Need help with finishes on C96

[DonVoigt]

Originally Posted by alvin

I bought a worn C96 extractor which lost its finish. Planned to use it on a range gun.

Also bought a bucket of nitre blue salt and a temperature meter. If my spouse allows me to borrow her cooking stove, I will have some fun in Christmas. She told me that she will think about that and let me know.

Surely, I will experiment on scrap steel first.

But there is a strength problem -- does this 570F - 670F temperature anneal the steel?

Alvin,
can you explain exactly what you bought as "nitre" blue salt and how it is to be used?

I have a lot of experience with hot salt bluing and it is not something you will want to do in the kitchen.

Yes, anytime you heat metal it "anneals" it, but the temperature you will reach in hot salt bluing will not significantly affect the "strength".

Annealing will remove any stresses built up from machining, forging and forming steel. Your part has already likely experienced those temperatures.

[Olle]

The color you see on the on original parts is not meant as a finish, it's actually a byproduct of the tempering process they used at the factory. The good thing about it is that the color is a great temperature indicator, so if you heat it to the factory color you won't exceed the temperature the factory used to temper it. You can do any part like that without hurting it, and some say you might even prolong the life of parts that have become "work hardened" by years of use. However, if you heat it too much, you have to reharden and temper again.

Annealing is a totally different animal. This is done to soften the steel before you work on it, and you have to heat it to a much higher temperature (glowing hot). The only color you get is steel grey.

[alvin]

Originally Posted by DonVoigt

Alvin,
can you explain exactly what you bought as "nitre" blue salt and how it is to be used?

I have never done things like this before. From youtube.com video, midwayusa showed how -- the nitre blue salt is a pink color solid stuff, he put the salt into a pan and "cook" it. The salt melted. Then, he dipped small parts into the melted salt, the metal color changed, then he took it out. That's it.

He did not explain the theory. Here is my guess -- that salt melts around 600F. So it's just temperature control media. This is obviously more controllable than using a torch to create fire blue on small parts -- using torch, you have no control on temperature, using this type of salt, it's much easier to control, and the metal is heated much more uniformly. Unlike regular bluing process, I don't think there is a chemical reaction between the nitre salt and metal. Just a guess.

Here is the video:

https://www.youtube.com/watch?v=-vF3zKiUIkY

[Olle]

Originally Posted by alvin

I have never done things like this before. From youtube.com video, midwayusa showed how -- the nitre blue salt is a pink color solid stuff, he put the salt into a pan and "cook" it. The salt melted. Then, he dipped small parts into the melted salt, the metal color changed, then he took it out. That's it.

He did not explain the theory. Here is my guess -- that salt melts around 600F. So it's just temperature control media. This is obviously more controllable than using a torch to create fire blue on small parts -- using torch, you have no control on temperature, using this type of salt, it's much easier to control, and the metal is heated much more uniformly. Unlike regular bluing process, I don't think there is a chemical reaction between the nitre salt and metal. Just a guess.

Here is the video:

https://www.youtube.com/watch?v=-vF3zKiUIkY

That's correct. The nitre salt is just a media that allows you to apply the heat in a controlled manner, otherwise it's the same process and the same finish as you'd get with a torch or in the oven.

One reason why I don't use nitre salt (and bluing salt) is that I don't like the idea of working with salt heated to high temperatures. I figure it can make for some interesting steam explosions if there's rinse water left on the parts. Still, many videos show the guy dipping in the nitre bath, dipping in a cup of water, then back to the salt again etc. I might be overly cautious, but I figure the torch and the oven are much safer.

I do extractors on a thin piece of shim stock, I heat with a torch from underneath and when I have the right color I drop it in quenching oil to cool off. The color doesn't appear immediately and it can continue to develop after you quit torching the part, but the quenching will stop the process right at the moment it's submerged and keep it at the right color.

[DonVoigt]

Originally Posted by alvin

I have never done things like this before. From youtube.com video, midwayusa showed how -- the nitre blue salt is a pink color solid stuff, he put the salt into a pan and "cook" it. The salt melted. Then, he dipped small parts into the melted salt, the metal color changed, then he took it out. That's it.

He did not explain the theory. Here is my guess -- that salt melts around 600F. So it's just temperature control media. This is obviously more controllable than using a torch to create fire blue on small parts -- using torch, you have no control on temperature, using this type of salt, it's much easier to control, and the metal is heated much more uniformly. Unlike regular bluing process, I don't think there is a chemical reaction between the nitre salt and metal. Just a guess.

Here is the video:

https://www.youtube.com/watch?v=-vF3zKiUIkY

Alvin
What you describe is Not typical hot salt bluing. Which is a chemical reaction, not just heat. Salts consist of sodium hydroxide and sodium nitrate, and each contains some crystalized water.

Tempreature is controlled by adding back water as it boils off, or the temprature will increase as time goes by.

It is a hazardous and corrosive mixture, rubber gloves, rubber apron, face mask, etc are recommended as the salts will splash.

You will corrode your stove and vent if you do it much!

Salt bluing is best left to an expert with the proper set up and know how.

It is not a do it in the kitchen operation.

Metal preparation is the most important step, followed by degreasing before putting into the salt.

Poor prep = poor results.

What is shown in the video is the same as what the others are describing using the oven or hot sand in the oven, he is using a salt bath to
control the temperature to 600 degrees+/- to achieve the pretty blue. It is heat treating as Ollie has described.