Retained austenite and impact toughness

samuraistuart

samuraistuart

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Interesting paper I came across titled, "Effect of Heat Treatment on the Microstructure and Mechanical Properties of Sintered Stainless Tool Steel", easily found on the web. Elmax was the steel studied. The paper shows Elmax being hardened from 1975F (1080C), and quenched in either oil vs compressed air, sub zero vs no sub zero (-100F, not cryo), low temper vs high temper. If you need impact toughness, looks like the best route to take is to skip the sub zero steps, and use low temperature tempering. For example, oil quenched with 350F temper, no sub zero, the impact strength was 72.3 J/cm3, with sub zero dropped it down to 33 J/cm3. The higher the tempering temp, the lower the impact toughness. Compressed air quench also was favorable to impact toughness over oil quench, as a little more retained austenite was left in the mix. 88.3 J/cm3 (350F temper) no sub zero compared to 35.3 J/cm3 with sub zero. Makes sense, but the spread was much greater than I thought.
 
Stuart, I know yo know most of this, but allow me to make a statement here for other readers:

One problem with metallurgical reports and charts is that the testing is rarely aimed at knife blades. It is generally for industrial cutting tools or aerospace parts. Impact resistance on a knife blade is vastly different that the requirements for other industrial purposes. Knifemakers spend far too much time worrying about % of RA and all sorts of microstructures, when the predominant factor of edge retention and impact resistance is in the edge geometry. A great HT with a bad edge may test worse that a poorer HT with a proper edge.



My advice to newer ( and older) knifemakers is to not spend so much time on specialized HT parameters. Use the standard proven methods and numbers and get the maximum hardness and martensite content. This will be the platform for a good blade (Really - you can't improve on it much more than a tiny amount by tweaking the HT).

1) Do the quench as closely controlled in time and temperature as possible with your equipment and immediately temper. If sub-zero or cryo is an aid to the steel type, do it immediately upon the blade reaching room temperature after quench, then temper immediately after the steel warms back to room temperature.

2) It is important to temper properly to get the desired hardness. I bet 80% of blades made by knifemakers are actually too hard. A Rc of 60 is HARD. Unless the blade is a dedicated slicer, a Rc above that is unnecessary. It will only shorten the edge life. For a steel like 1084 and 1095, 450°F is about perfect for the temper, with 500°F being a great temper for a tough use blade. I read all the posts where a blade was tempered at 300F and can tell you that the edge will have tiny chips with any real use. Up until about 10 years ago, Rc 58-59 was considered a great hardness for a user knife .... and it still is. If you carry a commercial knife, the odds are that the blade is only Rc 56-58. Up until the 1960's, most knives were only Rc54-56. As a boy, there were few good knives that I couldn't sharpen with a file. Better steel and better equipment has pushed that up, but there is no need to go beyond the practical limit.
It is like putting a 400HP V-8 engine in a mini-van - Yes, it will go faster quicker ... but it won't get you there any sooner than a small six cylinder engine will at 70MPH on a long trip. All it will do is waste gas and money. It will also tempt you to exceed the parameters of the road (both legal and physical) and may end up costing you a ticket, or wrecking the car.

3) Once the steel is properly hardened and tempered, get the right geometry. Most new maker blades have an edge that is one of two things - far too thick or far too thin. Making a hunting knife from 3/16" steel means the edge will have to be very small before the final sharpening to get any good cutting action. 1/8" would be just as strong and easier to cut with. 1/16" would cut like a laser. Measure some popular commercial blades that are sold by the millions .... very few are over .065". The opposite happens when the edge is taken down to a razor edge and ten a tiny sharpening edge added. This will be so weak that it chips, rolls, or just dulls quickly. It has no strength.

4) Lastly, the magic word in edges - GEOMETRY!!!!
Geometry is what cuts. I can sharpen a soup can to do surgery with, and sharpen a bar of welding steel into a wood splitting maul. If the geometry is right, they will work right. They may not last as long as one made from proper steel, but they will do the designed task perfectly. Pick the edge shape and angle right, and the blade will cut right. Choppers at 10° per side and skinners at 20° per side don't work as well as the reverse. A convex edge is great for chopping, and a FFG to a near zero edge is great for slicing. Again, the reverse does not work as well.
 
