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Posted: 2/18/2006 5:37:59 PM EDT
I didn't do well in my Material Science classes at USNA, and in any case they were so long ago that I really don't remember what little I DID learn, so forgive me if this is one of those "duh" questions.

What, exactly, is the difference between forged and cast parts? I'm fairly aware of the difference of how they are made, but I'm confused as to why one seems to be so much stronger than the other.

So, for myself, as well as the other metallurgically-challenged among us, can somebody explain this in plain English?

Thanks.
Link Posted: 2/18/2006 5:44:06 PM EDT
Cast: Pour molten metal into a mold to get desired shape.
Forged: Press a chunk of metal into desired shape using ass-loads of pressure.

Forging is stronger because it aligns the grain pattern of the metal or some such shit.
Link Posted: 2/18/2006 5:44:15 PM EDT
Forged parts entials placing the heated metal into a die and hammering it until it conforms to the die. This aligns the molecules and gives strength to the object. Casting involves pouring liquid metal into a mold and allowing it to cool. While casting is less costly to produce, versatile in terms of shapes, it can be porous and weaker than a forged part that requires machining to tolerences.
Link Posted: 2/18/2006 5:44:40 PM EDT
Isn't cast metal poured into a mold?

Great question Zaph, I have been wondering the same thing, but I was too afraid to ask the stupid question.


Of course, I always say, there are no stupid questions just stupid people. And I admit to being one of those in this case.
Link Posted: 2/18/2006 5:45:55 PM EDT
So what is "cold rolled" steel, then?
Link Posted: 2/18/2006 5:46:42 PM EDT
ok, so does stamped=ghetto forged?
Link Posted: 2/18/2006 5:48:10 PM EDT
tag for answers.
Link Posted: 2/18/2006 5:48:13 PM EDT
[Last Edit: 2/18/2006 5:52:15 PM EDT by LVMIKE]

Originally Posted By Zaphod:
So what is "cold rolled" steel, then?



I'm guessing that is by pressing the steel into a shape with large amounts of pressure and no use (or very limited) use of heat.

My guess is that the quality of stamped parts would generally be very low as the stamp is basically like a big ass cookie cutter. The metal couldn't be too hard, because the need to change plates for the stamp would go up from dulling on hard metals and thus negating the benifits of cost reduction in this process. I'm not completely sure, but I'd think forged parts start out this way and end up undergoing the final forging process from a rough stamped piece of metal.
Link Posted: 2/18/2006 5:49:20 PM EDT

Originally Posted By Zaphod:
So what is "cold rolled" steel, then?



Just like it sounds. The sheet, bar, wire, etc. is formed with the steel being cold.
(cold is a relative term here, consider cold to be other than red-hot)

There is hot-rolled steel too.

Link Posted: 2/18/2006 5:51:44 PM EDT

Originally Posted By Zaphod:
So what is "cold rolled" steel, then?



Even if you take a block of steel at room temperature, by the time you beat it into shape (rolled bar, flat plate, ect) the pressure required will have heated the steel to quite a not cold temperature. perhaps not red hot, but definately not hand friendly

even softer metals get very hot when stamped. for example, when coins get stamped at the mint they get hot enough to slightly liquify the surface, allowing for the minute detail you see on the face.
Link Posted: 2/18/2006 8:22:03 PM EDT
"Say, any of you boys smithies? Or, if not smithies per se, were you otherwise trained in the metallurgic arts before straitened circumstances forced you into a life of aimless wanderin?"


----Ulysses Everett McGill
Link Posted: 2/18/2006 8:48:52 PM EDT

Originally Posted By Zaphod:
So what is "cold rolled" steel, then?



Cold rolled steel is generally easier to machine (than hot rolled) but has a good bit of induced stresses due to the cold rolling process.

By easier to machine, I mean it will give you a nicer finish. Hot roll can be a bit gummy.

