Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues. In this installment, Dan Herring explores the heat tint colors that form on stainless steel during heating and cooling — how the surface oxide layer thickens and shifts through straw, bronze, peacock, and blue hues at specific temperatures — and explains the factors, such as chromium content, oxygen levels, time, and surface roughness, that influence how and when these colors appear.
This informative piece was first released in Heat Treat Today’s June 2026 Sixth Annual Buyers Guide Issue print edition.
Most heat treaters, engineers, and clients are familiar with temper colors on steels (Herring 2014, ASM International 1991) and often assume that these color tints (hues) are the same for stainless steels (Table A). However, there are subtle changes that are worth noting. Let’s learn more.
The Science Behind Heat Tint Colors
When stainless steel is exposed to an air atmosphere, or a high dew point moisture-laden atmosphere during heating or cooling, its surface changes color; that is, a thin oxide layer forms on the surface (Figure 1). This heat tint color (aka temper color) is caused by a progressive thickening of the surface oxide layer.
As most of us know, an invisible (aka passive) layer occurs naturally on stainless steels. It is extremely thin, typically in the order of 1 to 3 nanometers (3.93 x 10⁻⁸ to 1.18 x 10⁻⁷ inches) thick.
Upon exposure to air during heating or holding at temperature, this oxide layer grows in thickness. When it is approximately 20–30 nanometers (7.87 x 10⁻⁷ inches) thick, it starts to become visible to the human eye as a light-yellow or straw yellow color tint.
As the oxide layer becomes even thicker it transitions from almost transparent to a variety of different colors (e.g., bronze, peacock, blue).
As the oxide layer thickness increases from 20 nm to roughly 50–100 nm (1.97 x 10⁻⁶ to 3.94 x 10⁻⁶ inches), the colors deepen changing to a golden yellow, to a deep straw, to a bronze or golden brown, to peacock (a purplish-blue), to full blue, then light gray, and finally dark gray.
Above 100 nm (3.94 x 10⁻⁶ inches,) up to approximately 850 nm (3.35 x 10⁻⁵ inches) the tint transitions from dark blue/gray to black.
Such shallow oxides are known to enhance corrosion resistance on various stainless steel grades.
Factors Influencing Color Change
Several factors influence the type of oxide that forms on the surface, its adhesion to the surface, and how quickly the thickness of the oxide will grow (BSSA).
Chromium
From a purely material standpoint, the single most important element is the chromium (Cr) content of the stainless steel. To be classified as a stainless steel, it must contain a minimum of 10.5–11% Cr. The higher the chromium content, not only is the alloy more heat resistant, but the heat tint color formation mechanism is retarded.
Oxygen Content
Another factor that influences the rate of oxide formation and the thickness of the oxide is the oxygen content of the atmosphere (Figure 2). Air is approximately 21% oxygen. Nitrogen, however, contains between 0.01–5% oxygen depending on its source, while argon has less than 0.0005% oxygen. By contrast, water vapor contains around 89% oxygen.
As anyone who has run stainless steel in vacuum furnaces knows, stainless parts can be discolored due to such factors as an air leak during heating or cooling, a pinhole water leak in a heat exchanger which opens during cooling in one temperature range and close again at a lower temperature, or air infiltration in the backfill gas supply.
Time
Time plays a factor as well. The longer the exposure time, the deeper the heat tint color.
Surface Roughness
Finally, surface roughness influences both the rate of oxidation and the heat tint color formation. Rougher surfaces tend to oxidize at a higher rate and with all other factors remaining the same, deeper colors are produced.
Final Thoughts
Knowing the color tints that may form on the surface of stainless steel is invaluable in helping the heat treater explain this phenomenon to their clients and/or troubleshoot their equipment and processes in an attempt to minimize or eliminate undesirable surface tints on the stainless steel parts that they run.
References
- Herring, Daniel H. 2014. Atmosphere Heat Treatment, Volume I (Section 5.8). BNP Media Group II.
- ASM International. 1991. ASM Handbook, Volume 4: Heat Treating. p. 960.
- British Stainless Steel Association (BSSA). bssa.org.uk.
About the Author
“The Heat Treat Doctor”
The HERRING GROUP, Inc.
Dan Herring has been in the industry for over 50 years and has gained vast experience in fields that include materials science, engineering, metallurgy, new product research, and many other areas. He is the author of six books and over 700 technical articles.
For more information: Contact Dan at dherring@heat-treat-doctor.com.
For more information about Dan’s books: see his page at the Heat Treat Store.
