OK, Mr. Mark, it is a good idea. I will copy my three relevant posts here.
It may be good to briefly recollect the heat transfer concept. Heat conduction means that heat is transferred from high temperature to low temperature by molecular agitation within a material without any motion of the material as a whole.
Heat conductivity describes this material property. In metals, heat conductivity is quite high and is primarily due to free electrons. The same mobile electrons which participate in electrical conduction also take part in the transfer of heat.
Heat conductivity in non-metals is much smaller than in metals and is mainly due to lattice vibrations. Such heat transfer is often described in terms of "phonons", quanta of lattice vibrations. (Do not confuse with "photons", which are quanta of electromagnetic radiation.)
The rate of heat transfer is proportional to the heat conductivity and also to the temperature difference between the spots with different temperatures.
P.S.: An example:
Assume that the edge was heated up during sharpening from the room temperature 70°F to 470°F (yellow colour).
The mass of the heated edge was 0.1% of the total mass of the whole blade. How large will be the equilibrium temperature when we neglect blade cooling by the air?
Answer: Equilibrium temperature caused by the heat transfer within the blade will be 70.4°F. Really an imperceptible difference.
Steel composition affects the heat temper colour. In stainless steel, the chromium content is the most important factor influencing oxidation. The higher chromium content, the more heat resistant the steel. This results in delayed development of temper colour.
Other factor affecting heat temper colour is time. Standardised colour charts assume heating for approx. 1 hour. When the steel is heated for longer time than the heat temper colour falsely indicates that the steel was exposed to higher temperature.
Jan
It is good to distinguish between the tempering colours discussed in previous posts and the red hot colour sequence valid for steel at temperatures above 750°F.
Tempering colours are caused by a tiny oxide layer formed on freshly ground steel surface. Day light interferes in this film and the colour of the reflected light tells us about layer thickness and indirectly about steel temperature. This method works up to some 700°F.
The red hot colour sequence is caused by thermal radiation - glowing of the steel. It starts with heat red (more than 750°F) and continues to cherry red (1000-1400°F) to orange (1500-2000°F) to yellow, white and bluish.
![[Image: attachment.php?thumbnail=377]](http://bessex.com/forum/attachment.php?thumbnail=377)
Colours were successfully used to judge the temperature for centuries by master blacksmiths working with wrought iron, but it is good to know how imprecise they can be.
Jan
It may be good to briefly recollect the heat transfer concept. Heat conduction means that heat is transferred from high temperature to low temperature by molecular agitation within a material without any motion of the material as a whole.
Heat conductivity describes this material property. In metals, heat conductivity is quite high and is primarily due to free electrons. The same mobile electrons which participate in electrical conduction also take part in the transfer of heat.
Heat conductivity in non-metals is much smaller than in metals and is mainly due to lattice vibrations. Such heat transfer is often described in terms of "phonons", quanta of lattice vibrations. (Do not confuse with "photons", which are quanta of electromagnetic radiation.)
The rate of heat transfer is proportional to the heat conductivity and also to the temperature difference between the spots with different temperatures.
Jan
P.S.: An example:
Assume that the edge was heated up during sharpening from the room temperature 70°F to 470°F (yellow colour).
The mass of the heated edge was 0.1% of the total mass of the whole blade. How large will be the equilibrium temperature when we neglect blade cooling by the air?
Answer: Equilibrium temperature caused by the heat transfer within the blade will be 70.4°F. Really an imperceptible difference.
Steel composition affects the heat temper colour. In stainless steel, the chromium content is the most important factor influencing oxidation. The higher chromium content, the more heat resistant the steel. This results in delayed development of temper colour.
Other factor affecting heat temper colour is time. Standardised colour charts assume heating for approx. 1 hour. When the steel is heated for longer time than the heat temper colour falsely indicates that the steel was exposed to higher temperature.
Jan
It is good to distinguish between the tempering colours discussed in previous posts and the red hot colour sequence valid for steel at temperatures above 750°F.
Tempering colours are caused by a tiny oxide layer formed on freshly ground steel surface. Day light interferes in this film and the colour of the reflected light tells us about layer thickness and indirectly about steel temperature. This method works up to some 700°F.
The red hot colour sequence is caused by thermal radiation - glowing of the steel. It starts with heat red (more than 750°F) and continues to cherry red (1000-1400°F) to orange (1500-2000°F) to yellow, white and bluish.
Colours were successfully used to judge the temperature for centuries by master blacksmiths working with wrought iron, but it is good to know how imprecise they can be.
Jan