4340 Grain Structure of Plate Versus Round Bar After Hardening

[MechyMarco]

Hi all,
I'm hoping someone can back me up here. I'm making some extreme stress parts out of 4340 round bar per AMS 6415 (air melt). They are machined in the normalized condition then send for hardening. Tempered at 475F to 50-52 HRC. Tensile tests on some test coupons done with the parts put the strength in the 220-240 ksi yield range with some reasonable ductility. So all good there. The issue is cost. The parts are quite rectangular so machining from round stock is very wasteful. I can save a lot of money by using plate instead. However, machinists and some other engineers say I need to consider grain structure. Normally I would agree if I wasn't hardening the parts as the grain structure would be different. Round bar superior in this case. But since the parts are being hardened, the grains will be completely reformed during heat treat and relatively isotropic so any directionality issues from the raw stock in plate form versus bar form are no longer an issue right? The only thing I can think of is the inclusions that are present will still be a factor. So round bar transverse still worse than longitudinal. Same with plate. The through thickness properties will be worse than the length / width properties.

Am I thinking this through logically? Anyone see any issues substituting AMS 6359 plate for AMS 6415 round bar?

Thanks for the insight.

 

[mfgenggear]

Mechy
The grain structure will not be the same. Direction of grain of bar VS plate will be different. However please take in consideration of what is required for the part. Since we have no idea of fit, form or function. And the stresses required.
High stress parts will be made from forgings and grain direction is critical.

 

[EdStainless]

What is the thickest dimension of this part?
There is a reason that most AMS spec prohibit the cutting of bar from plate.
Plate has "L", "l-t", and "s-t". And the toughness and ductility tend to decrease in that order.
Bar just has "L" and "R" (or T if you like).
Have you looked at rectangular bar?
Did you select your temper temperature based on the as-quenched hardness?
Your temper sounds very low.

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[Compositepro]

Are not most high stress steel parts made from forgings, to eliminate material defects inherent in rolled bars or plates?

 

[mfgenggear]

Compositpro

It is well documented that due to the grain direction of forgings are high strength and high tensile than bar. However it is very expensive.
Depending on material there is vacuum melt which is generally better quality material. Defect free.
There is recommended stock removal to remove surface defects and decarb.
High stress parts require NDT to verify defect free.

 

[MechyMarco]

Let me clarify. If the parts are going through a full quench and temper then aren't the grains completely reformed anyway? So even if you started with an amazing forging where the grains follow the shape of the part perfectly, the benefit of the forging is gone after heat treat as the grains were reformed during austenitization. This means grains shouldn't play a role. Only the inclusions should. Things like slag and impurities that interrupt the grains.

 

[MechyMarco]

Hey all,
Found the answer to my question. Short answer is grain structure is mostly retained from whatever wrought process you started with. Grains don't regrow isotropically. They retain the wrought anisotropy. Reasons being materials science that I don't fully understand. At least I learned something today. Here's a link to post that helped me.

https://www.reddit.com/r/metallurgy/comments/rp5oev/grain_flow_and_heat_treatment/

 

[EdStainless]

No, Q&Y does not erase all prior grain texture.
After all you can still go back and find the prior austenite grain size.
If you are really stressing the part this hard go to 4330V or 4335M that has been remelted.
I wouldn't use non-remelted steels at strengths above 220ksi as an upper limit.
There is an AMS steel heat treating spec that tells you how to adjust temper temperature based on the as quenched hardness.

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[MechyMarco]

Yeah my primary material is 300M since it's a vac melt and have had great results at 220 ksi and up. But when you get to parts over 8 inches in diameter finding material is very difficult. And even if you find it, the price is tough. I know there's vac melt 4340 out there too, but it's almost the same price as 300M so sort of pointless in my eyes.

I got the AMS specs that give great guidance on tempering based off as quenched hardness too. Thanks Ed.

 

[EdStainless]

If you are moving from 300M because of availability, then the cost of remelted 4335/4340 shouldn't be an issue.
This isn't a place to try to save a few bucks on material.
I used to buy 4-6" bar in these grades.
If I wanted full UT I might as well go remelted as the cost difference wasn't much.

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[MagBen]

i see no issue to cut parts from plate vs from round bar. I think this maybe a better choice since the uniformity can be better with plate. The center of a round bar is literally zero deformed, and difference between L and R direction is maximized.

 

[WKTaylor]

My jet uses 300M at HT280-to-HT320[max] for many strength critical parts... BUT we try to use 4340M for HT275-to-300[max] applications... and use 4330M/4330V or 4340M for HT150-HT240[Max] applications.

