303SE or 30323 Free Machining SS for fittings?

[Canucker006]

Does anyone have any experience/knowledge about using a free machining stainless steel such as 303SE (aka UNS 30323) for a pressure fitting (i.e. a check valve).

It's for pressurized liquid, up to 6000 psi.

The check valve I'm looking at is currently made from 304 SS and production wants to change it to 303SE for easier machining.

any advice is appreciated.

 

[btrueblood]

303 is a no-no for pressure vessels, pressure piping, etc. The 303 alloy has unpredictable strength and ductility due to the sulfur and/or selenium additions, worse corrosion resistance, and no weldability relative to type 304. It's a loser from the start.

 

[Canucker006]

Thanks for the quick reply.

Are there any other free machining stainless steels that would be acceptable?

Is there any documentation that substantiates not being able to use it? (I know its not listed as an acceptable material in ASME, but there are a ton of materials that aren't listed but can still be justified)

thanks again!

 

[Canucker006]

like 17-4 H1150 for example? anyone?

 

[israelkk]

btrueblood

Can you provide reliable resources, specs, etc, that forbid the use of 303 and 303SE ?

 

[btrueblood]

Nope. Just wouldn't do it for 6000 psi service. I know it is not a listed material in the ASME piping and pressure vessel codes, which doesn't preclude it from being qualified for use anyhow...but can make the process a bit more difficult. And, have had specific experience with highly variable corrosion rates and strengths of different lots of the material.

 

[israelkk]

The reason I asked is that we have a good experience even for 6000psi and up. To what spec you order the material and how are you verify it?

 

[israelkk]

The only thing I found is from NASA-STD-(I)-6016 STANDARD MATERIALS AND PROCESSES REQUIREMENTS FOR SPACECRAFT

Here it is:

4.2.2.2.3 Corrosion Resistant Steel Unstabilized, austenitic steels shall not be used above 700° F (371° C). Welded assemblies shall be solution heat-treated and quenched after welding except for the stabilized or low carbon grades such as 321, 347, 316L, and 304L. Service-related corrosion issues are common for free machining alloys such as 303, and these alloys shall not be used in applications where they can get wet.

 

[btrueblood]

israelkk, we ordered a generic 303 originally, IIRC, i.e. no spec. I inherited the part, the fittings were made by a local machine shop. Failures were in fittings used for hot water at relatively modest pressures (below 1000 psi), so your comment about wet service applies. We tried sourcing "cleaner" varieties of 303, but were more successful with garden-variety 304 stainless, and working on machine processes (lubricants, cutting tools) to get good parts. We DID see inclusions and stringers in the original 303 parts, that disappeared when we started purchasing from a reputable source vs. cheapest source. But never got rid of field failures until we went away from 303.

 

[israelkk]

We use 303 for dry inert gases with no problem

 

[monkeydog]

israelkk,
We had issues with 303 due to the sulfide inclusions. We ended up getting rid of all 303 unless we could show we were at some% or less of tensile stress.
Can't share technical data within the company files, and could not find any public domain data to point to.

 

[israelkk]

We only purchase 303 with C.O.T from reputeable manufacturers that include heat and batch numbers and assurance that it conforms to specs such ASTM, MIL, AMS etc, as needed. We do not accept dealers C.O.C.

In one of the places I worked we had a warehouse to stock for all materials before approval. Every batch was metallurgically tested according to the spec and compared to the C.O.T before approved for moving to the supplying warehouse.