Introduction
The colour formed when stainless steel is heated, either in a furnace application or in the heat affected zone of welds, is dependent on several factors that are related to the oxidation resistance of the steel. The heat tint or temper colour formed is caused by the progressive thickening of the surface oxide layer and so, as temperature is increased, the colours change. Oxidation resistance of stainless steels
However, there are several factors that affect the degree of colour change and so there is no a single table of colour and temperature that represents all cases. The colours formed can only be used as an indication of the temperature to which the steel has been heated.
Factors affecting the heat tint colours formed
Steel composition
The chromium content is the most important single factor affecting oxidation resistance. The higher the chromium, the more heat resistant the steel and so the development of the heat tint colours is delayed.
Atmosphere
The level of oxygen available for the oxidation process also affects the colours formed. Normally heating in air (ie approx. 20% oxygen) is assumed. In welding, the effectiveness of the shielding gas or electrode coating and other weld parameters such as welding speed can affect the degree of heat tint colour formed around the weld bead.
Time
Laboratory tests done to establish the published heat tint colour charts have usually been based on heating for one hour. As exposure time is increased, the temper colours can be expected to deepen ie make it appear that a higher exposure temperature may have been used.
Surface finish
The original surface finish on the steel can affect the rate of oxidation and the appearance of the colour formed. Rougher surfaces may oxidize at a higher rate and so could appear as deeper colours for any given set of conditions. As the colours formed are by light interference, then the smoothness of the surface can also affect the appearance of the colours formed. There is no specific data published that compares the effect of surface finish, but it is worth noting that surface finish can influence the conclusion on heating temperature, from the colours seen.
Heat tint colour chart
The table below represents the temper colours that are likely to form on stainless steel type 1.4301 (AISI 304) if heated in air.
THIS INFORMATION MUST BE USED WITH CARE WHEN INTREPETING THE HINT TINT COLOURS OBSERVED ON STAINLESS STEEL SURFACES AS THE HEATING CONDITIONS ARE NOT SPECIFIED.
| Colour Formed | Approx Temperature C |
| pale yellow | 290 |
| straw yellow | 340 |
| dark yellow | 370 |
| brown | 390 |
| purple brown | 420 |
| dark purple | 450 |
| blue | 540 |
| dark blue | 600 |
Source: http://www.bssa.org.uk/
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