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Creating Heat for Hardening

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Tim Stevens10/10/2019 14:45:25
1155 forum posts

The link from John F includes an excellent colour chart of the various colours involved in the hardening and tempering processes. But, what is does not explain is that the lower half of the chart (up to 800F) is what you see by reflected light, and the steel surface needs to be cleaned and bright to show the colours properly. In reality, the coulrs are due to the thin layer of oxide on the steel surface (thicker as the temperature rises) - rather like the reflected colurs of a soap bubble. Above that temperature (top half of the chart) shows the colours of the radiant light (glowing) from the steel. These colours do not rely on a shniy surface, and can be seen in pitch darkness because the heat itself is producing the light. If this is not explained it is difficult to see how or why the colours change from red-ish at 500F, through blue, and back to red-ish again at 1200F.

The top half of the temperature chart is for hardening colours, and the lower half for tempering colours.

Cheers, Tim

Martin Dilly 222/10/2019 17:24:58
29 forum posts
3 photos

A bit of a pause there while other projects came along. However, I've made a fairly crude version of the two-bricks- and-a-torch hearth shown here:

However, I have a problem. When I fire up the propane torch and introduce it into the hole the flame goes out.The torch's air intake holes are outside and clear of the brick. The main internal width is around 2 inches, but it's not actually circular in section. I suspect it may be connected to the flame point 'bouncing back' from the wall it faces; if I leave the top half of the hearth off all's well till I put it back, at which out goes the flame. Suggestions welcome.

not done it yet22/10/2019 19:37:17
4482 forum posts
16 photos

Unfortunately it is a fact of life that the flame requires oxygen to burn the fuel. Disrupting the flow will cause the flame to extinguish. Either simply because products of combustion are being drawn in - instead of air - or that increased pressure is slowing the combustion air being drawn in by the ’venturi’ effect of the fuel flow. Improved design of the combustion chamber - shape, size - and improved exhaust exit from the combustion area are required. Slower heating is another option, utilising more heat from the hot gases generated before they exit the chamber.

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