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.
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.
Stainless Steel at Low Temperatures
Austenitic stainless steels are extensively used for service down to as low as liquid helium temperature (-269 deg C). This is largely due to the lack of a clearly defined transition from ductile to brittle fracture in impact toughness testing.
The distance a swinging hammer travels after impact on a sample of steel is usually used to measure its toughness. The shorter the distance, the tougher the steel as the energy of the hammer is absorbed by the sample. Toughness is measured in Joules (J). Minimum values of toughness are specified for different applications.
Steels with ferritic or martensitic structures show a sudden change from ductile (safe) to brittle (unsafe) fracture over a small temperature difference. Even the best of these steels show this behaviour at temperatures higher than -100 deg C and in many cases only just below zero.
In contrast austenitic steels only show a gradual fall in the impact toughness value and are still well above 100 J at -196 deg C.
The choice of steel at low temperature is often determined by its ability to resist transformation from austenite to martensite.
Stainless steel at High Temperatures
The choice of grade depends on several factors:
- Maximum temperature of operation
- Time at temperature of operation
- Type of atmosphere eg oxidising or reducing
- Strength requirement
Heat Tint Colour Chart
To aid you with your heat treatment of steel, we offer two temperature color charts.
Color Chart for Forging & Hardening : -
The steel should be regarded in a dark or faintly lighted room and must not be exposes to direct light. The colour chart should be regarded in normal diffused daylight and not in sunlight or artificial light.
Color Chart for Tempering : -
This colour chart applies to a tempering time of about 30 minutes