17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment 3

[moskalik]

My company works with a lot with 17-7 PH Stainless Steel, Condition CH900 to make various wire products. The wire products are formed as Condition C, degreased, and then heat treated in open air to Condition CH900.

The resulting color of the 17-7 after the heat treatment can vary greatly. Sometimes it is brown, sometimes blue, sometimes purple and sometimes a combination of all of these colors. We have chalked up these color differences to "heat treat differences" but never really understood why this is occurring. Since we are meeting the chemical content, physical properties, and heat treatment of AMS 5529, we don't worry about the color.

However, some of our more sensitive customers really want the same color for the parts we supply and would prefer the blue color. They question if color change is due to a difference in the material or heat treatment, but we assure them the material and processing is the same.

Has any of you experienced this? Is there an explanation as to what it causing the color differences? Is it changes in atmosphere, chemical composition, heat treat time, external chemical influences or something else...?

Thank you in advance for your input. This is quite puzzling...

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Ben Moskalik

 

[redpicker]

The color difference is due to a difference in thickenss of the temper scale layer that forms on the surface of the part. It could depend on a lot of factors (amount of pieces in the furnace, electric or gas fired furnace, actual heating rate of each piece, etc...). You could age them in a vacuum furnace (or back-filled with an inert gas) or clean them after aging.

rp

 

[moskalik]

I do agree that it may have something to do the oven load. We DID try the purge atmosphere using Nitrogen and that just made the parts look slotchy. That was not exactly what we were looking for. Vacuum is a possibly, but not a capability that we have in house and we would rather try to figure out the solution that we can do ourselves.

As for cleaning ... that would just make they silver colored again, wouldn't it? Again, we are looking for the blue color to remain and we want blue every time....

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Ben Moskalik

 

[redpicker]

Getting blue everytime might be a bit tricky. You might try steam (unsaturated, of course), that might be a bit more predictable. I think that is one way they achieve gun-metal blue on firearms, drill bits, etc... Another option is that, once cleaned, try a chemical conversion coating. There are quite a few out there, and I am pretty sure they have some applicable for 17-7. Years ago, I found one for 17-4 to try and address a galling problem. I don't think it helped the galling, but they all turned a deep blue.

rp

 

[CoryPad]

To do what you are asking will require very close control of the oxygen in the furnace and the temperature uniformity, and even then, you will still see straw, blue and magenta colors. You can search the internet with the words steel, temper, scale, straw, blue and magenta and find a lot of info on time and temperature, etc.

 

[EdStainless]

And of course you have to control dew point as well as oxygen since they are in equilibrium.
That combined with any little difference in surface texture and processing times will give wide variation in results.
Look at using steam/sir or steam/nitrogen for an aging atmosphere. Since you are trying to get some oxidation it shouldn't be too hard to fins conditions that will give reasonably consistent results.
As I recall you don't need to age in steam, but rather cool in steam. You might try nitrogen purge on the furnace just to minimize oxidation and then transfer into a steam box to cool.

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Plymouth Tube

 

[swall]

I think your best course of action is to color the parts for those customers that require it. As redpicker suggests, find a suitable conversion coating. Or, you might be able to get by on the cheap with Dykem or other dye. Get samples made and submit to customer for approval.

 

[tbuelna]

swall makes a good point. You can remove the color from heat treat by pickling or passivation, and the finished parts should have a uniform dull gray color. I would imagine that if you are using steel tooling to form/shear your 17-7 wire parts, then a final passivation would be required.

 

[israelkk]

Removing the colored heat tint using chemicals will results in hydrogen embrittlement.

 

[EdStainless]

Yes, you would have to bake them after the pickling.
I still like the idea of cooling them in steam.

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Plymouth Tube

 

[israelkk]

Removing heat tint using chemicals may remove too much material and it is difficult if possible at all to control how much material is removed. Since we are dealing with a wire form probably for a spring application, the spring mechanical properties may vary greatly because it depends on the power of three or even power of four of the wire diameter.

 

[jzanter]

We're dealing with similar issues on a number of materials. Customers want a uniform appearance. Our suppliers who do this well have a deliberate surface treatment - like mentioned above, use a conversion coating -like a "bluing" treatment to form the oxide you want. I think you'll find more consistent, but probably not perfectly consistent results.

