Hello, Chip et al.
* Regarding the preference between 15-5PH and 17-7PH - it would depend on the configuration and application of the component;
* Both alloys have good properties and characteristics, as do 17-4PH and 13-8 Molly.
* 13-8 Molly is a stronger alloy if you need higher strength, but it is a bit more expensive.
* Perhaps contacting Carpenter Technology in PA or ARMCO Steel in OH would help selecting the most optimal alloy, considering formability, weldability, machinability, HT distortion, cost…
* Important to note that the specifying the apptopriste tempering temperature is as important as the alloy selection itself - it typically ranges between 900F and 1100F in 25-50 degrees intervals.
* The lower end of the range assures higher UTS (ultimate tensile strength) and low RA and EL (reduction of area and elongation), thus poor fatigue properties.
* The higher end of the range provides decreased UTS and higher RA and EL, thus much better fatigue characteristics.
* Also, would be good to know if your application is high-cycle fatigue (over 100,000 cycles or low-cycle fatigue (below 100,000 cycles).
* 15-5PH would probably be better than 17-7PH for low-cycle fatigue.
* Puzzled by your min/max stress (-0.5/0.0)?
* If that is indeed correct, and there is no tensile stress - there is no concern for fatigue.
* Tensile stress is needed to initiate a fatigue crack and propagate it to catastrophic failure.
* Ultimate stress analyses for compressive yielding and column stability analysis in case of long slender components would be sufficient.
* Typically fatigue curves are constructed for:
R=-1 (fully reversed tension/compression of the same magnitude, like -1/+1);
R=0 (zero-to-tension, like 0/+1);
R=.33 (one-third tension to full tension, like 0.33/1.);
* The noted range (-0.5/0.0) would result in an infinite R-ratio? Never seen that in fatigue analyses.
* May want to double check; if indeed there is no tensile stress - fatigue analysis wouldn’t be a critical consideration.
** Good Luck, Chip!
Best regards.
-Jack.
Sent from my iPhone
Original Message:
Sent: 4/21/2022 7:45:00 AM
From: Nihad Ben Salah
Subject: RE: Relative fatigue strengths of 17-7PH and 15-5PH stainless steel
Hi Chip,
17-4 PH and 15-5 PH are actually very close. 15-5PH is the "little brother" of 17-4PH with slightly less chrome and higher Nickel in order to minimize the formation of the delta phase which is detrimental for fatigue. This could be very important for welding, and has its advantage for additive manufacturing.
Thus, it is easier to have thicker products in 15-5PH without cracking. 15-5PH toughness in the transverse direction is better, and crack growth rate lower linked to the finer and cleaner microstructure.
Corrosion resistance is similar.
You can find the properties of both materials in the Mil-handbook 5 (superseded by MMPDS-01), but you can still find it here:
http://everyspec.com/MIL-HDBK/MIL-HDBK-0001-0099/MIL_HDBK_5H_1804/
Hope this helps.
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Nihad Ben Salah
President
NBS- M&P Consulting
Canada
https://www.nbsmpconsult.com/en/home
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Original Message:
Sent: 04-20-2022 14:27
From: Chip Haynes
Subject: Relative fatigue strengths of 17-7PH and 15-5PH stainless steel
I'm looking to evaluate the two alloys mentioned above. similar HT (180-200ksi UTS)
Is there any additional information available outside the ASM handbook, volume 19?
Which of these two alloys works best (higher fatigue life)in a non- high temperature environment under variable cyclical loads with fmin/fmax ratio ranging between -0.5 and 0.0.
Info or resource is appreciated.
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Chip Haynes
Structures Technical Fellow
Aviation Partners Boeing
Edgewood WA
2062294943
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