IME, RA provides bulk impact toughness but only good for 1 time edge impact toughness. Once displacement/slipped occured on edge (RA -> Mart : work hardening), next damaging impact would fracture the apex. I guess scope of damage would depend on geometry & magnitude of impact. When RA% exceed percolation threshold, toughness will greatly increase in step function curve.
 
good stuff Stacy. i would add pick your steel to match the knife's job. O1 and 1.2519 do very well at Rc 62-64 and do very well as fine edge kitchen knives. to temper these steels to less hardness IMHO is a sin. use 1084 or 80CrV2 for knives that will be tempered to Rc 58-60
 
I think hardness should be based on the steel and the application. W2, 52100, and O1 all do very well around Rc62. They have extra alloying that helps with toughness and edge stability. 1084, 1075, 1080 don't have the benefit of the extra alloy, so they need to be run a bit softer.
 
Stuart, please don't give up :)

I apologize for placing a post not directly related to your point.


For the others, Stuart was trying to get a discussion going about the large difference in the results for different HT regimes on elmax.
 
Very useful information. In My experiences I found that hardness about 58 Rc is far enough for good knife. Thank You for confirmation. :) I agree with Stacey about geometry, it will do a lot. Especially on Elmax is blade prone to chipping at about 60+ Rc.
 
Thanks Stacy! I just came across the PDF online, thought it was interesting, thought others might want to check it out. I have always thought that one should reduce RA as much as possible, regardless of the type of knife you are making. But it "may" be that impact knives could benefit from some RA, whereas knives that need to excel at wear resistance over impact "might" need to have RA reduced as much as is practical. For the sorta new comers to knife making, let me be clear, retained austenite is so far down the list of how to make a great knife, please don't get hung up on RA. My post was menat sorta like, "Hey you geeks and nerds like me, check this out". Geometry is by far the #1 thing to understand, and a solid heat treat practice is #2, maybe steel selection a WAY distant #3, and RA reduction is probably something like #38. My experience is limited to carbon steel, and the HT I/we use, there is essentially no RA to worry about to begin with. Certainly only when you jump up to a steel with enough alloying like A2 and above, RA may/may not be an issue, but I've read that A2 should probably get the sub zero treatment.
 
Jesenius said:
Very useful information. In My experiences I found that hardness about 58 Rc is far enough for good knife. Thank You for confirmation. :) I agree with Stacey about geometry, it will do a lot. Especially on Elmax is blade prone to chipping at about 60+ Rc.
Click to expand...

Is it ? All my elmax knives are 60+ and slicers (0,2-0,3mm) behind edge and of course tested on hard boar bones and deer horn...no chips at all, same with D2 cryo , so doubt your HT.
And interesting topics :thumbup:
 
This makes me think of the industry heat treat for 52100. Larger grain, larger carbides, more retained austentite. In a bearing, wear resistance and toughness have higher priorities than fine edge stability.
 
Jesenius said:
Very useful information. In My experiences I found that hardness about 58 Rc is far enough for good knife. Thank You for confirmation. :) I agree with Stacey about geometry, it will do a lot. Especially on Elmax is blade prone to chipping at about 60+ Rc.
Click to expand...
I have no personal experience with Elmax, yet (this year I did make myself a hunter from Elmax, very thin of course, heat treated by Peters to 62HRC, and will have more to comment about it once I use it well), but have heard by others who HT Elmax all the time and have done extensive testing with it, that the performance is not there until it reaches 60.5HRC. 58 Elmax, from what I hear, is nothing compared to 61+ Elmax.
 
chumaman, samuraistuart: Maybe I do something wrong, maybe I am too careful with Elmax. I am just in testing stage with this steel but I do little progress every time (I hope). I do testing My knives by hitting to antler or brass rod (usually brass rod) and Elmax has always some chipping except last one (unfinished yet). By specification Elmax is not developed as a true blade steel, it is made for perfect industrial forms. I never tried 2100F HT temp as I read recently here on forum. In material specification is highest temp used 2010F so I am really curious about HT above that. If I figure out antiscale protection I will surely try it.
 
