Discussion of 17-4ph stainless steel

[Jim Allen]

Stainless steel 17-4ph ( stainless grade #630; UNS #S17400) is a well known precipitation hardening martensitic steel whose name comes from the addition of 17% Chromium & 4% Nickel. It also contains 4% Copper & 0.3% Niobium. It has been used for boat shafts & propellers for many years. The alloy cast well in thin sections & can be precipitation hardened easily. The advantage of a precipitation hardened steel is that in the annealed or solution treated condition they can be readly worked or machined. After working or machining, a low temperature heat treatment can be applied to greatly increase the mechanical properties of these steels. Since a low temperature is used for this ageing or age hardening, the component undergoes no distortion. After solution anealing (1925+-50 deg. f.) for 30 min. to 1 hr.; then air cooling to 76 deg. f., 17-4ph is aged or tempered at 896 deg. f. for 1 hr followed by air cooling. Typical tensile strenghts of 190,000 psi. & yield strenghts of 175,000 psi. are attainable.

Cryogenic processing, the gradual lowering to -320 deg. f. & soaking for 20 to 60 hrs.; then raising the temperature to 375 deg. f.; then gradually reducing the temperature to 76 deg. f. is doing what for model boat propellers? Cryogenic processing certainly has been used to increase wear resistance on cutting tools such as drills; broaches; & molds. Can someone prove by using this process on model boat propellers what mechanical properties (tensile or yield strength; hardness; Charpy impact valvue; compressive strength; Young's modulus; fatigue life; etc.) will be improved? Aren't boat propellers already expensive enough without the added cost of processes that will add little if anything to the mechanical properties required?

Jim Allen

 

[Chris Wood]

jim,

would the cost and time to do the Cryogenic process be worth it? I personally would like to try it if i had the equipment.. with all the work and time that go into props, it sure would be nice to be able to keep them in shape a little longer...

Have you had any experiences with doing this??

 

[Jim Allen]

Chris,

My experience with this process involved the use of cutting tools that had the cryogenic process done to them. These tools made of M-2 & D-2 steels showed increased wear resistance over untreated tools. I also have information that this process has improved the wear resistance of AISI S-7, which is the alloy I use for my connecting rod & crankshaft. I don't know of what value an increase in wear resistance would be to a model boat propeller. Therefore, I think the time & cost of this process for model boat propellers is unecessary!

Jim

 

[izitbrokeyet?]

I Agree!

The materials are well within their respective application regimes without the Cryro ($cha-ching$)

If memory servs…..the stress-strain diagrams for the materials take on a more brittle shape once they’ve been precip-hardened. I’d opt for the more ductile material that meets the strength requirements.

Also:

At high RPM an Impact scenario will quickly get into the peak of a brittle material exceeding it in an impact situation. If the material is ductile it will tend to transmit some of the impact energy into deformation (plastic and elastic) whereas the more brittle material will peak at ultimate strength and fail catastrophically.

I would use the impact scenario on a prop where you dont know what it'll come in contact with. I would feel more comfortable using the material on an output shaft or connecting rod. But the first statement still applies...why bother?

 

[Terry Keeley]

Wow, some of the minds around here amaze me. :blink:

I haven't had many problems with the C20 BeCu props when properly heat treated, threw a blade only once on a 2170 I thinned just a little too much.

 

[DON MAHER]

This is off subject but, Jim if would not mind sharing some of your vast knowlage I would like to know what you make your sleeve's & piston's out of ? I am currently running CMB 67's & 80's at about 27,500 rpm. Have had only one failure. ( stuck piston, too lean) May try to make my own & would appreciate any info.

Thanks Don

 

[Jim Allen]

Terry, it looks as though C-20 BeCu blades, that have been properly heat treated, when thinned to -.080 at the blade root back edge will possibly fail under load. This does not happen to 17-4ph blades, that have been properly heat treated, even when thinned to -.065 at the blade root back edge. This is because of the higher tensile & yield strebgth of 17-4ph & its lower hardness.

Jim

 

[Jim Allen]

Don,

I have used #360 free machining brass & 4032-T651P aluminum for liner material. From extensive dyno testing, it appears that liners which have a wall thickness of .100 & a lip thickness of .200 with the head bolts going through the lip, are essential design criteria. I have applied this to the .65 to 1.00 cu. in. motors built to date. Maybe these things are not necessary in motors of a smaller size, but I am sure they increase the performance potential of larger motors.

I have throughly tested A-390 (14% silicone); Mahle-138 (18% silicone); Dispal-250 (21% silicone) for piston material in both brass & aluminum liners. Oil contents varied from 5 to 15% & are now held at 6 to 8%. Once the "correct liner taper" (ground not honed) & the "correct piston top tapper" was established, piston scuffing disappeared; even when running motors overlean for extended periods of time.

