Broken blade question

blgoode

blgoode

Knifemaker / Craftsman / Service Provider
Joined
Oct 3, 2003
Messages
7,145
Well...I just had to do it. I am trying to learn what not to let happen during HT so I pushed this blade till it cracked. then I broke it to see what I can tell from the inside.

Can someone shead some light on this for me. What I have learned through this forum is that shiney steel(on the inside) means grain growth...I think.

Was it me overheating the blade that caused the cracking?

Any one have any insight from looking at these images?

Thanks in advance....
 
I should reword the first post...I didnt intentionally try to crack the blade but I was trying to pour the heat on. I am still using my o/a and I am learning to get the feel for heating the steel. What I wanted to do was not be afraid of overheating and see what would happen.....

I forgot to mention that I didnt preheat my quench. This may be the cause of the cracks.......

Also I read that when you heat treat a blade it is important to quench the blade as the heat is rising. It stated that a blade wont harden as it is cooling...

Any input is great.
 
First the photo must be clearly focused and there should be side lighting.Second " won't harden as it is cooling " is not anything you heard from me !..When using a torch it is very easy to over heat the edge and tip since they are thin ,it would be better to put the heat to the spine and let the heat conduct from there to the edge. What steel , how did you heat it, what was the quenchant ?? Intentionally breaking things can be a good learning tool but you have to know exactly how you did it so you don't do it again.
 
What I was doing was focusing the heat on the first .5" of the edge area. I was using 0-1 in peanut oil. When I look at the broken segment the steel is a matt gray in color then its a black along the first .25" or so. I went in the shop and intentionally just grabbed the blade and went at it like I didnt care....thats why I didnt preheat the quenchant. I think that may have caused the cracking and slight warping but I cant be sure......
 
If it's black for the first 1/4" that means that the fracture occured at a temperature high enough to oxidize it immediately, I wonder , ~1000F ???
 
So I still needed to get it about 600 more degrees??? By the color it was in the hardening range.I may need to as they say...practice, practice....and more practice....
 
blgoode said:
So I still needed to get it about 600 more degrees??? By the color it was in the hardening range.I may need to as they say...practice, practice....and more practice....
Click to expand...

I'm thinking that mete means 1000 or higher where you'll get some carbon burn off .

if it cracked in the quench (to cold an oil quench) the cold oil can trap the heat causing it not to disperse it fast enough...correct me if I'm wrong
 
I do think it cracked because of the cold quench. This is the best shoy of the blade I can seem to get...Any clues here?
 
Ahhh..the colder oil not dispersing may be the reason ED Fowler used his quench around 160 degrees. I have been using mine around 130 to 140 degrees. Why would a 100 degree quench be less useful than a 140 degree quench?
Now I need to gring some more blades :)
 
blgoode said:
Ahhh..the colder oil not dispersing may be the reason ED Fowler used his quench around 160 degrees. I have been using mine around 130 to 140 degrees. Why would a 100 degree quench be less useful than a 140 degree quench?
Now I need to gring some more blades :)
Click to expand...

Ed's oil is not peanut oil and 160 would be to hot for yours
viscosity in different oils at different temp's
maybe I missed,, it you edge quenched it?
and did you temper it after HT and before you broke it?
 
The normal hardening temp is 1450-1500F. The Ms temp is 400F.What I'm saying is that the crack must have occured at a high enough temp to form black oxide on the crack surface if that is what's there. again it's too hard to tell from the photo....The curved crack on the right is typical of a quench crack. If there is black oxide on the big crack could it have been there before quenching ??
 
What I am doing is heating the edge portion with an O/A torch. I am not edge quenching per say but I am only getting the edge to critical, then quenching the whole blade.



Also, what I think happened to cause the black is...I pulled the blade out of quench to reHT for a multiple quench and saw the crack. I just went ahead and brought it to temperature, quenched, then broke the blade. So you were right mete, the crack was there before the last quench, and I think it happened due to the cold quench prior. This has helped me alot.....just to understand what not to do....


What temperature would you recomend for penut oil? 125 degrees?

Thanks guys....
 
I'm a newbie to knifemaking, I've probably only heat treated about 20 or so knives (I'm counting multiple heat treats that I've done on one knife to get it correct). This is just my 2 cents...

I've never had a problem with knives cracking, I've quenched knives in cold water, (I didn't intentionally do this, but I just filled a bucket with the hose and afterward I read that this is really stressful) and I don't preheat my oil for edge quenched knives... it's a combination of motor oil, beeswax, and bacon fat. Basically whatever I could throw in there.

