Jump to content
Sign in to follow this  
shane justice

ferric etch and grain boundaries.

Recommended Posts

Good Morning! Snow on the ground today.Decided to have and another cup of tea before I break the ice off my quench buckets and grind a few before I go to my real job.

 

I had an interesting wreck in the shop that got me to thinking. I had a blade taped up while I was working on  getting the shoulders square for the guard. Somehow it must have caught some overspray from a water  bottle I was using to wet a stone. Anyway you guessed it...when i unwrapped the blade...rust. What got me was that it only seemed to attack certain parts of the blade. This blade had a ten minute etch. It showed typical 5160 formation. The rust fromed only in and around the elongated lines that always seem to run lengthwise on my 5160 blades.

 

I took the blade back to the grinder and ran it down on 240 grit. I was amazed to see how far the rust had pitted in a very short time. Again it was only on what I can call "grain boundaries"

 

My guess is that this must be  part of the "Pearlite Matrix" around the other carbides and that it is highly suceptible to rust.  

 

If that is true then...Does etching tend to "weaken it's immunity" to rust by leaving the more open grain structure free to catch dirt and moisture, and deposit it in the low spots which have been eaten away?

 

Also, would these micro fissures (the relieved portions of pearlite) cause micro stress risers and cause weakness in a blade?

 

Guys, forgive me if I am pulling these terms out of my arse. I don't have much of  a science back ground. I am only applying what little knowledge I have picked up and applying it in a genral way. Possibly I am attaching the wrong terms to the wrong things.

 

Anyway, what do you guys think?

 

Shane

If that is true...

Share this post


Link to post
Share on other sites
Was this a stupid question?

Share this post


Link to post
Share on other sites

hi

the question wasn't stupid..  

- i've noticed that etching steel seems to provide a good surface for rust..

- and i also think that the surface is coarse due to the oxidation

- a buffed piece of steel on the otherhand seems to resists rust a little bit better

 

I wouldn't say that it weakens the blade significantly.... nor leads to risers...   unless the etching was extremely deep

-look at all the damascus blades... that are etched and they don't show any brittle traits resulting from the etch

 

but there is a such thing as "hydrogen embrittlement" .. that happens when steel is pickled for awhile

 

maybe some of the chaps in the know will chime in ?

 

 

Greg

Share this post


Link to post
Share on other sites

Hey thanks Greg!

 

Just wondered aabout the whole strength deal! Some folks say even a  grit mark will weaken a blade when it is under stress! I haven't been able to determinf it will of not.

 

My own thought is it has a bunch more to do with metallurgy than finish.

 

ON this blade it was like most of the rust was under the surface. Weird, huh?

GOod talking with you.

 

Shane

Share this post


Link to post
Share on other sites
i have noticed that buffing a blade tends to keep it free from rust longer than if it were, say, hand-sanded or machine ground and then put in the box for a show. my theory on this goes back to the metalurgy class i took where we had to fine sand and finish a piece of steel and then etch it to see the grains, meaning to me that it had something filling the pockets between the grains which the acid removed allowing us to see these grains, but which would logically also leave small pockets for water to collect and/or be traped in and allowing it to rust easier, where-as buffed steel tends to close these holes or gaps by pushing the high spots back into the low ones, making for a more even surface with less places for water to become trapped than if it were etched. based on this theory of mine i buff every blade i make even if i etched it, i just dont buff it as long if it was etched

Share this post


Link to post
Share on other sites

Etching does open up the grain in the steel and does make it prone to rusting, it also makes it imperative to completely rinse and neutralize the etchant or it will continue to work. Ever notice blades darkening with age? For ferric chloride I do a thorough water rinse and then spray with ammonia and the loose oxide with a wet paper towel and baking soda, rinse, scrub, rinse...you can't be too obsessive about this. Dry and seal with a water displacing oil. For a temporary oil, I like WD40 the best because it is cheap and available everywhere. You can also get it off the blade easily if you need to reetch.

 

There are so many rust preventative products out there that it can become confusing, but I make an effort to try them all. Some are better than others, but the ones that work best are the ones that fill the open grain and make a bond with the metal. Some of the best preventatives out there stink, lanolin comes to mind, but also polar oil and many of the industrial cures.

 

Ones that work and smell OK are BreakFree CLP and Camelia sword oil. I use these in combination with Renaissance wax. Wax provides a good protective barrier and works about as well as anything. We, Jimmy and I, were introduced to Renaissance wax by Mr. Pettibone the curator of the Higgins Armoury. He was in charge of maintaining the huge collection of arms and armor and never touched anything without white gloves and a can of Renaissance wax.

