Ni-Cr Bar v Forgings

Will a high speed rotor made of Ni-Cr alloy bar machine and hold up as well as
one made from a forging?
The forging rotor may have a more radial structure and be able to spin at a
slightly higher speed, but that's not at issue here.
Bret Cahill
Reply to
BretCahill
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There are a variety of issues here. The right quality bar can replace almost any forging, how high a quality bar you need depends on the application.
I will address one issue that is sometimes overlooked. The properties of a heat treated bar are usually measured near the surface, so even if the alloy has sufficient ruling section to obtain the desired mechanical properties at the starting size for machining, the properties in the central part of the bar may be much lower. The ends of the rotor, with bearing journals drive couplings etc, are probably made from this material. This can be overcome by heat treating the bar at the proof machined stage but this often increases costs significantly.
Reply to
David Deuchar
. . .
Could the bar stock end up more expensive than the equivalent number of forgings?
I got a quote for forgings 7 times higher than the equivalent amount of bar.
Forgings have the advantage of reducing the overall amount of metal that is removed / wasted but that might not be an issue for small rotor disks.
Sounds ok because heat and rotational stress increase in the radial direction.
I'm guessing by central you mean radial (r=0), not axial (l=L/2)
I'm guessing this means the harder outer case of the bar has been removed by, say, turning in a lathe exposing the softer inner metal.
Might not be necessary. Aren't bearings mounted on Al and titanium shafts?
Bret Cahill
Reply to
BretCahill
Be very sure the bar stock has the strength and all the other properties that the forgings have if the part is in a place that can cause a serious problem when it fails. The bean counter that fails to understand the engineering of the part they are replacing can be held liable for the damage caused when the parts fail. Sometimes the chance seems safe in every way yet is sure to fail. If possible get the engineer that designed the part in the first place to look at the problem if not get a quailed person to evaluate it.
You don't want to be immortalized in history for causing a failure that costs someone their life for a few dollars savings. There was a reason it uses a forging find out why.
Reply to
Gordon Couger
Depends on what you need in bar properties. Probably not if the application is suitable.
Yes, bar is generally heat treated in long lengths and then cut to length for you.
Yes
The last failed high speed (20,000rpm) rotor I looked at, had a bent central spindle. This central spindle was part of the same forging. Would it have been appropriate to make the replacement part from a bar where the centre is half the expected strength, I doubt it.
If you are making parts from drawings, you have to allow for engineering design assessment (management of change) costs in any changes you make. High speed usually makes a mess when things fail. There are other potential issues e.g. radial strength, fatigue etc these depend on design and answers are specific to the design. I would guess that, for small production runs (replacement parts), 60-90 % of forgings can economically be replaced by machined bar / profiled plate parts.
Reply to
David Deuchar
I'ld like to drive gas turbines down to the $20 - $50/kW range, maybe even install them in hybrids as a small, light weight, temporary power engine. If it is only used 5% of the time -- hauling the boat down to the dock -- who cares if is only 15% efficient?
For general aviation the engine might be more reliable and cost a little more, maybe $80/kW. Flying through hubs wastes time and money and fuel, not to mention those body cavity searches. It's better to just know all 8 people on your plane.
But right now I'm just trying to get an idea of the costs involved.
Some guy at DOE said it would cost 10 billion to start a gas turbine company. This is nuts. The wars of the 20th Century caused gas turbines to be developed with unlimited funding. As a result the GT is vastly overpriced.
Bret Cahill
Reply to
BretCahill
You should read Sir Frank Whittle's biography before you talk about "unlimited funding" !!
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Steve
Reply to
Steve Taylor
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Frank Whittle was the genius of British technology who ran the group that developed the Gas Turbine engine for aircraft prior to and during WWII.
As part of the trading of Wartime Secrets, the British donated their Radar and Gas Turbine work to the USA in exchange for some form of cooperation in the Atomic Weapons development.
There is some real fascinating technical history involved, if you respect technological history, that is.
Jim
Reply to
jbuch
Some guy at NASA said 60% of the billions in R & D costs of gas turbines was in materials science.
It may be possible to prove beyond any doubt that film cooling will always be necessary for a high efficiency engine.
Bret Cahill
Reply to
BretCahill

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