Hi,
Just wondering how I can tell a forged case hardened Trigger for a N frame from a MIM Trigger?

Not sure if it applies to N-frames, but in general I believe the forged triggers are solid, while MIM triggers have a groove along the backside of the trigger.

Forged case hardened will have a mottled blue finish with hints of rainbow colors - as long as it hasn't been polished.

MIM will be a very uniform near-black color.
Or at least that is what I have seen consistently.

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Besides the solid trigger back a forged trigger will have sharper lines. Once you see enough of each it’s easy to tell what is what from just a side picture

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-Matt

Forged hammers look like this:




MIM hammers look like this:

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VCDL, GOA, NRA

Here are pictures of both kinds of triggers (696 and 696-1).
Once you see these it's pretty obvious on other S&W revolvers.
The MIM case hardening colors can look just like the forged parts.
It's the same basic alloy, just assembled differently.

Attached Images

	Forged Trigger 1024.jpg (51.7 KB, 218 views)
	MIM Trigger 1024.jpg (55.9 KB, 230 views)

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NEMO
"Everything 44"

Do they case harden MIM parts? I look for a light grey surface in case hardening.

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"He was kinda funny lookin'"

I have to assume they are.
The look of the 2 triggers above is so similar and the function identical.
The alloys are very close so the heat treating will be as well.
You are going to want that trigger sear to be as hard as possible.

Once the colors of case hardening, which are only a few molecules thick, wear off you can get a finish that looks like a coin finish but is still hardened.

There are so many types of hardened finishes now I have quit trying to keep track as they all look somewhat similar except for the really colorful ones.
For instance I believe my Rizzini double rifle has a vacuum vapor/plasma deposition chromium nitride finish but it looks like any other chrome or coin finish I have seen.
Heck it doesn't look any different from some stainless steel either but is supposedly harder than just about anything except diamond.
I haven't tried scratching it to find out .

The Kuhnhausen 5th edition S&W shop manual has a lengthy rant at the end about how the MIM parts rust way worse than the old forged parts.
This would have been written about some of the first MIM parts.
I am not sure why that would be or even if it is true if the alloys are similar.
I keep any non-stainless parts lightly coated with some kind of protectant and have not had any problems with that.
Eezox is one I use lately even though it is quite pungent.
Previous ones have included RIG grease lightly applied by rag and then wiped off and some of the spray products.

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NEMO
"Everything 44"

Just look at the back of the trigger and if it is hollow, then it is MIM. Forget about colors and all of that because case coloring (I disagree with hardening because anything that thin cant be doing any hardening) can be colorful to gray.

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S&W factory revolver armorer

Speaking on J-frames, I always wondered why my 642-1 trigger was so nicely rounded and felt so good, while my 637-1 trigger was sharper and had edges that bothered my finger after an extended session.

Turns out the 642 is MIM and the 637 forged. Do they mold "nicer" contouring into all the MIM's?

I'm no expert, but my understanding is that one of the reasons why MIM was popularized was because it required less final machining, i.e., more-or-less ready-to-install out of the mold, with little, if any, polishing or fitting required. Could be why your 642's trigger has smoother edges than your 637's trigger.

Or, as S-W put it, with MIM the precision machining has to be done one time, when the mold is made, vs. every part has to be machined.
The engineer part of me loves the way the MIM parts are designed, metal where it needs to be and not where it isn't.

MIM is a cost saving measure, the by-product is a more uniform surface from one part to another, as opposed to forged and machined parts that relied upon a sharp cutter for a smooth, uniform surface. Once a cutter dulls or the cutting blades get little nicks, the cut surface becomes less uniform and has tiny ridges, which leads to a gritty trigger pull.

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Most MIM parts that I've looked at have small round ejector pin witness marks. Usually a dead give away unless they've been polished or machined off.

The engineer in me appreciates the cost reduction that can be achieved with parts that are appropriate candidates for MIM. I do not like the way it looks. You can always see the graininess of the molded and sintered product unless it has been machined. Forged parts are so rough out out of the dies that nearly all surfaces have to be machined or ground to a pleasing polished look. Forged parts have one metallurgical advantage in that grain structure orientation can be optimized for loading. Not so with MIM. MIM has come a long way and is now the near equal of forged parts in fracture toughness, but nothing beats the beauty of a forged, machined and case hardened part. I like MIM stuff buried inside the gun where it can't be seen. In my shooting experience I have broken one MIM part, a old Taurus 92 hammer. Still waiting on my first forged part failure.

In the pictures I posted you can see at the top where the MIM trigger enters the frame, a round mark that may be where the sprue pin was cut off.

Case hardening need not involve a colored surface and the colors are not the hardened part.
The colors are a by product of certain methods of carburizing.
Case-hardening - Wikipedia
I have to believe that the hammer and trigger at least where they meet at the sear are hardened otherwise they would rapidly wear out.
Probably easier to just harden the whole part as indicated by the discoloring.
Hardening also reduces the friction.
The hardened layer can be up a mm thick.
When there are colors they are just sitting on the surface.
Color Case Hardening - Turnbull Restoration

Color Case Hardening - Shooting Sportsman Magazine
Goes into detail how Purdey case-hardens in colors their guns. Quite interesting.

"Jonathan Irby, James Purdey & Sons’ gunroom manager, offered this: “The color finish was—and is—the by-product of the hardening process. This is favored as providing a hard surface with a certain level of flex underneath, rather than a glass-hard finish.” Hardening incorporates carbon into the molecular structure of the surface steel and occurs when hot steel is splashed into cold water. The process gives the outer steel an armored skin that prevents wear and corrosion while preserving the interior, ductile, shock-absorbing strength."

Next they describe bone charcoal color case hardening.

"The colors themselves are perishable from sunlight and wear. The remaining colors on vintage guns offer a “rule of thumb” indication of cosmetic condition. Rubbing, wear and wiping can result in fading. This fading is varied and occurs first along edges."

It does not effect the hardening.

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NEMO
"Everything 44"

MIM parts are the same hardness al the way through. VERY hard wearing.

"Still waiting on my first forged part failure."


Don't know if you would call it a "failure" or not, but I've replaced several J frame hammers after the revolvers have been dropped and landed on the hammer, snapping off the spur.

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You should be able to tell by the - number.