If you take the same alloy in hot roll, it will generally tend to be more ductile than the cold rolled version.



Link Posted: 2/18/2006 8:53:29 PM EDT
Link Posted: 2/18/2006 8:57:57 PM EDT

Originally Posted By Waldo:

Originally Posted By Zaphod:
So what is "cold rolled" steel, then?



Cold rolled steel is generally easier to machine (than hot rolled) but has a good bit of induced stresses due to the cold rolling process.

By easier to machine, I mean it will give you a nicer finish. Hot roll can be a bit gummy.

If you take the same alloy in hot roll, it will generally tend to be more ductile than the cold rolled version.






‘Cold Rolled’ is work hardened in the forming process. ‘Hot Rolled’ is pretty much annealed and is easier form/bend.
Link Posted: 2/18/2006 8:59:43 PM EDT
[Last Edit: 2/18/2006 9:01:27 PM EDT by CAR_16]
Cast vs. Forged:

Cast parts have problems induced inherent to the casting process.

Porosity: tiny air pockets get trapped in the material, causing weakness.

Oxide films: liquid metal is very reactive, and forms a layer of oxide when exposed to air. This oxide film can end up in the cast part, also causing weakness.

Forgings are stronger, have a grain pattern that follows the coutours of the part, and do no have porosity or oxide film issues. However, forgings are much more expensive to make because of the cost involved in making the required dies.

Hot rolled vs. cold rolled:

Cold rolled metals have strain hardening induced into them by the cold rolling process. Strain hardening is when imperfections called dislocations in the metal crystal lattice "pile up" and are not easily moved. This makes the material stronger, but also more difficult to machine.

Hot rolling does not get the strain harding effect, but still reduces the thickness of the metal to the desired dimension.

Sheet metal forming is a manufacturing process. Hot and cold rolling are bulk material processes.


Of course, it is never that simple, but that is a basic explanation.
Link Posted: 2/18/2006 9:05:05 PM EDT
Why is good if you check forgings but it's bad if you're forging checks?
Link Posted: 2/18/2006 9:05:16 PM EDT
milling=a type of forging, or something different?

when i think of forging i think of taking a piece of metal and hitting it to make it look like it is intended to look

but milling is taking the same piece of raw metal and cutting away what you dont want

what's the +/- of milling vs forging if they are indeed different processes at their root?
Link Posted: 2/18/2006 9:11:53 PM EDT

Originally Posted By USAF_Hop_N_Pop:
milling=a type of forging, or something different?

when i think of forging i think of taking a piece of metal and hitting it to make it look like it is intended to look

but milling is taking the same piece of raw metal and cutting away what you dont want

what's the +/- of milling vs forging if they are indeed different processes at their root?



A forged part may have to be milled some after it is forged to get the desired surface finish and tolerances. For instance, take our beloved AR15.

Here is the raw forging:



Which is then milled and anodized to become this:



A part can also be a "hog out" though, in which case the whole thing is milled from a solid block.
Link Posted: 2/18/2006 9:17:34 PM EDT

Originally Posted By CAR_16:
Cast vs. Forged:

Cast parts have problems induced inherent to the casting process.

Porosity: tiny air pockets get trapped in the material, causing weakness.

Oxide films: liquid metal is very reactive, and forms a layer of oxide when exposed to air. This oxide film can end up in the cast part, also causing weakness.

Forgings are stronger, have a grain pattern that follows the coutours of the part, and do no have porosity or oxide film issues. However, forgings are much more expensive to make because of the cost involved in making the required dies.

Hot rolled vs. cold rolled:

Cold rolled metals have strain hardening induced into them by the cold rolling process. Strain hardening is when imperfections called dislocations in the metal crystal lattice "pile up" and are not easily moved. This makes the material stronger, but also more difficult to machine.

Hot rolling does not get the strain harding effect, but still reduces the thickness of the metal to the desired dimension.