Is it? http://www.bucorp.com/knives.htm ...but on the one hand you are right most of the steel has been developed for industrial use .:thumbup:

And you say so....try better foil (309), 1150 C , LN etc...You have too much to improve :thumbup:
 
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There is an important distinction that I think should be made between impact toughness (joules of energy absorbed in a fracture impact) and edge durability in impact. It's tempting to look at the one number and ascribe it meaning that isn't always applicable. For example, mild steel has excellent impact toughness but doesn't hold up well as a knife. And a relatively soft high carbide steel may have good impact and abrasive wear numbers, but a mushy crumbly edge that fails at the slightest clack in a knife. The numbers mean something, but they don't always mean what we think they do.

RA may be beneficial in increasing ductility and toughness in a stamping tool application where sections are thick and geometry is obtuse. But RA does not improve the durability of an edge in a knife. Quite the opposite.

Current metallurgy text show more stabilized RA in common materials than once thought.

I wouldn't speak too broadly about appropriate Rockwell hardness, but in many modern materials a hardness of 62 is not fragile (a thin hard blade tolerates prying and can be hammered through almost anything without breaking) so reducing hardness simply reduces strength and edge retention with no meaningful benefit.

I wouldn't want a chopper in a steel much under 60. The softer weaker material ripples and buckles in chopping, requiring a thicker edge and obtuse primary grinds. A modern steel with an appropriate heat treat can and should be 60 or more in a chopper application.

Also, edge retention goes up with hardness until you reach a point of low ductility and you see the chippy edge. A modern steel with a heat treat optimized for a hard thin application doesn't shine until you're over 61 and many times your sweet spot for edge retention and edge stability (particularly with thin geometry) will be in the neighborhood of 62 or even 63 and I wouldn't discount those going even higher. This is my experience.

If you're a hobbyist, that's one thing. But if you make and sell knives, I think you should push yourself and don't just settle for an industry standard HT that was likely more concerned with reducing dimensional growth and risk of cracking in weird sections than supporting a knife edge. Particularly if you're working in complex modern steels.
 
samuraistuart said:
Interesting paper I came across titled, "Effect of Heat Treatment on the Microstructure and Mechanical Properties of Sintered Stainless Tool Steel", easily found on the web. Elmax was the steel studied. The paper shows Elmax being hardened from 1975F (1080C), and quenched in either oil vs compressed air, sub zero vs no sub zero (-100F, not cryo), low temper vs high temper. If you need impact toughness, looks like the best route to take is to skip the sub zero steps, and use low temperature tempering. For example, oil quenched with 350F temper, no sub zero, the impact strength was 72.3 J/cm3, with sub zero dropped it down to 33 J/cm3. The higher the tempering temp, the lower the impact toughness. Compressed air quench also was favorable to impact toughness over oil quench, as a little more retained austenite was left in the mix. 88.3 J/cm3 (350F temper) no sub zero compared to 35.3 J/cm3 with sub zero. Makes sense, but the spread was much greater than I thought.
Click to expand...

So the higher the tempering temp the lower the toughness? Even in the 700 F range? So 350 F with both an air and oil oil quench is something of a peak in toughness for Elmax, and it doesn't recover the toughness below the standard embrittlement range of around 800 F found in most all martensitic stainless steels?
 
Higher the tempering temp the lower the toughness? I think so. That is my understanding, in a general sense. The higher the tempering temp the more wear resistance due the the carbide precip, but toughness suffers at higher tempering temps. Ask Nathan. I think he figured that out during his testing as well. Better apex stability using the lower temps anyway. Peak toughness for Elmax would probably be at it's max attainable hardness, I "think".
 
I just finished reading the article. Looks like the notion that softer steel is tougher doesn't hold again. It's also very interesting that the air quenched parts have lower RA than the oil quenched specimens. So, air quenched I'd tougher and has less RA. Contradictory?
 
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