I've heard that if you don't get all the grinding marks out before quenching you can create stress risers which will cause cracks, so I usually finish the knives to 220 grit or so before quenching. Maybe this is the problem? I also use a propane forge and check temperature every few seconds to prevent overheat... I don't know how feasible this is with a torch, it sounds like using a torch is a lot more difficult.

Also, The only time I've heard of people doing multiple quenches is for 52100, and even then I think you are supposed to leave a day between quenches.
 
B.Goode what you did is called differentially hardening...I can't help you with info about peanut oil, bacon fat etc.
 
I think your cracking problem is from over heating the edge. I havn't cracked a blade in years, but when I was learning I cracked a few until I learned not to over heat. Quenching a blade that is too hot will cause more problems than anything else you can do. I did some test a few years ago, I took a blade and quenched it at what I thought was the right temp and check it with a file, it was hard but did I over heat it? I did this several times, lowering the temp ( by color ) everytime and then checking for hardness, I was able to get a fully hard blade at a much low temp than I thought was possible. I learned to heat my blades slowly and evenly in a dim light and find the critical temp before I quenched, it solved a lot of probs for me, no more cracking and very little warping. Use a magnet at first to help find critical and if you over heat, let it cool down to critical before you quench.
I hope this helps.

Don Hanson lll
 
Did I miss the part where the kind of steel you used was mentioned?

I was reading kind of fast.

I won't offer much advice...except that every combination offers it own challenge.

There are so many variable to the equation that it gets tricky.

I have used Texaco Type "A" on 5160 and 52100 with great results. It has become increasingly clear that oil temp is very important.

I used Brownells Tough Quench and had good results. But not with poorly forged blades. Funny angles or tight corners always seem to give me hell. My bad.

Peanut Oil is a media that I don't know well. But it stands to reason that the temp is imporatant.

As far as how you are heating the blade. It seems like maybe you are pretty close to the edge. Was it a half inch? I try to stay a third of the way up the blade and let the torch move the heat toward the edge and point. My last pass will bring the yip to heat. Like Mete said, heat is damn hard on the edge and tip.

Good luck! Breaking blades is something we have all done. I got a bucket full. Learned from every one of them. Some times I look at that old bucket and smile, and give it a kick.

Shane
 
Brian,

Good for you. It hurts alot to loose hard work but after time down the road you will remember the lose as something good.

Above all else, listen closely to anything Mete has to say to you or any of us concerning heat treating and all things relative to that.

I have been watching you from your beginning here. You are a rising star with great enthusiasm.

RL
 
I proudly admit to the statement that blades while cooling can remain non magnetic to a temperature that is too cool to harden when quenched. On the way up the magnet is an excellent indicator of the hardening temp. On the way down you have a different situation, just because the blade is still non magnetic doesn't mean it will harden when quenched. the greater the degree of grain refinement you have achieved through manipulation of the steel the more significant becomes variables like temperature of the steel and the quenchent. Try timing the events I call bladesmith normalizing cycles, when fine grain is present, each cycle down will take less time from bright to dark to bright again.

I strongly recomend that you either experimentally determine the quench rate of your peanut oil or take a short cut and let the ASTM folks guide you to a quenchant that has known properties. It is a terrible waste of time to allow a variable that is easily managed to discuorage your enthusiasm.

The photos you supplied are interesting, but hard to interperte. I see the hard cone pointing down, which could indicate either the upper portion over the edge was hot and the edge cool or the edge had been over heated and
no longer responds like it should, or maybe someting else. Keep this blade and try to explain it should it ever happen again.

Good Luck
 
You brought the blade up to critical on the edge and quenched. You thought to yourself....hmmm, let's do a multiple quench and see what happens. You immediately reheated the edge and quenched it again and CRACK. Thermal shock has now kicked your butt. The second picture on the first post shows that you did get it too hot. See all the little dots. Way too hot. :). Fun stuff huh?
 
shane- I am using 0-1

Thanks for all the input. I was surprised at how far up from the edge my file skated. This tells me that the steel is responding at a lower temperature than I thought. I will expariment with lower temperatures and see what I get. I know that the torch is a tricky way to learn but I do like a challange :D

I just love the idea of having soft spine and a hard edge all in one handheld tool. I dont have a forge yet so the torch is all I have plus the torch is an easy way to get a hammon which I like. I just need to perfect my ability and eye to know what is going on. I am sure I will update you all on my progress.

Thanks guys.
 
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