Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

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  #1   Report Post  
Leo Lichtman
 
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Default Steel color change - how come?


"Peter Grey" wrote: I know that as steel (or some steel anyhow) heats, the
surface changes color (clip) Can anyone explain what's happening to cause
this?(clip)
^^^^^^^^^^^^^^^
As the steel is heated, an oxide layer is formed on the surface. Light
falling on the surface is partially reflected from the front of this layer,
and more is reflected from the back of this layer/front of the steel. If
the thickness of the layer is 1/4 wavelength thick, the light from the front
and the light from the back will interfere and cancel. The eye will then
see a color that is the complement of that wavelength. (The same mechanism
produces the colors in an oil slick.)



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Peter Grey
 
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"Leo Lichtman" wrote in message
...

"Peter Grey" wrote: I know that as steel (or some steel anyhow) heats,
the surface changes color (clip) Can anyone explain what's happening to
cause this?(clip)
^^^^^^^^^^^^^^^
As the steel is heated, an oxide layer is formed on the surface. Light
falling on the surface is partially reflected from the front of this
layer, and more is reflected from the back of this layer/front of the
steel. If the thickness of the layer is 1/4 wavelength thick, the light
from the front and the light from the back will interfere and cancel. The
eye will then see a color that is the complement of that wavelength. (The
same mechanism produces the colors in an oil slick.)

Thanks. To your knowledge, are there any steels that are more prone to
forming oxides on the surface as they're heated? This is not a structural
application so the mechanical properties of the metal are secondary to its
appearance.

Peter


  #3   Report Post  
jim rozen
 
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In article , Harold and Susan Vordos says...

Controlled coloration was often practiced in heat treatment. S&W, for
example, used to color the hammer of their hand guns in that fashion. You
can expect a wonderful range of colors, blues, greens, reds. It's
chemically induced. I have no information on the process, but one of Guy
Lautard's (sp?) Bedside Readers has a formula contained within.


Starrett also does so-called 'color case-hardening' of their tools.

This was "The Bullseye Mixture" story in the bedside reader.

The idea is you pack harden items, and then dump them right out
of the retort into brine with oil in it, agitated by air pumped
through bubblers in the bottom.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================
  #4   Report Post  
Harold and Susan Vordos
 
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"Peter Grey" wrote in message
nk.net...

Hi all,

I know that as steel (or some steel anyhow) heats, the surface changes

color
and if one removes the heat the color will stay on the surface. I've done
this using a propane torch, but isn't easy to do in an even manner. Can
anyone explain what's happening to cause this?

I've got some projects where I'd actually like to maximize and control the
surface coloration. What I can do to make it as pronounced as possible?
Are there different types of steel that would be more or less prone to

this?
Any techniques where this coloration can be more easily generated or

saved?
Any websites that discuss this?

Thanks,

Peter


Controlled coloration was often practiced in heat treatment. S&W, for
example, used to color the hammer of their hand guns in that fashion. You
can expect a wonderful range of colors, blues, greens, reds. It's
chemically induced. I have no information on the process, but one of Guy
Lautard's (sp?) Bedside Readers has a formula contained within.

Harold


  #5   Report Post  
spaco
 
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Blacksmiths use these colors to determine tempering temperatures.
Take this link: http://www.anvilfire.com/FAQs/temper_colors.htm
You can pretty well get as far as "full blue" in a kitchen oven.
Some experimentation will be necessary since ovens aren't always that
accurate. When I typed "temper colors" into Google, I got 275 hits.
My own eye sees some prettier reds than that particular chart shows, but
I'm sure you get the idea.
If you spray your completed project with clear laquer, etc., the
colors will stay a long time. But if you leave the part bare,
eventually thicker rust will overtake the original thin oxide.
I once made a set of hammers, fullers and hot cuts that had beautiful
temper colors, just where I wanted them. They stayed that way until one
day when I was demonstrating on a rainy day. The next day it was all over!

