Is tool steel, and 4140 alloy stainless?

4140 is not stainless. Tool steels are not stainless. They will rust in the high temperature and moisture. Nice job on the rotors. milo

Just to append Milo's correct response, 4140 is an oil-hardening alloy steel with the following specified chemistry:

.38/.43C, 1.60/1.90Mn, .20/.35Si.

Typical Ladle: .40C, 1.77Mn, .027P, .016S, .25Si, .10N, .12Cr, .01Mo.; grain size: 6-8.

Not my field, but I do know nice work when I see it. VERY NICE!

Randy

Hi,

Very nice work!

To improve the strength at high speed I would like the radius enlarged where the blades match the plate, there is a stress concentration! Nearly best form would be if the radius has the same value as the plate thickness and if it starts as a radius at the plate and evolves into an ellipse - larger axis parallel to the blade and 1.5 times the radius.

Also: the free outermost edges of the blades contribute significantly to bending stress by centrifugal force. I have no sound estimate how much to cut back but I would try with 45°.

Material: 17-4-PH has very high strength and is not rusting in usual conditions but if you operate this turbine with pure steam at elevated temperature then anything may be different.

RHABE

Aluminum is not a suitable material for high temperature steam. It will rapidly corrode as the atomically thin oxide layer is eroded away by the high velocity steam or air. The high temperature strength and creep resistance of aluminum also do not match the requirements. I would expect flex fatigue cracking at the blade to hub joint in a few minutes or hours of operation.

Thanks. So here's what I'll try: use high temperature paint to coat it, and either a more supported aluminum rotor or a steel, or 4140 rotor.

With Al-alloys you have to limit the temperature to below 120°C else creep will be a severe problem.

RHABE

Hard anodizing will be much better than high temperature paint. It will actually provide a much harder surface than the base aluminum.

have you considered inconel? or do you not like the price?

Aluminum loses mechanical properties quite fast at elevated temperatures, i.e. the tensile strength of A357T61 will drop from 45,000 psi @ 100 degrees F to 10,000 psi @ 400 degree F. Aluminum is NOT a safe material to use with practically any steam condition. It gains strength at lower temps so maybe you could change your plan and make it an expansion turbine, reducing natural pressure gas or refrigerant so the temperature never exceeds 0 degree F.

Even a 7075 aircraft aluminum will suffer at elevated temps. Aluminum alloys start to melt around 1050 F so the closer you get the greater the reduction in physical properties.

Also the use of 4140 should not be a problem since I believe that a proper system would be operating "in the absence of oxygen" which, after all, is needed in order to have oxidation (rust) occur. That is the reason that turbines have specific methods of shutting down and maintaining when not operating.

However, 17-4 is an excellent choice of material I do believe.

I need something I can buy in a small order. At metal supermarkets stainless or bronze is almost too expensive, they don't have 17-4, is it more expensive?

i would like to know, what hp are you transmiting, and steam conditions, normally the shft is steel 4140

right now the shaft is SS 304, but will be either SS 301 or 4140. I don't have it on a load yet, but it will probably transfere 1/4-1hp.

Here is a video of it running. Theres a defect on the rotor that makes it slightly unbalanced, that makes it vibrate and the unperfect sound. I can tell that the rotor is streching because it hits the nozzle when I have it in there, does some damage to the rotor but only where the over balance is. In this vid the nozzle isn't in it.

http://www.youtube.com/watch?v=Xp0keOh-6kE

It looks as though you used a billet to machine the rotor from, on which you did a very nice job. I once worked on a building expansion for a turbine rotor manufacturer in Cleveland, Ohio, and they made their rotors using the lost wax casting method. I don't know what material they used - it could well have been Aluminum.

Perhaps you could source a turbine that fits your needs, one that is made for, say, a turbocharger that would be of suitable design and material to satisfy your needs. It might also be more efficient as they are manufactured with more sophisticated tools that permit machining compound curvatures.