Forging and Investment Casting Explained

I’ve seen a number of posts recently asking about the differences between investment casting and forging, so I typed up the following summary of investment casting and forging processes, and the structural differences between each for reference. I did not create any of the pictures, only the text, and credit for the pictures goes to their respective websites and/or organizations.

If anyone has additional questions about the processes, feel free to ask here. I’ll try to answer them as best I can, and invite any lurking individuals with experience in these processes to join in as well.

Investment Casting

While there are various types of casting processes such as die casting, investment casting, and sand
casting, each involves the same fundamental steps in one way or another - melting of a metal or alloy,
transferring the molten metal into a mold, then allowing it to cool and solidify. Casting provides a
relatively low cost method of producing a part that is close to its final shape and requires little or no
machining after casting. Of the aforementioned types, investment casting is probably the most
commonly used casting method in the manufacture of firearm parts.



Investment casting, also known as the “lost wax” process, is one of the oldest known casting processes
and has been used since antiquity, and is probably the most commonly used casting method in
production of firearm parts. A simple summary of the process is as follows – create a wax part, build a
ceramic shell (mold) around the part, melt the wax out of the shell, pour metal into shell. The individual
steps will be described in more detail below, with focus on how investment castings are typically made
using modern manufacturing processes. The image above from CustomPartNet illustrates the process
sans wax burnout.



The first step in investment casting is creating the wax for the desired part. This wax part is going to be
the same shape as the desired final cast part. A common means of making these parts in a production
setting is with a hollow metal die. Wax is injected into this die, allowed to cool, and then removed,
producing a wax part. Each one of these wax parts allows for one casting to be made. The process is
repeated until the desired number of wax parts is made. All images from here forward are from Protocast JLC (http://www.protocastljc.com/).



At this time, the wax parts are be joined together into a “tree”. The wax tree consists of a system which
will provide a place to pour the metal into the mold (gate), a channel to transfer metal down the tree
(runner), and little channels to further direct metal to each individual mold (sprues).



At this point in the process, the wax tree is be used to create a mold for casting the final parts. The tree
is carefully dipped into a ceramic slurry and sand a few times the shell becomes thick enough so support the weight of the molten metal.



Once the shell is built up, the wax must be removed in order to make room for the metal. The shell is
inverted and heated, generally in an autoclave or furnace, to “burn out” the wax. After the shell is free
of wax, it is removed from the autoclave and allowed to cool.



The molten metal for casting the part is created by melting the base metal and mixing in alloying
elements (if necessary) to create the desired alloy. The mold is also heated to a temperature slightly
under the temperature of the molten metal reduce the risk of cracking due to thermal shock. When
everything is ready for the actual casting, a sample of the molten metal will generally be taken and
checked for chemistry to verify that the alloy meets requirements – this will be the foundry’s last chance
to adjust the alloy before the parts are made. Once this checks out, the molten metal is poured into the
gate on the mold until the tree is filled.



Once the metal in the mold has cooled and solidified, the shell is removed from the parts. This leaves a
metal tree identical to the wax one created earlier. The individual parts are then cut from the tree and
sent on their way for additional inspection/processing.


Good video on the process, with a little touch of the 80’s:
Investment Casting Process

Forging

A simplified description of forging would be bulk deformation of solid metal. Most metals are heated to
a high temperature, but still a ways under melting point, for this type of process. Heating is necessary
so that the metal is more easily deformed, and the “hot working” provides advantages which will be
described in another section. Like with casting, forging includes a variety of different processes such
closed die forging, open die forging, and ring rolling. Forging requires a substantial up-front investment
for the forging dies, and requires a large quantity of parts to amortize the cost of the die and make it
feasible to get these high-quality parts. Hand forgings (e.g. bar, plate, billet) use standard flat dies, and
can provide stock with which to machine a part for much lower cost. For firearm parts, closed die forging and open die forging are most frequently used.

Like the name implies, closed die forging creates parts which are impressions of the closed dies. Open
die forging, also known as hand forging, relies on more rough/bulk deformation to create simpler
shapes through the use of open dies that do not come in contact with one another during processing.
The former produces forgings which are shaped similar to the final part, while the latter is mainly used
to make bar or billet from which parts will be machined.