Pete Stanaitis
----------------------------------------------------------------

Peter Grey wrote:

Hi all,

I know that as steel (or some steel anyhow) heats, the surface changes color
and if one removes the heat the color will stay on the surface. I've done
this using a propane torch, but isn't easy to do in an even manner. Can
anyone explain what's happening to cause this?

I've got some projects where I'd actually like to maximize and control the
surface coloration. What I can do to make it as pronounced as possible?
Are there different types of steel that would be more or less prone to this?
Any techniques where this coloration can be more easily generated or saved?
Any websites that discuss this?

Thanks,

Peter





  #6   Report Post  
David Billington
 
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IIRC these colours occur about 100C higher for stainless steel. Not what
you asked but it might be worth noting.

Peter Grey wrote:

"Leo Lichtman" wrote in message
...

"Peter Grey" wrote: I know that as steel (or some steel anyhow) heats,
the surface changes color (clip) Can anyone explain what's happening to
cause this?(clip)
^^^^^^^^^^^^^^^
As the steel is heated, an oxide layer is formed on the surface. Light
falling on the surface is partially reflected from the front of this
layer, and more is reflected from the back of this layer/front of the
steel. If the thickness of the layer is 1/4 wavelength thick, the light
from the front and the light from the back will interfere and cancel. The
eye will then see a color that is the complement of that wavelength. (The
same mechanism produces the colors in an oil slick.)

Thanks. To your knowledge, are there any steels that are more prone to
forming oxides on the surface as they're heated? This is not a structural
application so the mechanical properties of the metal are secondary to its
appearance.

Peter



  #7   Report Post  
Chuck Sherwood
 
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I've got some projects where I'd actually like to maximize and control the
surface coloration. What I can do to make it as pronounced as possible?


clock hands are blued in this manner. Willian SMith, a famous clock maker
suggests to put the hands on a bed of brass filings collected from the
lathe or band saw and heat then on the stove top until they turn blue.

The metal must be clean because you are dealing with oxides and you
want the metal to all react at the same time. Therefore the metal must
be clean and uniformly heated.

chuck

  #8   Report Post  
Newshound
 
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There are loads of proprietary and home brew formulae for different colours
on steel and brass. Try googling for "chemical blacking"


  #9   Report Post  
Richard J Kinch
 
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Leo Lichtman writes:

As the steel is heated, an oxide layer is formed on the surface.


More to it than just that. The thickness (1) equilibrates instead of just
growing, and (2) differently so with differing temperatures, and (3)
reversibly so since the thickness follows the temperature both up and down.
  #10   Report Post  
Fred R
 
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Peter Grey wrote:

Thanks. To your knowledge, are there any steels that are more prone to
forming oxides on the surface as they're heated? This is not a structural
application so the mechanical properties of the metal are secondary to its
appearance.

Peter



Just anecdotally, the most brilliant coloring I've ever seen was on a
chromed motorcycle exhaust pipe 'heat treated' by hard riding.

--
Fred R
________________
Drop TROU to email.


  #11   Report Post  
Leo Lichtman
 
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"Richard J Kinch" wrote: More to it than just that. The thickness (1)
equilibrates instead of just growing, and (2) differently so with differing
temperatures, and (3) reversibly so since the thickness follows the
temperature both up and down.
^^^^^^^^^^^^^^
Your additional explanation answers some of the questions I have had, but it
raises others that I wish I had answers to. We know that steel oxidizes in
contact with air. We know that chemical reactions are accelerated at higher
temperatures. So, I always assumed that as the steel is tempered, the oxide
film grew continuously, but more rapidly as temperature went up. According
to your esplanation, this is not true--instead the film thickness reaches an
equillibrium thickness depending on temperature. Now I think I understand
why the temperature can be judged by the color, and time does not enter into
it.

Now the part that baffles me. If the film thickness tracks the temperature
reversably, as you say, why doesn't the film/color disappear as the steel
cools?


  #12   Report Post  
Richard J Kinch
 
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Leo Lichtman writes:

If the film thickness tracks the temperature
reversably, as you say, why doesn't the film/color disappear as the
steel cools?


