Billet & Forged Crankshafts

[ronniejoe]

The billet piece has an added advantage - any weak points in the forged piece can be strengthened in the billet piece by leaving extra stock - this is easily seen in the aftermarket transmission shafts, where the shaft diameter is increased in critical areas, and splined areas are lengthened for more 'bite', or spline length is reduced for increased strength in the uncut diameter.

And often the billet piece is 'tempered', where the forged piece is not, or is to a lesser degree

The only reason people advertise "billet" parts is to sucker gullible folks into thinking they're getting something that they aren't. The word actually comes from the fact that large chunks of raw material are called billet...hence, "machined from billet" which has been shortened to "billet" for advertising purposes. By making a redesigned piece from solid stock (billet), one can alter sizes and gain some strength in some areas. If one were to design and make a new forging that incorporated the increased section sizes and make a part from that, it would be by far stronger.

The reason that new forgings are rarely made by the aftermarket folks who really like to use the "billet" advertising advantage is because a forging is expensive and their small production runs cannot justify it economically. In this case, a "billet" piece becomes a reasonable compromise, but it is a compromise none the less. To sell their parts, they crow about it being a "billet" piece that is "stronger than a forging" and leave the impression that forged pieces are somehow inferior. This is completely not the case.

For the subject at hand, I would be very skeptical if someone claimed that a "billet" crankshaft for the 6.5 was very much stronger than the current cast cranks that we are using.

As for your comments about tempering... What is your source for information here?

[grape]

we use cola, crower, callies, and bryant in the race stuff, they are machined from a billet and are $3800 for a v-8 or a straight 4.............

[ronniejoe]

we use cola, crower, callies, and bryant in the race stuff, they are machined from a billet and are $3800 for a v-8 or a straight 4.............

Again, that's because they can't justify the cost of producing new forgings for such small numbers. If you think that price is high, try amortizing the cost of a forging over the same number of pieces. In reality, the same thing is true for castings.

The "billet" pieces are the only economical way to get some of these low quantity parts. They've just spun the marketing angle so that gullible people think they're getting something better.

[gmctd]

I agree - you'll never see a billet down-pipe on my truck, or billet brake hose, or billet fuel-level float - even tho they are cheaper than the forged versions.

Have been considerin' one o' them billet radar detectors, tho, and a forged Rolex ain't too bad, fer the price.

But, I digress - I agree - I'd rather have the forged tuf-trided piece, maybe with a different firing order.

The ease of making design changes in the billet piece is another strength over forged.

But I 'spect the question is which will survive better than the cast-piece now in use.

[ronniejoe]

I agree - you'll never see a billet down-pipe on my truck, or billet brake hose, or billet fuel-level float - even tho they are cheaper than the forged versions.

Have been considerin' one o' them billet radar detectors, tho, and a forged Rolex ain't too bad, fer the price.

So, you're shown to be blowing smoke and you start mocking... I tried to be gentle.

The ease of making design changes in the billet piece is another strength over forged.

This is it's only advantage...not "another" one...as I already stated in my earlier post.

I've attached an image of a section of a crankshaft forging for your viewing enjoyment. Notice how the grain follows the contour of the shape. This is what makes a forging stronger than a "billet" piece or a cast piece. This is called "preferential grain flow".

Even if you cold work the piece of billet stock or even if you forge the billet stock into it's bar shape, the machining operations to get the shape of the crank will cut through the grain so that the grain pattern is interrupted. This yields a loss of strength in the finished part.

Bottom line: Make two identically shaped parts (in this case crankshafts), one from a forging and the other from "billet". The forged one will be stronger every time. End of story.

[More Power]

Let's not get testy... I just asked a simple (in retrospect maybe not all that simple...) question that I needed more info on....

I'm surprised someone didn't ask why I posted this in the "6.5L Turbo Diesel Forum".

I'm in the process of gathering more information... I received a couple of really cool photos (which I'll share soon), and I asked a few folks what they all meant. Aside from a factory girdle, billet cranks came up in the conversation (I had heard last year they would be forged - more later).... I still need to talk to a few who really have the scoop on what I saw...

Jim

PS... Did I mention an electric fan clutch?

[ronniejoe]

Let's not get testy... I just asked a simple (in retrospect maybe not all that simple...) question that I needed more info on....

I'm surprised someone didn't ask why I posted this in the "6.5L Turbo Diesel Forum".

I'm in the process of gathering more information... I received a couple of really cool photos (which I'll share soon), and I asked a few folks what they all meant. Aside from a factory girdle, billet cranks came up in the conversation (I had heard last year they would be forged - more later).... I still need to talk to a few who really have the scoop on what I saw...

Jim

PS... Did I mention an electric fan clutch?

I apologize for getting "testy"...

I've explained the differences and provided justification for the technical content of my comments. You choose who to believe.

It seems to be a safe assumption that GEP is developing some new stuff. Their choice of "billet" over forged is nothing more than an economic choice. They will probably be able to engineer a machined from solid piece that will out perform the cast pieces presently used. However, their decision mirrors that of GM in the first place when the decision was made to use cast cranks in these engines.

