Magnetic 316L and galling (investigation)

[BiPolarMoment]

So, we have some machined parts of somewhat dubious history where the drawings indicate the parts are 316L but due to the fact one of the components exhibited ferromagnetism and was not galling in a same-material pair, we suspected the material may actually be hardened 400 series (e.g. 416/420) but simply did not have that correctly documented. Fast-forward to one metallurgical test later and it confirmed that the suspect material did meet UNS S31603 (hardness 86 HRB).

So the main question is: If the magnetism was present from the raw form (due to condition furnished?), would that be associated with any appreciable/expected galling resistance when mated against another 316L item?

History: we have some equipment that has been in use for >10 years and the major 'functioning' component of interest is a ~5/16" cylindrical post that is rotated a few times (loosely mated) within a bore in the part in question (e.g. 2-3 turns and is then held in that position by a ratchet/pawl mechanism)--there is a side load on this post applied above the 'joint' while it's being turned that increases to ~10lbf . Galling has not been an issue in the equipment presently in use to my knowledge.


While we were attempting to make design 'improvements' on a new purchase, efforts were made to minimize the number of changes to try to avoid issues such as what we are experiencing (lol), but nevertheless several changes did occur and perhaps are the contributing factors. For example: the post/bore clearance was reduced (still a clearance fit) to limit 'rattling'; the ratchet/pawl mechanism (not shown) was made more robust which incidentally applies it's own lateral force on the post ('Into' the view, near the interface of the components) not indignificant, but not quantified; it's likely the surface finishes for all components are not identical (and were never well defined). Suffice to say that once we began testing this new iteration of the equipment, galling occurred at this interface almost immediately which has halted implementation plans while we investigate.

It was perhaps 'hoped' that because these other changes were considered/thought to be inconsequential that the raw material of the 'magnetic' component may have been the solution/problem. Now we are not so sure.

 

[EdStainless]

Two options come to mind.
What is the mating material???
Make the post from cold finished 316L bar.
Have it drawn (or rolled depending on size) to a higher hardness than usual.
This could be why your sample is magnetic, it has been cold worked.
I question your HRB86, was that on a nice flat surface?
If so, then this material is junk.
It either saw almost no reduction from the original casting or it was mis-heat treated (or some combination, or the chemistry is messed up, or about 5 other ways to do this).
Or the worst combination, poor casting/reduction/HT procedures leaving a lot of residual ferrite, and then lightly cold working it.

The second (and my preferred) option is to look at using cold finished Nitronic 50 or 60 for the post.
I would aim for 120ksi min yield/140ksi min UTS/35% min elongation for properties.

 

[BiPolarMoment]

All materials in this equation are nominally 316L; the form of the actual raw materials is not specified on the drawings, however, according to the material certs, the 'new' (non-magnetic) plate was made from 1.5" smooth turned cold-finished round bar; the 'new' mating post appears to be made from 7/8" annealed cold drawn round bar. (Certs from North American Stainless)

The 'old' equipment material form and condition is completely unknown but chemically should be 316 for both mating components.

The second (and my preferred) option is to look at using cold finished Nitronic 50 or 60 for the post.
I would aim for 120ksi min yield/140ksi min UTS/35% min elongation for properties.

This is a probable material mating solution we are considering, but due to the sheer number (and machined complexity, relatively speaking) of 'posts' already manufactured (each 'plate' has four posts), the proposal is to make the 'plate' out of Nitronic 60 to salvage the existing 316 posts. Is there a particular concern for which component is Nitronic?

It either saw almost no reduction from the original casting or it was mis-heat treated (or some combination, or the chemistry is messed up, or about 5 other ways to do this).
Or the worst combination, poor casting/reduction/HT procedures leaving a lot of residual ferrite, and then lightly cold working it.

One point of curiosity is that these 'old' components have a hazy yellow look to them... reminds me of heat tint (not quite 'TiN' gold). After use they are routinely exposed to steam... but that shouldn't be relevant.

I question your HRB86, was that on a nice flat surface?

The hardness was measured by the same metallurgical consultant that performed the material determination -- nearly all the surfaces are flat on this 'plate' item so I would have to presume 'yes'. Keep in mind I'm referring to the 'old' magnetic plate that within its particular use case has not experienced galling. The mating 'post' in this situation is supposedly also 316, however we have not yet performed any analysis/testing on it.

Again, the confusion stems primarily from why this was somehow never a problem in the existing parts... we (I) would not have chosen to go with a 316/316 mate if it hadn't already seen over a decade of use without incident.

Thanks for your response as always, Ed.

 

[EdStainless]

I see no real issue with making the plates from Nit60 and using the 316 posts.
The discoloration could easily be surface oxidation.
Another long-term solution would be to make the posts from LDX2101 (lean duplex).
It is stronger than 316L, has slightly better corrosion resistance, a more stable price, and it machines very well.

 

[BiPolarMoment]

Another long-term solution would be to make the posts from LDX2101 (lean duplex).

Thanks for the suggestion, although this might be harder to 'sell' since some may ask "is that [surgical] stainless steel?" -- if we can say "Yes, it meets chemical requirements of ASTM F899" then great, but...

 

[EdStainless]

F899, who cares?
That spec is just used so that you don't have to look up six other ones.
It has no special properties, processing, testing, or anything.
All regular stainless plate and bar meet it.
A duplex would be covered by some of the same ASTM specs referenced in F899, but depending on the product form there may be other specs involved also.

 

[BiPolarMoment]

F899, who cares?

Regulatory agencies/bodies that see "FDA recognized consensus standard" and stop asking additional questions?

 

[EdStainless]

There is an F spec for duplex, but I don't know it off of the top of my head.
At one time the FDA recognized a number of ASTM A and B specs also.
Check the list.
Yes, I have encountered this when doing pharmaceutical work.