Engine parts: Forged vs "Billet" Rods

dstryr · Oct 28, 2012

Seems like "forged" rods are the way to go when it comes to modifying high horsepower engines but how would a set of rods machined out of solid fair in comparison? I'd like to start a discussion to learn some more about the automotive side.

casement · Oct 28, 2012

Have some Carillo:

Home

gvasale · Oct 28, 2012

Forged if of the same alloy should be stronger. Forging compacts the metal. Carillo rods are supposed to be top shelf.

In the machine shop I used to work in sometimes the packing list would state "forged..."

Cole2534 · Oct 28, 2012

Forged.

message is too short

dstryr · Oct 28, 2012

Besides forged being superior...how would a set of rods machined out of solid fair in comparison to cast stock rods?

Machines! · Oct 28, 2012

From BME's website:

"Following extrusion, BME’s die-forging process shapes the aluminum and compresses its grain structure by heating it to 700°F then applying pressure of 2200 tons. Compared to rods which are cut out of ordinary flat stock, Bill Miller Engineering's forging process: 1) enhances grain flow and increases grain density, 2) forces the grain of the extrusion into the connecting rod shape and, more importantly, aligns that part of the grain which makes up the rod's tapered beam with the direction of highest stress the rod will sustain and, 3) forces the grain around the rod bearing bore, creating a "hoop stress" phenomenon which provides maximum strength for the limited cross-section available at the rod’s big end."

casement · Oct 28, 2012

Interesting Porsche rod comparison:

Powder metal forged rods • Speed Talk

SeymourDumore · Oct 28, 2012

gvasale said:
Forged if of the same alloy should be stronger. Forging compacts the metal.

This is a question I also would very much like to get a somewhat scientific answer on.
Virtually every single aircraft component I make, the material is specified with an AMS number.
For each and every material, let it be SST, Nickel or TI alloy, the AMS number is identical for bars and forgings.
For example, AMS 5646 is 347SST, bars, wire, forgings, rings and tubings.
AMS 5666 is Nickel alloy 625, Bars, forgings and rings
AMS 5613 is 410SS, bars forgings, rings and tubes.

So far I've only found forging-only specifications for Titanium, as far as I can see all steels, stainless or nickels share the same AMS number between forgings and bars.
where Bar = round bar only!!! No rectangles, sheet, plate, or any other extrusion, those have separate AMS numbers, which in all cases allowed to be substituted
by a bar-spec alloy of the same.
Ditto for castings, bar can be used in place of a cast part but never in reverse.

Note, I am not stating anything, rather very curious in the official answer!

gvasale · Oct 29, 2012

Is this enough of an explanation? Forging - Wikipedia, the free encyclopedia
If not, there are many more web references.

johnoder · Oct 29, 2012

You opened the thread comparing forged to machined from solid - now you want to compare machined from solid to cast stock rods. Do you know for a fact they are cast? From what?

dstryr said:
Besides forged being superior...how would a set of rods machined out of solid fair in comparison to cast stock rods?

casement · Oct 29, 2012

Some rods are cast steel. I've not heard of cast iron being used in recent years (or seen any in the many engines I've worked on or parted out) but I'm not an automotive machinist.

Here's a forum which might have your answer:

AERA Community Forum

WILLEO6709 · Oct 29, 2012

a bit of non scientific data, but in light duty diesel engines one has had both forged rods and powder metal rods in its life. The ford/ navistar 7.3L/ DT444e. In actual applications horsepower levels above 400HP are about twice as likely to be a death sentence for a powder metal engine but a forged engine the same odds start occurring at 500 HP. So add that to the data base.... wives tails from diesel forums.

MBG · Oct 29, 2012

It's all about the direction of the grains. Under "billet" the grains all flow I one direction. With forgeing the grains are all perpendicular to the surface.

johnoder · Oct 29, 2012

With forgeing the grains are all perpendicular to the surface.

Seems unlikely - how would they get that way? And it isn't "grains". its grain.

Easiest way to visualize "grain" is in a hot rolled round bar. They all got to be reasonably straight and generally aligned with the axis of the bar - this happened in the process of manufacture.

If I take that bar, heat it to forging temperature and bend it into a "U", all the grain will then be "U" shaped - and such a piece would make a better U-bolt than a u-bolt machined from a solid block of the same material.

lazlo · Oct 29, 2012

SeymourDumore said:
Virtually every single aircraft component I make, the material is specified with an AMS number.
For each and every material, let it be SST, Nickel or TI alloy, the AMS number is identical for bars and forgings.

