Heat Affected Zones (HAZ) and Heat Tints caused by welding

The chromium in stainless steel when exposed to oxygen in the atmosphere forms a thin invisible layer called chromium oxide. This invisible layer covering the entire surface gives stainless steel its ability to resist stains and rust. If this layer is damaged by mechanical impact, by impurities or by input of high tempertures caused by welding rust can be formed on the surface at the point of that damage.

When stainless is heated above 260^oC (500^oF) the oxidation reaction is enhanced, and the layer becomes thicker. As the thickness of the layer increases, the wavelength of light transmitted changes, and hence the color we see can be yellow, brown, dark purple, blue or rainbow tints on the stainless steel surface. These discolorations are calles Heat Tints and the discolored surface is called Heat Affected Zones (HAZ).

Color  Tint                    Temperature
Light Yellow                 290 °C / 550 °F
Straw Yellow               340 °C / 640 °F
Yellow                             370 °C / 700 °F
Brown                             390 °C / 735 °F
Purple Brown             420 °C / 790 °F
Dark Purple                 450 °C / 840 °F
Blue                                  540 °C / 1000 °F
Dark Blue                      600 °C / 1110 °F

Various methods can be used to remove discoloration from stainless steel after welding. The most common among them are chemical pickling and powered mechanical grinding and polishing.