Heat Treating Colors Of Steel

[Mark Reich]

Is it possible to transfer the relevant posts to this thread? 

This subject is worthy of it's own thread. Let's see if we can get some "Color to Temperature" illustrations put up.

[Jan]

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.

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.

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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

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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

[Mark Reich]

Thanks a bunch, Mr Jan! Sorry I've been laid up for couple days.

Your scientific explanations are always incredible! That's like making science come alive. You're a great teacher!

Most of what I know relates to hardening and tempering of tool steel. The colors are supposed to be very accurate. You just need to know what color you're looking for. That's were it gets subjective, but experience will allow your eye to be very accurate.

For anyone heating steel in anything other than a knife kiln, there is one thing that is critical. It's actually so critical, that it's called "Critical". It's the austinization temperature, or the temperature you're looking for when you're trying to harden or normalize steel. Surprisingly, that color becomes easily recognizable.

I differentially harden most of my blades by heating the edge with an A/O torch, and a large welding tip. I have a a couple relevant videos that a friend took not long ago. One is of hardening the edge, and another shows how I draw the spine to spring steel. I'll have to try find to post them.

I just got a new laptop recently, and I didn't want to lose any information from the two cheap laptops it is replacing. I noticed I have way more pictures than I thought I did, so I'm trying to get them coordinated. I will try to get them posted as soon as I can.

I've seen color charts that assign very exact temperatures to the colors of steel, but I assume they are all for simple carbon tool steels. I know other steels have different temps, but I'm not familiar with them.

Especially now with all the new powdered steels, the applicable temperatures are quite a lot higher.

Of great importance to me are tempering colors, because I do need to know how hot a blade can get before it starts tempering and losing hardness while grinding.

I *think* tempering colors are only significant with hardened steel, and really have no relevance when grinding steel before hardening.