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Anyone done hot caustic bluing?

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  • Erich
    replied
    Originally posted by tlfamm View Post

    Nobody has knocked yet ...
    That happens when you add item 2. XXX gallons Diesel fuel.

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  • J Tiers
    replied
    The acid wash took off the black spot right away, again. Cleaned off the part, acid washed it again. It is currently in a new batch of potassium nitrate solution, this time mixed with condensed water (same as goes into the battery bank).

    When thinking about temps etc, remember, this is not the NaOH solution, but the "markx method" from the link. It will be different from your method, your results. timing, etc will not apply directly, and per the info in the link, the 285 F is not needed.

    Cold blue is worthless, in general, it wipes off pretty easily, at least it has for me, although I did not do the bluing.

    As for the oil, you can harden the parts in the oil, and get the same result. No need to anneal the parts in the process.

    O1 parts, I heat to red-orange and dump in. Out they come black, and it is a tough coating. A2 parts I let cool to just below glowing, and then dump in. They also come out hard, and black.

    My input to this process has been moved to the "black oxide experiment" thread in which markx first explained it.
    Last edited by J Tiers; 09-26-2020, 04:18 PM.

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  • JoeLee
    replied
    Originally posted by J Tiers View Post
    So, what I get after 40 minutes is this. A little rusty, one black spot that may have been there already, now that I consider the matter, and that's the lot. I think this one is going to have to go up to 800F and get dunked in oil. I know that works, but did not want to do it with this.

    Dirty oil quench also works, but this part would be better blued much colder.

    I will guarantee you that this is not at 285.... boiling point is much lower. Maybe with a pressure pot.

    It was "regular clean steel color" originally. Air is getting to it, because it has rusted. it boiled for 40 min to get here. It's clean and has been acid dipped, it's in the acid again.

    The dark spot is probably in the steel, cause unknown, turned from 2" rod stock.



    That spot looks like it may be pitting or corrosion in the steel. 40 minutes in the bath is way too long..... something is wrong all you need is 15 min. max in the proper solution and temp to get the steel as black is it's ever going to get. There is no way your going to get proper blackening below 285. When I first started doing this I talked to several people from companies that supply the salts. They all stresses the importance of the bath being at a rolling boil at 285 deg. and maintaining that temp.

    You don't need a pressure pot to reach 285 boiling point. The salts raise the boiling point. Approx 8 Lbs of salts per gallon, from what I remember.

    I never liked oil blackening.... the temps needed for that can start to mess with the hardness of your parts. The clean up is messy and the stuff flakes off too easy.

    You can try cold blue just to see what happens. That's a good test to see if the part will take to hot blue. I do it on sometimes if I have something I'm not sure of.

    Most recently were some screws. They wouldn't take to cold blue, then I realized that they were hard chromed....... a quick dip in the muriatic acid took care of that.
    You can also try rust bluing..... it's a repetitive process I'm not crazy about it.

    The brown coating is typical.... I get that most of the time. I think that comes from pulling the parts out of the hot bath through the surface scum.
    Most of it goes away with the cold water rinse and the dip in the water displacing oil. Any traces wipe right off.


    JL................
    Last edited by JoeLee; 09-25-2020, 04:13 PM.

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  • J Tiers
    replied
    So, what I get after 40 minutes is this. A little rusty, one black spot that may have been there already, now that I consider the matter, and that's the lot. I think this one is going to have to go up to 800F and get dunked in oil. I know that works, but did not want to do it with this.

    Dirty oil quench also works, but this part would be better blued much colder.

    I will guarantee you that this is not at 285.... boiling point is much lower. Maybe with a pressure pot.

    It was "regular clean steel color" originally. Air is getting to it, because it has rusted. it boiled for 40 min to get here. It's clean and has been acid dipped, it's in the acid again.

    The dark spot is probably in the steel, cause unknown, turned from 2" rod stock.



    Last edited by J Tiers; 09-25-2020, 03:32 PM.

