Why did my O-1 blade curve down?

C_Claycomb

C_Claycomb

Joined
Dec 11, 2000
Messages
1,128
I am sorry to be posting this since I KNOW it has been talked about before.

I just finished heat treating what was intended to be a cutting competition type knife made of O-1 tool steel (0.25" stock, 2" deep, 10" blade). Due to various screw ups, the blade got quenched five times (edge first, full depth), the last two happened a good couple of months after my first attemps.

The blade has bent / curved down towards the cutting edge by at least 1/4". I didn't notice this after the first three unsuccessful cycles, but it was definitely happening after the fourth.

Can someone remind me why tool steel bends this way when simple carbon steel curves the other way? :confused: With katanas I understand that the steel expands slightly when it hardens, so forcing the curve to happen. As I say, I know I have read this in another thread recently, but cannot find the thread, nor the correct search combination to turn it up again.:(

The only thing that comes to mind is that the entire blade hardened and the relatively more massive spine exerted more force than the thinner edge. If that is the case, is it a problem for the durability of the knife?

All the best.

Chris
 
I think your kinda right. When I worked with 1095, and files, I quenched edge first, and always got a slight down curve. After talking with Glen Mclain, I changed to point in first. This helped, but after getting an oven, and thereby a more even heat, I get none in either direction, but I still go in point first.
 
First I want to thank you for sharing an account of this happening with O-1 as the simplistic explanations never accounted for this, and it is the fact that O-1 will do it that made me look harder at the problem and develop a much more in-depth explanation. For years bladesmiths have struggled with the fact the blades that would normally stand still or curve away from the edge in water will instead curve into the edge in oil. On a very simple blade shape like a Scottish dirk I have seen this happen to the extreme. Edge down was bad, but point down (vertical) quenches were often the same if not worse for me on blades of such shape, this is why I took to spine quenching long ago.

Now as for the causes, it has to do with the timing of the shift from fcc to bcc and the massive expansion that accompanies it. If you form any pearlite there will be an expansion much quicker at around 1000F in the cooling, while the martensite will expand more massively and later at round 400F or below. If the spine and the edge were allowed to cool evenly there would be no distortion at all since the expansion would be symmetrical. But if the edge cools fast enough to harden before the spine (as in water) there will be a rapid expansion on the edge side that will curve the blade into the saber configuration while the spine cannot catch up. At least this is the common explanation that happens to be quite inadequate or even wrong in its simplicity.

My observations have shown me that there are at least three different mechanisms at play here, there are indeed expansions due to allotropic phase shifts, there is also basic thermal expansion and contractions going on, but most importantly there are varying degrees of ductility of the phases present in the timing of the quench. Watch a katana quenched in water under slower replay and you will notice that there is not just a simple curving into the saber that we expect to see. Upon entering the water there will first be a dramatic downward plunge of the tip, causing immediate panic in the heart of anybody who has not seen it before. This is only temporary and as the boiling lessens one can watch the tip rebound and come back up with a vengeance as the martensitic edge forms. What is going on here?

First there will be almost imperceptible contraction of the faster cooling part (the edge) until there is an allotropic shift to bcc in some part of the blade. This will actually occur sooner in the spine as pearlite precipitates out at 1000F due to insufficient cooling to avoid it. It is at this point that one observes the point dropping. But what most have completely ignored in their attempts to explain things is the ductility of meta-stable austenite allowing the edge to adapt to the expansion and recover when Ms is reached and a much greater expansion occurs in the edge. But his time there is little ductility in the bcc spine so it acts as an anchor, pulling the blade into a permanent saber.

So one can see how the who process depends on a very involved balancing act that require just the right timing in order for it all to work. The problem is that oil has a drastically different cooling curve from water. When we quench in oil things get inverted and the edge can actually reach Ms before the spine has dropped low enough to go bcc and the edge will act as the anchor point instead and the much more massive spine will turn the blade into a sickle when it finally catches up. This easily explains why even deep hardening steels like O-1 will still suffer from the problem even thought they are entirely martensitic throughout. This is also why I have found success in quenching spine down to over come it. Convert that spine first and you should have success, just about every other explanation I have heard explains it the opposite.

Not often do I ask this but this is one of those things that I would appreciate a credit on when passing the information along. It seems every knowledgeable smith out there has been working with an over simplified model on this one and I put the effort and time into figuring it out a little more thoroughly, to hear somebody presenting this three months from now like everybody has always known it and they have it all figured out would be quite disappointing. I am going to do some writing on the phenomenon, but I am more than happy to share my work now as long as folks do the right thing when passing it along.
 
Good stuff, Kevin. Generally speaking, we always give you credit, because if we actually quote what you post, people would know that we couldn't have come up with anything like that on our own anyway or arguably even understood a lot of it....lol.:D So when are you going to write the comprehensive bladesmithing book? I know that you are busy during the daylight hours trying to make a living as a smith, but it is pretty clear that you don't require a lot of sleep and stay up late a night peering through your microscopes anyway, so writing time shouldn't be that much of an issue;)
 
Thanks for that Kevin. I was going to try to answer but I couldn't have come close to a clear (easy to understand) explanation as you did.
 
