Issue with a 52100 blade

This is like a thread on ex-wives. It's important to know, but trying to understand will just make your head hurt. :biggrin:
 
Doug Lester said:
Ok, Ed, I do understand your position that you are progressively adding more carbon into solution to form adequate martensite to harden the blade. My concern is that with each subsequent quench you are also stressing the steel and forming more microscopic cracks with the plate martensite, which is 100% with the carbon content found in 52100. My feeling is that it is better to shorten the soak time, do it once, and stress the steel once with a single quench. Of course this would take a scanning electron microscope to test completely.

Doug
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"slow quench oil" Ed Fowler
 
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Ed, how was it determined that the last blade you tested broke along the grain boundaries? I ask because I have not seen any information that does not point to grain boundary fracture as an indicator of a problem, at least in knives and related ambient temperature uses.
 
Lerch,

sounds like your blade did not reach full hardness during the quenches, or your tempering oven is WAY off if you tempered it at 350 and the edge stayed bent during your brass rod test.

you said that you quenched 3 times back to back. were you doing just the one knife or several? what was the temp of your oil at the time of the last quench? could be that your oil was too hot to cool the steel right. also how old is your canola? could it have broken down over time and not be quenching correctly? or could it have been contaminated?

you said that your blade was ground thinly. what did the scale pattern show after you quenched? I had a problem for awhile when I was leaving the edges too thin in an effort to save on grinding after heat treat. what I found was that by leaving extra meat on the edge, it not only protects the future cutting edge from carbon loss, but it helps to keep the edge at the proper temperature during the trip from the forge {or kiln or torch} to the quench. if the edge is too thin, it may cool enough during that trip that the edge may not harden like it should. a tell tale sign of that is your scale pattern. if there is some scale hanging on to the blade at the edge after the quench, but not higher up the blade, then the edge cooled to below the hardening temp before it was quenched. even though that trip may be short, going from a nice warm kiln to room temperature at even a 100 degree day is quite a shock.
 
Lerch said:
Hi guys

I made up 3 little EDC skinner knives for me and two buddies of mine for a recent hunting trip. The knives worked well on the one buck we killed but after we have been back i just dont feel like these 3 knives hold a edge as long as they should.

HT info is as follows

Normalized by heating to 1675deg and air cool, then 1550 deg and air cool, then 1475deg and air cool.

Triple quench by heating to 1490deg and holding for 4 min then edge quenching in canola oil heated to 150deg. I did this process 3 times in a row with the blades cooling in the oil, i did not let them rest over night, all three heat treats were done back to back.

I tempered them twice at 350deg.

I tried to do a edge flex test over a brass rod and after having to push VERY hard i think the blade has flexed just a little but not gone back to straight, there seem to be just the smallest bend to the cutting edge.

A previous knife i had done a triple quench with while letting it rest overnight in my freezer in between each heat treat seems to hold a sharp edge for ever. and a single quench blade i did holds a great edge, i chopped down about 15 cedar limbs and it would still pull hair afterwards.

These knives just dont seem to hold the same edge as the previous blades. all of these knives are 52100 steel from ALdo. On these 3 knives i ground them down a little thinner than i normally have before heat treat, could this be carbon loss on the cutting edge ??

any ideas ??

thanks
steve
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Joe Calton said:
Lerch,

sounds like your blade did not reach full hardness during the quenches, or your tempering oven is WAY off if you tempered it at 350 and the edge stayed bent during your brass rod test.

you said that you quenched 3 times back to back. were you doing just the one knife or several? what was the temp of your oil at the time of the last quench? could be that your oil was too hot to cool the steel right. also how old is your canola? could it have broken down over time and not be quenching correctly? or could it have been contaminated?

.
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That's an interesting question, if you are doing three blades and ttipple heat and quenching them, that's nine quenches.
What's the volume of your oil? especially if you are using a shallow edge quench container.

I see you fully heated the blade in a kiln, then only edge quenched.

I suspect that maybe the heat retained in the spine may have travelled down to the edge and auto tempered the edge

I'm only guessing, but looking for the simplest possibilities.


I'd much rather full heat, full quench and differentially temper or a full homogeneous temper
As I understand it a hardened blade is stronger than a full soft blade and that is more important to me than having one that bends easily.

I'm guessing you followed this method because you are following EF's advice - ( the 52100 choice, heating and quenching three times...)

If so then why not torch edge heat like he does ? You would be guessing at the temps, but also avoid that auto tempering effect. ?
 
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ME2: I am sorry for not responding to your question sooner, just noticed it today.

Rex is the man who who brought this to my attention. When evaluating a blade failure he saw the tear along the grain boundaries, it takes a microscope to see it and a knowledgeable man to read what he sees.
A break is different because the fracture runs through the grains.

The finer the grain you can develop in a blade the more likely it will tear instead of break, thus potentially providing a hardened and properly tempered blade that is tougher. The way I understand it, the tear has to travel further as it proceeds around the grain boundaries rather than traveling in a straight line through the grain.
 
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Interesting. I'll have to investigate this further. Everything I've read up to this point has pointed to some sort of brittle failure when grain boundary fracture occurs. Impurities that segregate to the boundaries, prior austenite grains with carbide not fully disolved, liquid metal embrittlement, excessive carbide precipitation, brittle fracture surfaces in Charpy specimens, and more, lead to or indicate grain boundary issues. Grain boundaries tend to be pretty active, as they have room for things that won't fit into the more ordered arrangements inside the grains.
 
I think it has something to do with the differences between, intergranular cracking, cleavage fracturing,... and “quasi-cleavage”, which may sometimes be referred to as a “tear“, and I think it is associated with fine grain sizes. You’d have to check on it for yourself though.
 
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... as a side note I think the word "tear" might also be associated with high temperatures or "hot fracturing", which is in turn is associated with trying to hot forge a part too fast or too hard.
 
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Edited by Kevin R. Cashen. I am more than happy to disscuss any topics that pertain to this forum, on this forum. But linking and dragging conversations form other sites here in order to stir up trouble I will not tolerate. What I or anybody else does on other sites isn't a Knifedog issue.
 
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So when is a fracture not a fracture? I guess it's when it is a tear. So I guess that from that point of view a pane of glass actually tears when when struck with a hammer and, conversely, a sheet of paper breaks when an edge of it is pulled in opposite directions at the same time. I would imagine the two terms, in relationship to material science, do have distinct meanings. Still I don't see how intertwined fibers being pulled apart equates to atomic bonds between crystals becoming unbound due to the application of force. I think that stating steel tore rather than broke is modifying the normal use of the term to make it look like something happened that didn't. It's like saying that an African Violet didn't die when it's owner failed to water it for a month, it merely failed to go on living. As a rose by any other name still makes me sneeze.

Doug
 
Hey again Ed. Do you have any micrographs of what Rex described? Did he say anything aboutseeing differences between the different regions/microstructures of the blade?
 
Kevin Gray's work will be published in the future, right now we don't know when, I will post where it is published when it comes out. It is a very inclusive report, but it is Kevin's, not mine.
 
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