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  • JoeLee
    replied
    Originally posted by J Tiers View Post
    I know the purity the same way you do, read what it said on the container "contains potassium nitrate". I know the other ingredients that same way, " " it said nothing about them. Sure , we have hard water. OK, and?

    It's been in there for 25 minutes again, after an overnight soak in purple, and an acid dip. The gray spot was removed, but now is back, and is approaching black, in the same location as before. The rest of it is nearly as shiny as stainless. And, no, it is NOT stainless. I'd know if it was..... After machining the entire thing, I believe I would "probably have noticed"...........

    That "use potassium nitrate" thing is beginning to sound like "fake news". The only thing saving it is that a scraggly spot is indeed turning color.
    The mystery spot could be a hard spot in the material, or a spot that was welded at one time.... different material in that area?? don't know what your part is...any pics?? Just guessing here. I've ad a few surprises but I've always found the problem.

    25 minutes in the hot salts seems a bit long. Usually the recommended time is about 15 min. Temp issue?? what is your boiling point??

    It's typical for the solution to turn brown after a few uses and teh foam and scum is also typical, sometimes it goes away with the rolling boil.

    How stable is your temp?? small shallow tanks with large surface areas difficult to keep stable because of the evaporation rate. You have a little leeway with the temp variations, like + - 5 deg, of 285 degrees.


    JL..................



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  • JoeLee
    replied
    Originally posted by tlfamm View Post
    @JoeLee: "I wouldn't advise using distilled water"

    ???
    Because distilled water could be contaminated from aluminum or copper condenser coils compounded by the large volume of air flow over the coils introducing other contaminates into it.. Traces of these elements can cause problems with the process / results.

    Water with too much iron, can also cause issues. But there have been few reported issues with tap and well water.

    JL.................



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  • pinstripe
    replied
    Originally posted by tlfamm View Post
    Nobody has knocked yet ...
    You haven't ordered enough yet

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  • DennisCA
    replied
    I honestly wouldn't know, I've never tried it, you're in unknown territory now

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  • J Tiers
    replied
    So the "markx method" will ONLY work with AN? 'Cause it's putting a killer black spot on this tool. Just not touching the rest, and I've cleaned and acid dipped it.

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  • DennisCA
    replied
    I was talking about hot caustic bluing, there you can swap the AN for easier to find potassium nitrate. I don't use the other process that was linked because it requires AN and I can't find it, so I fume blue or use hot caustic bluing. Or I phosphate (parkerize).

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  • J Tiers
    replied
    I know the purity the same way you do, read what it said on the container "contains potassium nitrate". I know the other ingredients that same way, " " it said nothing about them. Sure , we have hard water. OK, and?

    It's been in there for 25 minutes again, after an overnight soak in purple, and an acid dip. The gray spot was removed, but now is back, and is approaching black, in the same location as before. The rest of it is nearly as shiny as stainless. And, no, it is NOT stainless. I'd know if it was..... After machining the entire thing, I believe I would "probably have noticed"...........

    That "use potassium nitrate" thing is beginning to sound like "fake news". The only thing saving it is that a scraggly spot is indeed turning color.

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  • tlfamm
    replied
    @JoeLee: "I wouldn't advise using distilled water"

    ???

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  • JoeLee
    replied
    Originally posted by J Tiers View Post
    Started with potassium nitrate blackening today. Getting the nitrate from stump remover, which I found some of in the shed.

    Put in something over 10 g of the material, heated the solution and the part in it. The part had been cleaned with purple cleaner. Container is an old SS cooking pot.

    After 45 minutes, the solution was a brownish tint, with a hint of some scum, but the part had only a few "spots" where there was a sort of gray discoloration.

    The source for this idea was https://bbs.homeshopmachinist.net/fo...ide-experiment

    Is the AN the only stuff that works for this?
    What was your boiling point?? gray discoloration is usually a sign on the bath not being hot enough and oil or solvent film still on your parts.

    I always clean my parts in lacquer thinner, it cuts everything and evaporates fast. If the part fails to take the blue then I etch it in muriatic acid, wash it off and then go to the thinner wash.