:eek:
I think I am going to have to print that out and try to digest it over a period of days!!

It was a lot more explanation than I thought, and what is more, if that is the "why", I sure hadn't read it anywhere else. Like JDM61 says, I can't see anyone trying to pass that off as anyone else's work!:D

THANK YOU!

Not only an explanation, but a possible fix for the next big blade that I do. I might have worked around to trying a spine first quench, but I think I would have had to produce a couple more bananas before I got there.

There is one thing I have not managed to piece together from the above explanation. Do you think that that bend reduces the strength of the blade at all? The increase in down curve doesn't hurt either the asthetics, or the useful shape of the blade.
 
Kevin, I believe everything you said above, although I do not completely understand all the terms used. Please explain why I get no noticiable change in either direction. Sometimes a side warp, but not up, or down. I use PG 01, stock removal, I pre heat at 1250o for 20 minutes, soak at 1500o for 20 minutes, quench point first in 130o oil. When I was using a forge for heat, and an edge quench, my 01 would turn down slightly. Since I went to an oven, and point first, this has stopped.
Also, I read a piece on 01 a while back, that said if 01 is tempered above 450o, it will be brittle. I don't think I believe that. What do you say, and if true, why?
 
Kevin , thanks for an excellent explaination of a very complex subject !!....LRB, again it's a very complex subject and varying your details may change things...Side warp is not part of this and indicates other problems...Torsion impact tests of O-1 indicate a rise in strength to 400 F then it falls to a minimum at 600 F then it rises. However this is not the case for other types of impact tests !! And I don't quite understand why. This is exactly why I caution about use of impact data !! Sample preparation is also very critical. Use impact data as a guide not an absolute !
 
Well said Kevin.

This is where I harp on matching the steel, temperatures, and quenchant.

The temperature needs to be exact (the whole HT needs to be exact), and done completely to allow all the elements to do their metallurgical dance
(Fiddles and banjo playing wildly)....Let's go now - Iron men ,leave your partners and all join hands... Make a square now ... Circle around the ladies in the center... They could have been a diamond, but don't be blue, times might get hard but they'll stick with you..... (music fades, but the dance goes on )

The quenchant needs to be matched to the steel for the structures you desire to form. Too fast and you get things you don't want...too slow and you get things you don't want....just right and you get the planned results (well, in a perfect world you would).
(Mellow sax playing)....I want a lover with a slow hand.I want a lover who will understand.I need a lover with a gentle touch. I want a lover who will spend some time, not come and go in a heated rush.....(Music fades to a dark and steamy stillness, all is quiet)

As to the steel....
Blade shape and proportions will lend to different cooling rates. Consider these when deciding on tapers, thickness, etc. Steel type, hardenability, alloy type, etc. all give specific results. Take advantage of each steel's special uses to blade makers. 1095 = simple mix, high in carbon, shallow hardening = hamons, high hardness, easy HT, fast quench.
5160 = mid carbon content,moderate alloy, deep hardening = slower quench, low distortion, high toughness.
(Drums, horns, and electric guitar)...'Cuse you've got, Personality, Charm, Personality, Grace, Personality, style, Personality..... (music out)


Now that that is said I'll leave you with this last song dedicated to Kevin:

( Six string, smooth piano, and a string base)...
Hey folks gather round, Let me tell you what I'm puttin' down. I fix broken hearts ; Yeah,I'm your answer man. ( a warped blade can be heart breaking)
I'm not the kind who rushes in like a fool, I'm handy with a thinker's tool , I make all my blades according to a plaaaan.
If you want good results ,and are willing to learn...Yeah then, I'm the man to see.
But if you don't give a damn, and say ,"What the heck", well, you'll go running from meeee.
Now, here is the main thing that I want to say, Keep learning 24 hours a day.
If you want to succeed then I know you really can........

My sincerest apologies to Stan Winchester, The Pointer Sisters, Lloyd Price, and James Taylor
Stacy
 
Ok now im flippin scared to harden my blade! Thanks Kevin... glad I saw this thread and info.

I never even considered warping problems. I am going to have to research this more as I am currently making a pretty thick bladed knife and I will have to see if this will have a more detrimental effect or if it will keep it more stabilised. I always thought the pure reason for quenching edge first was to get the edge to optimal hardness and leaving the spine slightly more tough. Would spine quenching not be counterproductive to this idea? And wouldnt tip first be very prone to cracking?
 
