Why does steel change it's colour through heating

[antonantal]

Hi. I'm just curious why did the colour of the heating element of my soldering iron change (see the picture attached). I mean I know it's because of the heating but what exactly makes the colour change? Is it oxide? Or is it because of a change in the crystalline structure of the metal?

 

[scott_alexsk]

I think simply more oxidation in those parts. From experiences I have had recently the part that gets hottest becomes darkest or changes the color the most. That's the hottest part right?

-scott

 

[GCT]

As scott alexsk said, the differences in colors probably has to do with the temperature variation on the surface and interaction with gas molecules in the air. Gokul may be able to give you a more comprehensive explanation of what's going on with relation to this phenomena.

 

[antonantal]

It doesn't seem to be oxide, since the colour differs in different regions. If it was an oxide i guess there would've had to be different shades of the same colour. But the colours vary from yellow to purple and then blue in the middle.

 

[Gokul43201]

Yes it is from oxidation - specifically the thin layer of magnetite that forms on the hot steel as oxygen diffuses into it. The colors formed are called "temper colors", and are typically indicative of the (usually sub-micron) thickness of oxide formed (which in turn is a function of time at temperature). At different positions, the time at elevated temperatures is different, making the oxide thicknesses different.

http://www.anvilfire.com/FAQs/temper_colors.htm

Note: A chunk of magnetite is back in color, so this isn't really the color of the oxide that you are seeing, but rather, the effect on the color due to the thin film.

 

[antonantal]

I see. Thanks.

 

[Borek]

That's effect similar in nature to the colors you can see when oil/gas drop falls on the water.

 

[turbo]

Yes it is from oxidation - specifically the thin layer of magnetite that forms on the hot steel as oxygen diffuses into it. The colors formed are called "temper colors", and are typically indicative of the (usually sub-micron) thickness of oxide formed (which in turn is a function of time at temperature). At different positions, the time at elevated temperatures is different, making the oxide thicknesses different.

http://www.anvilfire.com/FAQs/temper_colors.htm

Note: A chunk of magnetite is back in color, so this isn't really the color of the oxide that you are seeing, but rather, the effect on the color due to the thin film.

I never got really proficient at it, but I have "accidentally" tempered a few knife blades to the point where they are fairly tough to sharpen, but hold an edge really well. I got some tips from an old machinist who was quite proficient at making cutting tools.

 

[Gokul43201]

I never got really proficient at it, but I have "accidentally" tempered a few knife blades to the point where they are fairly tough to sharpen, but hold an edge really well.

When you say 'accidentally', you don't mean 'by grinding', do you?!

 

[turbo]

When you say 'accidentally', you don't mean 'by grinding', do you?!

No, I used steel from saw blades or other cutters, softened the materials to make them amenable to shaping with bench/belt grinders, shaped them into blades, and then heat-treated them. I made a slim letter-opener in the profile of a Buck Duke with linen Micarta scales that will slice you like a razor if you're careless. The host material was a cutter blade made for slicing paper on a high-speed paper machine, and it certainly couldn't have been shaped without softening. The host blade was made by the Hyde Knife company, and the quality of the finished blade was a function of my intuition and the guidance of a really sharp old guy who had a great "feel" for the way heat can temper the finished product.

 

[jmnew51]

You have to be careful with the temper you put on steel. Normally the harder you make it the more brittle it becomes. I found this out tempering chisels. When I made them real hard the sharp edge at the tip was extremely hard, but would fragment instead of just getting dull over time. Also the alloy of the steel will denote what kind of temper it can receive (wrought iron cannot be tempered at all, not enough carbon).
Jim

 

[lightarrow]

Hi. I'm just curious why did the colour of the heating element of my soldering iron change (see the picture attached). I mean I know it's because of the heating but what exactly makes the colour change? Is it oxide? Or is it because of a change in the crystalline structure of the metal?

This effect is called "thin film interference". The colour is given by the light reflected from the two interfaces (air/oxide and oxide/metal) interfering with itself. The oxide's thickness changes with temperature.
It's the same reason we can see colours in a pond with a thin oil spot on it, as Borek said.
See, e.g. :
http://dev.physicslab.org/Document.aspx?doctype=3&filename=PhysicalOptics_ThinFilmInterference.xml