17-4 PH vs 420 Stainless Steel 3

[Viroos]

Hello,
I have to design some miniature stainless steel parts like shafts and linkages which should withstand very high dynamic loads during very short time. Which steel is better: 17-4 PH or 420 SS?
Thanks,
Max

 

[israelkk]

More information is needed.

 

[Viroos]

What kind of info?

 

[tbuelna]

Stress and load cycle details would be helpful.

 

[EdStainless]

With 420 the heat treatment is a quench and temper, so a lot of different properties are available, but there is distortion and surface cleaning to worry about.
The PH grades (any of them) will have better corrosion resistance. And their properties can also be varied (trading strength for toughness).
In general for similar strength levels, a PH grade will have superior toughness and fatigue resistance.
Add to that such things as 13-8PH super-tough (remelted) and the PH alloys offer much.
Depending on the parts and processing required you may want 17-4 for some, but 17-7 or 13-8 may make sense for others. And then there is Custom 465. Lots of options.

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P.E. Metallurgy, Plymouth Tube

 

[israelkk]

As I understand you are trying to design (or re-design) linkages where the 420 or 17-4PH may be used a the pins (axis) of the link. An additional information will be for example: the link materials, environment, how short is very short?, what about the long qualification testing periods that links will endure compared to the "short" time during actual use time?, etc.,

 

[israelkk]

Viroos, may I ask where are you from?

 

[Viroos]

tbulena,
Unfortunately, I cannot disclose too much info, but we're talking about tens of thousands of g's during counted milliseconds.

EdStainless,
Thank you so much, this is very helpful!
What would you say about the machinability of both alloys?
Also, shouldn't I worry about the deformation of 17-4 during the heat treatment?

israelkk,
Correct. Both the links and the axes will be made from some tough SS. The life span of the product is very short, counted minutes, but the storage is very long, tens of years, so galvanic corrosion is also important.
P.S. Sure. Currently I live in Canada, but I'm from Israel.

 

[israelkk]

Viroos, I wasn't talking about long storage periods which is obvious. I was talking about the long time that it takes to test the linkages at the lab and outdoor tests before approving the design. Therefore, the links have to last sometimes for many minutes or even hours under the loads during testing in the subsystem before installed for final tests in the complete system.

From your posts I think I have fair understanding what you are designing. I am also sure that the designs you are probably familiar with are using 420 for such cases. From my experience and others experience I suggest to stick with the already proved design including the pin/shaft/axis hardness as was specified.

The main goal is to use the thinnest pin/axis diameter to minimize friction torque, but it requires very thoughtful design of the links to avoid breakage of the pins during qualification test and life use.

https://sites.google.com/site/israelkk/

 

[Viroos]

israelkk,
In our case a lab test which can simulate the real loads isn't available, so we do our best and hope the design will work IRL. For a while, we succeeded doing this
Anyway, some friction wear here isn't an issue, if I understand you correctly, because the lab test doesn't apply even a small portion of the real loads and the real life span is very short.
We're a electromechanical subcontractor company with little experience in this particular area, that's why I'm asking for some help. Do you think 420 is more suitable than 17-4? Why if so?

 

[israelkk]

See MIL-H-6875H page 20 footnote 11 regarding 420 special heat treatments. To my best understanding it was added for exactly cases as yours.

http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-H/download.php?spec=MIL-H-6875H.021377.PDF

I have to disagree with your design approach as I understand from your last post. If I was the customer I would insist that you will show in a lab tests that an actuator you design (if this is the case) has to provide and withstand the maximum torque/force it will see/apply during real mission including any position/speed/acceleration accuracy needed. Therefore, just to show it, you will need tests to fine tune the control system. The actuator has to last during these test before it will be allowed to be assembled for the mission test. I am not sure what is the governing specification your design has to be designed to but, for aerospace/military uses such specs will surely ask for such tests.

 

[Viroos]

Just to make things clear-of course, the device is tested, but not on the desk in the lab. Sorry that I cannot disclose more information...

 

[israelkk]

If it was tested why have you doubts about the material. You must stay with the tested solution otherwise, you will have to retest from scratch to approve the new design.

 

[EdStainless]

In reality people temper 420 either 500-600F, or 1000-1100F.
The size changes in 17-4 (or any PH) when you age is known and very predictable and repeatable.
It is much easier to machine a PH grad in the annealed ( A or T condition) and then final age, than trying to machine 420 fully hardened. If all that you are doing is a centerless grind then it does not matter much, but if there is any drilling, milling, or turning it will matter.

Actually you could play mix and match. The alloys (just about all of the 4xx and PH grades) will be galvanically compatible. If there is any sliding or rubbing using two different alloys will reduce the risk of galling.

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P.E. Metallurgy, Plymouth Tube

 

[Viroos]

EdStainless,
Thank you so much!

Actually, I can show some screenshots, the length of the link is about 12 mm.
Do you think it's a good idea to make the orange piston from C86300 bronze and the rest of the parts from 17-4 PH?

 

[EdStainless]

If the plunger is bronze will it be strong enough?
I take it that the links (yellow) will bear directly on the pin (pink?).
If so I would consider making the links in 420 and the pin in 17-4PH.
You could temper the 420 at 1000F and age the 17-4 at 1050F, this would put both of them at about 150ksi UTS (35-40 HRC). If you need more toughness you should higher temps (and lower strengths).

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P.E. Metallurgy, Plymouth Tube

 

[WKTaylor]

Viroos...

Max and max-stabilized service temperature may also make a significant difference in materials selection. Are You running very hot or very cold?

IF bearing-like properties are important... high hardness, wear and galling resistance is attainable using CRENS Corrosion Resistant Nitrogen Steel alloys such as AMS5898, AMS5925

Also there are new copper-nickel-tin alloys that push strength, toughness and wear to the limits normally the realm of HT copper-beryllium alloys in bearing/wear applications.

Regards, Wil Taylor

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[Viroos]

EdStainless,
And what alloy would you choose for the "cylinder" (the transparent part) and the "piston" (the orange one)?
One of the most significant factors in this design is the cost, any dollar we can strip off the production costs is important.

WKTaylor,
The temperature range is a standard mil-spec: -40 to +80 degC.
I'd like to limit myself to some more conventional alloys-mainly because of the production costs.

 

[EdStainless]

How tight is the fit in the cylinder?
If someone expects this to sit for a decade and then activate without a fault then cost does not really matter to them.
How big is this thing? Can you hold it in the palm of your hand?
How much structural load is there on the cylinder?

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P.E. Metallurgy, Plymouth Tube