Straw effect - how to?

[SIGP2101]

I would like to play with small gun parts and achieve that straw semi yellow-golden-brown effect. I would like to setup something in my own garage but need some guidelines. Maybe some good info to read or someone else’s experience shared. How difficult is something like that to achieve, I know it will take lots of practice but this is where all fun is hiding.

Any input from you guys? What equipment and what ingredients to start with.

Thanks

 

[mooncoon]

To harden you can use oxy propane or oxy acetylen and quench in oil or water as appropriate. To draw to straw, I would suggest an electric lead pot filled with wheel weights and a lead thermometer to you can use repeatable temperatures. Further would suggest a copy of Machinery's Handbook as a source of technical info.

cheers mooncoon

 

[guntech]

You can draw to a various colors with a propane torch and doing it very gently... a light straw is the first color, then a darker straw onto purples - blues - and too much...

Practice on a scrap of polished steel and simply cool and re-polish to use it again.

 

[tiriaq]

As mooncoon has pointed out, straw colour is from tempering. The piece is hardened, polished, and then reheated until it acquires the colour. The colour depends on the heat. Heat controls the tempering. As clean steel is heated, its surface changes colour because of oxidation. If you want to experiment, take a piece of steel and polish it. Clean it, so there is no polishing residue. Use a propane torch and gently heat it. As the steel gets hotter, you will see a variety of colours appear, straw being toward the cooler end of the range. Soaking a part in a medium - melted metal, hot sand, melted salts, etc. can give more even control over heating. Fairly difficult with a torch.

 

[guntech]

The question was never asked about heat treating parts...

He stated, "I would like to play with small gun parts and achieve that straw semi yellow-golden-brown effect."

He only asked how to get "that straw semi yellow-golden-brown effect".

To do that the part does not need to be heated and quenched, it only neeeds to be polished and then heated enough to get the colour desired.....

 

[spi]

Another technique to get more even coloring on small parts is to put them on a plate of steel and heat the plate as opposed to the piece.

(Especially if using a torch)

I'm going to re-write this as I was kind of responding to the other writers, not specifically to the OP'er.

Anyway, if you are doing it to small parts, you don't need anything more complicated than one of those torches that you attach to a disposable propane bottle for soldering pipe. I think you can still get them for $9-10.

As I said, for temperature control, you will be best served to heat the part on a flat plate to avoid over heating the part and having stuff turn blue/purple/black. Parts will turn out best if they are as highly polished as possible before heating. Just as the small part is getting close to the color you desire, you will want to tip it into water, oil, or onto a cold block of steel to keep it from over heating.

You will want to practice a few times first to get it right on a part that you have spent time on.

**IMPORTANT**

Remember that what you are doing is not merely decorative. Color is used as an indicator in tempering steel and parts may be "strawed" not just because it looks good, but because they need to be tempered to that hardness. The hotter the part gets, the softer it gets, so if you go past yellow into brown, you may have made the part unnacceptably soft for it's purpose, and in a machine like a firearm this could be a FATAL error. Do a little more research and know what you are doing before you attempt it.

 

[mooncoon]

Why would you want to achieve a straw colour unless you were hardening/heat treating the part. Perhaps Sigp did not realize what the straw colour somewhat implies, when he posed the question.

cheers mooncoon

 

[bronco_mudder]

Just as the small part is getting close to the color you desire, you will want to tip it into water, oil, or onto a cold block of steel to keep it from over heating.

**IMPORTANT**

Remember that what you are doing is not merely decorative. Color is used as an indicator in tempering steel and parts may be "strawed" not just because it looks good, but because they need to be tempered to that hardness. The hotter the part gets, the softer it gets, so if you go past yellow into brown, you may have made the part unnacceptably soft for it's purpose, and in a machine like a firearm this could be a FATAL error. Do a little more research and know what you are doing before you attempt it.

If you are heating chrome moly, or carbon steels and quench them, be it in water, oil, or what ever they will not be softer they will become harder, and more brittle. To become softer the part has to be cooled slowly, ie. wrapped in some type of insulation, or placed in an annealing oven.

