17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
(OP)
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...
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





RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
rp
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
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
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
rp
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
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
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
I still like the idea of cooling them in steam.
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Plymouth Tube
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
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.
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
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_stand...
http://en.wikipedia.org/wiki/Tempering_(metallurgy)
http://en.wikipedia.org/wiki/Thin-film_interferenc...
The best option is to explain to customer that its the physics and it will cost more if they want to change it.
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
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
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
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.
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
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.
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
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
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
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Ben Moskalik
RE: 17-7 PH S.S., Condition CH900 - Need Consistent Color After Heat Treatment
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