Materials for Turbins

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ddun
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Materials for Turbins

Post by ddun » Wed Aug 18, 2004 2:55 am

Hello, I have only one question. Reguarding thrusting jet engins as used in aircraft. What are the turbins made from? I would first assume Titainium but it can not be welded. Is there another high temperature allow that can take the stress and flexure that would be required?

Any help would answer a great question.

Thank You

NanoSoft
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Re: Materials for Turbins

Post by NanoSoft » Wed Aug 18, 2004 3:29 am

To your surprise titanium is not what turbine blades are made from. I may be wrong but i think they are made from a nickel based metal. I'll look into it.

Nanosoft

steve
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Re: Materials for Turbins

Post by steve » Wed Aug 18, 2004 3:45 am

ddun wrote:Hello, I have only one question. Reguarding thrusting jet engins as used in aircraft. What are the turbins made from? I would first assume Titainium but it can not be welded. Is there another high temperature allow that can take the stress and flexure that would be required?

Any help would answer a great question.

Thank You
why would it need to be welded? most blades are designed to siplly slide into machined grooves in the turbine wheel. this allows easy maintinence if a single blade needs to be replaced. I do think it is made from some nickel aloy (in many cases) though.
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skyfrog
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Re: Materials for Turbins

Post by skyfrog » Wed Aug 18, 2004 6:20 am

The reason of why Titanium cannot be used to build turbine wheels is because it is flammable when working temperature reaches 1000 deg F, and when the working temperature reaches 400 deg F, the strength of Ti will be degraded very quickly. Titanium can be welded by TIG process.

Stainless steel can be used to build turbine wheels but the service life is short. The early FD3/64 wheels are made from stainless sheet that is slit and twisted to certain angle and profiled by hand grinder.

Nickel based alloy like the Inconel, which has more heat-resistance capability than stainless, is usually used to build the turbine wheels.
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Al Belli
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Re: Materials for Turbins

Post by Al Belli » Wed Aug 18, 2004 11:40 am

Hi,

Commercially produced turbine blades are investment cast from a nickel based alloy similar to Stellite-21. This material is used for tools used to machine steel alloys; so You can see that it cannot be bent or formed since it is brittle. I would recommend using a precipitation hardening stainless steel like 17-4 Ph., that can be bent and formed in the annealed state and then heat treated.

Al Belli

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Re: Materials for Turbins

Post by skyfrog » Wed Aug 18, 2004 12:24 pm

Al Belli wrote:Hi,

Commercially produced turbine blades are investment cast from a nickel based alloy similar to Stellite-21. This material is used for tools used to machine steel alloys; so You can see that it cannot be bent or formed since it is brittle. I would recommend using a precipitation hardening stainless steel like 17-4 Ph., that can be bent and formed in the annealed state and then heat treated.

Al Belli
Hi Al,

You sound like very familiar with investment casting process,so I'd like to ask you a question if you don't mind.

WRT casting process for turbine wheels, do you think its necessary for the Inconel ingots to be vacuum melted, how about melted under insert gas ? I am talking about building model jet engines only, not about a passenger jet engines, which should be regulated by the strict aviation standard.
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rafa
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Re: Materials for Turbins

Post by rafa » Wed Aug 18, 2004 2:49 pm

Hi guys.

Inconel and other special steels can be melted in a carbon arc furnace with argon or hidrogen atmosphere.

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Re: Materials for Turbins

Post by skyfrog » Wed Aug 18, 2004 2:54 pm

Oops, I mean inert gas, not insert gas.

Thanks for the info, rafa

I read from a Japanese investment casting book, it says Inco 718C, Inco 713C are usually vacuum melted. In compliance with strictest aviation code maybe ?
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Al Belli
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Re: Materials for Turbins

Post by Al Belli » Wed Aug 18, 2004 5:42 pm

Hi Skyfrog,

Vacuum melting not only keeps the metal clean, but keeps adsorbed gasses from being trapped in the metal. The parts are also cast in a vacuum to help the molten metal fill the thinner areas in the mold. Some of these alloys are surprisingly viscous in the molten state.

Al Belli

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Re: Materials for Turbins

Post by skyfrog » Thu Aug 19, 2004 3:54 am

Al Belli wrote:Vacuum melting not only keeps the metal clean, but keeps adsorbed gasses from being trapped in the metal. The parts are also cast in a vacuum to help the molten metal fill the thinner areas in the mold. Some of these alloys are surprisingly viscous in the molten state.
Really grateful Al, for imparting those useful info.

Yes, think of a wheel turning at a speed of more than 100K RPM, and every steps making the wheel should be very careful. The RPM figure is itself a strict code.
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Al Belli
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Re: Materials for Turbins

Post by Al Belli » Thu Aug 19, 2004 12:01 pm

Hi Skyfrog,

Here is a useful formula for calculating the stresses in a disk.( From Machinery's handbook 17 Th. edition page 342 )

Al Belli
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larry cottrill
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Re: Materials for Turbins

Post by larry cottrill » Thu Aug 19, 2004 12:26 pm

Al Belli wrote:Hi Skyfrog,

Here is a useful formula for calculating the stresses in a disk.( From Machinery's handbook 17 Th. edition page 342 )

Al Belli
Skyfrog -

In high-speed disks with thick hubs it is important to not have a sharp "break" between the disk surface and the hub, but rather a well-rounded 'fillet'. It is in fact possible to design a 'disk of equal stress' or 'disk of uniform stress' where the entire disk surface is a carefully calculated long fillet from the hub out to the edge. Such a disk has the same unit stress [such as lb/sq.in) at every radius, so there is theoretically no stress concentration anywhere! That's the way to go if you have the tools to lathe turn a non-circular curved face on each side of the disk.

Note also that it is important to have the two sides of a disk symmetrical with one another. Otherwise, you will have a disk that tries to 'flatten' in one direction or the other, creating increased tensile stress in unexpected places within the disk because of torsional forces.

Such considerations become critical at high radial velocities.

L Cottrill

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Re: Materials for Turbins

Post by skyfrog » Fri Aug 20, 2004 4:05 am

Many thanks, Al and Larry.
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