Recover ductility after cold working 316L / 1.4404 1

[Nobert_47]

Hi,

We have a stainless steel plate of 6mm thickness that we are bending during fabrication, thus the material will be cold worked.
For this particular application, the increased strength / hardness is not something we want, actually we would like to keep the excellent elongation properties for this material as it is.
(Literature have shown reduction in elongation @ break down to 5% after cold working)

1) How can we recover the ductility of such a material (316L / 1.4404) after cold working, if even possible?
2) If possible, which values (elongation @ breakage) can we expect?


Notes:
- the material needs to be austenitic, due to possible presence of Hydrogen, but not necessary 316.
- corrosion resistance is not a big issue, a least not the reason for selecting this material.
- cold working is unavoidable

 

[TugboatEng]

316SS can be annealed by heating to ~1900°F. The material will have it's maximum %elongation in its annealed state.

 

[Nobert_47]

Thanks for replying TugboatEng.

These cold worked parts will be part of a welded assembly => should the annealing treatment be done before or after welding of the complete assembly?
(The assembly consist of both cold worked and pre-machined parts, all of the same material, which will be machined after welding).

By reading some DNV rules (DNV rules for ships), they say that heat treatment of welded austenitic material is not required. So that leads me to think that its best to do the annealing process before welding.
But I'm afraid that this heat treatment could distort the shape of the parts, so we could have difficulties joining them during welding.

Anyone having experience on such a process?

Notes:
- the cold worked material are "thin" sheets (6mm) which are bended / formed into a box of approx. size 500x600x1500mm

 

[BiPolarMoment]

they say that heat treatment of welded austenitic material is not required. So that leads me to think that its best to do the annealing process before welding.
But I'm afraid that this heat treatment could distort the shape of the parts, so we could have difficulties joining them during welding.

(Emphases mine)

I don't understand your line of logic; if you have the opportunity to anneal after welding (is it all the same material?) you eliminate the potential of the "distortion" (i.e. stress relieving) affecting assembly. If it is not feasible to anneal after welding (e.g.: due to the assembled size) then you can tackle the question of "distortion". No?

 

[TugboatEng]

On top of stress relief, annealing after welding and then quenching through the sensitized range will improve the corrosion resistance as it sounds like this will be exposed to a marine environment.

 

[dik]

if you have the opportunity to anneal after welding (is it all the same material?) you eliminate the potential of the "distortion" (i.e. stress relieving) affecting assembly.

Heat treatment does not reverse distortion, it reduces residual stresses.

If it is not feasible to anneal after welding (e.g.: due to the assembled size) then you can tackle the question of "distortion". No?

That would require requalification of the welding procedure, and any Welding Engineer will go to great lengths to avoid that scenario. If you must anneal (do you really need to?) then anneal it before welding.

For L grade austenitics with C < 0.02%, sensitization is a very low risk. If corrosion is not a big issue, go with 304L. Most steelmongers will offer dual certified with the straight grade, so a pure L grade is harder to get.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[TugboatEng]

This is a shipboard application, there will be chlorides. Do avoid 304.

I know in some cases, structures can be fabricated on a rigid jig. Welding creates residual stresses that will cause the structure to spring when removed from the jig. Before removal, the structure goes into the oven with the jig to be normalized. Afterwards, it should hold the correct shape after removal from the jig.

 

[Nobert_47]

Hi all, and thanks for replying to my thread.

The annealing is needed primary to recover ductility, but increased corrosion resistance is a bonus, but not needed. Even 304L material for this application would be just fine.
(Note: the availability of 314 is not better than 316 in Norway, so this would not make things simpler or cheaper). Elongation before break, together with possible presence of Hydrogen is the primary reason for selection 316L material.

Since the bended sheet is the largest part of the complete welded assembly, heat treatment of part(s) vs welded assembly would still require an own of a certain size.

So this leads me to think that the following production sequence is the best choice:
- pressing / bending of parts
- assembly by welding (bended parts and pre-machined parts)
- annealing (of complete assembly) to recover ductility lost during cold working
- final machining

@ironic metallurgist
You mention requalification of welding procedure: Not sure I understand you. Would welding of an annealed material trigger anything "special" compared to the raw material?
We don't have settled the fabricator yet, so which welding procedures they have in place in not known at the moment.

 

[dik]

Because a change in an essential variable triggers requalification, and heat treatment is an essential variable.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

You need to make sure that the post-anneal properties are still acceptable.
While this is hard to get wrong you have changed the process so you need to verify this.
Just make sure that it is clean after annealing.
Do not grind. Just acid pickle or blast and pickle will get what you want.
Any heat tint seriously lowers the corrosion resistance.

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