First, you need to understand why steel changes color. When steel heats up, its structure changes. When the surface of the heated steel meets the atmosphere, it reacts with the elements in the air and becomes oxidized. The resulting color depends on the composition of the metals, the composition of the atmosphere, the temperature at which they meet and the time the metal is exposed to high temperatures, etc., in addition to surface oxidation.
Oxidation of the surface of the
stainless steel plate will cause pores. This is where shielding gas or flux enters, as both are designed to shield the hot weld area from atmospheric pressure until the weld bead/HAZ cools enough that the steel/atmosphere mix does not impair the properties of the steel. So when someone tells you that your welds are a certain color because you're welding at a certain temperature, they're only partially correct. Many factors go into it. Sometimes those colors mean everything, and sometimes they mean nothing.
For example, on stainless steel, any color in the weld or heat-affected zone indicates the formation of an oxide layer, which affects corrosion resistance. The darker the color, the thicker the oxidation. Colors follow a predictable pattern, from gold to blue to purple. In some industrial areas, such as pharmaceuticals, any color other than chrome is unacceptable in welds, but in other sanitary welding situations, such as dairy, light blue is allowed to pass. These colors can be removed mechanically or chemically, or both, and corrosion resistance can be restored.
But sometimes, a beautiful weld color is sometimes needed. With the chemical composition of 308 stainless steel for example, a little heat can result in some very vivid colors. But mild steel can also develop great color, and even mixing the two can produce interesting effects.
However, in this case, not only will the corrosion resistance of the stainless steel plate be reduced, but the pollution from the atmosphere may actually seriously affect the integrity of the weld. Titanium is a strong material, but at high temperatures it likes to inhale hydrogen, nitrogen and oxygen, and if allowed to, it can become brittle. While it's not an absolutely foolproof indicator, the resulting color is a good indicator of the strength of the weld. Again, this depends somewhat on the industry and use.
