Plate vs. Oil Quench

[Mobius1]

Well, if my mind was made up, I wouldn't be here posting about this.

There's too much that I do not understand, which certainly gives me certain doubts about my process.

For example, as I said earlier, why is an interrupted quench recommended for oil? Once you get down to 900F, why pull the blade out of the oil? At that point it can't be cooling much faster than plates? And why would you want it to cool less rapidly once it reaches 900F? This is something I wish I understood. I can't know for sure when my blade is at the right temp to interrupt the quench, and I always wonder if quenching slightly too long or slightly too short makes any difference.

But then I also wonder - If air hardening steels have a maximum hardenability, and cooling them more rapidly really makes no difference, then why do people bother blowing air through their plates? If I can just slap together some simple set of plates and cool it down in a minute and a half vs 30 seconds, it's not like I'm losing a lot of time. There's got to be more to it than that, otherwise I doubt people would bother trying to plate quench as rapidly as they do.

 

[Hubert S.]

But then I also wonder - If air hardening steels have a maximum hardenability, and cooling them more rapidly really makes no difference, then why do people bother blowing air through their plates? If I can just slap together some simple set of plates and cool it down in a minute and a half vs 30 seconds, it's not like I'm losing a lot of time. There's got to be more to it than that, otherwise I doubt people would bother trying to plate quench as rapidly as they do.

I am curious about this myself as I have seen guys blow air between the plates on youtube. Does it actually change the cooling rate significantly or is it just something people do because they have seen it on youtube? Personally, I don't bother with the air as I cannot imagine it could have a significant effect and the blades get hard without it.

 

[A.McPherson]

Blowing air helps keep the plates cool if nothing else. this becomes important if you're doing more than one blade per session...

 

[HSC ///]

I don’t use air to cool . I just stick the plates in a bucket of water between cycles

 

[Willie71]

Length has just about everything to do with it. I never have much if any warp in small short blades no matter how I quench them. But once you get into the 9+ length it can be an issue. I have seen long thin blades come out of oil curved as much as a banana, not joking. bowing comes from uneven cooling and any stress or other things baked into the steel. A lot of times heat treatment will remove stress but if there is something funky going on inside like alabama damascus it will bow. Another thing is how are you pulling a 1725-2050f blade out of the oven and cutting off the foil and pulling out the blade without bending it. And then getting it into the oil faster then slamming it between 2 plates. I'm not saying it cant be done but it is tough and especially so with thin blades. I have some .0395 thick aebl that might change your mind about oil quenching and not getting bows lol. it's so sensitive that when I tried to gas shield instead of foil i would find weird warps on the tang. come to find out the cooling effect of the tongs grabbing them caused it to bow.

I also don't see how oil quenching on air hardening steels has so much less retained austenite then plate quenching that your that much harder. Maybe if your not giving the plate quenched blade a proper cryo treatment but that's a must for just about any stainless or high temp carbon steel. Also we know from papers and other research that you set up your toughness you want in the austenite phase. I think it was M4 i was reading about that stated that a blade austonited at a lower temp and tempered to say 60rc would be tougher then the same blade austonited at a much higher temp and tempered down to that same hardness. What i'm saying is final tempered toughness is affected by the temp it was austonited at.

This is exactly why I plate quench rather than oil quench. Most air hardening steels are fine with just cooling in air, but certain steels like m390/20cv, z-wear, V4e do better with an accelerated initial cooling, then slowing the rate down. I have been confident with my thin knives plate quenching, but if I regularly did over 1/4”, I would look into different processes. I would find a way to plumb an inert gas into my oven, or do a high temp salt pot. You can get them for high alloy steels with the right salts. If I went that far, I’d get set up for marquenching into salts.

I have some steel here I can do coupons for, and quench with plates, air, and oil to see how they respond and send them to Larrin.