Hand forging a piece of bar, plate, or billet frequently begins with a cast ingot. The ingot is heated and
shaped bit by bit along the length using large dies. Attaining the final shape may require multiple
passes, rotating the part, etc. The resulting product is large forging with simple geometry. Without
getting into specifics about the parameters, this forming process is fairly straight-forward. This process can be used bar from which parts may be machined, but is not the only method of producing bar. Above image from University of Cambridge.

Closed die forging (referred to from here out as simply “die forging”) utilizes a set of dies to create an
impression forging from a piece of material stock. Each half of the die conforms to half of the final part,
so that when they are closed, the material inside flows to the desired final shape. Generally this process
is performed using a pair of dies, but more complex shapes can utilize several inserts, wrapping dies, etc.



Dies are created from a block of tool steel into which the desired impression is machined, or “sunk”.
Thickness of the die can vary for several reasons such as the amount of pressure/force required to forge
the part, the temperature at which forging will occur, or for addition of spare material for re-sinking the
die when it becomes worn. Above image from University of Cambridge.

Die forgings typically begin as a piece of round bar stock, which itself is forged or worked to some
extent. The forging stock is placed into a furnace, and the forging die is heated so that the material does
not cool too much during forming and either fail to fill the die or become too resistant to moving. Once
up to temperature, the forging stock is removed from the furnace, placed into the die, and struck.

For simple parts with little deformation, the forging part of the process can end here. For more complex
parts which require more working, an additional set of dies may be necessary to incrementally shape the
part. The intermediate die is known as a “blocker die”, and serves to apply rough shape to the forging.



In order to fill the die, the forging stock generally consists of more material than the actual forging. This
results in some material being ejected from between the dies. This material is known as “flash”, and
comes out where the dies separate, which is known as the “parting line”. Between steps, the flash may
be trimmed off or punched off in a special die. Above image from prosna.com.

After the shaping is complete, the flash is trimmed off and the forging is sent on for further processing.

Good video describing forging:
Forging Process

Structure

In general, when in solid form, most metal atoms like to arrange themselves in an orderly fashion. Let’s
use water as an example. When water is frozen, small crystals begin to form and grow until the water is
completely solid. The driving force behind this is moving the atoms to the position which uses lowest
energy – the freezing point being where it suddenly becomes easier for the atoms to line up instead of
bouncing about randomly as a liquid. A very similar process happens when metal solidifies from being
molten. In metals, the crystals are known as grains. Grains of zinc can be seen on galvanized steel
surfaces are quite large, and can be seen with the naked eye. The grains in the steel of your firearm will
be much, much smaller – usually on the order of microns in size. The image below shows the structure
of a heat treated 4140 steel part.



In a casting, the liquid that touches the cooler mold will, of course, solidify first. Many grains form near
the surface, but since there are grains beside them, they can only grown in the direction of the liquid.
As the liquid cools, the grains continue growing inward, forming a coarse columnar structure that can be
seen in the picture below of a cast aluminum ingot. All castings will have a similar structure, with the
surface being finer than the center. However, in thin or small castings cooling happens quickly enough
to avoid very large grains.



While liquid, the molten metal for casting has the tendency to aborb gas. When the metal cools, the gas
comes back out of the liquid and ideally should be let out. This is the cause of the big pocket in the
aluminum ingot above. If the gas in unable to leave, it will form little pockets inside the casting –
porosity. Most castings will have some amount of porosity, which does weaken the part. Die castings,
while capable of producing an intricate and accurate part, are especially notorious for having porosity.
Below is an example of porosity in a cast aluminum part.



Forging can remove porosity and align the grains to produce additional strength. As mentioned before,
the metal used in wrought products like plate, bar, and forgings began somewhere along the line as a
casting. Heating the metal and deforming it breaks up the coarse grain structure, welds shut internal
defects, and aligns the grains, giving it a lamellar (layered) structure.

If designed properly, forgings can use this aligned structure to provide strength where it is needed most.
Many people have used the wood analogy to describe a forging – it is easier to break wood along the
grain than against it. It’s much the same with a forging. Again, like wood, there will be some part of the
forging where the grain direction is exposed to the surface. While this is easily located in a low-stress
location with good design, bad design or forging process defects can result in this spot being in a
high-stress area. This will drastically reduce the strength and fatigue life in that location when
compared with a good part. Machining a part from forged billet, bar, or plate provides the advantage of
having a fully solid part, but does not provide the full benefit of grain flow. The picture below illustrates
how grain flow in a die forging can be aligned with a forging to produce a stronger part.