I suppose the process "freezes" at relatively cool temperatures, and if
cooled quickly enough you freeze the film thickness characteristic of the
recent hot temperature.
  #13   Report Post  
 
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The oxide doesn't change back to steel.

  #14   Report Post  
Richard J Kinch
 
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The oxide doesn't change back to steel.

Correct. It fumes off.
  #15   Report Post  
Steve Smith
 
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Leo Lichtman wrote:

"Richard J Kinch" wrote: More to it than just that. The thickness (1)
equilibrates instead of just growing, and (2) differently so with differing
temperatures, and (3) reversibly so since the thickness follows the
temperature both up and down.
^^^^^^^^^^^^^^
Your additional explanation answers some of the questions I have had, but it
raises others that I wish I had answers to. We know that steel oxidizes in
contact with air. We know that chemical reactions are accelerated at higher
temperatures. So, I always assumed that as the steel is tempered, the oxide
film grew continuously, but more rapidly as temperature went up. According
to your esplanation, this is not true--instead the film thickness reaches an
equillibrium thickness depending on temperature. Now I think I understand
why the temperature can be judged by the color, and time does not enter into
it.

Now the part that baffles me. If the film thickness tracks the temperature
reversably, as you say, why doesn't the film/color disappear as the steel
cools?



Leo, the film thickness does not track the temperature. The oxide grows
faster at higher temps as you say. The oxide stays around--it doesn't go
away as the temperature drops.

One factor confusing this issue is that the colors don't keep going.
Once the oxide gets thick enough, the colors go away, but the oxide
keeps getting thicker.

Steve


  #16   Report Post  
Steve R.
 
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I used to do this, as many antique clocks and watches that came through my
shop were missing hands. I used a stainless steel container for the brass
filings, which slowed the process, giving better control over the colour.

Steve R.


"Chuck Sherwood" wrote in message
...
I've got some projects where I'd actually like to maximize and control
the
surface coloration. What I can do to make it as pronounced as possible?


clock hands are blued in this manner. Willian SMith, a famous clock maker
suggests to put the hands on a bed of brass filings collected from the
lathe or band saw and heat then on the stove top until they turn blue.

The metal must be clean because you are dealing with oxides and you
want the metal to all react at the same time. Therefore the metal must
be clean and uniformly heated.

chuck



  #17   Report Post  
Peter Grey
 
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Do you know what's happening with the interaction between the brass and the
steel that accentuates the "blueness" (insert Yellow Submarine joke here) of
the steel?

Peter

"Chuck Sherwood" wrote in message
...
I've got some projects where I'd actually like to maximize and control
the
surface coloration. What I can do to make it as pronounced as possible?


clock hands are blued in this manner. Willian SMith, a famous clock maker
suggests to put the hands on a bed of brass filings collected from the
lathe or band saw and heat then on the stove top until they turn blue.

The metal must be clean because you are dealing with oxides and you
want the metal to all react at the same time. Therefore the metal must
be clean and uniformly heated.

chuck



  #18   Report Post  
Peter Grey
 
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"Richard J Kinch" wrote in message
.. .
Leo Lichtman writes:

If the film thickness tracks the temperature
reversably, as you say, why doesn't the film/color disappear as the
steel cools?


I suppose the process "freezes" at relatively cool temperatures, and if
cooled quickly enough you freeze the film thickness characteristic of the
recent hot temperature.


It seems to remain even if one lets the steel cool down by exposure to room
temerature air. I haven't had to quench the piece in order for the color to
remain.

Peter


  #19   Report Post  
Peter Grey
 
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"Richard J Kinch" wrote in message
. ..
The oxide doesn't change back to steel.


Correct. It fumes off.


That explains why the color disappears as the temerature of the piece goes
up.

Peter


  #20   Report Post  
Bugs
 
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Those oxidation colors were used by blacksmiths to judge the drawing
temperature to temper steel tools. They range from straw yellow to
Blue-black.
The hardness remaining ranges from razor steel to spring temper.
You can get a uniform color coating in a well controlled drawing oven.
Bugs



  #21   Report Post  
Chuck Sherwood
 
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Do you know what's happening with the interaction between the brass and the
steel that accentuates the "blueness" (insert Yellow Submarine joke here) of
the steel?


