Need help selecting a suitable material. 1

[RNDROB]

I have a mechanism in an assembly that is causing a problem and I need some suggestions. The application is on the wet end of a paper making machine. Chemical environment is typically alkaline. We prefer to use 316 SS when we can. 316SS has a tendency to gaul when used for threads. We have successfully used a material combination in the past of a drive nut that is 70/30 copper Nickele alloy and a drive screw that is 17-4 PH ss with a heat treat spec of (900 full hard after machining)thread size is 3/8-16. Recently an assembly was returned because the drive assembly had stripped out. I removed the assembly and had our shop split the assembly with a wire EDM (photo attached). It appears as though the threads on the nut were weakened by corrosion and stripped out.
One though would be to make the nut have the same spec as the screw. I just don’t have any experience to tell me if it would be a good idea or not. All input is appricated.

Thank you,

 

[Metalguy]

I'd try using Nitronic 60 for one of the pieces, and wouldn't use 17-4 in the H-900 treatment. Consider H-1100 instead. Nitronic 60 and similar stainless steels are very resistant to galling.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[RNDROB]

Thanks for the reply. Can you explain your comment about 17-4 heat treat at h1100 rather than h900?
Thank you,

 

[btrueblood]

H1100 is less susceptible to stress corrosion cracking. See AK data sheet for 17-4 PH steel.

 

[BiPolarMoment]

Asking for the sake of Rob, would anyone here recommend any of Carpenters alloys instead of 17-4? It seems there may be opportunity to maintain the strength of H900 17-4 but with similar gains to corrosion resistance. I can't speak from experience so I thought I'd pose the question for Rob's benefit.

 

[Metalguy]

He hasn't reported a corrosion problem with the 17-4, but if he really does need the higher strength a switch away from it would be in order.

Another possibility would be to use the H925 heat treatment. Slightly less strength/hardness, but would probably not suffer SCC since the H900 apparently hasn't.

It's just not a good idea to let 17-4 in the H900 cond. get wet!

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[EdStainless]

He can't need the strength of 17-4 in the H900 condition since his mating part only had 1/3 of the strength level.

Use Nitronic 60 or 2205 and a bolt of 17-4 in the H1100 condition.
They will be galvanicly similar, offer good strength, and be available.

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Plymouth Tube

 

[unclesyd]

Take a look at Nitronic 50, 22-13-5.

 

[EdStainless]

N50 may be a better option. You want the female part to have lower corrosion resistance than your bolt.
In the past your 90/10 was providing some galvanic protection to the 17-4. Without that you might bet enough corrosion to have hydrogen cracking issues in the 17-4.

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Plymouth Tube

 

[BiPolarMoment]

Ed: I don't disagree with you, but I was trying to understand why he was using it in H900 in the first place--if it was for strength or hardness there are other options that won't have the same potential issues which is why I presented the question.

Thanks.

 

[RNDROB]

Thank you all for the continued ideas, all good stuff. Why the male part is H900 I don’t know. It has been since initial design in 1993 and the people involved are not longer around.
I have a very reasonable quote for the Nitronic 60 and will probably go that way.

 

[Metalguy]

The big advantage N60 has over N50 is anti-galling. N60 and 17-4 in a better heat treated condition should work fine--probably no corrosion, no galling.

I think the high pH is what saved the day for the H900 part.

I learned way back ~1960 the US Navy forbade the use of the H900 for aircraft use--a lesson I never forgot.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[tbuelna]

RNDROB,

Besides Nitronic 60, both PH13-8Mo or 15-5PH would give strength and corrosion resistance as good or better than 17-4PH without the SCC issues. You might also consider 718 Inconel.

Nitronic 60 is not a heat treatable alloy, so must be worked in its final hardness condition. If your part's threads must be rolled, this may be an issue with Nitronic alloys at high strengths.

Good luck.
Terry