Ductility issue_excessive grain growth at both surfaces of strip
Ductility issue_excessive grain growth at both surfaces of strip
(OP)
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!
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!





RE: Ductility issue_excessive grain growth at both surfaces of strip
RE: Ductility issue_excessive grain growth at both surfaces of strip
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.
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P.E. Metallurgy, Plymouth Tube
RE: Ductility issue_excessive grain growth at both surfaces of strip
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.
RE: Ductility issue_excessive grain growth at both surfaces of strip
RE: Ductility issue_excessive grain growth at both surfaces of strip
RE: Ductility issue_excessive grain growth at both surfaces of strip
RE: Ductility issue_excessive grain growth at both surfaces of strip
RE: Ductility issue_excessive grain growth at both surfaces of strip
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.
RE: Ductility issue_excessive grain growth at both surfaces of strip
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.
RE: Ductility issue_excessive grain growth at both surfaces of strip
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.
RE: Ductility issue_excessive grain growth at both surfaces of strip
So these were coil annealed?
the outer strands are clearly overheated and decarbed.
You have furnace issues.
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P.E. Metallurgy, Plymouth Tube
RE: Ductility issue_excessive grain growth at both surfaces of strip
RE: Ductility issue_excessive grain growth at both surfaces of strip
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.
RE: Ductility issue_excessive grain growth at both surfaces of strip
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)
RE: Ductility issue_excessive grain growth at both surfaces of strip
RE: Ductility issue_excessive grain growth at both surfaces of strip
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?
RE: Ductility issue_excessive grain growth at both surfaces of strip
RE: Ductility issue_excessive grain growth at both surfaces of strip
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.
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P.E. Metallurgy, Plymouth Tube
RE: Ductility issue_excessive grain growth at both surfaces of strip
Skin effect is absoutely a good point for duplex structure!
RE: Ductility issue_excessive grain growth at both surfaces of strip
Why cannot I give star per post, but only per person?!
I noticed that, when I gave a star to Edstainless' most recent post, the previous star disappeared.