My Knife Fractured, need Metallurgic expertise

M

MoTRoN

Joined
Jun 18, 2016
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A couple of weeks ago i made a outdoors knife for myself out of O1 steel, and after quite a lot of abuse while batoning it snapped in half.
First i just thought i abused it to much, but then out of curiosity i did some research, and found that the fracture is probaply a intercristallin fracture.
These fractures, as i read, only occure due to grain boundry embrittlement. And this has only negativeconsequences for the steel.
If the steel had been heatreated correctly the fracture should have ben transcristalline, that usually results in a much smoother surface.

I have to admit that i quenched the steel in oil at about 1000°c meshured with a IR-Thermometer where it should have been somthing like 830°.
but if someone with the metallurgic expertise could tell me the mechanism at work i would be very greatful.

Further online research also told me, that there are only 2 reasons for such grain boundry embrittlement.
1. Hydrogen grain boundry embrittlement
2. Carbide grain boundry embrittlement.

Hydrogen grain boundry embrittlement happens when atomic hydrogen contacts the surface of a steel. this could have happend during heating, since i used a propane torch. one of the porducts of burning propane is water and that can react with iron to hydrogen and ironoxyde.
Nevertheless i think it is unlikly to be the cause for this faliure since a lot of people use this heating method.

Carbide grain boundry embrittlement can be caused by flawed heat treatment. So by quenching but also by tempering. (I tempered 2x 1h 200°c in a kitchen oven)

If someone could confirm that the error is defenetly in the to high quenching temperature, i would be verry grateful.

No matter what i think o learned my lesson on extreme batoning and listening to the temperature recomendations after all this was only my 5th knife.

Sorry for my bad english and by the way this is my first post here, so hello guys!

SOURCES:
https://tu-dresden.de/die_tu_dresde...rofessuren/wpc/risse_und_fraktographie_teil_1
http://www-e.uni-magdeburg.de/wiwf/...Schadensanalyse 2014 - 2015 - Vorlesung 9.pdf
https://books.google.de/books?id=-U...r+Schadensf%C3%A4lle.html?autr="Günter+Lange"
http://aquensis-verlag.e-bookshelf....r+Schadensf%C3%A4lle.html?autr="Günter+Lange"
https://en.wikipedia.org/wiki/Fracture
 
It is impossible for us to tell you exactly why your knife failed. It could be a non metallic inclusion for all we know, but because you did austenize O1 at such an elevated temp, that would be suspect. Carbides in grain boundaries can indeed cause embrittlement, but no way for us to really know if this is the issue. Carbides in grain boundaries can become problematic especially in hypereutectoids, especially if a slower than "still air" cooling was done prior to austentizing. Can you take some pictures of the fracture, especially at the place where the crack propagated from? BTW, Welcome to the forum!
 
Photos of the fracture would be helpful. It sounds like carbides formed in the grain boundaries due to the excesssive teperature .That makes forn a very brittle steel !
 
I would read a bit more regular metallurgy books and less "on-line" metallurgy. Most of your info isn't accurate, or mis-applied.

Your knife broke for two main reasons.
1) It was being abused beyond the scope of the steel type and HT. Even a good blade can break in batoning. A tougher steel, like 5160, and a spine drawn down to the low Rc50's will survive better.

2) It was heat treated wrong. O-1 needs good temperature control and a 10-15 minute soak to properly distribute the alloying. The greatly excessive temperature was the biggest problem. It created large grain growth and made a path for fracturing along the grain boundaries. The lack of sufficient soak would also make a weaker blade.

You did not mention the tempering specs, but that also can create a brittle blade.

If you look at the break, I bet it is sort of rough and crystalline looking - like coarse sand or broken concrete. A properly hardened blade will look somewhere between a frosty surface and fine sand or salt. If part of the break is dark and part is shiny, the dark part shows a crack in the blade from the quench.

Close up photos of the break would help. - OK, you just posted them, and they confirm the above.
 
You can even see where places were burned down from pits in the blade and created dark spots al, the way through the blade from the pit on the surface.

The grain is massive. Break a file in half and compare what the proper fine grain should look like.
 
