastic limit. Things that fail out here above five percent strain are clearly ductile. They stretch like this, the Silly Putty. Things that are in between are sort of beauty in the eye of the beholder.

Physical demonstration. Cast duplex stainless centrifuge (~1 ton, 3000 RPM, 4 ft diameter) for separating starch in corn-methanol production. Embrittled by iron-chrome intermetallic phases. Shows orange-peel surface where large casting grains stretched before fracture.

Duplex stainless steels: they increase the chromium and lower the nickel to get higher strength. You can double or triple the strength by going to a duplex. A duplex stainless is merely a mixture of about 50/50, or 30/70, or 70/30, of austenite and ferrite — ferrite being a body-centered cubic magnetic phase. [Tom holds up a piece of duplex stainless steel.] This came off a centrifuge that was used to separate starch from other things. They started selling a lot of these centrifuges when they started making methanol from corn, because they had to separate the starch from all the other liquid. The centrifuge weighs about a ton, spins around like 3,000 RPM, about four feet in diameter. Made out of cast duplex stainless steel because they needed high strength.