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Hunter Lottsfeldt

Problem getting a full hardness using a kiln

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Hey so in preparation to hardening a 1095/15n20 damascus bowie knife I have been running tests on hardening small sample cut of pieces from the billet. I suspected I wasn't getting a full hardness from the quench which was reinforced by a 40-50c range on the rockwell hardness. I ran the test with an 800C (1475F) austenitizing temperature and quenched in cooking oil. Im wondering about some advice to getting a full hardness.

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What are the thicknesses of the coupons you are working with? They may be too thick to beat the pearlite finish line at the nose of the curve. I know that you have 15N20 in the mix but as I understand it the nickel doesn't have that much effect on the depth of hardening. Try forging a coupon down to under 1/8" and see if that gives you any better luck or just put an a rough edge on it and see how it holds up. You might also want to try a fast commercial oil or even brine.

 

Doug

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are you grinding after hardening - kilns are notorious for producing a thick de-carb layer? are you testing both the edge and spine? that mix in cooking oil from that temp will probably only harden to between 1/16th and 1/8th thickness, depending on normalisation...

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Hunter,

 

1095 could be giving you trouble on a number of fronts. Your mix of 1095 and 15n20 is still hyper-eutectic and I will just use 1095 for the sake of simplicity.

 

You have something like 0.5 seconds to get from the austenitization temperature (in this case 1475 Fahrenheit) to below the Ms (Martensite start) temperature. I have seen numbers ranging from around 500 degrees F to 1200 F for the Ms temperature of 1095. The point is, are you certain that the cooking oil you are using will cool the steel, that much, in less than half a second? This is the reason quenchants like Parks 50 or Tough-quench, from Brownells, are recommended for shallow hardening steels. Another, riskier, option is to quench into hot water for a few seconds and then into oil until cool enough to touch. You could also really take chances and just quench into water.

 

Next, are you certain that the steel was at the temperature the kiln was set at? This may sound elementary but, different heating methods take different amounts of time for the steel to reach temperature. Electric kilns are among the slowest. I am making the assumption that your kiln is calibrated and accurate.

 

1095 also requires a soak time, at temperature, once it has been reached. This is necessary with hyper-eutectoid steels, like 1095, to ensure there is enough carbon in the matrix to harden. Around 10 minutes should do the trick. If you are taking your steel, putting it in the kiln, waiting 10 minutes and then, quenching. The steel may have just reached the Austenitization temperature but, not had any time to soak, and not enough carbon is in the matrix to reach full hardness. There should be no appreciable grain growth at your austenitization temperature of 1475, leaving it in there longer will not cause an unwanted state in the steel but, not leaving it in long enough will.

 

~Bruce~

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Thank you all for your responses!

 

A little clarification, the cut off pieces being used are on the thicker side of a little under 1/4" but along with the cut off of 1095/15n20 I also put a little kitchen knife that is very thin that didn't harden as well. Though the knife I believe was 1075 (I'm not sure if the HT is analogous enough). Using the kiln actually produced very little scale. And the soak time was an hour. I know the nose is .5 seconds for 1095 but does 15n20 going to adjust what I should be doing for the HT? And I also will be taking a precise pyrometer down to test the kiln tomorrow, but given the amount of use it gets I think it should be precise. I also bumped the heat up to 1500 with an 1/8" sample piece of 1075 and still couldn't get a full hard.

 

My plan of action is to increase the size of my quench tank and try a low temperature oil quench. But if that doesn't work my next plan is to forgo the kiln all together and use the forge. I am still very unfamiliar with the kiln and think it might be better to stick with what I know better here.

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Hunter,

 

A few possibilities:

 

1) The kilns temperature is not accurate. Perhaps the lack of scale is an indicator of not reaching temperature? Did you check with a magnet before quenching?

2) Your quench is too slow or does not have enough volume. Lowering the temperature of your oil will not help, hot oil is less viscous and circulates more readily, cooling a blade faster than cold oil. If you are lowering the temperature of the oil because the volume is low, you may not cool the steel quickly enough during that crucial half second. You can easily check if the quench is the problem by quenching a test coupon into warm/hot water and seeing if it hardens. Be sure to use a test piece that, should it crack, will not upset you!

