[Steve Grisetti]

I have seen references to blackening and bluing steel, particularly for hilts. Never really thought much about how it was done. But then, I saw the following statement at work, and thought that some of my fellow forumites might be interested. Maybe the application to swords is limited - this specifically relates to tool steel. I am not a metallurgist. Perhaps some of you who know more about materials would care to comment. To put the statement in proper context, someone had been operating a gas turbine, and wanted to know why the color of the bolts on the outer casing had changed color after 200-300 hours of service.

"Steel exhibits different colors depending on temperature. Temperatures above 800F produce incandescent colors; the atoms in the steel are so energized by heat that they give off photons. Temperatures below 800F produce oxidation colors. As the steel is heated, an oxide layer forms on the surface; its thickness (and thus the interference color as light is reflected) is a function of temperature. These colors may be used in tempering tool steel."

[Greyson Brown]

Steve,

I am not a metalurgist either and blacksmithing is only a hobby for me, so I may not be the best to answer this. Nonetheless, I'll try to help you out. Since your question really involves iron or steel, I try to explain why those show the colours that they do. Other metals (bronze, aluminum, etc.) behave differently, and I am not sufficiently knowledgable on those to comment.

Obviously, when iron or steel are heated sufficiently, they glow various colours from a dull cherry red to a bright white. I think that is what the top half of your chart is intended to show. When a piece of steel is heated to the correct temperature (usually that point at which it is no longer magnetic) and quenched, it hardens the steel. This makes it hard (I'll bet you were able to guess that one), but it also becomes brittle. In order to make the steel more flexible, the metal is tempered. In this process (for which there are many different techniques) the metal is reheated. It is not (at least in any technique of which I am aware) heated to thermal colour (i.e. the colours mentioned previously), but as the heat affects the metal, you can see various colours on the surface of the material. This is where you will see the blues, and yellows. I can't tell you what causes them, as I don't know, but each colour shows up within a certain temperature range (I don't know if the ones on your chart are correct, I'll have to double check), and can be used (with varying degrees of success) to determine whether a piece is correctly tempered.

The short answer to your question is this: bolts are hardened (I think there is an episode of CSI that mentions this), and the heat from the turbine was probably sufficient to "draw out" some of that hardness. I doubt that it is a serious concern in terms of the turbine case's structural integrity or anything like that (I would hope such things are taken into account during manufacture), but that is why the colour changed.

Hope that tells you what you wanted to know. I'll go check my "Blacksmith's Primer," and see if I can't get you a copy of the metal forging colours and tempering colours charts in that.

-Grey

[Sam Barris]

I remember from when I was much younger watching the man who made shoes for our horses at work. He had a small anvil and gas furnace in the back of his truck, so he was completely mobile. Once, he showed me the effect that color could have on steel. He could make a rainbow in bar stock just by using heat. It was really impressive to see.

It comes as no surprise that a gas turbine would produce sufficient heat to have a similar effect on the metal. I'll have to pop down to our engine rooms and see if ours have done that. But even if they have, I'm sure that the engine is still quite sound. There are ships that have had the same LM 2500s in service for decades. If there was an issue of structural integrity, I'm sure we would have seen engines fail because of it before now.

[Steve Grisetti]

Thanks for the responses, guys.

I should clarify that there is no concern whatsoever with the integrity of these engines. We have been refining the design of the frame in question for many years, and they offer excellent reliability. They are also big enough (up to 300 tons) to make an LM2500 look like a toy. Of course, this explains why the LM2500 excells at shipboard propulsion, while one of these would sink the ship.

The bolts that I was commenting on had taken on a blue-grey color in a cold state after having been run up to operating temperature. So, what I was trying to relate this to was the process for heat-bluing steel, if it relates at all. Does it?

[Jean Thibodeau]

More decades ago that I should mentioned the instructer of a machining course I was taking had us, as a project make steel center punches. After quenching he had us roll the punches on a large block of heathed steel and instructed us to watch the color change from straw to blue to draw back the hardness of the punch by eye .

So depending on how hard you wanted the finished product to be you stopped at the color corresponding to the hardness / brittleness desired. (And maybe get some differential tempering: Harder tip / softer body.)

Now I am very far from an expert at this but this seemed to be the way they thought us to do it 35 to 40 years ago.

And I vaguely remember a color chart like this in our machinest's handbook.

( At the time I passed my machinest course but was sort of intimidated by the equipment: Today I would be happily learning this stuff to make all sorts of interesting things. Unfortunatly, they didn't encourage making swords as school projects or I would have been a lot more motivated ! )

[Greyson Brown]