Isn't the AMS designation the alloy? You would specify the heat treat and/or condition separately?

I know that gets into a grey area with the age hardening aluminum alloys, but AMS 5666 is an Inconel alloy, and while it's not hardenable by heat treat, it cold works (forges) to much higher hardness than annealed. In the same fashion, AMS 5613 is 410 stainless, so you can have AMS 5613 annealed, quenched and tempered...

As far as the grain compaction, Wiki has a very cool high-resolution picture of a forged con rod that's been etched to show the grain. You can't see it well with this thumbnail, but in the full sized image the grain flow is pretty amazing:

http://upload.wikimedia.org/wikipedia/commons/5/5c/ForgedConrodShowingEtchedSection-s.jpg

SeymourDumore · Oct 30, 2012

lazlo said:
Isn't the AMS designation the alloy? You would specify the heat treat and/or condition separately?

Lazlo, not exactly.
IOW AMS5666 specifies Nickel Allloy 625, bars forgings and rings, along with the obvious material content.
It also specifies temperatures and quenching process during the actual manufacturing of the material, but ONLY during the manufacturing of the material.
Any heat treatment or hardness specified on the print is for the part only and has little or no correlation to the material.
So, in a case of a print specifying AMS5666 or AMS5613, you must use Inco 615 or 410 SST round bar or forging
to manufacture your part. You may not - unless explicitly approved - use flat bars, rectangles, plates, sheet or any other forms of material of the same alloy.
From my experience ( however limited that may be ), if your print specifies a plate, sheet, rectangle etc AMS number, you can always substitute it with the appropriate alloy in bar ( again: round bar ) form.
Same applies to a cast part where you'd rather machine it out of a solid, a bar is always allowed.
Interestingly, to date any and all forged parts I've made were always allowed to be substituted with solid machining out of a bar.

And judging from the link posted above, the compactness of the material has littlle to do with the strength or preference of forgings, rather the fact the forging process does not severe the material grain flow, instead it forms it to be continuous and to follow the shape of the part.
Sort of like rolled threads, which ARE quite often required for aerospace parts. No single point or die cutting allowed.

MBG · Oct 30, 2012

johnoder said:
Seems unlikely - how would they get that way? And it isn't "grains". its grain.

Easiest way to visualize "grain" is in a hot rolled round bar. They all got to be reasonably straight and generally aligned with the axis of the bar - this happened in the process of manufacture.

If I take that bar, heat it to forging temperature and bend it into a "U", all the grain will then be "U" shaped - and such a piece would make a better U-bolt than a u-bolt machined from a solid block of the same material.

http://www.nickelinstitute.org/~/Media/Files/TechnicalLiterature/StainlessSteelForgings_9016_.pdf

2nd page down

JWaldo · Oct 30, 2012

Aircraft engines usually only turn in the 2,500 RPM range. (Reciprocating, not turbine)

Thanks,
Jim.

SeymourDumore · Oct 30, 2012

lazlo said:
Isn't the AMS designation the alloy? You would specify the heat treat and/or condition separately?

Lazlo, just got back to the shop and looked at the AMS index book.

Here is an example ( without detailed description )
AMS5589 - Inco 718 - Seamless tubing, induction melted @ 968 C
AMS5590 - Same as 5589 but melted @ 1066 C
AMS5596 - Inco718 sheet, strip, foil, plate - induction melted @ 968 C, heat treated
AMS5597 - Same as 5596, but melted @ 1066 C
AMS5662 - Inco718 - bar, forging or ring - induction melted @ 968 C - solution heat treated and precip hardenable
AMS5663 - same as 5662 but precip. hardened ( you cannot buy it off the shelf in this form, usually processed from 5662 prior to machining)
AMS5664 - same as 5662 but induction melted at 1066 C

This is just a small sample of the variation of the same alloy ( Inco718 ). Much the same applies for stainless material specs as well.

Looking at MBG's link, a whole lotta nice info, but I still can't find if there is any difference in tensile strength for rolled bar vs. forging.
Interesting to note that wire is typically the strongest form in all materials, sometimes by a large margin!

adama · Oct 31, 2012

Wire is often the strongest in most shapes simply because its worked so much to get it to size, It makes it a lot denser and tougher than most bars - flats that only get rolled.

Wire often goes from billet, to rolled round then drawn through dies to size it, hence it sees massive compressive forces in it making several times.

Engine parts: Forged vs "Billet" Rods

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