Cast parts can be repaired by welding. Ideally the part would be fully re-heat treated after welding. Areas adjacent to the actual weld which are heated, but not melted ("heat affected zone"), can be slightly weakened or lose their corrosion resistance (in the case of some stainless steels) - re-heat treating restores the properties.

Powder Metallurgy / MIM

Parts formed using metal powders are pressed and formed into the shape of the final product. Pressing works without the use of any additives, but like with forging, it uses two dies, and the part can only be so complex. More intricate powder parts can be formed using MIM (Metal Injection Molding). MIM works by adding a binder to the metal powder. The resulting liquid mix is injected into a die, which allows the part to be a bit more complex. Once the MIM part is removed from the die, the binder can be removed using solvents and a furnace.

When these parts are only compacted, they can appear solid, but are full of pores. The rounded metal powder particles are very close together, but there are still gaps between them. A process known as sintering is used to compact the parts and attain high density.

Sintering requires heating the material up near the melting point, but does not melt the material. This process relies completely on diffusion to densify the part. The technical explanation of how this works is that the driving force is the reduction of surface area. All of the particles (even though they are packed together) have large surface area compared to volume, which is high energy. So, when heated, the atoms are able to move - when given this chance, they move/diffuse as to minimize surface area and fill in the pockets. This will result in the part shrinking a bit as the atoms fill inward. The below image shows the progression of sintering over time (from European Materials Research Society).



Proper powder metallurgy parts can have very high densities that approach 100%, but is much more difficult (read: costly) to make fully dense. MIM can allow the fairly rapid production of intricate, dimensionally precise, high quality parts that take a slight drop in strength compared to a wrought product. If they are designed and used properly, they can be very useful in a mass production setting.

MIM Video:
http://www.youtube.com/watch?v=zCfUd7ZEOjg

The connecting rod drawing is misleading. Steel doesn't have that kind of grain structure so you won't get that kind of result. The only metal that does react as shown in the drawing is wrought iron. "Forged" parts are termed so because they're made in a drop forge, i.e. a stamping press. With steel the alloying elements and heat treating, if required, play a much greater role than the process which forms the part (forging or casting). The only grain alignment, if any, comes from the steel manufacturing process during rolling.

Click to expand...

Die forged steel will have a structure like shown in the drawing. Steel, titanium, aluminum, iron, etc. will all behave this way when die forged as a result of the way material flows to fill the die. Heat treatment has the greatest effect on strength of material, but any wrought material will have different properties in the longitudinal and transverse directions. Proper forging will make potential cracks grow across the grains, making it more difficult to fail.

Heat treatments for forgings do not involve leaving them at high temperature long enough for the grains to grow enough to affect the aligned structure. If you were to leave a forging at high enough temperature for an extended period of time, new grains would form and/or grain growth would occur (depending on the material), which could remove some or all of the effects of hot working.

I would assume your Craftsman wrenches have been heat treated. Most steel forgings at least receive a heat treatment called normalizing, which refines the microstructure and reduces any internal stresses that could be left over from forging. Air cooling after normalize would make the tools soft, but tough, whereas quenching and tempering would produce a strong (but not quite as tough) tool.

While excessive grain growth can occur if the forging if left at normalizing temperature for too long, the growth isn't going to undo the larger grain flow structure. 4140 would be forged somewhere in the realm of 1800-2000°F. It’s important to note that despite being at these temperatures in a forging furnace waiting for its turn on the press, possibly on multiple occasions depending on the number of steps, the structure still retains alignment from the previous operations. Aluminum, on the other hand, does not get grain refinement from heat treatment like steel, and excessive heating, particularly during forging, can result in very large grains that don't offer much in the way of grain flow.

Of the treatments you listed, the first carburize may result in additional grain growth in the receiver, however the temperature is not high enough or duration long enough to make the worked structure go away, but the grain structure would be coarser. The second treatment is lower temperature and time, so it certainly wouldn’t affect the overall worked structure. The third heat treatment looks more like through-harden and not a carburize to me.

I don’t know the history behind the development of their hardening, so I can only speculate the reasons for each of the methods used. It looks like they started with surface harden, weren’t entirely satisfied with how it went, then gradually shifted to a through hardening heat treatment – hardening the entire part instead of trying deal with the carburize. The alloy is also different for the third one (WD 2340); it is similar to 4140, but has some nickel added, which increases hardenability (ability to strengthen the part throughout during a quench).