I don't agree about vacuum. 900F is a very low temp for a vacuum furnace - you would probably have to do a few purges to keep your parts clean and run in a high partial pressure to get good heat transfer, if that's the route you pursue.

 

[TVP]

I don't know of any applications where the temper color has been specifically controlled on bulk-processed parts, only for pre-tempered spring steel such as that produced by Blue Blade Steel, etc. These pre-colored steels are processed on highly controlled continuous heat treating lines, rather than some type of batch process. You should contact manufaturers of continuous annealing furnaces about controlled oxidation of discrete parts. Abbott makes these for steam treating of powder metal parts, but there are many manufacturers of continuous mesh belt furnaces that may be interested in this type of application, e.g. Can-Eng, Surface Combustion, Pyromaitre, etc. that would probably use air rather than steam.

 

[kumkumvijay]

I would like to share my opinion as below:

Science behind the color: Thin-film interference is the phenomenon for the coloring. The thin oxide film formed during heating and cooling cycle allows the light to pass through the film and give different color based on the thickness and other several factors.
Those factors are :
Chemical composition: Eg high Cr give high resistance to oxide layer growth and delay the heat tint color.
Atmosphere : More oxygen and oxide layer growth.
Surface finish
Time
Temperature.

If you want to get consistent color then you need to ensure the heating temperature, atmosphere, chemical composition. Each chemistry will follow its own trend, so to make consistence color, you need to restrict more parameters which will be more costly.
The link below give an idea for

http://www.bssa.org.uk/topics.php?article=140

Refer the link below for the last sentence.

http://www.smalley.com/engineering/materials_standard.asp

http://en.wikipedia.org/wiki/Tempering_(metallurgy)

http://en.wikipedia.org/wiki/Thin-film_interference

The best option is to explain to customer that its the physics and it will cost more if they want to change it.

 

[moskalik]

These are all interesting posts and there is definitely some further testing I need to do. Just to clarify some points: The customer is looking for a consistent color and prefers blues because a competitor of ours seems to be supplying consistently blue colored parts. The customer wants consistent colored parts from us as well.

These parts are price sensitive, so the option of adding extra processes like passivisation and then adding color is just out. We need to concentrate on the one process that needs to be done anyway: the heat treating.

You have all opened my eyes to the fact that different variables like surface finish, chemistry and atmosphere can all affect this. This seems to make it rather daunting to get all the variables correct every time.

I still want to try some of the things that Ed mentioned. Maybe steaming during cooling will help...

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Ben Moskalik

 

[kumkumvijay]

If you need to bring the customer expected color then i would suggest to analyze the following parameter for the prefered color formed process:
1. Temperature with tolerance followed
2. Microstructure analysis for the thickness of the oxide layer
3. controlled atmosphere followed eg: steaming
4. Chemistry - Cr,Ni mainly

If you can chart the details above for the blue color or prefered color part, it can be easy to set up the process parameter for same color.

 

[tbuelna]

moskalik-

Your customer's requirements seem quite unusual. If your company were to deliver to me some custom parts made from formed 17-7 wire, I would expect them to have a uniform dull gray appearance, because that would indicate they had been given a final passivation/cleaning/polishing/deburring. If the parts were delivered with the blue or brown tint from heat treating, that would indicate to me that there was nothing done to the parts after heat treat.

 

[EdStainless]

A proper passivation will not touch the tint from aging.
That tint does significantly lower the corrosion resistance of the material as Cr in the oxide cannot help with corrosion resistance.
but if there is no requirement for bright parts then I don't see why they would care at all.

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Plymouth Tube

 

[moskalik]

Ed - You bring up an interesting point. I have always noticed that heated 17-7 will corrode much faster than unheated 17-7 which agrees with your point exactly. Why is it that the Cr in the oxide can no longer help the corrosion resistance? What is the technical explanation? I have always been curious why this happens...

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Ben Moskalik

 

[EdStainless]

Because it is not in the alloy. If the oxide layer is thick enough to have color (and actually even if it is thin enough to not be visible but more than a few atomic layers thick) then the metal right below it will be depleted in Cr as it gave up Cr for the oxide.
It is this Cr lean metal that corrodes easier. To make matters worse the oxide layer can trap solution and shift you from pitting corrosion to crevice corrosion which is much more severe.

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Plymouth Tube