Steve Sells

Intro to Heat Treating

Recommended Posts

Introduction to Heat Treating simple steels.

I would like to start off by stating that this is in the blade section because thicker items need different procedures than the thin cross sections we are dealing with in blades. Mill specs that come with an order of new steel are fine for dealing with cross sections that are inches of thickness, but leave the poor blade smith in the dark as to where to begin. Example of 1095, while fine with a water quench for a large tool, when we use it for a knife, it can crack with a standard water quench.

In order for our newly made blade to hold an edge, we must get it hardened. Many have asked how to heat treat a blade. So here is a very basic primer on that process.

When we finish forging our blade, we have a knife shaped object. To make it hard enough to hold an edge and actually be a blade, we use thermal treatments to change the molecular arrangement of the metal crystals. The following processes only work on steels with a carbon content of 30 points (0.30) or higher, if lower amounts of carbon are present the steel will not harden enough to make a difference.

At room temperature steel is in a body centered cubic structure. The cubes of steel have an iron atom at each of the 8 corners of the cube, and one in the center of the box. This is called ferrite.

By heating the steel we can change the arrangement of the carbon and iron, into a face centered cubic where iron is at the 8 corners, and carbon is on the six sides of the cube. This form is called austenite. The temperature that this occurs is close and slightly above the curie temperature. Lucky for us there are a few easy ways to identify this point. One is by using a magnet, as steels are non-magnetic at this phase transformation point. Also when you see the color from the glow of the hot metal, there will be a color shift, as the color progresses from orange into yellow from the added heat, at the curie point the color will shift back to a darker orange color as it changes state, then progress toward yellow again. This is very subtle, so trust the magnet.

Then remove the steel from the heat, and slowly allow to cool. This is normalizing. It allows stress to relax in the blade. Many do this 3 times to cycle the steel for grain refinement as well to insure no warping. The next time we raise the steel to this curie point, we want to cool it fast enough to freeze the carbon atoms within the cubes of iron. Most of the time this will be done by quenching in Oil, tho some some lower carbon steels will need water to be fast enough, We are only dealing with simple steel here, so will not address air hardening or high alloy steels.

 

When Quenching in a liquid, vapor pockets can form around the steel, so agitation in the form of a gentile up and down motion can break this up andd allow even cooling. Do not use a side to side or any fast motion as this can lead to warping.

 

This quenching creates another form of steel known as martensite, where the carbon is trapped in the iron 'cube'.  This is the hardest form steel can take, but it is also very brittle, so we must immediately temper this. I have had blades crack while waiting to temper, so heat the temper oven before you start to harden and you wont have this issue.

Tempering relaxes the thermal shock we caused during the hardening process, all hardened blade should at least get a hour or more to relieve the stress. Most take only 320F or so, and I prefer 350F two times at two hours each, with a rest period of one day between. When we harden the blade, there may be retained austenite, rather a small amount of the austenite that has not yet converted to martensite. This will convert into martensite after some time as room temperature or lower, and that is the reason for a second temper cycle, to relieve that also. This is also one reason many use a sub zero quench, for advanced steel treatment, to force the change into martensite.

Using a higher tempering temperature will result in a softer blade that takes more beating before breaking, but reduces the edge retention, as with the steel choices, its all a compromise, there is no one size fits all.

for further References see PB 0078, and read "Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel", by Prof. John D. Verhoeven links at the top of the knife forum

Dr Bain's paper on Bainite http://www.msm.cam.ac.uk/phase-trans/newbainite.html'>http://www.msm.cam.a...newbainite.html

Share this post


Link to post
Share on other sites

I assume you posted this for comments; so:

Could you add that the alloy should be a hardenable one to the beginning of that write-up?

Also: "The temperature it occurs" is the austenitizing temp that happens to be *close* to the curie temp for *some* alloys; particularly the simple Fe-C ones.

 

added,  thank you for pointing that error out

Share this post


Link to post
Share on other sites

When tempering in an oven I've saw where it was recommended suspending the knife via wire so it's not resting on metal.  I don't have this option due to space, but wondered about setting the knife instead on a firebrick in the oven vs. a metal rack.  Any preferences?