 

[guntech]

If you are heating chrome moly, or carbon steels and quench them, be it in water, oil, or what ever they will not be softer they will become harder, and more brittle. To become softer the part has to be cooled slowly, ie. wrapped in some type of insulation, or placed in an annealing oven.

Steel will not harden when quenched unless it is first heated to the correct temperature for hardening... (usually glowing red hot).

....and the straw colour desired is a very low temperature and is not considered annealing..... if it is hardened steel it will still break like glass....

 

[poweredbybeer]

Maybe try plating them? Cadmium plating is sort of light gold in color, and zinc coatings can appear in several colors like black, green or yellow. Cadmium plating is very corrosion resistant, commonly used on aircraft grade fasteners. Zinc coatings are also pretty corrosion resistant, as well as being wear resistant. Zinc plating is also cheaper, can commonly be found on steel fasteners like wood screws at the hardware store.

I googled "zinc plating steel" and found these guys, they sell kits for applying zinc coatings; http://www.caswellplating.com/kits/zinc.htm

 

[bronco_mudder]

Steel will not harden when quenched unless it is first heated to the correct temperature for hardening... (usually glowing red hot).

....and the straw colour desired is a very low temperature and is not considered annealing..... if it is hardened steel it will still break like glass....

Chrome moly and high carbon steels don't require heating to red hot to be hardened when quenched, but the hotter you make them before quenching the harder and more brittle they'll become.

And yes, you're right, the straw colour comes at very low temps.

 

[SIGP2101]

Great input guys, I really appreciate all ideas and suggestions. Just for the record, I would like to achieve straw effect solely for aesthetic reasons. Following your hints I manage to produce all sorts of colors on small parts of highly polished steel, using only propane torch. Results are amazing. On one longer rod I have whole spectrum of rainbow colors. If I could only make a chart which color correspond to which temperature?

So far worked like this:
I heat up a part, and while is still white and just started turning color I removed it from the flame. You can observe then how color changes from white, yellow, brownish, violet, blue, dark blue, etc. Trick is to quickly quench part when it reaches desired shade. So far so good. My idea is to restore parts that were previously straw, and they lost it due to wear and tear. Question is how compromising to the properties of original metal is this procedure.

Thanks

 

[guntech]

Great input guys, I really appreciate all ideas and suggestions. Just for the record, I would like to achieve straw effect solely for aesthetic reasons. Following your hints I manage to produce all sorts of colors on small parts of highly polished steel, using only propane torch. Results are amazing. On one longer rod I have whole spectrum of rainbow colors. If I could only make a chart which color correspond to which temperature?

So far worked like this:
I heat up a part, and while is still white and just started turning color I removed it from the flame. You can observe then how color changes from white, yellow, brownish, violet, blue, dark blue, etc. Trick is to quickly quench part when it reaches desired shade. So far so good. My idea is to restore parts that were previously straw, and they lost it due to wear and tear. Question is how compromising to the properties of original metal is this procedure.

Thanks

If you are simply trying to get a colour, quenching when you get it works, however if you are actually drawing the metal in a heat treating process, it is important to slowly draw only to the colour required and not quench it..... heating it more quickly and quenching would not give the desired properties to the metal.

Drawing a hardened piece of steel to a straw leaves it very brittle and hard, good for the tip of a fine wood working chisel... drawing to blue may work better for a hard punch and springs.... it really depends on the steel composition and you need to know if the steel is oil hardening or water hardening as well...

There is a lot to heat treating. You may want to do a google search on oil hardening steel, etc.

 

[spi]

If you are heating chrome moly, or carbon steels and quench them, be it in water, oil, or what ever they will not be softer they will become harder, and more brittle. To become softer the part has to be cooled slowly, ie. wrapped in some type of insulation, or placed in an annealing oven.

Well, you got that sorta right, and ass backwards at the same time.

