A small part in my now discontinued pistol broke, and I was looking to machine a replacement part since its pretty simple in design. However, I'm not sure what grade steel would be best for this application. For reference, I have access to a full size mill and I took basic machining classes back in college, and have machined some pretty basic parts here and there since then so I'm not a total newbie, but I am by no means an expert machinist.

The part seemed to have cracked due to stress from normal operation, as far as I can tell I didn't do anything unusual with it before it broke. Factory ammo only. The part itself seems to be some sort of coated steel, but beyond that I can't tell what it is.

From my research, it seems that 416 stainless and 4140 steel are fairly common materials used for gun parts, however I don't know which would be ideal. I don't have access to any sort of heat treating or finishing process, so I was concerned with the strength of untreated 416 steel, and the rust resistance of untreated 4140 steel.

Any suggestions for what to machine this part out of?

The part itself is small, around .2" X .175" X .65", see pictures below. You can see the part in the part diagram by the red arrow, and is listed as part #23 "Disconnector Wedge".







Edit: Better photos of the part breakdown out of the manual.

Yeah that wall thickness makes me uneasy, pretty obvious why it broke I suppose. Unfortunately due to the way it fits into the slide I can't move that hole at all or increase the thickness. I plan on making several since the part is pretty simple, I'm just stuck on what material to use.

Edit: For reference, here's is how the wedge installs on the bolt, note the silver pin that is nearly flush with the bolt surface that has to insert into the off center hole.

I went ahead and took some cleaner photos of the manual, and included the other half of the parts breakdown if that helps.

It's kind of difficult for me to tell what that part does,   so help that in mind while reading the following:

You can get 4140 in a pre-hardened condition from many places, including McMaster-Carr.  It comes in around 28 Rockwell C.  Its listed yield strength value leads me to believe that it has been normalized rather than quenched and tempered, but it's still quite a bit more durable than annealed 4140.

You can get ETD-150 (also from McMaster). This is basically 4140 with the addition of selenium, and is rather easy to machine despite having a high yield strength value, higher than 4140 pre-hard.  However, it's stated ductility is rather low,  and I suspect it's impact toughness is not very good.  I can't find any values for this, so that is supposition on my part.

You can make it out of a tool steel (S7 would perhaps be a good choice), but you would have to heat treat it, which you said you can't do.

You could try to heat-treat it with a MAP torch, but I don't recommend it because overheating could make the part a lot less tough than just using it as-is.  You don't have the precise temperature control that you would have using a kiln or a molten salt pot.

Thanks for the input, I think the pre-hardened 4140 steel from Mcmaster carr seems like a good starting point. I called around to some local guns stores and one of them can do cerakote on parts this size for pretty cheap, which should help with the corrosion resistance. Not sure if the cerakote will wear off or not, but I figure I'll just make a handful of them and see how they hold up, if need be I can try again with other materials. Using bolts is an interesting idea, I might give that shot if the 4140 isn't strong enough.

It never occurred to me to make a part out of a bolt - that is a good idea.  A grade 8 bolt is going to be pretty hard - high 40 Rockwell, I think.   A grade 5 should be more manageable.

For small parts like this made from various steels, I will treat them with an acid blackening solution called "Tool Black" if I don't want to heat up the Parkerizing tanks.

McMaster Tool Black  Though I'm sure it can be found elsewhere. I've had very good results over the long term with this product.

I just noticed that McMaster has 4140 hex bars that are stress-relieved, with a yield strength that roughly corresponds to a normalized specimen.  They are hexagonal bars - not the most useful shape, but I am going to order some for some parts I am making.  That must be a new offering, because I pretty much live to browse that site, and I never noticed it before.  They also offer 300M (an alloy similar to 4340) that is normalized, but I think that 4140 is much more forgiving.

This is a little stronger than the other pre-hard varieties of 4140, and a normalized and stress-relieved material is a preferable material condition, v.s. annealed, or v.s. non-stress-relieved material, especially if one intends to heat- treat the part.  I know that you said you weren't planning on it, but it's still a better material even if you use it as-is.

Sure you do....
I have made & heat treated hundreds of small parts over the years, including odd-ball springs by using the kitchen stove, with a small can of oil at the ready.

Heat them slowly until orange, dunk them. (hardening)
carefully polish them back to normal steel color.
heat them even slower until they turn blue, and dunk them again. (tempering)

I managed to machine a handful of the parts, 3 in pre-hard 4140 and 3 in 416 stainless, and got the 4140 samples into the gunsmith for cerakote. Although with everything that's going on I suspect I may be waiting a while for the local ranges to open back up and actually do some test fires. I'll be sure to keep all the ideas you have been posting in this thread in mind if these parts break again when I finally do get a chance to test them, thanks for all the help.