Heat Treatment, how / why does it work?

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lasersailor184

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Sep 15, 2003
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I tried to use the search, but it wasn't working.


So I was wondering what the process was to heat treat a knife and also why it works.
 
Oh boy, I am not going to touch this one. I refer to the experts. ;)

FYI - You will need to tell "us" what type of steel you are using to get a decent quantitative answer.
 
It changes the chemical arrangement of the steel for the purposes intended. That is the what for and the why. It is basically simple in concept but can be complex in practice. All the time not out of reach for those that wish to do it properly on a minor production scale. In knife making the heat treating is the knife. Without it there is only something that looks like a knife.

Hang around here if enough interested and you will learn much more than I could tell; and in shorter order than you might expect.

RL
 
lasersailor184 said:
I tried to use the search, but it wasn't working.


So I was wondering what the process was to heat treat a knife and also why it works.
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I'm gonna reply with a simplistic answer, but one which is relatively easy to understand.

Metal is made up of mollecules. In actual fact, there is a lot of "space" between mollecules in anything. Even in a solid object. Heat it up, and the mollecules dance, so much so that if you heat them up far enough, they show the properties of a liquid, and even more, and they turn into a gas. All that is happening is that the "space" between them is increasing. (Think in terms of ice / water / gas ... OK so far?)

Heat up metal and things happen, the mollecules dance, but when they cool down, they settle themselves into random patterns, or not. Hammer them hard, and they shuffle themselves into different patterns.

Copper metal is an interesting one for example, heat it up slightly and let it cool naturally, and then the mollecules will have settled themselves down into a random-ish pattern which makes the metal really soft and easy to hammer. Hammer it, and the mollecules dance into patterns, which makes it harder.

Steel isn't quite like that, it needs to be red hot to soften it enough to be workable..... but can you imagine what happens when it cools down, or if you hammer it when it is cold or too cool?

Therefore, after all the work has been done, a heat treatment is necessary to get those mollecules back into line. In fact into a specific regimented order, like little soldiers. Heat it up to a specific temperature (different for each type of steel and the type of hardness you want), so that the mollecules shuffle themselves into a certain pattern, and then quench them in oil (or water depending on the type of steel), and you literally "freeze" them there.

Viola! Steel that is of the hardness (and what is called "temper", without being brittle) that we need for the knife.

Rod
 
Metals do not have molecules , they have atoms, and crystals. In the heat treating of steel you are heating the steel to form a crystal structure called austenite and when you quench it the austenite changes immediately to a crystal structure called martensite which is strong and hard.
 
Sorry Rod, as soon as I saw the first "M" (molecule) word in your post I rushed to save you before mete saw it, but too late, he had already sunk his teeth in ;) :D

The original question is like asking for "War and Peace" in a sentence or two. The closest I can come is to say it is all about where you put the carbon atoms, and temperature is your tool for moving them around.
 
Kevin R. Cashen said:
The closest I can come is to say it is all about where you put the carbon atoms, and temperature is your tool for moving them around.
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Dang, Kevin, that is about the neatest reduction of HT to "first principles" I have seen!!!! Thanks!
 
So for a good heat treatment, you would need to compress the atoms and crystals into place first, then freeze them? Or do they get in order just be freezing them?
 
It's not about compressing the crystals, it's about making them a different shape and stuffing them full of carbon. In practical terms, you heat the steel until it's X temperature, soak as long as may be needed, then cool it off quickly enough that the carbon doesn't have time to escape the crystals. Air hardening steels have the most complex and strict requirements about what temperature, and the cooling can be done in open air, the most gentle quench. Most other steels are a little simplier, heat to non-magnetic (which is the curie point, and indicates that the steel is turning into austinite), let it soak until the temp evens out, then quickly dunk it into water or oil to cool it. After that you want to temper it at a lower temperature to relieve strain and brittleness. The specific temperature and times required will vary by steel alloy, but the oil and water hardening steels have a little more fudging room on the specifics.