I don't think there is any interaction. I think the brass just adds
thermal mass to avoid temperature swings.

chuck
  #22   Report Post  
Tim Williams
 
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"Peter Grey" wrote in message
nk.net...
The oxide doesn't change back to steel.


Correct. It fumes off.


Whaaa?! You do realize ferrous oxide has a *melting point* in excess of
2,000°F, don't you?

That explains why the color disappears as the temerature of the piece
goes up.


Maybe under heavy reduction, where the hydrogen and, more importantly,
carbon in the flame are able to actively reduce the oxide from FeO to Fe
(such happens readily with copper metal), but not under any other
circumstances.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


  #23   Report Post  
Tim Williams
 
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"Steve Smith" wrote in message
...
Leo, the film thickness does not track the temperature. The oxide grows
faster at higher temps as you say. The oxide stays around--it doesn't
go away as the temperature drops.


Here's the deal. Take a piece of iron. It has a perfectly clean surface, I
mean the oxygen and nitrogen molecules of the air are bouncing directly off
the pure iron (and occasionally iron carbide and others) surface. Now at
room temperature, a few of these do bust up on the iron surface and oxidize
it. Like aluminum, this layer is invisible, but because it's somewhat
indifferent, it doesn't change the chemistry of the iron and you don't
notice it. This layer is only as thick as it is because the oxygen
molecules can't get past it at this temperature.

Ok, so let's raise the temperature. Radiation heats the air molecules near
the steel to the same kinetic energy, that is to say, hot air's molecules
move faster. So they hit the iron surface with more energy, and
occasionally one will pass through the oxide layer and oxidize more iron.
(It probably passes by diffusion, where the surface oxidizes to magnetite -
Fe3O4 - or rust - Fe2O3 - which is then passed backwards to the metal, which
reduces Fe3O4 and Fe2O3 back to FeO, at the price of more Fe metal being
burned.) Just as carburization can diffuse hardening carbon (or nitrogen in
some cases) only so far, likewise the oxygen only goes so far through the
oxide. It's always passing through, even at room temperature, so the
response of oxide growth is probably logarithmic - it tapers off quite
quickly as thickness rises, but never stops completely. It's just that
thermal response is exponential, so it'll take about two million years to
eat a tin can, while at orange heat, your tin can will hold molten aluminum
for only about fifteen minutes!

A temperature of 350°F for a few minutes produces a nice light straw color
(hard to spot because the oxide takes time to grow, and you can't anticipate
it because this is the first interference layer, around 80 nanometers
thick?), but the same temperature extended to an hour gives a purple
coloration.

One factor confusing this issue is that the colors don't keep going.
Once the oxide gets thick enough, the colors go away, but the oxide
keeps getting thicker.


Actually, they come back several times, but each time harder to see because
the light has to travel through more oxide thickness. If you heat a shiny
bar from one end in plain air, you'll see the first layers, yellow, purple,
blue; then a darker run of yellow, purple and blue, and so on for maybe
three or four total modes. What's happening is light is spending 1/2, 1
1/2, 2 1/2, ... wavelengths inside the thickness of oxide (1/4, 3/4, 1 1/4,
.... wavelength thick layer, for the wavelength of light *passing in the
medium* (light slows down per the index of refraction), of the *canceled
frequency*). Thicker layers attenuate more, so it quickly (500nm?) starts
looking black. Fe3O4 is a wonderful electromagnetic-absorbent material,
after all. No wonder the military uses it...

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


  #24   Report Post  
Peter Grey
 
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"Tim Williams" wrote in message
...
That explains why the color disappears as the temerature of the piece
goes up.


Maybe under heavy reduction, where the hydrogen and, more importantly,
carbon in the flame are able to actively reduce the oxide from FeO to Fe
(such happens readily with copper metal), but not under any other
circumstances.


Just using a propane torch the blue will disappear as the I continue to hold
it to the steel piece I'm heating. It doesn't come back as it cools.