Share this post


Link to post
Share on other sites

theres not enough space to suspend from wire, but you can fit a firebrick in? :)

 

if you have enough room to add clearance to the bottom could you make a thin trivet set or cradle of wire like a mini hammock to set the knife on, rather than hang from?

Share this post


Link to post
Share on other sites

Thanks Chinobi, I realize now I need to change how I was approaching the question.  Instead of hanging up and down it could be suspended horizontally so just the blade isn't making contact.

Share this post


Link to post
Share on other sites

MAy want to try a couple of ways,,,I preheat my oven till it is the correct temp then lay blade on rack flat,,,if you choose to hang or prop it up think about wot you will use and how that may affedt time and temp. For instance a room temp fire brick with a slot to hold the blade will do wot you are thinking,,,,do you know how long the brick will take to come up to temp? Untill it does, the blade will not either.You could preheat the brick ot wotever or jusr add time to the process....wonder how much time?

Heat treat oven makers sell a kind of rack..a flat ceramic with verticle rods spaced to hold several blades so the blade does not lay flat,,,I use mine when I heat blade for hardening,,,  not tempering, And then I do that i leave the rack in while I preheat the oven before placing blade inside.

Share this post


Link to post
Share on other sites

Good point on the pre-heating.  I was thinking of welding a rack of sorts out of scrap steel that would hold the blade horizontally by the tang.  Would that sort of contact if minimalized be a concern? 

Share this post


Link to post
Share on other sites

I wonder how many people wish they read this before they ruined their blades, trying to teach themselves with you tube?

Share this post


Link to post
Share on other sites

I have noticed over the years, most posting the simple questions dont look, the ones that have looked dont have to post  lol

Share this post


Link to post
Share on other sites

Being new to blacksmithing (but not to common sense) I read all the stickies that I could when I first joined and still refer back to them. Not sure why so many don't do the same but my guess is that they have initial misconceptions about the complexity of blacksmithing to begin with due to movies and video games.

As for myself, I could have never forged that last set of dragon scale armor of impending sarcasm nor my +5 mithril longsword of unavoidable mockery without doing some homework first.

Share this post


Link to post
Share on other sites
On 12/22/2015 at 3:17 PM, Steve Sells said:

I have noticed over the years, most posting the simple questions dont look, the ones that have looked dont have to post  lol

Glenn: Is this in the "Gems and Pearls" thread? It's as true as most anything we read here.

Frosty The Lucky.

Share this post


Link to post
Share on other sites

A word, as someone who has asked questions that are answered in the forums (pinned or not).  I asked because what is logical placement isn't obvious to those who don't know  as much as the authors.  After all it used to be that information was lost when the last person or copy died out, but now it's lost because it is buried in the masses of posts.  That said I quit asking while I try to digest the pinned posts at least =-)

 

Share this post


Link to post
Share on other sites

Steve,

Your sage observation,

namely,

"I have noticed over the years, most posting the simple questions don't look, the ones that have looked don't have to post".

bears a little suspicious relationship to the Latin, phrase "Reductio Ad Absurdum".

Taken literally is there no need to post, no need for the forum?

Somehow I do NOT think so.

Regards,

SLAG.

Share this post


Link to post
Share on other sites

Dan: Your Latin phrase caused me to think a little. :) Here's an ancient Latin saying I just thought up. "Reducio Via Nauseum." 

Yeah YEAH I know, bolemia was an accepted practice back then and I'm sure has a proper Latin name. What fun's that though?

Okay gang, back to your regular programming.

Frosty The Lucky.

Share this post


Link to post
Share on other sites

"Reducio Via Nauseum?" err ... I have another one ... Latinus grossus facit tremare pilastros ... :)   (*)

Not that Reductio ad absurdum is any better. It may be spelled correctly but it's applied incorrectly. 

To say that those who post a simple question most of the time have failed to look for the answer, and that those who have found the answer need therefore not to ask a question, is a simple statement of fact and not an argument to prove or disprove anything much less the need for the forum to exist.

Since we always talk about 'how to' perhaps the most pertinent latin quote would be this from the poet Virgilio

"Vires acquirit eundo" or ... for the common man ... you gain strength (or you get better) as you go. 

 

* Maccaronic Latin makes the columns tremble ... (or rattles the building's foundations) 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now