Quenching spine first will convert the spine to martensite first and the thinner blade immediately afterwards. Since there will be little thermal mass in the edge ,it will convert quickly,too.The whole thing should become martensite, and any force applied by the edge converting last will be resisted by the much stronger spine. If any curvature is created, it will be a sori (upcurve).
In an edge first quench, the force applied by the spine has more metal to create the force and is resisted by a much weaker edge,thus the curvature.
Stacy
 
C_Claycomb said:
:eek:
There is one thing I have not managed to piece together from the above explanation. Do you think that that bend reduces the strength of the blade at all? The increase in down curve doesn't hurt either the asthetics, or the useful shape of the blade.
Click to expand...

I don't feel the downward curve effects the strength of the blade at all and when it happens to one of my blades I usually like the look. Can look very wrong on a Japanese style blade though.

I also usually don't get the downward curve from a single quench but when doing more than one quench (rarely anymore) to get a decent hamon. After the second or third quench, it becomes very pronounced.

Kevin, I didn't know about the spine first quench and probably wont use it on my blades, but when I share this info, your name will be attached.
 
bladsmth said:
...The quenchant needs to be matched to the steel for the structures you desire to form. Too fast and you get things you don't want...too slow and you get things you don't want....just right and you get the planned results (well, in a perfect world you would)....
Click to expand...

Good point Stacy, the prevailing attitude that too many have taken is that the fastest possible quench is best, as with everything- too much of a good thing does not automatically equal better. How many times in the improvised quenchant debate have I heard "well my XXXX seems fast enough", if fast is all that matters why even have the debate, everybody could just use brine!

Now I really need to add some extra points to my above post:

1. Although I pointed out the flaws in the oversimplified explanations out there it must also be pointed out that my explanation above is also quite simplified in order to fit it all in one thread and not write an entire book on the subject (which could be done). There are an infinite number of possible combinations of phases and dimensional changes that could occur at any given point in countless variations of cooling curves. Heck due to unseen variables one can get entirely differing degrees of sori using the same water.
In some instances any one of the influences of thermal contraction, expansive phase shifts, degrees of ductility etc...could end up being the dominate factor. If your edge is forming martensite at a time that happens to coincide with the greatest amount of simple thermal contraction in the spine, it is not hard to see how you will get exaggerated sori.

2. Unless stated otherwise always assume I am talking about a bare blade with no clay on it! Something like clay can change everything and put the added variables right through the roof into astronomical levels. I have commented on my techniques before, only to see elsewhere somebody quoting me while pointing out that what I said was bunk because when they did it with a clayed blade...:rolleyes: I kind of find it an irritant that even though that heat treating method was used on one little Pacific island group by a few people, a few centuries back, everybody on the planet is now assumed to be working with those same techniques :mad: There is only an entire planet of people and cultures that did it in different ways, and I know of no I-T diagram that was developed using clay on the steel!
 
Yeah, this is an old thread but.... I took some pics of this problem and was going to ask for advice when I ran across this thread in my last ditch search for an answer. Did not find it the previous 2 days of searching, must have finally typed the correct search phrase during this last attempt.. Anyway, I thought I would bump this old thread in case anyone else has struggled with this issue.

Below are pics of a blade I am working on where the tip is more than 1/3 inch lower than the original grind. The first photo shows the paper template placed over the blade. I ground the clip to exactly match this paper template, really wanted the concave sweep along the clip on this blade profile. I had noticed on my last couple of blades that I had convex clips and was wondering how I let that happen when I ground/filed them. Those blades were intended to have a straight clip profile. I specifically put a concave profile on this blade and was very carefull when I ground the clip to exactly match my design. Imagine my surprise when I had the same problem with this blade. I had always assumed I would get a slight upward movement of the tip. Not down. This thread answered that question.

This W2 blade was quenched 4 times to get the hamon I wanted (a frustrating ordeal!!$#@^$#!!). It was quenched in parks 50 and I assume the amount of hardened area at spine of the blade added to this effect. I am using the blade anyway, it looks fine and is for a hobby project without a specific owner. Just not what I was shooting for. Will try the spine first quench method on future blades.

DSCF0002.JPG


DSCF0075.JPG



Barry
 
I somehow missed this thread. Kevin made a statement on another forum recently about quenching spine down, and now this helps to explain the reasons.

Good stuff, although I expect this thread, like many others, will be closed shortly due to the "age" of the thread.

Robert
 
Was just thinking, I see a lot of WIP posts where experienced folks are using a cylinder to hold quench oil. This requires that the blade be quenched TIP first. I always assumed that this was done for ease of storage, I.E. the cylinder can be capped and stowed easily in an out of the way place.

Upon reflection of the info above, is this tip first quench also done to reduce the effect above (as LRB alluded to in the 1st response)? Both sides of the blade would experience the quench at almost the same time..... You would still get perlite formation in the spine, meaning downward forces on the tip, but the time difference would be much less subjectively. This would reduce the amount of perlite formed before martensite forms to counteract it.


Barry
 
Last edited:
You must log in or register to reply here.
Back
Top