As for the electric fan clutch... I saw such a device on pre-production prototype gas pusher motorhome chassis using Chevy 8.1 liter engines last year. I thought then that this could easily be adapted to the 6.5. At that time, I was not allowed to speak about what they were doing for competitive reasons.

As for sections with carburize etc., look them up in your engineering texts. This stuff is usually covered in second year engineering classes. I've posted photomicrographs of nitrided surfaces in the past. That generated more questions than it answered.

Briefly, carburizing, nitriding, induction hardening, "tuftriding", nitrocarburizing, etc. are processes that increase the surface hardness of a part while leaving the core in a softer state. This increases the fatigue resistance of cyclically loaded parts because of the residual compressive stresses produced near the surface. This is due to the change of phase of the material from the hardening process (reference your metallurgical texts for a discussion of the phase diagrams for various materials) that increases the volume of the material at the surface.

These hardening processes all have advantages and disadvantages that must be carefully considered. Esentially, materials that are used with nitriding are different from materials that are used with carburizing and are different from materials used with induction hardening. The processes are not interchangeable.

The "best" process, from a strength standpoint, is carburizing. However, there are many other considerations that dictate the use of the other processes in many situations. These other considerations make the other processes "better" in those circumstances.

This discussion, though, is off the topic of this thread.

4130 steel was mentioned in a previous post. Can anyone explain what the numbers in the designation mean? How about 9310 steel or 1010 steel or 8620 steel or 5150 steel? What are each of these materials commonly used for? Which heat treat process or processes should be used with each?

[gmctd]

Just as I suspected - those things have changed hardly at all since back in the late '50's- early 60's, when I first saw several comparisons for cast, forged, and 'cut', or totally machined from 4130 or some such steel, cranks.

Odd how those 'cut' cranks could survive <1500hp for ~4secs in a motor that consumed 1.5gals fuel per second

Nice pic, ronniejoe, very informative - got one with similar slice thru a billet version?

Also be interested in seeing sliced inductive hardening, carburizing, and also nitriding examples - notice how I separated 'tempering' examples from nitriding.

Then there's tempering by 'quenching', but that's very smokey - as is burning weeds........

[bcbigfoot]

Billet parts have become popular in recent years due to CNC machines that throw alot of metal chips in a hurry with a minimum of supervision/cost.

I agree that a forged crank is likely to be better than a billet crank as long as the metal used in the two processes are of equal quality/design for the job at hand. I suspect the material used in the billet cranks that Grapes speaks of is of a superior quality than what GM uses in thier forged cranks. So it is very possible to have a superior billit crank to that of a forged crank.

Other issues to consider when assessing the durability of a crank is surface finish, and the size of radius corners. Smoother the surface, and larger radius corners, better the durability.

[tommac95]

{ i penned this after only reading the first page of the thread ... sorry }

Metallic properties are determined by various means: alloy composition, heat treatments, and physically distorting/forming the grain structure.

I would agree with R.J. that (for the same/comparable) composition alloy, forged pieces are far superior due to the fact that the grain structure follows the shape .

R.J. is also clearly correct that for small runs {especially small shops} a forging process is unmanageable expensive (particularly large item like a crank).

I will unequivocally assert that the increasing prominence [to current dominance] of so-called "billet" manufactured articles represents the wide acceptance of CNC or computer-controlled tooling and cheap computers and software . The software program is much easier to pay off than a forging mechanism, and the tooling+software are more versatile to apply to form many different products.

Enter the variable of composition.... Being mindful that most pure metals have extremely limited applications [as gold and copper for electrical conductors] , and that most metal components are employed in the form of engineering alloys [as 14-Carat gold for strong jewelry, or silicon-bronze for corrosion-resistance]... note that there are whole classes of alloys [ie, commercially proven (initially proprietary) admixtures of metals] which can be cast , but are not capable of being forged , or cold-worked.

There are some heat-resistant alloys [also corrosion-resistant , etc] which cannot be forged/hotworked/coldworked ... yet have very, very fine mechanical (or other) properties in the AS CAST condition. It is quite possible that niche manufacturers can use CNC/billet fabricating techniques to create small production runs of very high-quality crankshafts {possibly superior to production forged cranks incorporating cheap commercial alloys} . These would command the $3000 price (or whatever).

Automobile manufacturers can only sell competitively-priced articles in the mass-markets. Thus the first (proven) cranks were forged ... then creative/experimental research developed cast steel cranks , now cast iron cranks are adequate for many purposes {and use only CHEAP cast iron (with appropriate heat treatment) instead of the superior alloys in the forged cranks ... or the even more dear admixtures in the quality billet cranks}.

And soooooo ..... One cannot a crankshaft properly classify , by production process alone. Composition matters .

I always wanted to have stainless alloy wire welded onto production (forged) cranks prior regrinding , for corrosion-resistance , but ... never got that far!

[moondoggie]

"Gonna take a metric buttload of anything to impress that little girl. Wow 192 at 16. By the time she is 21 she'll be making warp speed " She went 6.61 @ 204 last weekend.