Heat Tint (Temper) Colours on Stainless Steel Surface Heated in Air

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, (i.e. 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, i.e. 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 oxidise 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 INTERPRETING THE HINT TINT COLOURS OBSERVED ON STAINLESS STEEL SURFACES AS THE HEATING CONDITIONS ARE NOT SPECIFIED.

Colour Formed Approx Temperature oC pale yellow 290 straw yellow 340 dark yellow 370 brown 390 purple brown 420 dark purple 450 blue 540 dark blue 600

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Product Description

Color steel and stainless steel parts by heat treating and oil quenching

Possible colors include yellows, browns, purples and blues.

Our Heat Coloring Kit Includes Casweld™ Keep Kool™ Putty, which prevents heat from travelling up your pliers or other part holder and burning you.

With the use of a blowtorch or gas burner (not included), steel and stainless parts can be colored by heating up to different temperatures:

On Stainless, color changes start at approx 550 deg F for a pale yellow and continue to change until a dark blue will occur at approx 1100 deg F.

On mild steel, color changes start at 350 deg F and continue through 750 deg F.

Never heat color knife blades or other tempered steels, as this will damage them.

Rainbow/Prism effects can be achieved by heating up different areas of the part to different temperatues. This can be achieved by using the Keep Kool™ to "mask off" areas of the part to prevent them from heating. Patterns are limited only by your imagination.

The Kit Includes:

• 1 x 2lb Tub Casweld™ Keep Kool™ Putty
• 1 x Quart Quenching Oil
• 1 x Quart Liquid Degreaser
• 2 x Cans For Degreasing and Quenching
• Instructions with color chart

 

Instructions For Steel & Stainless Steel Parts

• Parts Must Be Polished or sanded first
• Never heat color knife blades or previously heat colored or tempered parts
• Part must be perfectly clean and dry before heating (no fingerprints, dust, oils)
• If using a blowtorch, make sure you have lots of fuel. Once you start the process, you can€™t stop
• Different alloys of steel will have slightly different colors
• We recommend practicing technique and timing on scrap parts first

Steps:

1. Remove all combustibles, paper, rags etc from the heating and quenching area
2. Place the Quenching Oil outside, or in a well ventilated area
3. Form a large blob of The Keep Kool Putty around your part holder, pliers etc to prevent the holder from heating up and burning your hand. 
4. NOTE You can also use the putty to mask off certain areas to create patterns. The metal under the putty will not heat up.
5. Fully degrease the part in the degreasing solution
6. Avoiding touching the part with your bare fingers, attach to your holder or pliers etc
7. Begin heating the part, rotating the part over the flame to heat evenly.
8. After the color begins to change, back off the heat a little and watch the part closely. It will progress through the colors quickly.
9. Once the correct color is achieved, quench the part in the oil. Do not allow the part to touch the sides or bottom of the container.
10. Once it has cooled down, remove it from the oil and allow to dry for approx. 20 mins
11. Wipe off excess oil and enjoy your new colored part.