3) Has the decarbeurized layer been removed prior to testing for hardness? This was mentioned previously but, you did not mention anything in this regard.

 

~Bruce~

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An hour in a kiln with no scale does show you're not getting hot enough. Of course, if you were getting hot enough an hour soak might totally decarb a thin blade of 1095 anyway, since a kiln is the absolute worst atmosphere for heat-treating unless you put a lot of charcoal in it to provide a less-oxidizing atmosphere. Or run a slow inert gas purge, but charcoal is cheaper ;) . Remember the industry soak time recommendations are PER INCH of thickness. Adjust accordingly for lesser thicknesses. Personally I'd never soak 1095 more than a minute or two, but I know some people do it for up to ten.

 

The 15n20 won't affect the 1095 HT enough to notice.

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Ok, this is making me think that the kiln is off. There was some de-carb but very very little witch makes me think that the blade is not reaching critical temperature. I think to be safe I am going to increase the size of the quench tank and run an inert gas purge through the kiln as well (the place I am using the kiln makes this cheap and easy). And yes I am testing hardness on a polished surface.

 

Thanks Bruce and Alan! I'll update my findings as I get them.

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So I have some updated info. I ended up increasing the size of the quench tank, and using a gallon of canola oil at 130F I quenched some 1095 sample coupons. I also tested the kiln and its within a degree of accuracy. I quenched at the stated temperatures. I put the pieces in the kiln while it was at temperature, waited for the kiln to get back up to temperature ( on average about 3 minutes) and set my timer for 10 minutes.

(Took 3 Rockwell C tests of each sample)
Unhardened -- 22, 25, 28
Sample 1 at 800C -- 41, 38, 41
Sample 2 at 810C -- 45, 45, 44
Sample 3 at 825C -- 45, 45, 44
Sample 4 at 835C -- 65, 65, 63
So my test spawns a few more questions. When we refer to soak time we are talking about the piece at temperature, but how long is the actual time in the kiln? Do you preheat the piece? And also can someone explain to me the science of why the hotter oil quenches faster or point me in the right direction?
Thanks again for all you help! I am going to run another set of experiments to confirm my findings.

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How thick were your test coupons? How long it will take to heat up in the kiln is dependent on how big the sample is (thermal mass). This is why industry standard has heat treat soak times set at 1 hour per inch (or 1 hour for the first inch and 0.5 hours for every inch thereafter) with a 1 hour minimum. Basic steels do not need that soak time, but it ensures that the entire part is up to temperature. Pre-heating your pieces will help speed things along, but isn't really necessary. As a ball park for 1095 at those temperatures (BTW, Alaska is still the US, we use F not C! :P) 10 minutes is plenty of soak time to distribute carbon (dissolve carbides, and fairly homogeneous austenite) but another 5 minutes won't hurt to ensure you are at temp long enough. Make sure you've normalized properly before hand for grain refinement.

 

Soak_at_Temp_Effects_Reduced_Size.jpg

 

 

As to the hot oil working better: The heat transfer is is better for the material, as is the viscosity. You have probably noticed that with heat the oil flows a lot easier. This allows convection currents to more readily stir the hotter oil away from the blade, to be replaced by the cooler 130* oil. As this chart indicates, temperature is not nearly as critical what the material is (water, oil, polymer). Yeah, I know the chart uses C not F. :angry:

 

Quench_Oil_Temp_Effect.jpg

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So I got reliable results using 1/8 thick 1095 strips (2"x3/8"x1/8") I was getting an as quenched hardness of 64-65c. Though I tried hardening a piece of damascus (1095/15n20) the coupon is like an inch square and about a quarter inch thick and had at 825c for 10 minutes and then a 130f oil quench didn't harden it. I then tried upping the time to 15 minutes, nothing. I then upped the time to 25 and still got nothing. I didn't normalize in between. Is that the reason I was't getting a full hardness? I didn't think the piece would be that much greater volume. I'm kind of lost in what to try next... I also tried getting a tiny piece to harden to no avail using the same process as the 1095. So Im not sure what to try next. The one thing I noticed with the damascus pieces was a lack of decarb when I pulled both pieces out of the kiln. Is the 1095/15n20 mixture going to require a high temperature?

 

Thanks for the charts Jerrod! As for the mixing of F and C I do all my work in an on Campus Lab, so most measurements are in C. Is grain refinement necessary to achieve a full hardening?

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I think that you are still looking at the coupon being too thick to cool quickly enough to beat the nose of the cooling curve at the nose and it's crossing the pearlite/bainite start line. If you have fine grain size a shallow hardening steel may not want to harden much above 1/8-3/16". With you getting hardening with a 1095 1/8" thick it really makes me think that the damascus coupon is just too thick. Try forging or grinding it down to about 1/8" and trying it again.

 

The austinizing temperature of 825°C should be hot enough to get thorough austinization with a 10 minute soak. It's actually hotter than is usually recommended. Upping the time to something longer will only help you if for some reason the steel is not coming up to temperature. Time is less critical than temperature. Reviewing your test coupons for 1095 I would have expected full austinization with test #1. I hate to say it but I still think you have multiple problems here.

 

Doug

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Are you sure that your samples aren't cooling off too much between the kiln and the quench? I could see cold tongs sucking the heat out of a 1"x1"x0.25" chunk pretty fast, thereby making you miss your chance by the time it gets in the oil.

 

Your "failed" quench cycles are refining grains better than a normalize would, but also leaving higher stresses. Your soaks before each subsequent quench should be removing at least the cast majority of these stresses. A refined grain structure is necessary to achieve optimal properties, not just hardness.

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It may also be that your oil is simply too slow for 1095. With that alloy you have less than one second to drop the temperature from critical to below Ms, which for 1095 means removing heat fast enough to go from 1425 f to around 750 f in under one second. Try brine at room temp.

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Ok, I will try the brine quench and see if I can harden my sample. The weird thing I noticed was that the damascus sample didn't have any scale on it before I quenched it, unlike the 1095 which had a thick layer.

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Change to a mix of 1084/15N20 and most of your troubles will go away. Get some Parks 50 quench oil if you stick with the 1095 mix.

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Change to a mix of 1084/15N20 and most of your troubles will go away. Get some Parks 50 quench oil if you stick with the 1095 mix.

Absolutely.

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1. thinner sections (I couldn't harden a significant amount of a sword that I tried to ht prior to grinding. It was W2 and 1075 - which should be almost identical to 1095/15N20 mix in ht characteristics). It was in a kiln

soak 10 min

at 1480F

1/4" thick

3 trials

first quenched in room temp Parks 50

second quenched in 120F water

third trial quenched in 120F brine.

 

I wanted an autohamon, so I left it thick, thinking there would be a nice line of transition. The hardening happened a little, but it was sporadic, and counter-balanced by the decarb.

 

Just the slightest bit of hard skin on the outside is what I got.

 

I had to grind to thinner section, and then I got the amount of hardening that I wanted, in Parks 50.

 

My suggestion from this experience: use brine or Parks 50 or another fast quenchant.

 

Also, use knife-shaped cross sections and max thickness of just over 1/8" (if you are aiming at making a knife in the long run).

 

good luck!

kc

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Is it possible you decarburised the billet in the welding and forging process?

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Is it possible you decarburised the billet in the welding and forging process?

Possible sure, I don't believe so though. I m pretty sure I kept it between the proper forging temperatures when forging. My team mate I am working with did the forge welding with a very experienced smith here in OR, so I don't have a reference if they operated at too hot a temperature.

 

 

Change to a mix of 1084/15N20 and most of your troubles will go away. Get some Parks 50 quench oil if you stick with the 1095 mix.

 

I totally agree, I have been hearing that since I bought the 1095. I am defiantly going to try it next time I go to weld up a billet.

 

 

Thanks Kevin! Always appreciate the advice.

Edited by Hunter Lottsfeldt

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So I don't have all the data with me currently, but as part of an update. We ended up do the HT in the forge, for some reason when we did and quenched in canola oil that was fairly hot we got not glass hard, but still fairly hard (skidded across the blade for the most part). The problem we ran into though was a delam in the middle up the blade that puffed up like a blister. Our competition is almost over and afterwards I plan on making a more complete post on the events to let everyone know about the project and to thank those who have helped me and my team along the way. Look forward to seeing it in the coming weeks.

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