    How ever with all these back yard concoctions I have no idea where the optimal temperature would be. Are the chemicals your using pure?? do they contain other additives that may hinder the process. a lot of unknowns there.

    I just did a batch yesterday, everything came out perfect as usual except for some old wood screws that were bare steel. they didn't take at all. I gave them a quick dip in the muriatic acid and tried again. they came out perfect, must have been some petina that the thinner wouldn't remove. There was also one 1/4" machine screw that didn't take at all, while all the others from the same machine did. I thought it might have been stainless. I gave that one a muriatic dip also and it came out perfect.

    Even the commercial salts contain other elements.

    One other thing to take into consideration is your water source??? What minerals might it contain?? and what are their percentages. This might sound like were going too far but it can make a difference. Also.......... I wouldn't advise using distilled water.

    JL................
    Last edited by JoeLee; 09-25-2020, 12:42 PM.

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  • J Tiers
    replied
    Originally posted by SLK001 View Post


    10 grams? That's about 1/3 of an ounce. KNO3 might work, but with this small of a quantity, you'll need to get your mixture perfect. Also, using a SS pot is said to poison the mixture (because of leeching?).

    NH3NO3 works and so does NaNO3.
    Ummmmm..... Not YOUR formula, but the straight ammonium nitrate version (just using potassium nitrate as recommended) in the link shown. Check the link (extract below)

    Originally posted by markx View Post
    Perhaps this be interesting for some viewers here....

    I have been doing some development work on black oxide coating process to apply a visually appealing appearance to the machined steel parts. The classical approach for a hot blackening bath usually involves saturated solutions of lye and nitrate with some additives like nitrites. Although it works well and gives a durable finish I do not like the highly corrosive composition and hot temperatures involved in the process. Spill the contents of the hot bath and all hell shall break loose.
    So perhaps it can be done in milder conditions and with less danger of permanent chemical burn.....

    I propose the following bath composition:

    Ammonium nitrate : 10g
    Distilled water : 1000ml (1L)

    *AN content is not critical, bath remains operational from 1-15% AN content by mass (10-150g per liter), but higher AN content causes more instability and sidereactions.


    Optional accelerators for the bath:

    Option A- Potassium or sodium chlorate : 160-240mg /1L
    Option B- Potassium or sodium perchlorate: 400-800mg/1L

    The bath operates with ferrous alloys at boiling point of water or slightly lower (95-100C) due to the dilute solutions and has no caustic components that can cause chemical burn or damage the surroundings when spilled. A decent black coating is obtained within a timeframe of 5min (for simple carbon steels) up to 2 hours (for heavily alloyed tool steels). The coating time is individual for each alloy, but usually 30-40min shall guarantee a nice coverage for most ferrous alloys. The accelerators are optional and do not have to be included into bath composition. They do provide faster oxide coverage of some alloys and slightly impart on the coloration of the coating (bluish purple coatings can be obtained, but the effect is a bit vague). For simplicity sake they can be left out of the composition.....as when overdosed the acceleration effect is reversed and oxide formation is slowed significantly. That is especially true for "Option A", upon overdosing of chlorate the bath becomes inert.

    Regarding plating vessel material and storage of the plating solution:

    Stainless steel is best. Aluminium could theoretically be used, but I have not tried it. A.....................

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  • SLK001
    replied
    Originally posted by J Tiers View Post
    Started with potassium nitrate blackening today. Getting the nitrate from stump remover, which I found some of in the shed.

    Put in something over 10 g of the material, heated the solution and the part in it. The part had been cleaned with purple cleaner. Container is an old SS cooking pot.

    After 45 minutes, the solution was a brownish tint, with a hint of some scum, but the part had only a few "spots" where there was a sort of gray discoloration.

    The source for this idea was https://bbs.homeshopmachinist.net/fo...ide-experiment

    Is the AN the only stuff that works for this?

    10 grams? That's about 1/3 of an ounce. KNO3 might work, but with this small of a quantity, you'll need to get your mixture perfect. Also, using a SS pot is said to poison the mixture (because of leeching?).

    NH3NO3 works and so does NaNO3.

    Leave a comment:

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