To "harden" steel you need to heat it up to what is known as the "critical range". The critical range is around 1500°F for a steel like 4140. Visually the steel is a bright red at this temperature. You can also tell when it is in the critical range because it becomes non-magnetic. If you then quench it - in whatever media, oil, water, brine, air, etc... is appropriate - it will be hardened to it's maximum hardness. Too hard and brittle to use for most purposes.

To reduce the hardness to a point where the steel is both tough and hard, we heat it again in a process called tempering. This is also referred to as "drawing the temper" or "drawing back" the steel.

Tempering is where the rainbow of colors that we are talking about comes into play. By heating steel up to approximately 300°F, we can make it a pale yellow color. At a little higher than that the steel will turn bright yellow, which is the "straw" yellow you are talking about - it is also the common temper for knives. Between 300°F and 450°F the steel will transition between pale yellow to darker shades of yellow until it is a light brown. The darker the color gets in this range, the less hard and more elastic the steel becomes. As you continue to heat it, it will turn from brown to purple to blue, becoming progressively less hard and more elastic. "Blue" is the classic temper for springs. At around 600°F the steel will begin to turn gray and at that point the metal is not "hard" anymore, even though it is not fully relieved. At this point you would need to re-harden it, if hardness and temper are a required function.

So in TEMPERING, which is what we are discussing, the hotter you heat up a metal, the softer it becomes.

SIG, the color/temperature goes roughly like this:

Pale yellow 300°F
Straw yellow 400°F
Purple 475°F
Blue 550°F
Gray 600°F

 

[wyrhare]

If perfect results are required use Niter Salts heated to the required temperture. All that is needed are the salts, a container, thermometer, and heat. I have used this process for many years both for cosmetic colour and heat treating

 

[Anvil]

If perfect results are required use Niter Salts heated to the required temperture. All that is needed are the salts, a container, thermometer, and heat. I have used this process for many years both for cosmetic colour and heat treating

I was going to post about nitrate salts but you seem to have it covered.
Is there anyone in Canada that sells heat treating salts in small quantities? I'm looking for high temperture salts at the moment.

 

[wyrhare]

I was going to post about nitrate salts but you seem to have it covered.
Is there anyone in Canada that sells heat treating salts in small quantities? I'm looking for high temperture salts at the moment.

I got 2.5 gallons from Brownells over 20 years ago and its still going strong after being used hundreds of times. I don't know if they will ship it now because it is a haz. mat item. I could never figure the haz.mat. deal because when it is cold its inert and solid. In use its as dangerous as hot lead and a lot of the same safty rules apply. I like it because odd shaped parts are soaked in the molten salts and thin area receive the same heat as thicker areas, also small parts such as screws and pins are very simple to do. A blue typical of the old fired colt finish can be achieved on a highly polished surface and cast parts blue without problems. The only downside is that the salts absorb moisture in a damp climate when they are solid and this requires a airtight vessel to store the in.

 

[tiriaq]

Many years ago I spent some time in the plating shop at Prestolite (Electric Autolite). One of the finishes we applied was the bluing on spark plug shells. The bath was a molten salt, sodium nitrate, if my memory serves. The shells went through a cleaning dip, a rinse, the salts, a cooling dip and then an oil dip. Don't know what salts are in Brownells' nitre bluing pack, but in all likelihood the chemicals could be obtained from a chemical supply house. As far as safety goes, the bath is extremely hot. One of the other workers had an accident, dropping a bucket of damp shells into the salts. The resulting steam explosion showered him with molten salts. The pattern of hair growth on his head was very odd, as a result.

 

[Anvil]

Brownell's Nitre Blue is a low temperature salt, I have a supply of a very similar product that came from APCO industries in Toronto many years ago. The salts that I want are for much higher temperatures, beyond the critical point of most tool steels. There are quite a few suppliers in the US but since the salts are hazardous materials they are a pain to ship. I will likely pick some up on my next trip to the US if I can't find a supplier here.

Any combustable material dropping into the salt pot will start some immediate excitement as well. Nothing like an oxidizer fire to get your attention. You really have to pay attention when playing with these chemicals, there usually isn't a second chance.