How is your bar made? forged-bar... Extruded/drawn/rolled... drawn/rolled/stretched assuming. We must assume all this is done 'hot' in some specified temper that would be considered 'annealed'.

Be careful... 8-inches cannot be thru HT... obviously, machining the annealed to make it equivalent round 'thickness' for HT is required... and then a really good/smart/experienced HT vender to 'get-it-right', thru-N-thru.

NOTE1. As I understand it oil-field drilling pipe is 4330M or 4335m commercial variant... so that is a fairly common alloy... less-so are the aerospace spec grades AMS or MIL. Oil-field pipe is made by the ton... the potential for easy availability of 'cleaned-up aerospace grade' is pretty solid.

NOTE2. 300M should be restricted to ultra-high strength aero applications...

FYI1, VERY Important to review the following spec... it will raise the hair on Your neck... SAE ARP1631 Manufacturing Sequence for Fabrication of High-Strength Steel Parts 300M or 4340 Modified Low-Alloy Steels 270,000 psi (1860 MPa) Tensile Strength and Higher

FYI2. Request oil field pipe guys chime-in...

Regards, Wil Taylor
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o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
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[MechyMarco]

Guess I'll describe the part and why I'm trying to use plate / rectangular bar instead of round bar.

The part is essentially a blade that is much taller than it is wide. The dimensions are roughly 8.25in tall, 2.75in wide and 12in long. It's rough machined to this approx size and then hardened and finished machined. The input force is on one of the 2.75x8.25 faces. The blade tip is on the 12x2.75 top. I've done FEA and the part sees tremendous triaxial stress states. Though the main load path is 2D along the transvers and longitudinal axis of the part. Not much along the short transverse.

Machining from round bar means I need minimum 8.25 rounds and then waste a ton of material. It's just painful. The parts are consumables so if they fail an MPI after a job it's not the end of the world. However, I don't want them chipping and fracturing on the load carrying features. Which has happened. I suspect because the plate has the grains lined up like a deck of cards on it's side and the stress shears the cards apart along the T-L and S-L directions.

I've played with S7 plate, 4340 air melt plate and 300M round bar. The 300M wins every time. It's premium round bar ARC-VAR remelted from Carpenter and comes normalized, tempered, peeled and polished.

Cost does matter here. It's not going on an airplane and it can be thrown out after use.

Anyone play with rectangular bar and see any significant drop in transverse properties? If not then I could look at a mill run to get some rectangular bar that's at least a bit more economical to machine.

Forgings are not completely out, just very expensive.

 

[EdStainless]

If you go the plate route (3"?) you should pay attention to which direction you cut them, T or L.
Perhaps cutting them transverse out of the plate makes sense given you loading.

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[MagBen]

you can even specify the requirements in your PO on both L and T directions. qualified supplier should be able to test and certify mechanicals on both directions for such a large mill stock.

By the way, are you working with Carpenter? S7 was for experimental order Carpenter places, it seems Cartech still uses the term?!

 

[EdStainless]

Knowing Carpenter they are probably still trying to deliver some of those orders from a decade ago.

Marco, You can decide whether you want to have the plate UT'ed or if you want to rough machine the pieces and then UT.
For Plate you are likely working with air melted material. Re-melted plate is uncommon.

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[MechyMarco]

I typically get material through a third party distributor like SSA. Mill runs are looking more and more attractive everytime we go to make a batch of parts as the shelves are drying up more and more. To make matters worse, I was just quoted 40 week lead time on 300M rounds from Carpenter.

Are the mills normally like this or is it worse now for lead times on raw material? At least in larger rounds.

 

[mfgenggear]

Mechy

half the issue is how to hold a part in a fixture. while it may be wasting less material
verify with the shop. how they can hold and fixture the thin wall part for roughing and finishing.
also with plate water jet cutting the rough out part is a viable solution.
I like to rough machine, Normalize harden and temper in the rough condition, then if thickness allows, with a semi finished part, allowing for final machining such as grinding, vacuum heat treat with gas quench, very little distortion.

 

[EdStainless]

mfgenggear, I suppose that you could high pressure (3bar+) He quench 4340 to through harden 3" material, but not normal gas cooling.
Actually, I would guess that with a gentle oil quench you could get through hardness and no distortion.
By using N&T prior to machining you will improve machinability and help minimize distortion.

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[mfgenggear]

Hi Ed

4340M and 300M which is the same material.