Sheet metal forming is a manufacturing process. Hot and cold rolling are bulk material processes.


Of course, it is never that simple, but that is a basic explanation.




Finally some one who knows their shit.

I'll add in addition to porosity, you can get inclusions (ex al. oxide) in cast parts along with micro cracks that act as stress risers.

Forged parts can suffer from secondary shear forces which can lead to cracking also. In either case, the process plays an important role in the quality of the final part.


Cold rolling as stated above takes advantae of strain hardening. Once again, a double edge sword because if you strain harden into the plastic range, you end up with a brittle part. Once again, process is the key.


Bomber
Link Posted: 2/18/2006 9:29:39 PM EDT
[Last Edit: 2/18/2006 9:31:39 PM EDT by freeride21a]
yall are generalizing casting. there are MANY different types of castings, and grades within those types. oxidization can be stopped by pouring in an intert atnosphere. Computers can also control the pour and temp of the metals as well as the mold iteslf to allow for proper flow for grain strenght with all the runners and gating computer designed for creating the best pour.

you can hold very high tolerances with modern investment casting methods.. +-.005 easily.
Link Posted: 2/19/2006 4:01:59 AM EDT
Go here for more info...
Link Posted: 2/19/2006 5:11:24 AM EDT
[Last Edit: 2/19/2006 5:14:09 AM EDT by AeroE]

Originally Posted By doorgunner:
I don't buy off on the BS that forged steel is any stronger than investment cast. For example, take a hard look at a Ruger M77 receiver, and compare it's strength to a Remington 700. You might just be very surprised.



You might be right in certain circumstances.

What everyone tends to overlook that the performance of metals and their selection for an application is more than just outright strength, it also depends on the materials ductility, the stress where it yields, its stiffness, fracture toughness, density, and a few other considerations.

I can heat treat a mild carbon steel such as 4130 and similar alloys to 280 ksi and even higher tensile strength, but the part will be so brittle that it will literally shatter like glass if the temperature is cold (not sub-zero) and it's hit with a hammer. There are probably cast steels that can be heat treated to similar stress levels, but they are a novelty and not in practical use at those strength levels.

One of the benefits of wrought metals is impoved strength and ductility in the same direction as the grain alignment, so we exploit that property to make better parts. Forging causes grain alignment to flow around the details such as stub outs where a lug or trunnion will eventually be machined into the part, rolling causes grain alignment parallel to the direction of roller motion.

Ain't nothin' free - the mechanical properties of wrought materials are different in the directions transverse to the rolling or forging operation, many times so much so that careful design is required to accomodate that difference if you want to avoid problems with product performance.

Investment castings have low ductility compared to the same alloy after it has been worked by rolling, and it is directly related to the random orientation of the grain structure. Recent casting technologies have improved the problem, but the resulting parts still cannot perform on par in real life applications with wrought materials, no matter what the sales brochures say.

doorgunner,
You can't compare the structural integrity of those two parts by looking at them, or even by handling them and bending and mashing them. They both have sufficient strength for the application as proven in service in the field, and that's the only conclusion you can make without a lab test.
Link Posted: 2/19/2006 5:13:47 AM EDT

Originally Posted By freeride21a:
yall are generalizing casting. there are MANY different types of castings, and grades within those types. oxidization can be stopped by pouring in an intert atnosphere. Computers can also control the pour and temp of the metals as well as the mold iteslf to allow for proper flow for grain strenght with all the runners and gating computer designed for creating the best pour.

you can hold very high tolerances with modern investment casting methods.. +-.005 easily.




I agree. I kept my answer simple and mentioned twice that in any metal forming operation, the process is very important.


Bomber
Link Posted: 2/19/2006 5:21:13 AM EDT
Aero,


I'm glad you brought up "toughness" (the area under the stress/strain curve). Good explaination. Depending on the application, you definitely have to balance UTS with elongation.

