Heat tint of stainless steels 1

[apiguy]

Does anyone know where I can obtain a chart that would describe the heat tint discoloration of stainless steels.

What I am concerned about is a line that is welded to a thermal oxidizer which operates around 600 deg.C which is around 1112 deg. F. According to ASME Section II part D it appears that the maximum temperature for SA-312 TP 316L is 850 deg. F.

The line has been in operation for only a couple of weeks and has turned a dark blue fading to reddish copper farther awy from the heat source.

Please advise.....
Richard Schram
Mechanical Integrity Specialist
Pharmacia Global Supply Arecibo-P. Rico
rschram@pharmacia.com

 

[mcguire]

The Stainless Steel handbook gives the maximum temperature for intermittant service for 316 to be 870C and for continuous service, it is 925C. This is purely from an oxidation point of view,it says nothing about structural considerations.
The heat tint is normal and to be expected. Heat tint is deleterious to corrosion resistance, but that doesn't appear to be a major consideration, I assume.
The heat tint variea in color with oxide film thickness. It is an interference effect. The higher the temperature and the longer the time, the thicker the film. Straw colored films first form around 300C and turn to red, blue and black as the oxide thickens.