Here is a part of an article I found on Investment Casting. The higher "notch strength" this article talks about cast frames having makes me wonder if it wouldn't have improved the low 12,000 round frame life colt's rail gun had in testing trials before developing Frame and slide cracks. And if you do a little research you will notice the rail gun cracked precisely at these "notch" points on dust cover. I personally don't know if colt should move to cast or not. Just saying after reading the article below and seeing the frame cracks colts forged frames experienced in testing at all the "notch" points it just makes me wonder??
Article Start-
I became very familiar with the investment casting process when I
worked at Sturm-Ruger. To describe all the facets of it would fill a
book, but I can give you a brief description. The parts to be cast
start as an injection moulded wax model of the part, which is done with
great precision, with the dimensioning of the die calculated to control
the shrink rate of the wax and the size mould it will make when the wax
is burned out and the silica shell surrounding it is fired. The wax
pieces are gated together in assemblies of maybe several hundred small
parts, or about 12-15 rifle receivers or revolver framed, sometimes
combining smaller parts in the assembly which is called a "tree",
because it looks somewhat like a tree when finished. This assembly is
first dipped in a thin slurry of silica dust and water, then
alternately coated with progressively larger sizes of silica and more
slurry until a heavy shell is built up around it which will stand
normal handling in the foundry. Groups of the trees are cured until
dry, and after a correct period of time they are fired in a kiln which
burns out the wax form, hence the name "lost wax" used to describe the
process. The fired shells are stored until needed, and prior to casting
are heated red hot in a gas furnace, while the steel alloy is melted in
an electric induction furnace. Stainless steel parts are often cast
with vacuum melted steel in a controlled atmosphere such as nitrogen or
argon to control oxidation. Common alloy steels such as 4140 chrome
moly are cast in normal ambient atmosphere, but the open mould sinks
are covered immediately with a de-oxidant compound. After casting the
silica shell is broken off and the individual parts cut away from the
connecting gates. The parts are then cleaned, usually by sandblasting,
then visually inspected, and criticial parts such as rifle bolts,
revolver frames and rifle receivers are inspected by x-ray and/or
ultrasonically, before being annealed, straightened, and lotted up in
groups to be sent to the machine shop. In my experience I feel that
investment casting as done at Ruger is far superior to forging because
it reduces the stock removal necessary to make the finished part, and
permits use of alloys which cannot be machined by common stock removal
methods. Consequently, Ruger can use materials of a very low sulphur or
selenium content with a high hardenability which provides greater
tensile and compressive strength than the lower alloys other
manufacturers much use because their manufacturing processes require
use of additives like sulphur or selenium to obtain acceptable
machinability. I feel that resulphurized steels should not be used in
thin sections or in applications where extensive machining is required,
because of their greater notch sensitivity. We routinely subjected
Ruger rifles and revolvers to testing which destroyed competitor's
products.
