303SS not recognized by ASME B16.34 ?

[quizzical1]

Dumb question time....
Does anyone know the reason why 303SS isn't listed in that standard?

 

[EdStainless]

It is a free machining grade, read 'made deliberately with a lot of trash in the microstructure so that it will make chips'.
It will have inferior impact and fatigue properties.
And they can be highly direction dependent.


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P.E. Metallurgy, consulting work welcomed

 

[bcd]

For a material to be listed, it must comply with 2 criteria.

1. There must be a commercial need to use this material versus one of the others already listed.
2. The allowable stress must be listed in ASME BPVC Section II Part D or in a Code Case to the BPVC. Without allowable stress values, the Pressure-Temperature rating cannot be established.

 

[quizzical1]

Thanks Guys!

 

[gtaw]

The "Trash" Ed mentions is typically sulfur. The sulfur content is about five times higher than the upper limit (to allow for ease of welding)for most steels. The sulfur is a low melting point constituent meaning that it is still in the liquid state long after the surrounding metal has solidified. Much of it gets rejected to the center line of the weld bead (solute rejection), resulting in a longitudinal center line crack once the weld cools. Sulfur solidifies around 235 degrees F if I remember correctly and it doesn't easily alloy with most metals, so it is rejected along the grain boundaries during the manufacturing of the "Free Machining" steel. The presence of the sulfur makes machining easier and it causes the chips to be small rather than one long ribbon. This makes disposal easier for the shop trying to machine a shaft as an example. The sulfur plays havoc when it comes to attempting to weld.

You can try to select a filler metal with a manganese content of about ten times the sulfur content. The manganese will combine with the sulfur, thereby taking it out of solution as a spherical nodule and help to mitigate the potential for cracking. Use stringer beads at low amperage and you might stand a chance of success.

Best regards - Al

 

[dik]

Sulphur causes 'hot shortness' in the HAZ...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[EdStainless]

There are also free machining options with Ca or Se in them. Just as bad.

S in SS is interesting. SS with very low S (<0.002%) is difficult to get smooth weld with.
In the range of 0.005-0.017% you get better weld fluidity and it helps.
By the time that you get to 0.03% it is nearly impossible to get good sound welds.
Nearly everyone in world making welded SS tube and pipe is trying to buy roughly 0.01% S.
This does not apply for thin wall (<0.030" or so) or thick material where you would weld with plasma.
And it doesn't apply to the laser weld people either.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed