17-4 H900 usage

[mighoser]

Can anyone give me reason on why I shouldn't use this alloy/HT in a low intermittantly stressed (less 50% of yield) part in marine air environment (worst case, part is for Air Force Jet). I'm familar with the Kure Beach SCC studies which were sustained stresses much higher than this part would experience. I've got a bunch of parts made and found that my customer prohibits precipiation hardening treatments below 1000F. This correspondes with verbage from "Structural Alloys Handbook" by CINDAS/Purdue University stating "17-4 is quite immune to SCC as aging temperatures above 1000F the only samples to fail were H900"

Any thoughts/cautions/warnings???

 

[btrueblood]

Is this a critical item (would its failure compromise mission, life of pilot, etc.)?

 

[mighoser]

Failure would cause the system to stop functioning but not risk the pilots life.

 

[israelkk]

Why would anyone will treat 17-4PH to H900 if the stresses are low as you say?

Here is the quote from MIL-HDBK-5J

"Design and Environmental Considerations — For tensile applications where stress corrosion is a
possibility, 17-4PH should be aged at the highest temperature compatible with strength requirements and at
a temperature not lower than 1025 F for 4 hours minimum.
The impact strength of 17-4PH, especially large size bar in the H900 and H925 conditions, may be very
low at subzero temperatures; consequently, the use of 17-4PH for critical applications at low temperatures
should be avoided. For non-impact applications, such as valve seats, parts in the H925 condition have
performed satisfactorily down to -320 F. The H1100 and H1150 conditions have improved impact strength
so that parts made from small diameter bar can be used down to -100 F with low risk. For critical low
temperature applications, a similar alloy, 15-5PH (consumable electrode vacuum melted), should be used
instead of 17-4PH because of its superior impact strength at low temperature."

 

[mighoser]

The alloy and HT selection wasn't my choice but I'm try to decide what to do with existing parts on hand. Are you saying there isn't any reason to use 17-4 H900. Our environment could have higher stresses than our predictions and we try to design with a good amount of margin from previous experience. Do you have any suggestions or additional information?

 

[israelkk]

If you looked at the quote from MIL-HDBK-5J and have any doubts that the 17-4PH H900 may fail then you should go with stronger alloy such as Custom 455, 465.

There isn't any reason for a too high safety factor if is not required. If it is for a military aircraft then as far as I know the factor of safety is 1.5 till complete fail. Therefore, there is no point to use too strong alloy in tensile where it may fail in SCC or inpact at low tempratures.

"Our environment could have higher stresses than our predictions...". This is a vage definition of your load and environment case. Only the spec define the system. If you suspect higer stresses, then the spec should say it specifically. There in no place for hypothesis. If test should be conducted to define the spec values then do the tests to correctly define the spec values. If analysis can define the spec values then you have to do it before designing over designed systems. If you have to start designing and building hardware before the spec is completely defined then go with better (more expensive) alloys that will have higher tensile strength but will not be sensitive to the environment.

 

[mighoser]

israelkk,

Thanks for the input. My problem is the scope of this statement from MIL-HDBK-5, "For tensile applications where stress corrosion is a possibility..." This would be almost every application. I haven't seen any information which shows 17-4 H900 failing due to SCC in stress states less than 75% of yield. Furthermore, these failures were for sustained load conditions. I don't plan on using this alloy/HT going forward but I would like to understand my risk before scraping 80K of parts. Do you have any info on this alloy/HT failing at low stress due to SCC? I'm aware of the low temp impact sensitivity and have data for this. I don't have any data for SCC crack growth for various load conditions. My data only covers the higher temp heats of this alloy.

 

[israelkk]

I do not have any specific data regarding SSC of 17-4PH. In the company where I worked as a policy we used only 15-5PH and Custom 455/465. We never used the H900 for parts under tensile or shear stress.

 

[mecoman]

Hello,

Just a thought, if someone can correct, refine or just call me wrong, please do. Cannot the 17-4 H900 parts be re-heat treated by first normalizing to return to the annealled condition. Then heat treat to a H1150 or other spec above the 1000F threshold?

Cheers,
Mecoman

 

[mighoser]

True,

Only problem is the film lube, and molded PEEK part which are attached. If my study indicates that using H900 for this application is not acceptable (internally or externally) I may try and salvage them. Not my first choice since it would be labor intensive and could introduce other issues.

 

[EdStainless]

The other problem with TH900 is the high level of internal stress. This only adds to the SCC issues with this heat treat condition.
If it were not for the PEEK you could simply re-age the parts at a higher aging temp.

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[mighoser]

EdStainless,

Why would you say H900 has a high level of internal stress? In the case our this part, we are adding stresses by cold forming (stamping). Are there internal stresses when the material is delivered in Condition A? It was my understanding that the aging process relieved stresses.(higher temp aging is better).

 

[EdStainless]

My view (based on work with wire) is that the lower age temp is too low to give much stress relief and the aging to the high strength causes significant stress.
The higher aging temps result in less internal stress. In spring work sometimes springs are re-aged after forming.
I hate the A condition. It is so variable and has low dictility.
If you are stamping I would overage, stamp, anneal and age. You may have to coin the parts afterward to hold shape and tol.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm