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Getting under 25 HRC for my blades after quench


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I have been trying different things to fix this issue, but nothing seems to net me a blade higher than 355 HL, which is somewhere under 25 HRC.

My general routine for quenching blades is:
Normalizing once -- Heating to 2000F and letting cool in air

Quenching -- Quenching around 1800F to 1900F (i've tried as low as 1500)

Tempering -- one to two cycles at 450F for 2 hours

Any advice?

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First and foremost, Leeb is a less-than ideal test method (for anything, ever).  I am on record as noting that it is not actually a hardness reading, it is a bounce reading.  Are you sure that you are using the correct settings?  For any blade you should be using the C impact device, which still has a minimum part weight of 4 pounds.  Since your part is almost certainly less than that you need to couple it very well to something heavier, which is very difficult to do right.  Any blade worthy material is going to be difficult to get down to a 25HRC on accident, not going to happen when doing anything that you think should harden it.  

 

What you use for temperatures is going to be highly dependent on alloy, so you definitely need to tell us what you are using.  If you are doing anything other than a few specific stainless type alloys, 2000F is way too hot.  You need to normalize 3 times much closer to the critical temperature (50-100F above it).  Then Quench from about 50F above critical.  tempering should be 2 cycles at 1 hour each.  Temper temperature will be dictated by alloy and style of blade.  

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Theres a whole lot of variables there.  The best place to start is by asking what steel are you using?  That will narrow down temperatures and expected results.  For simple carbon steels, it sounds like your temperatures are way to high.  Which brings another question of, how are you measuring your temps?  

 

 

Never mind, Jerrod types faster than I do. Lol

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Sounds like you're trying to harden a low carbon alloy.

 

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Hello, thanks a lot for all the replies

I currently use mostly 1095

I was worried it had something to do with my hardness tester, so I tested my previous blades and they all had a lot of edge rolling

This is the meter im currently using for hardness

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14 hours ago, Tristan_stone said:

This is the meter im currently using for hardness

That says it comes with the D impact device, which requires a 15 pound weight minimum.  Not good for blades.  We use a type E impact body at work, which is pretty similar, so I am quite familiar with it.  Though we use a much nicer unit in general; that appears to be a $300 made in China unit, ours is a high end $6,000 Equotip 550 with a $3,000 impact body, and it would still not be very good for blades.  For those interested, the ASTM spec covering the Leeb impact test is A956; my most recent copy is 2012, but there is a 2017 version out there, too.  

 

1095 should be heated to about 1475F before quenching, maybe as high as 1500, but really no more than 1550 for sure.  You should even be able to get 1425 to work if you are really quick from heat to quench.  What is your quench medium?  A really fast oil should work for thinner blades to get full hardness, and an auto-hamon on thicker blades.  

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You say you use "mostly" 1095?  Is the blade in question definitely 1095?  What did you quench it in?  How long did it take you to move the blade from the forge to the quench tank? Also, where did you find those temperatures recommended for any carbon steel?

 

The thing with 1095 is that it needs a very fast quench, as in it needs to go from 1475 F to under 900 F in less than one second or it won't harden.  If you pull the blade from the forge and admire the color for the count of one, you've already blown it.  

 

The one thing perversely working in your favor is that 2000 degree soak.  That would have grown the grain to huge size, which increases hardenability. However, quenching from 1800 means you won't be getting it below 900F in under a second, either.

 

Finally, 450F is pretty hot to temper a knife blade.  For 1095 on a small blade I'd go to 325-350 max.  

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Sorry for the late reply, i've been out of town. 

Quote

You say you use "mostly" 1095?  Is the blade in question definitely 1095?  What did you quench it in?  How long did it take you to move the blade from the forge to the quench tank?

Yeah, I also work with 5160 from time to time which is why I said this. I quench it in oil. Though I'm not sure what kind, we bought it specifically for quenching. I get it from the forge to the tank in about a second or two.

 

Ill try quenching it at much lower temperatures and come back with the result. When you all measure temperature, what do you use. I've been using a rather innacurate temperature sensor and color, so im not even sure if that's right.
 

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You didn't mention how you are judging the temps. For 1095 table salt works well it melts at 1474 degf. normalize three times right around this temp to reduce grain size, then quench from the temp the salt melts. heating slowly helps keep everything heating evenly  turn the forge down and go slow.  1095 should be quenched in something like Parks 50, but In thin cross sections like a knife medium speed oils will work though you may get an auto hamon, in a pinch canola at heated to 125DegF or so should be fast enough for 1095. 

 

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The most important thing to note here is that Jerrod is completely correct about the hardness tester. You'll never get accurate results on something as small as a knife. 

We have an impact style hardness tester, similar to what you linked to (I believe its SPI brand) and a standard Rockwell tester at work.  One of the first things I did after I started working there, was test the same blade with both.  The impact tester simply will not give accurate results.  I tried holding it in a vise, holding it firmly against a granite table, etc.  Just isn't useful.  

I would be more trusting of Rockwell files than I would an impact tester for our knifemaking purposes.  

 

 

While you may not have blades as hard as you want, you certainly don't have 25 HRC blades.

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10 hours ago, Tristan_stone said:

Ill try quenching it at much lower temperatures and come back with the result. When you all measure temperature, what do you use. I've been using a rather innacurate temperature sensor and color, so im not even sure if that's right.

 

If you have a slow oil  designed for O-1 and 5160 like Parks/Heatbath AAA, taking more than a second to get from the forge to the oil will result in not hardening on 1095.  

 

For temperature, in the coal forge I go by decalescence. Look it up here on the forum, it's an infallible way to tell when it's time to quench. Be sure to use a muffle tube (a length of pipe or tubing with one end sealed) in the forge so you can see the steel.  Matt is right about common salt, though.  That's a good way to judge for 1095 and O-1, but not 5160.

 

In the gas forge I use a type K thermocouple probe and thermometer.  This is accurate to +/- 5 degrees F or so at 1500 F.  

 

The Evenheat oven has its own thermocouple, and it's programmable.  It's not something many beginning smiths have access to, however.  

 

Don't go by color, we all see it differently, and in a bright shop you can be off by 400 degrees or more.  And absolutely don't use a non-contact IR thermometer.  They do not work at incandescent temperatures.  In other words, if the object is hot enough to glow, the IR thermometer can't read it.  

 

For cheap (free!) and reliable, decalescence always works on non-stainless steels.  You don't even need to know the actual critical temperature, you can see the transformation happen in the steel.

 

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In addition to the things that have already been mentioned above, and which should be addressed first, another thing that could potentially throw you off here is decarb. 1095 will lose carbon from its surface unless heated in an oxygen-free environment (over heating and over soaking will exacerbate this too). You'll need to grind the decarb layer before you can accurately determine the hardness (this is separate from needing to clean up the scales).

 

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  • 3 weeks later...
On 3/12/2021 at 12:19 PM, steven smith said:

have you tried testing the hardness of your quenched blades with a regular file? 

So I'm getting ready to do my first blade and its from 1095.  I'm going through this thread to learn about heat treat and quenching.  I have 2 questions.  First how long do you leave the blade in the oil ((I'm using canola)?  Second how soon after quenching can you file test?  Does the blade need to cool or can it be done as soon as you come out of the oil?  Thanks for any help.  

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First, be sure the canola is heated to 130-150 degrees F to ensure it will harden the 1095.  Put the blade straight into the oil edge-first, taking care not to tilt it to one side. You can move it lengthwise in the oil if you must, but never side-to-side. Give it about three or four seconds, then pull it out and check for warps. If it has warped, bend it back straight before it drops below around 450 degrees F. (wear heavy gloves), then cool completely, or at least to below 100 F.  Water is fine at this point.  Wipe the oil off and try to file the edge.  

Ideally, the quench will have blown the scale off the edge, leaving clean steel.  If there is scale, scrub it off with a broken grinding wheel or something, then try to file the clean steel. If you have decarb, the file will bite for a couple of strokes and then start skating.  Once the file skates, congratulate yourself and put the blade into a preheated oven to temper.

This whole process (quench, straighten, cool, file test, put in oven) should take about a minute to a minute and a half.  Don't leave an untempered blade sitting around, they can spontaneously crack.

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A little warmer how much?  The idea is to lower the viscosity of the oil, which then transfers heat faster, which means you get full hardness out of shallow-hardening steels like 1095.  130-degree canola will pull the heat out of steel about twice as fast as room temperature canola.  There are other things going on as well, vapor jackets and convection and so on, but hot oil is a faster quench than cold oil in general.  You do reach the point of diminishing returns when you get over around 150 degrees F with canola.  You also get more flash fires with hotter oil, since it's closer to its ignition temperature.

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