Single-crystal turbine blade growth in gradient furnaces

GE and Pratt cast 16–24 blades per investment sprue; Rolls-Royce uses 4. Ten-hour pull through a 1000° gradient, $3M furnace per ~16–32 blades/day. Cost per blade ~$6,000 but justified by life-of-engine fuel savings. Used to teach that the steep gradient suppresses constitutional supercooling and gives planar-front solidification.

I think I told you Rolls-Royce uses four blades on a little investment casting sprue. General Electric and Pratt, their casting shops use about sixteen or twenty-four on an investment casting sprue. They grow it as a single crystal in a gradient furnace. It takes about ten hours, because you have about a thousand-degree temperature differential over five or six inches in this furnace, and you slowly pull it through over about ten hours. You don't get dendrites because you suppress them with the steep gradient. You don't get constitutional supercooling that allows the instability for the cell to grow up into the liquid. You can get nice planar-front solidification — a very perfect structure. But it costs a lot of money. This gradient furnace is about three million bucks to grow sixteen blades a day or maybe thirty-two blades a day. You can start to see why these things start to cost $6,000 apiece. But if they allow you to use half the fuel over the life of the engine, it's worth it.