I think you are on the right track, but it doesn't necessarily have to be an embritalment issue, it could simply be due to the fact that you have such large grains. Because you were able to tell that the fracture was inter-granular this tells me that you are able to see the grains with the naked eye which means they are far far to large. Large grains even without any grain boundary issues lower the strength and perhaps more importantly the ductility of the metal. If your temperature measurement is accurate you are way too hot which would definitely explain the large grain structure, and maybe the unexpected failure.
 
Don't worry too much about XYZ yet. Take care ABC: correct aust temp, quench, temper. Along with basic blade profile & edge geometry.

edit: time took me to post 1 line. Stacy published 2 book chapters :)
 
i tempered 2x 1hour at 200°c and i know that a less hard steel will be tougher, but my very limited research lead me to belive, that something was wrong despite the abuse. and i belive the dark spots are just dirt from the wood i was batoning.
 
Nebulae said:
it could simply be due to the fact that you have such large grains. Because you were able to tell that the fracture was inter-granular this tells me that you are able to see the grains with the naked eye which means they are far far to large. Large grains even without any grain boundary issues lower the strength and perhaps more importantly the ductility of the metal. If your temperature measurement is accurate you are way too hot which would definitely explain the large grain structure, and maybe the unexpected failure.
Click to expand...

Thx very much, this was what i wanted to hear. i noticed that the graines are very large, but wasn't shure weather it was normal or not. And i wanted to know for shure that the error was in the to high temperature.
but just out of interest, why does high temperature create big grains. and in my very small metalurgic education i was told, that large graines cause steel to be soft, because deformations aren't stopped as quickly on the grain boundris. in other words the defects of the steel can moove freely inside the big grains and allow for some movement.
 
a long shot - you can ask this ht junky on youtube for help: OJK Custom knives

He has quite a bit of ht experience with O1.
 
Well guys, thank you verry much for your quick and helpful answers. you will defenetly see more of me on this forum.
This was defenetly a lesson learned the hard way for me.
and i will post some SEM images next weekend if i remember to.
 
Motron, you are correct about the large grains being softer and weaker, this correlation between grain size and strength is called the Hall-Petch relationship. Smaller grains also increase ductility primarily because they impede inter-granular crack growth. High temperatures encourage grain growth because they increase the mobility of various flaws and grain boundaries allowing grains to consume each other. Grain growth is controlled by time and temperature, but while time is more of a linear dependence temperature is exponential so it is a much more important factor.
 
Hey guys, as promised, some SEM pictures of the fractrue.
http://postimg.org/gallery/267b0m7sy/
As expected, the condition of the steel is pretty horrific, with ginormous grains and cracks in between them.
surprisingly some grains are fractured in the middle, so it's not a 'pure' transgranular fracture.

also surprising is, that the edge, which felt pretty dull, is still below one micron, as far as i could tell. but i only looked at it from the side, so i'm not really sure about that.
 
Those are not good photos to illustrate the problem !! Good photos would show distinct grains ,shape completely defined along the fracture surface and you would also see cracks perpendicular to the fracture surface..That's brittle !!
 
Nice SEM images :thumbup: Cool you have access to SEM (and maybe BSED too)!

Your O1 microstructure is nifty messed up, especially cracks at ten of microns in length. As I mentioned before that OJK Custom Knife guy can help you with O1 ht. He commented on my today yt video - [video]https://youtu.be/HH_EYiL0zF8[/video]. Maybe ignore hrc, just consider how an edge at that geometry supports such chopping activities. Aaahh, I brough this up because I could send you a small 66+rc 3V paring (no handle) to keep & snap a few SEM & BSED pics :cool:


MoTRoN said:
Hey guys, as promised, some SEM pictures of the fractrue.
http://postimg.org/gallery/267b0m7sy/
As expected, the condition of the steel is pretty horrific, with ginormous grains and cracks in between them.
surprisingly some grains are fractured in the middle, so it's not a 'pure' transgranular fracture.

also surprising is, that the edge, which felt pretty dull, is still below one micron, as far as i could tell. but i only looked at it from the side, so i'm not really sure about that.
Click to expand...
 
Same question/curiosity raised - read here: 3V 65+rc
In this video - it's a working/testing and happened to at max rc 66+rc (~66.5rc). It would helpful for me to see a few BSED of my 66+rc 3V, so I can give a better shot at getting 67rc 3V.

butcher_block said:
i woudl love to hear more about thi 66RC i didnt think possible on 3v even right from quench
Click to expand...
 
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