Ductility issue_excessive grain growth at both surfaces of strip 2

[MagBen]

We reheat and then soak a specilty steel for 3 hr at a temperature about 100C below austenitizing temperature, and then quench in a water spray tub. It is coil strip with a thickness of .1''.

The material is breaking during re-coiling at the take up reel. SEM anaysis of fracture surface showed coarse cleavage facets that appeared to occur during an intergranular fracture. LOM image showed the presence of blown grains and cleavage cracks. The blown grains were as large as ASTM 1 but only at or near failure cracks. I attached a picture for your review.

It seems clear the cause of failure is related to large grain surface structure. But my questions are 1. what caused the coarse grains at the very surfaces? 2. could re-quench bring back fine grains (need samller than astm 6).

Any inputs, no matter if or not making common sense, are appreciated!

 

[MagBen]

Sorry here is the attachment of grain structure

 

[EdStainless]

Is there any cold work on the strip? A small amount of cold work would accelerate the surface grain growth.
And it may be that the surface is heating much faster (and hotter) than the bulk furnace temp due to radiation effects.

Now, if you could figure out how to do this the other way around you would have a product.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MagBen]

Ed, do you mean cold work before soaking? or after quenching, or during quenching?

We do quench right after hot roll, there could exist residual strain duirng coiling, but ht soaking would remove that.
After quenching: obviously the mechanical deform at re-coiling lead to fracture.
At quenching: we do have issue with substaintial mechanical work created during uneven rotation of reel in the soak furnace. That is one of suspected areas we are looking at,re-crystallization might occur preferably at the surfaces, but why only at surfaces? also note the soak temprature is relatively low.

It is true the surface is heating faster and hotter, but this limits only to couple of wraps on OD. big grains exist on both surfaces and in deep through the whole length of strip.

 

[metengr]

What was done to the specialty steel before a final subcritical thermal treatment? There could be several causes for this, I would go back and review the complete processing history.

 

[MagBen]

May not exactly the same locations, I didnot find any such duplex grain structure in hot bands. The grains after hot roll are pretty fine.

 

[weldstan]

There has also been a lot of decarburization at the surface. You also need to take a look at the coiling temperature.

 

[DeleteriousPhases]

any idea on chemical composition for this? you said duplex, but I don't want to assume.

 

[MagBen]

Decarburization is one of things we are examining. we do find heats with higher C less questionable. But Still, why the issue was not limited only to couple of outside wraps?

Do you mean the coiling temeprature after hot roll? so the cause was not just restricted to the quenching process, say the duplex structure was already there before quenching? The re-coiling at takeup reel was at room temeprature.

 

[weldstan]

In making hot strip, both the final rolling temperature and the coiling temperature greatly affect properties and grain structure.
One also must ask, why a three hour soak? If you are heating the entire coil, the outside wraps will experience the highest temperature for the longest time. They will also experience the greatest exposure to oxidation.

 

[MagBen]

Weld, 3 hr soak was adopted by simulation, taking into account of temperature profile of the coil and furnace as well as the size of coil(s). The location of coils and furnace were such designed to avoid direct and overheat. Thermal couples were replaced all over the coil, inculding inside the coil to make sure the uniformity of temperature.

Currently it is hypothesized that the cause of large grained structure has nothing to do with hot roll and coiling since there was no evidence of any blown grains in hot band structure of any heats. But we won't exlcude these factors until the problem is solved.

 

[EdStainless]

Small amounts of cold work can greatly accelerate grain growth.
So these were coil annealed?
the outer strands are clearly overheated and decarbed.
You have furnace issues.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[weldstan]

Change the furnace atmosphere.

 

[MagBen]

DeleteriousPhases,
sorry for confusion, by duplex I meant grain structure: fine grains at center, coarse at both surfaces. this is not a stainless steel.

Ed,
It is kind of laminar cool to get a high cooling rate to avoid a undesired phase. Damping the whole coil in any cooling agent could not obtain enough cooling. The strip will have to cold roll down to couple of mils, properties at this stage do not matter, breakage is the big problem.
I tend to believe you hit the key point, but still donot understand why only both surfaces have big grains, and why BOTH surfaces? Is anything related to recrystallization, second recrystrallization? In this case, I assume the blown grains were formed in hte funace before quenching.

Weld,
Reducing atmospher would be good, but not optional at the moment. but again, why both surfaces got coarse grains.

 

[MagBen]

There was a typo in my opening post, "reheat" should be "preheat". The coil was preheated to 550C, then soak, so the possible cold work due to coiling after hot roll seemed to be irrelevant.

Does anyone have new insight on the duplex structure?

We re-quenched the material using exactly the same parameters, the big grains at both surfaces disappeared, a unform gain structure was obtained, with an overall grain size increased. How the big grains got decreased is beyond my understanding! (note the soak was 100C below austenitizing temperture)

 

[weldstan]

Heating lightly cold worked steel above the recrystallization temperature will increase ferrite grain growth.

 

[MagBen]

Weld,
Thanks, but the preheat at 550C was supposed to be able to remove all cold work. Also, 550C is way below the recrystalization temperature for this alloy.

Could the uneven rotation of reel right before quenching into the tub cause dynamic recrystallization?

 

[weldstan]

All cold work is only removed when heated above the recrystallization temp. Most stresses are relieved at 550C.

 

[EdStainless]

If you have lightly cold worked material and want to remove the stress from cold work without grain growth you need to use low temperature/long time heat treatments.

If you roll a coil and just try to improve flatness and gage accuracy then you may just use a 10% reduction. At that low of a cold work you are not deforming the material all of the way through, it is really just a skin effect.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MagBen]

We didnot do a pinch pass. Do you think the coiling at finish hot roll caused a skin effect?

Skin effect is absoutely a good point for duplex structure!