 

[Mobius1]

Ok, so from what I have gathered after doing some more reading, is that the quench is interrupted just before the martensite transformation stage, because it allows it to more uniformly cool and results in greater toughness (If I am understanding it correctly.). If you interrupt too soon (Before it gets down to 1000F), then you can end up with more pearlite. So ideally you want to interrupt the quench just before 900F but below 1000F.

Honestly, I've often thought of trying to quench in salt, to assure that I am not improperly interrupting an oil quench. But unless I can toss a container of salt in my second oven at 900F and it be good enough, it sounds like a bit of a complicated setup.

This is exactly why I plate quench rather than oil quench. Most air hardening steels are fine with just cooling in air, but certain steels like m390/20cv, z-wear, V4e do better with an accelerated initial cooling, then slowing the rate down. I have been confident with my thin knives plate quenching, but if I regularly did over 1/4”, I would look into different processes. I would find a way to plumb an inert gas into my oven, or do a high temp salt pot. You can get them for high alloy steels with the right salts. If I went that far, I’d get set up for marquenching into salts.

I have some steel here I can do coupons for, and quench with plates, air, and oil to see how they respond and send them to Larrin.

I'd love if someone could do some testing! I'm waiting on a brand spanking new heat treat oven, and I am trying to get my hands on a rockwell tester, and then I will probably do some testing myself.

 

[Willie71]

Ok, so from what I have gathered after doing some more reading, is that the quench is interrupted just before the martensite transformation stage, because it allows it to more uniformly cool and results in greater toughness (If I am understanding it correctly.). If you interrupt too soon (Before it gets down to 1000F), then you can end up with more pearlite. So ideally you want to interrupt the quench just before 900F but below 1000F.

Honestly, I've often thought of trying to quench in salt, to assure that I am not improperly interrupting an oil quench. But unless I can toss a container of salt in my second oven at 900F and it be good enough, it sounds like a bit of a complicated setup.



I'd love if someone could do some testing! I'm waiting on a brand spanking new heat treat oven, and I am trying to get my hands on a rockwell tester, and then I will probably do some testing myself.

I’d have to check the datasheets again, but this sounds about right, you would need two salt pots, preferably three to do this right. One would have to be good to 2100f or a bit higher for steels like M4 or m390. This would be a multi thousand dollar setup. The benefit would likely be quite minimal in knife profiles/thicknesses. For turbines and other complex shapes, this would be the way to go.

My reliable process is profile, heat treat, cryo, low temper, grind, finish.

I use more belts, but I get better performance this way compared to any other I tried. I could not get steel out of the foil fast enough to oil quench. Quenching with the foil on in oil did not work. Couldn’t get full hardness.

 

[Stacy E. Apelt - Bladesmith]

Mobius1,
You have it a bit wrong. The pearlite nose is around 1000°F. We usually use 900°F as a safe point below the nose. This has to be reached in the time needed for the steel type. In steels like W@ and 1095, the time is around 1/2 second. In steels like O-1 it is 30 or more seconds, and in stainless and high alloy steels it is in tens of minutes to nearly an hour.

The martensite start point (Ms) is much lower than the pearlite nose. In carbon steels it is around 400°F. The martensite finish (Mf) point for carbon steels is between 100°F and 200°F. In stainless and high alloy steels the martensite start point is still around 400°F, but the Mf point is usually near -95°F. At room temperature these steels are still fully austenite.

In a stainless steel there is no need to interrupt the cooling rate from the quench. The quench doesn't need to be fast. You could just take it out of the oven and hang it in the air and it would harden.

I did a long butcher's knife done in .100" CPM-154 for a friend today. Blade was 14" and hidden tang was 6". I took the foil packet from the oven and put it in the aluminum quench plates for about 2 minutes. It was cool to the hand when removed. It was also dead straight. It went into a pan of dry ice and alcohol for about 10 minutes. Time is not significant. Just give it enough time to drop to the bath temperature. It was removed and taken inside to bake in the oven at 350F for two one hour sessions. It is still dead straight and flat.