Sword-hilted Broken-back Seax

Step 26 Heat treating

For most of my pieces I use a marquench. This is the process I use for marquenching.
First, I normalize 3+ times (bring the blade up to the critical temperature then let it air cool).
Normalizing relieves the stresses put into the blade during forging, filing, and sanding.
Then, I bring the blade up to temperature, about 1600F for this steel. Bringing it up a
bit hotter than needed allows me to get the blade to the quenching tank at the right temp.
Next, the blade is plunged into 450F oil and allowed to cool fully to that temp.
Once the blade is at 450F it is semi-plastic and can be molded by a gloved hand.
As the blade cools it "sings" making a shimmering noise as the martinsite crystals form
and the blade reached its hardend state. Finally, the blade is given 3 one-hour heat cycles
at 475F. This converts more of the steel to martinsite, and releives the stress of the quench.

*A side note: I lost three blades in the HT (heat treatment). Also, a blade of these dimensions
will develop a forward curve during the HT so the final blade was forged with a backwards curve

Comments

[brandon_a_boyer]

Some of your Information is incorrect.

Tempering converts some of the martensite back into iron and Iron carbide.

Also, why three heat treatment cycles? A single two hour cycle should be more than sufficient for this purpose.

[ben potter]

The tempering has to do with gain structure rather then actual composition. Three cycles are necessary to convert as much of the steel to martensite as possible, for a full HT it takes three cycles over a tree day period to insure that all of the steel that is going to convert has and that it has all been tempered. The transformation to martensite happens as the steel cools and more of the steel will convert if you heat it and cool it additional times (Three times being the best as additional cycles do not convert much more steel and have other less desirable effects). There is a fair bit more to it but it would take too long to type it all out.

[brandon_a_boyer]

Three cycles are not necessary for total conversion of martensite.

there are several different structures that occur in an iron and carbon solution

ferrite (iron)
Iron carbide
Martensite
Pearlite
cementite
austenite

the ones critical to our discussion are austenite and martensite. Generally speaking all martensite is, is an unstable form of of austenite that occurs at elevated temperatures. Rapid quenching freezes the molecules in place and causes the steel to remain in an austenitenitic structure. All you are doing by tempering is allowing some of the steel to return to it's austenitic state.

I'm sorry but whoever gave you your heat treat information was incorrect.

[ninja of suburbia]

Actually, Ben is right. To simplify what happens, when high temperature steel is quenched, it locks the grains into a high tension structure. This structure is very hard, but is very brittle. The tempering allows some of the crystalline structure to soften, making the blade softer, but far more malleable. Heat treating allows the steel to not only convert to a Martensitic structure, but makes it more resistant to wear and heat, as this structure is very solid and hard to break down. Austenite steel is only formed when the metal is heated to high temperatures, and loses its magnetic properties during forging. This is very short lived, reverting back to the high tension crystalline pattern when quenched. Therefore, Austenite is irrelevant except as a time gauge to use when forging, as this property of lost magnetism indicates the optimal time to forge, as well as the optimal time to normalize, a process by which one lines up the blade with a compass needle, forcing the magnetic particles within the steel to align with the Earth's magnetic field, forming lines down the blade, rendering the blade stronger, and not unlike a very weak form of pattern welded steel.

[brandon_a_boyer]

You're only partially correct.

If you quench carbon steel while it's at high temperature (900 C +) it forms the non-equilibrium phase of martensite. Which is hard brittle, and unstable. What tempering does is it returns some of this marstensite into austenite, then into it's other equilibrium phases. Resulting in a more durable blade.

Ben said that tempering turns the steel into martensite, which is just flat out wrong. The only time martensite is formed is during the initial heat, and quench.

Sorry, but your statement about holding the blade in alignment with magnetic north is just ridiculous. It does nothing to make the blade stronger. It's an old smith's tale. Proper heat treatment goes much further.

[ninja of suburbia]

Au contraire. The magnetic alignment may not affect the overall strength of the blade very much, but it does help to prevent chips in the blade, which is very desirable, for obvious reasons. We are in agreement about the tempering conversion, because tempering merely loosens the crystalline structure, giving a blade ductility.

[brandon_a_boyer]

It doesn't "loosen" the crystalline structure, it changes it so that the number of slip planes increases, making the material more ductile

Don't pretend that the magnetic alignment does anything for the strength of the steel. This does absolutely nothing. The closest thing to what you are talking about is a cooling technique that they use for turbine parts, and all that achieves is an increase in high temperature strength.

[ninja of suburbia]

I don't know what you're talking about with turbine parts, all I know is what I have seen in my own experience, and it has been my experience that aligned blades chip far less often than non-aligned blades.

[brandon_a_boyer]

And that's probably true, but not because of the earth's magnetic forces. All that is happening is you are allowing the steel to air cool a bit, which causes some of the martensite to change into softer states, making the blade less brittle (AKA Normalizing.

[ninja of suburbia]

Regardless, that's the way I learned from my master, and that is how I will continue to do it.

[brandon_a_boyer]

And that's fine. It's perfectly normal for someone to hold on to their beliefs. Although I would recommend either taking a materials science course or picking up a book specifically on materials science. Also the ASTM heat treater's handbook is a great thing to have.