Peter


  #25   Report Post  
Steve Smith
 
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Yup. What Tim said.

Steve

Tim Williams wrote:

"Steve Smith" wrote in message
...


Leo, the film thickness does not track the temperature. The oxide grows
faster at higher temps as you say. The oxide stays around--it doesn't
go away as the temperature drops.



Here's the deal. Take a piece of iron. It has a perfectly clean surface, I
mean the oxygen and nitrogen molecules of the air are bouncing directly off
the pure iron (and occasionally iron carbide and others) surface. Now at
room temperature, a few of these do bust up on the iron surface and oxidize
it. Like aluminum, this layer is invisible, but because it's somewhat
indifferent, it doesn't change the chemistry of the iron and you don't
notice it. This layer is only as thick as it is because the oxygen
molecules can't get past it at this temperature.

Ok, so let's raise the temperature. Radiation heats the air molecules near
the steel to the same kinetic energy, that is to say, hot air's molecules
move faster. So they hit the iron surface with more energy, and
occasionally one will pass through the oxide layer and oxidize more iron.
(It probably passes by diffusion, where the surface oxidizes to magnetite -
Fe3O4 - or rust - Fe2O3 - which is then passed backwards to the metal, which
reduces Fe3O4 and Fe2O3 back to FeO, at the price of more Fe metal being
burned.) Just as carburization can diffuse hardening carbon (or nitrogen in
some cases) only so far, likewise the oxygen only goes so far through the
oxide. It's always passing through, even at room temperature, so the
response of oxide growth is probably logarithmic - it tapers off quite
quickly as thickness rises, but never stops completely. It's just that
thermal response is exponential, so it'll take about two million years to
eat a tin can, while at orange heat, your tin can will hold molten aluminum
for only about fifteen minutes!

A temperature of 350°F for a few minutes produces a nice light straw color
(hard to spot because the oxide takes time to grow, and you can't anticipate
it because this is the first interference layer, around 80 nanometers
thick?), but the same temperature extended to an hour gives a purple
coloration.



One factor confusing this issue is that the colors don't keep going.
Once the oxide gets thick enough, the colors go away, but the oxide
keeps getting thicker.



Actually, they come back several times, but each time harder to see because
the light has to travel through more oxide thickness. If you heat a shiny
bar from one end in plain air, you'll see the first layers, yellow, purple,
blue; then a darker run of yellow, purple and blue, and so on for maybe
three or four total modes. What's happening is light is spending 1/2, 1
1/2, 2 1/2, ... wavelengths inside the thickness of oxide (1/4, 3/4, 1 1/4,
... wavelength thick layer, for the wavelength of light *passing in the
medium* (light slows down per the index of refraction), of the *canceled
frequency*). Thicker layers attenuate more, so it quickly (500nm?) starts
looking black. Fe3O4 is a wonderful electromagnetic-absorbent material,
after all. No wonder the military uses it...

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms






  #26   Report Post  
Steve Smith
 
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See what else Tim said. The color disappearing doesn't mean the oxide does.

Steve

Peter Grey wrote:

"Tim Williams" wrote in message
...


That explains why the color disappears as the temerature of the piece
goes up.


Maybe under heavy reduction, where the hydrogen and, more importantly,
carbon in the flame are able to actively reduce the oxide from FeO to Fe
(such happens readily with copper metal), but not under any other
circumstances.



Just using a propane torch the blue will disappear as the I continue to hold
it to the steel piece I'm heating. It doesn't come back as it cools.

Peter




  #27   Report Post  
Leo Lichtman
 
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"Tim Williams" wrote: Here's the deal. Take a piece of iron. It has a
perfectly clean surface, I
mean the oxygen and nitrogen molecules of the air are bouncing directly
off the pure iron ......

^^^^^^^^^^^^^^^
I followed your explanation with great interest. You seem to take me back
to some the beliefs I held originally, and abandoned briefly as this thread
unfolded. I take it that you do not agree that a certain color on the
surface is correlated with a certain temperature in the metal. As I thought
originally, the color DOES correlate with film thickness, which, in turn, is
dependent on the time/temperature history.