All of these motors had the same stroke (.902) & bores from .860 to 1.156. I have tested .90 & 1.00 cu. in. motors at rmp's beyond 35,000 during dyno pulls of 2 minutes duration with no problems.

In the future I will post pictures of how to make liners including fixtures; necessary math & tools required.

Jim

 

[J Solinger]

In this article is the name of a company that may be able to tell you what cryogenics will do for 17-4 SST.

The Big Chill

 

[Jim Allen]

Joe,

I e-mailed Silver Bullet Machine Manufacturing Company which is named in the big chill article. I asked about the effects of the cryogenic process on 17-4PH; S-7; M-2; O-1; C-350 & C-20. Received no response from these people.

Jim

 

[J Solinger]

Joe,
I e-mailed Silver Bullet Machine Manufacturing Company which is named in the big chill article. I asked about the effects of the cryogenic process on 17-4PH; S-7; M-2; O-1; C-350 & C-20. Received no response from these people.

Jim

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Hmmm, maybe they will get back to you later. I don't have any personal experiance with Silver Bullet Machine and don't know what to expect from them.

I did some searching and found some good news. Cryogenic treatment is being done on 17-4 sst. I think that hardening to an H900 condition then cryo stabalization would result in a very durable prop that you could thin down with out it folding or shattering.

Here is another outfit that does cryo treatment. They advertise processing marine propellers, among other parts. Are full scale props made from 17-4? Anyway, they do treat 17-4 and other precipitation hardening steels.

another cryo source

Hopefully you can get through. Maybe a direct call would do the trick.

 

[Jim Allen]

Thanks Joe. I'll try this company by e-mail & phone.

 

[Jim Allen]

Joe,

I received an E-mail & phone call from NW Cryogenics Cold Treatment Services. There are no test numbers available for this process; only testimonials about improved wear resistance on certain alloys; & testimonials about changes in the microstructure when samples are viewed under an electron microscope. No claims or numbers about increases in toughness; hardness; fatigue resistance; tensile or yield strenghts; etc.

They did offer to treat my samples for free. I would have to run my own test to determine what this process can do for engines or boat hardware. I don't think this process can do anything for a properly heat treated 17-4PH (H900 treatment) model boat propeller except add to its cost.

Jim Allen

 

[Ken Retallick]

Jim,

I wouldnt heat treat 17.4PH to the H900 condition, It is very prone to hydrogen cracking. Even in fresh water. Not something we need in a highly stressed model boat prop. H900 and H925 tempers have even been banned in many applications.

17.7PH is more resistant to hydrogen cracking in H900.

Cheers, Ken

 

[Jim Allen]

Very good information Ken. Heard about the hydrogen embrittlement problem, but have not experienced this yet. I have used 17-4PH (H900) for many years without any failures. Maybe this is just good luck. Will look into this further.

Jim

 

[J Solinger]

All I would expect cryo treatment to do to 17-4 in a H900 condition is toughen it a little.

One thing I'm curious about is the difference in treated cast 17-4 versus treated 17-4 bar stock.

 

[glenng]

Very good information Ken. Heard about the hydrogen embrittlement problem, but have not experienced this yet. I have used 17-4PH (H900) for many years without any failures. Maybe this is just good luck. Will look into this further.     
Jim

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Jim, I've machined parts from 17-4 for about 20 years and it's one of my favorite materials in the annealed condition or heat treated to cond.H900. I've found it to be a very stable material and not brittle even when heat treated to H-900. I've seen H-900 splined parts I've made twist about 20 degrees without breaking. Personally I don't think heat treating or cryo treating is worth the money as far as boat props is concerned. I won't pay 95 bucks for a r/c boat prop. If anyone has any info on heat treating BeCu props let me know, Thank's, Glenn

 

[glenng]

All I would expect cryo treatment to do to 17-4 in a H900 condition is toughen it a little.
One thing I'm curious about is the difference in treated cast 17-4 versus treated 17-4 bar stock.

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Hi Joe, 17-4 in the annealed condition(condition A) is somewhat softer than condition H900(that has been heated to 900 degrees for one hour and then air cooled). Condition H-900 has a rockwell C scale hardness of 44-45. Give me a couple days I'll send sheer/tensile strength for both. Glenn

 

[Jim Allen]

Glenn,

I have had no problems when properly heat treating 17-4PH (H-900 treatment) for model boat propellers. I have also used 17-4PH without heat treatment & experienced no problems. There is no additional cost involved because I do my own heat treating.

Propellers cast from C-20 (C82500) beryllium copper reach an ultimate tensile & yield when heat treated at 650+-5 deg. f. for 2.5 to 3.0 hrs. The time is not critical but the temperature is. Tensile strength should be 160,00 psi; yield strength 115,000 psi & Rc 40 hardness.

Thanks to everyone for the information posted on this subject.

Jim