17-4PH Material

[mtncrawler]

Quick question - well, hopefully.

We have a part which specifies using 17-4PH, heat treated to 34 R/C Min. Since we buy this material in Cond "A", already above the min hardness requirements, are there any advantages to post (or pre)-machining heat treat? Seems like we machine the Condition A, as is, and verify the hardness requirements. Am I missing something? I've had experiences in machining the H900 - H1150M grades,but they all had higher hardness requirements.

 

[israelkk]

17-4PH should not be specified by hardness but by the heat treatment such as H1025. At each heat treatment the hardness can vary up to 10RC. This is the reason why you will not see hardness values for PH steels in MIL-HDBK-5 but only strength values for the variety HXXXX heat treatments.

 

[mtncrawler]

So is it uncommon for a finished product, using this grade, not to be solution treated to one on the HXXXX grades?

Would it be acceptable to basically change the material spec to say "17-4PH, Condition A" .vs. "17-4PH Heat treated to R/C 34 Min"

 

[unclesyd]

What is the end use of the part?

I've have reservations about using 17/4 in Cond. A in dynamic situations.

 

[EdStainless]

Don't use it in the 'A' condition.
The properties are highly variable and the ductility may be low.
If you don't need it very strong use the H1150 or H1150M to maximize toughness.

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

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

 

[israelkk]

I do not think that anyone can guarantee that condition A will always give RC34 MIN. The fact that the current batch you have is RC34 is just luck. As unclesyd said using Condition A is not recommended. The PH17-4 should be heat treated as MIL-HDBK-5J specifically says "Heat Treatment — 17-7PH must be used in the heat-treated condition and should not be placed in service
in Condition A or T."

 

[kenvlach]

17-4PH in Condition A is susceptible to stress corrosion cracking.*
It is also dimensionally unstable if heated. Suppliers don't guarantee mechanical properties in Condition A, so evidentally experience has shown that it shouldn't be used in this condition, as pointed out in above responses.

Hardness will vary due to variations in composition, solution cooling rate, and aging heat treatment. Carpenter Stainless Steels gives 'typical' values:
Condition A HRC 36
H 900 HRC 44
H1025 HRC 38
H1075 HRC 36
H1150 HRC 33
H1150M HRC 29

Note that Condition A has the same typical hardness as H1075. I expect HRC's to vary within +10% (maybe what israelkk meant?)

* "For applications where stress corrosion cracking is a possibility, 17-4 material should be aged at the highest temperature compatible with the strength requirements and a temperature not less than 1000^oF (wrought), 935^oF (cast) and not less than four hours hold time."
-- MIL-H-6875H

 

[israelkk]

I meant 10RC. MIL-HDBK-5C used to publish this data in a table) which was removed in newer versions.

 

[mtncrawler]

Thanks for all the feedback everyone.

Seems like we would be better off with an age-hardening of the Cond 'A" material - before machining. We don't want to worry about dimensional changes AFTER a heat treatment. The application is for a disposable surgical device, so I don't know if stress corrosion cracking would really be an issue, but it certainly sounds like the material could not be received consistently enough in the Cond A, to meet RC34.

I also found the Carpenter technical website very helpful.

 

[rorschach]

If this is for a surgical device you'll be worrying about Chlorides, so yes SCC is a possibility.

 

[kenvlach]

For surgical devices, be sure to properly passivate. Carpenter Technology recommends Types II or VIII per QQ-P35C [Nitric 1 & 4, respectively, in ASTM A967] or citric acid (note: more rigorously than at the minimum ranges in A967).

http://www.mmsonline.com/articles/100304.html

SCC shouldn't be a problem for disposable items (unless someone sterilizes & recycles).

israelkk, thanks for info on the hardness variability.