Bomber
Link Posted: 2/19/2006 5:26:13 AM EDT

Originally Posted By USAF_Hop_N_Pop:
milling=a type of forging, or something different?

when i think of forging i think of taking a piece of metal and hitting it to make it look like it is intended to look

but milling is taking the same piece of raw metal and cutting away what you dont want

what's the +/- of milling vs forging if they are indeed different processes at their root?



Milling is a machining process used to remove material from stock. The stock could be rolled, forged, or cast, and it might not even be metal - when you route the edge of a wood plank, that is a milling operation.

There are no high performance net section forged parts in use, they all require post forging machine operations to obtain the final shape. In fact, I doubt that there are any net section forged parts in use except for possibly very simple shapes stamped from sheet metal stock, and those that are hand forged by blacksmiths.

Forging is an operation where the metal feed stock is gotten hot enough to be in a plastic state, and then pounded with a forging hammer - the metal might be forged on top of a blacksmith's anvil, a giant anvil weighing hundreds of tons at the mill (different context than above, use a dictionary), or into a die that shapes the part into a crude representation of the final product.

These forging dies are a principal reason that restarting the production lines for an aircraft program takes so long and has such expense - the dies are enormously complicated and expensive, they wear out, and at the shut down of a project they get scrapped by the vendor after a short while to eliminate inventory and storage cost. If the F-15 or F-22 line is shut down 100% for more than just a few months, the lead time for new dies will be at least 2 years and probably closer to 30 months to 3 years.

Link Posted: 2/19/2006 5:31:43 AM EDT
Best visual image I can describe is this...


Picture a gazillion tiny toothpicks in the metal. (grain structure)

With a forging, all those tiny toothpicks are "flowing" together, just like cars in traffic.

With a casting, all those tiny toothpicks are pointing in random directions.
Link Posted: 2/19/2006 5:34:40 AM EDT

gotten hot enough to be in a plastic state



SHouldn't that be elastic? I thought plastic was the point where it's reached it's strain limits.


Bomber
Link Posted: 2/19/2006 2:08:30 PM EDT

Originally Posted By thebomber:

gotten hot enough to be in a plastic state



SHouldn't that be elastic? I thought plastic was the point where it's reached it's strain limits.


Bomber



When a part is loaded in its elastic range, it will return to its original shape before the load was applied. When it's loaded such that part of the critical cross section has stresses larger than the yield stress, this is called an elastic-plastic state of stress, and when the entire critical cross section is yielded, that is called a plastic state of stress. In both of the last two cases, the part has permanent deformationation after the load is removed.

Some material, such as steel heat treated to a very high tensile strength, or carbon fiber and glass fiber products, have no discernible point of yield, or it's so near the tensile strength that it can't be exploited to cause graceful failures.

"Plastic" in terms of mechanics of materials means stress free strain. Visco-elastic might be a more accurate description of the material while being shaped. To forge a part, you want the metal in a state that will flow to a new shape under the smallest possible loading, so there is not an ideal state of perfect plasticity, but one of highly non-linear stress-strain behavior.
Link Posted: 2/19/2006 2:17:33 PM EDT
AeroE is a smart guy...

The differences between the forged and cast parts can be best seen in the size of the crystal structure and the (mis)alignment of the crystal lattice. Large crystals (grains) occur if the material is poorly cast. Similarly, if an object is forged in a poor manner, this can introduce defects into the crystal structure which potentially cause stress concentrations and ultimate cracking/failure, though this has less effect on strength than would, say, a poor casting vs a properly done casting

Forged parts generally have a better aligned, tighter crystal lattice than cast parts, but with good, modern casting techniques, there is really not much difference for your average lower receiver.
Link Posted: 2/19/2006 3:40:53 PM EDT

When a part is loaded in its elastic range, it will return to its original shape before the load was applied



Aero,


You are right! Not sure what I was thinking . My Strength of Materials Professor probably just rolled over in his grave.


Bomber
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