 

Some folks like the silicon based preventatives and they do seem to be very effective. I don't use them in the shop because they are very difficult to remove if you have to go back and work on the blade.

 

Before I tape a blade it always gets a soaking in WD40. I rarely get rust under the tape and when I do it is almost always from not neutralizing properly.

 

Buffed blades are more resistant to rusting just as Dragon says and it works because buffing actually smears the surface. I personally don't like buffed blades so I don't include it in my process, but it does have that benefit.

 

If anyone has the cure for rusting, the world will beat a path to your door.

Share this post


Link to post
Share on other sites

Thanks Guys!

I'm gonna hafta take a look at my process..

 

Don,

Your prolly right about the whole neutralizing deal....prolly need to work a bit more at that.

 

Thanks,

Shane

Share this post


Link to post
Share on other sites
I've also noticed that etching can lead to quick rusting; I etch, then sprinkle baking soda  onto the blade, rinse with cold water, then dry and immediatly douse with WD-40.  Only then will I start really removing the oxides and polishing to reveal the hamon.  On the other hand, once this process is completed I find that rust resistance is improved; after all, etching is a process of forming Iron Oxide on the surface of the steel, and rust is iron oxide, so what you're really doing is rusting the metal, but on your own terms.

Share this post


Link to post
Share on other sites

Interesting thoughts David...

 

I am thinking it depends on the depth of etch/etch time.

 

Too long and I feel like little pockets form here and there...and then doom on you...rust.

 

I like the etched blade, need more time to get it right.

 

Shane

Share this post


Link to post
Share on other sites
Long time, no post.  I used to give a blade a long etch, a hour or so soak in the Ferric Chloride, before polishing, but I've found that the time makes very little differance.  Once the etch is started the layer of oxide that is built up inhibits the further action of the etchant; the more you etch, the less affect the etchant has on the surface in question. Once you've etched for a few minutes the rest is really up to you.  The world is the metallurgical molusc of your choice

Share this post


Link to post
Share on other sites
"elongated lines " ? Are you talking about 'alloy banding' ? This is caused by segregation of alloying elements as the ingots solidify in the early stages of the steel making. I can't be changed by heat treatment.

Share this post


Link to post
Share on other sites
Guest Rick Baum

Hey Shane...

 

Don't know much about the metallurgy stuff. I'm affraid I'm in the same boat as you.  However, one thing that I picked up that workes real good for curbing the tendency for rust, was to soak the blade in melted parafin wax for about five minutes or until the blade came up to approximately the same temp as the wax.  Then let it hang and cool.  This works well on Damamscus when it comes to preventing rust.  I cleans up with a little laquer thinner.  Maybe it's an option for your etched carbon blades.  Food for thought...

 

Rick

Share this post


Link to post
Share on other sites
Guest Rick Baum

Mete...  What kind of heat treatments are you talking about.  I have experienced the exact phenomenon that Shane is describing.  And this is after many many thremal cycles at or just above critical.  The last blade I forged had a combination of at least 12 thermal cycles into this range and the bands were there when I etched the blade.  They look like little worm tracks and they follow the direction of forging.

 

Rick

Share this post


Link to post
Share on other sites
Alloy segregation [ alloy banding ] can't be changed by any heat treatment . It's there forever !!

Share this post


Link to post
Share on other sites
Guest Rick Baum

Sorry Mete... After looking more closely at your initial reply I see that you said "Can't" not "Can"... Oops!  Thanks for the reply though!

 

While we're on the subject.  Does the segregation that we are seeing mean anything in regards to the quality of the steel?  Can it affect performance in a blade  positively or negatively?  If so, please elaborate.

 

Rick

Share this post


Link to post
Share on other sites
Big Ingots cool slowly and segregation can be significant.  However, tool steels are more often cast in small ingots and heavily hot worked to break up the segregation.  Clean steel means fewer non-metallic inclusions in today's material and that means fewer stringers showing up in the final polishing step.  The segregation, if REALLY bad can lead to harder and softer areas in a blade but nothing as distinct as a patern welded blade.  I don't think I would worry too much about it.  ???

Share this post


Link to post
Share on other sites
I don't believe what I am seeing in my blades is alloy banding, I believe it a carbide structure because it can be wiped out and recreated by doing thermal cycling.

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
Sign in to follow this  

×
×
  • Create New...