Here's something written by Kevin a while ago to help explain the crystal stuff.
http://swordforum.com/metallurgy/ites.html
 
Kevin R. Cashen said:
Sorry Rod, as soon as I saw the first "M" (molecule) word in your post I rushed to save you before mete saw it, but too late, he had already sunk his teeth in ;) :D
Click to expand...
OK.... point taken. I was thinking of an alloy being an intermetallic compound, rather than a solid solution, or an intimate mixture of minute crystals of the constituent metallic elements.

Now where's one of those icon smiley things for walking away with tail between legs..... damn, this one will have to do :o

Cheers
Rod
 
There is probably no more misunderstood (especially by many who think they understand it) subject than hardening of steels.Mete's info is correct.Steel is composed of atoms that will form in many different crystal structures.Just as pure carbon can be soot,graphite,or diamond (all having different hardness and properties), so steel can crystallize in many forms.The main ones concerning blade smiths are austenite and martensite. All of this is due to the addition of alloy elements to the iron.Most carbon steel is about 97-98% iron.As more elements are added the chemistry gets more complicated (tool steels and stainless steels).The simple mix of 1% carbon and 99% iron produces a usable steel for hardening.Pure iron has only one crystal structure and cannot be hardened by itself.The final bit of confusion to all this is that the quenching temperature,rate of cooling,temperature that cooling stops at,and time held at these places all will affect the end result.Skilled smiths spend a long time learning the variables,and how to use them to their advantage.
Hope this helps a little. -SA
 
basically the iron matrix when cool is too small to let carbon in, when you heat the steel to the right temp. the matrix expands and allows the carbon to enter the matrix and dance inside. then when the steel is quenched the carbon gets stuck in the iron matrix, and becomes very hard. this rapid cooling causes severe stress in the steel, from the carbon trying to break out of the iron prison. so it must be tempered to make the carbon yhappy in its new home. worst case senario without tempering? the steel explodes from stress.
and if i know you guys, your gunna make a joke about "the matrix".
 
Sounds good to me ;) , I am even cool with "the matrix" , the array of atoms in the ferrite is often refered to as a matrix- so you are correct "Mr. Anderson" (O.K. sorry I had to slip one wise crack in ;) ).

Only one small detail, forgive me it can drive me crazy when somebody else nitpicks like this, but the matrix actually contracts (I know - confusing). At room temp the arrangement and stacking of the matrix is much less efficient and does not have as many clear interstitial spaces. When you heat, things get more efficient and creates the spaces for the carbon atoms to go.
 
So I was wondering what the process was to heat treat a knife and also why it works.


the reason it works is...
there r elves that live in steel
when u thermal cycle it this really makes them mad so they call in there friends the trolls
now these trolls r hight temp trolls and attack the metal on a molecular level
both hardening it and softening it at the same time

unless..
its damascus then all bets r off
if its damascus u gotta petition the woods gnomes for help!!
now these particular woods gnomes only live in space
not in outter space
but the space between the layers of steel:0
ive heard that if u forge REAL thick and not to well to shape and do a quench 5 times and draw it 12 times
then tell everybody its the best there is
then etch the blade u can see mars on a full moon night
if u have used the snot of enraged moles to quench in that is
or at least thats the way it was explained to me
i read it in a book
hey if i read it its gotta be right!!
right????
i mean afterall it was in a mag about knives
and nobody that writes in a knife mag would
espouse b.s.
just for there own edification !!right???
yea!! right:)
harley
luke sky possum
 
Now, is it best if you bang out with hammer, anvil and heat the shape of the knife?

Or is it possible to heat treat a knife then grind it down to what I want? Will this lose any strength?


The reason I initially asked is because of something a friend of mine tried. We have a pair of POS Fairbairn sykes rip offs. Just from sitting in the air, they have begun to rust.

Now, my friend heard the very basics of what happens to heat treat a knife. So he scraped off the paint, and stuck the knife in his wood stove. He heated it up till the blade was glowing red, then dunked it in Motor Oil. The blade seems a little harder, but the main thing I've noticed is that it hasn't rusted yet. Anyone have any comment on this?
 
When you heat it hot enough to glow, it forms scale on the surface. The scale is more rust resistant then plain shiny steel.
 
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