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Stainless Steel 316L Discoloration

I wish to know what temperatures stainless steel pipe of grade AISI 316L has reached if blue spots are observed on pipe when it is subjected to heat in limited air atmosphere for a duration of 75 minutes.




That is a weird thermometer, Abhijeet :-)

Please provide the background and why you want to know; it sounds interesting. Thanks and good luck!

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey


Hi Abhijeet,

I am sure you will find the answer at the Nickel Institute. Check out their web page

Have fun,

Tony Johnston

We are heating stainless steel 316L pipe/tubing in a nitrogen atmosphere to a temperature of 550 C, but we keep seeing discoloration on the 316L. The discoloration seems to start out as yellow or a light brass-like color then as it increases the discoloration turns a blue then purple.


We've assured there's minimal oxygen present (less than or equal to 1 ppm or 20 ppm from two different nitrogen sources). The ovens have been tested to make sure they are not exceeding 550 °C by more than 40 degrees. Any ideas as to what is causing this discoloration?

Stephanie Walkup-Birkhead
Lab Technician - The Woodlands, Texas
May 8, 2008

The problem is not with your nitrogen, it is with trying to displace all of the air (oxygen) that is in the furnace, which just flowing N2 thru it is not going to get rid of it in a timely manner.
A second cause is dirty parts.

James Watts
- Navarre, Florida
May 9, 2008

We start out by flowing nitrogen through the system, then we pull vacuum. The system is leak tight (we check it every day). With the vacuum, there should be no residual oxygen left in the system. Before we turn off the vacuum entirely, we flood the system with nitrogen. Also, everything is clean, the system and the stainless steel.

I've heard something about nitriding. Could that be a possible cause for the discoloration?

Stephanie Walkup-Birkhead
- The Woodlands, Texas
May 14, 2008

First of two simultaneous responses --

The vacuum may or may not be adequate. For weld build up on titanium vanes, one place that I worked at had an OEM spec that required the bubble to have a vacuum pulled 3 times and backfilled with Argon each time and then had a minimum flow rate of the argon. Anything less did not work.

I have wondered about N2 gas doing a poor mans nitride, but have never seen anything on it.

James Watts
- Navarre, Florida
May 14, 2008

Second of two simultaneous responses --

Do you know at what part of the process you are seeing the work change colors? If the metal is exposed to room atmosphere and temperature when the parts are above 350°F + - then you will see the start of discoloration.

Kevin W. Gallich
- Freemansburg, Pennsylvania
May 15, 2008

Concerning the references in this thread to nitriding.

Remember that air is 80% nitrogen. From the point of view of availability of nitrogen in the furnace atmosphere, there is no great difference between heat treating in a pure nitrogen atmosphere (whose purpose is really only to exclude oxygen), and heat treating in air.

Neither causes nitriding.

Nitrogen has to be in a more reactive state that just ordinary molecular.

Bill Reynolds
Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.

May 15, 2008

May 16, 2008

It would seem to me that Stephanie's post was with regard to material that is heat treated in a controlled atmosphere furnace. The purpose of the nitrogen is to displace oxygen that will react with the surface of the (heated) material and form an oxide coating.(Refer to JW's initial response). It does not take much O2 to produce an oxide film on stainless steels being heat treated. I would suspect the oven's seals, or the evacuation and purge process is insufficient for providing an inert atmosphere.

Nitriding on the other hand is often performed with anhydrous ammonia [on eBay or Amazon affil links] that is introduced into a gas furnace that provides a source of nitrogen for producing nitrided surface.

Carburizing can be performed in a similar manner by introducing methanol [affil links] (or other carbon fuel source)into the same gas furnace.

You can always send a sample out to a competitor for a comparison. Advise them of the issues you are experiencing.

Willie Alexander
- Colorado Springs, Colorado

May 19, 2008

I'm not sure when the discoloration is occurring. All I know is we seal it in a reactor that gradually goes up to 550 °C then holds for a few hours at 550 °C then it gradually comes back down. We never open the reactor until it has cooled to room temperature.

As to it may not have all of the oxygen out of the system...I suppose it is possible. We flush nitrogen or argon (we just recently decided to try argon to see if there's a difference, which there hasn't been)for about five minutes in hopes of getting oxygen at least out of our gas lines then we pull vacuum for a few minutes, usually brings the pressure down to around -20" Hg. Next we open the line back up to the nitrogen or argon and wait for the pressure to build back up before we open the vent line again. From there we leave the nitrogen or argon flowing at about 1000 cc while the system goes up to temperature. I would have thought this would have been enough to ensure all the trapped oxygen had been removed, but I really don't know.

I was also wondering, could there be a problem with stainless steel that once it has been oxidized, it is predisposed to oxidation? After we see oxidation, we polish it off the surface of the tube, but it seems to keep coming back in the same places with usually the same pattern.

Stephanie Walkup-Birkhead
- The Woodlands, Texas

June 20, 2008

As in my previous post of pulling a vacuum 3 times with an argon purge, we used close to 29" of vacuum, vs your 20". big difference.
Another thought is the part is not clean--really clean, which will contribute to the problem.
Your picture is a deep shade of blue, like parts that we stress relieved in an atmospheric oven.
We could get a dark blue with our vacuum furnaces when they had a leak or when they had not had a burn out done recently.

James Watts
- Navarre, Florida

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