Here are some excerpts from the 5 pages generously contributed
by renowned knifemaker and gunsmith Kevin McClung of Mad Dog
Knives and MD Labs. He knows more about steel, heat treating, etc.
than anyone else of my acquaintance.
I think his remarks bear serious consideration.
Much of what he has said merits serious criticism, if not outright rejection. His overall thesis, that
...cast receivers are not as durable or reliable due to the process they are created with: Vacuum Casting.
is nonsense.
The problem with cast high carbon steel (as opposed to cast titanium or aluminum alloy) is that the carbides precipitate in dendrites, rather than being evenly distributed throughout the steel as in forged material. The forging distributes the carbides properly and homogeneously, and develops suitable grain structure and direction, rather than the amorphous matrix and dendritic crystalline structures found in cast steel.
Rifle receivers are not made from high carbon steel. Instead they are usually nowadays made from a medium carbon CrMo alloy, such as 4140, or from a martensitic stainless like 416, which is fairly low in carbon (<0.15% C). Older receivers were typically made of plain carbon steel either of medium carbon content, or low carbon with a carburised surface.
Why is that important? well, for starters the fact that the author started out with an error like this sends the whole argument down the wrong road. You see, with the steels actually used (known as hypoeutectoid) it is the ferrite which starts to solidify first as the steel cools from liquid, not the carbides. You don't get dendrites of carbide at all. Further, as-cast steel doesn't have an amorphous structure (if it did it would be glass
), nor is dendritic segregation going to be seen in any finished casting - especially after heat treatment, any more than forging is going to "distribute carbides"
.
The principal advantage of forging is that if you take this as cast structure and hammer it into shape you close up any porosity and align any inclusions - it is these which give the so-called fibre structure. You also increase the number of dislocations in the crystal structure and, if you then heat treat, this helps refine the grain size. The as heat-treated grains aren't "aligned" though, only the inclusions, and with clean modern steel this is much less a factor.
Now with castings the principal issues of the past were such things as shrinkage, cracking and inclusions (whether solid or gaseous) weakening the structure. However with good clean steel and modern casting technology, and a product designed to be cast you can eliminate these problems, and alloy additions can serve to refine the grain structure, meaning the end product can be just as durable if not more so than a forging.
Fact: The cast receiver manufacturers use vacuum casting. Vacuum does not align grain structure, so far as the metallurgy goes. Neither vacuum nor centrifugal casting distributes carbides properly.
There's three "facts", or rather assertions there, actually. Vacuum casting is one technique, where air pressure is used to drive the liquid metal into an evacuated mould. As I understand it this is not the method actually used however. Ruger for example uses investment casting. This can be done by melting and pouring the metal in a vacuum chamber (for removal of gases and to assure purity, for really high-duty results - the usual method for Ti alloys), but is more usually done in air.
Casting actually can align grain structure, for good or ill, by virtue of the design of the gates and risers and the use of chills. Generally speaking though what is actually wanted is a fine grain structure, which is produced in forgings by the creation of dislocations to nucleate grains on recrystallisation, and in the case of castings largely by the use of good design and/or grain-refining alloy elements.
As for "distributing carbides properly" well, it is hard to make any sense of that assertion at all. the distribution of carbides is a product of grain size, and to some degree heat treatment.
Fact: Forging does align grain structure and properly distributes the carbides in the alloy.
No, forging aligns non-metallic inclusions not grain structure. If you eliminate the inclusions with a clean steel the point is moot. It is also a method of grain refinement, but the product is fine, randomly-oriented spherical grains.
Fact: Casting carbon steel and martensitic stainless produces carbide dendrites. These dendrites weaken the structure (as compared to a forged structure) by precipitating carbides in a fernlike lattice.
This is nonsense, particularly in view of the fact that we are talking about hypoeutectoid steels where it is the low-carbon ferrite which solidifies (precipitates) first.
Fact: Casting steel is merely a way to save time and money in obtaining a net or near net shape. Its sole benefit is LOW COST to net shape. The penalty for the cost saving is less strength, ounce per ounce, than a forged part.
Casting does enable savings in cost. That isn't necessarily a bad thing. It also allows the production of shapes which forging cannot, among other things. It is not true however that castings are necessarily less strong than forgings.
Fact: Serious high performance applications require that cast steel ingots are roll forged to sheet, plate or billet; or are hammer forged to near net shape after casting to develop ultimate attributes for a given alloy. This is true in everything from mild steel used in car bodies to structural steel used in aircraft and automatic weapons. Manufacturers of cast receivers skip all of that troublesome "middle part" where the best attributes are developed.
That was probably true up until a few decades ago. Nowadays very high duty components are indeed cast, because in some cases that is the best technique for developing the properties needed. Turbine impeller blades are a good example. Another I've been involved in is the production of handcuff components: the little chain between the handcuff wristlets being cast in one unit so as to eliminate any welds or joins in the links which might weaken the links.
Fact: This is not to say that investment cast frames and parts are not adequate for many functions, even in weaponry. But, merely "adequate" in the eyes of the manufacturer is often less than wholly desirable for the end user who may be betting his life on the gear.
I would rather stake my life on a product which has been properly engineered and tested than anyone's mere assertion myself. It is quite possible to make unreliable equipment with any technique, including forging.
I would also point out that were investment casting high strength steel to net shape a desirable method of making a truly superior firearm, this process has been available for over 200 years now and has never once been used to produce an American military shoulder firearm of significant caliber, nor has it been applied with notable success to high strength cutlery or edged tools.
Probably more to do with the inherent conservatism of military weapons procurement (hey, I've been involved!) and the fact that the current service weapons of your Army were designed so long ago. That isn't a criticism of the weapons either, but just a statement of fact. The M16 platform for example is fundamentally a 1950s design, and it was designed around the production engineering of the time. Investment casting has been around for far longer than 200 years, but as a means of producing high-integrity steel components its history is much shorter. It is becoming a well-proven production method for small arms though.
In the case of edged tools it is also gaining ground. Not perhaps in the area of custom knives - again perhaps a function of innate conservatism and the economic barriers, but is certainly is being used in the production of such items as drill bits for rock drills, shear blades, defibriators, surgical blades (as well as surgical instruments and implants), turbine blades and all sorts of other components.
There's altogether too many myths about casting and forging. Both have their place, and with good design and materials and heat treatment both can be used to make strong reliable components for firearms and other applications.