Since "drawing the temper" of steel, as done by blacksmiths, using the
surface colors, is a way of raising the steel to the desired temperature,
why does it work? Wouldn't a longer time at a lower temperature produce the
same interference colors as a shorter time at a higher temperature? Could
it be that the time/temperature history produces the same effect on color
that it does on hardness? Or am I completely off the track here?


  #28   Report Post  
Tim Williams
 
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"Leo Lichtman" wrote in message
...
I followed your explanation with great interest.


Thanks.

You seem to take me back
to some the beliefs I held originally, and abandoned briefly as this
thread unfolded. I take it that you do not agree that a certain color
on the surface is correlated with a certain temperature in the metal.


It can't be, since I've torch tempered and oven tempered metal myself. The
oven pieces come out considerably darker, in the purple range as I
mentioned.

Since "drawing the temper" of steel, as done by blacksmiths, using the
surface colors, is a way of raising the steel to the desired
temperature, why does it work?


Because it works in the short term. As you expand time exponentially, you
get more "out" of it, but after a while, to get even a small amount out,
takes a very long time. The endpoint is arbitrary; by eye, over a few
minutes, you'll probably spot between 300 and maybe 500°F (SWAG). In the
oven, you get a bigger change out.

Why does it work as far as the metal? Two reasons. For one thing, it's
just simple carbon steel, you can't really go wrong with it (short of
overheating before quench, which makes it crunchy no matter what!). Number
two, the reactions in the metal, where bainite and whatnot break down to
more stable phases during tempering, is the same kind of time-temp governed
reaction as the oxidation is. It might not proceed at the same rate (it
would be interesting to compare this!), but who knows.

Wouldn't a longer time at a lower
temperature produce the same interference colors as a shorter time
at a higher temperature? Could
it be that the time/temperature history produces the same effect on
color that it does on hardness?


Exactly! So it may be that my purple blades at 350°F for an hour are
overtempered, while the yellow-for-a-few-minutes torch tempered jobs are
undertempered. This is where some imperical evidence comes in handy. The
last blade I tempered was the brass handled knife he
http://webpages.charter.net/dawill/I...rassKnife2.jpg
Now I've sharpened this good enough to shave with (not real comfortable, but
it cuts the hair smoothly anyway..), and in the process I don't notice much
of a burr turned up so it must be pretty hard. That's fine with me since
it's so thick it'll "never" break.

I tempered that to 350°F for an hour and it came out purple (splotchy mind
you, fingerprints for instance are prime spots to prevent oxidation). It
seems to be simple 1080-1090 (used to be a chisel), nice yellow-white bursts
on grinding, same as a file.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


  #29   Report Post  
Steve Smith
 
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Leo Lichtman wrote:

"Tim Williams" wrote: Here's the deal. Take a piece of iron. It has a
perfectly clean surface, I


mean the oxygen and nitrogen molecules of the air are bouncing directly
off the pure iron ......


^^^^^^^^^^^^^^^
I followed your explanation with great interest. You seem to take me back
to some the beliefs I held originally, and abandoned briefly as this thread
unfolded. I take it that you do not agree that a certain color on the
surface is correlated with a certain temperature in the metal. As I thought
originally, the color DOES correlate with film thickness, which, in turn, is
dependent on the time/temperature history.

Since "drawing the temper" of steel, as done by blacksmiths, using the
surface colors, is a way of raising the steel to the desired temperature,
why does it work? Wouldn't a longer time at a lower temperature produce the
same interference colors as a shorter time at a higher temperature? Could
it be that the time/temperature history produces the same effect on color
that it does on hardness? Or am I completely off the track here?




As Tim mentions, the longer you leave steel in a tempering oven at a
*constant* temperature, the color keeps changing.

Tempering by colors works if you use a consistent timing. If you change
from warm forge exhaust to direct torch heat, I don't think the same
color indicates the same temper. Not that I've ever tried this, but it
seems to make sense. So in the end, the temper colors are a guide that
you have to experiment with to find what works for you.

Steve
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