More 1084 weirdness

[DevinT]

You know what I think what happen here ,well one of the possibilities? JT has long HT oven with heat elements/coils/ only on top of oven ....looking in this numbers on blade , tip of knife/deep in oven/ HARD ...then SOFT ..then HARD ........maybe ,maybe blade was hold to short in oven so HT elements heat blade in spots /zebra type/ in right temp , while rest is still not on right temp? Bad explanation but I think it is understandably what I think ?

Nope, not possible.

Hoss

 

[Cushing H.]

Nope, not possible.

Hoss

Hoss - why not possible? Understood the thermal conductivity of steel is pretty good ... but radiative heat thansfer at high temps is dominant..... not really doubting you, but hearing the reasoning would help...

 

[DeadboxHero]

Hoss - why not possible? Understood the thermal conductivity of steel is pretty good ... but radiative heat thansfer at high temps is dominant..... not really doubting you, but hearing the reasoning would help...

Because you're not grilling the knife directly on the heating elements in open air.

 

[DevinT]

Hoss - why not possible? Understood the thermal conductivity of steel is pretty good ... but radiative heat thansfer at high temps is dominant..... not really doubting you, but hearing the reasoning would help...

The insulation holds too much heat and those heating coils are not on continuously.

Hoss

 

[Cushing H.]

The insulation holds too much heat and those heating coils are not on continuously.

Hoss

Makes sense. Thanks

 

[Mecha]

You know what I think what happen here ,well one of the possibilities? JT has long HT oven with heat elements/coils/ only on top of oven ....looking in this numbers on blade , tip of knife/deep in oven/ HARD ...then SOFT ..then HARD ........maybe ,maybe blade was hold to short in oven so HT elements heat blade in spots /zebra type/ in right temp , while rest is still not on right temp? Bad explanation but I think it is understandably what I think ?

I discussed this type of phenomenon with an ABS master smith, who had seen such a thing happen due to the rack holding a blade in a HT oven. I had suggested what you said, above. I don't think the differences in hardness were nearly as wide as in JT's case here, but the coils and blade racks CAN noticeably affect blades' heat treatment imo.

 

[Natlek]

I discussed this type of phenomenon with an ABS master smith, who had seen such a thing happen due to the rack holding a blade in a HT oven. I had suggested what you said, above. I don't think the differences in hardness were nearly as wide as in JT's case here, but the coils and blade racks CAN noticeably affect blades' heat treatment imo.

That was my thoughts to , he HT lot of steel .I don t think that he HT one by one knife so I suppose that he has some kind of rack inside ,maybe double deck one . When HT elements are ON they radiate max. heat , right ? And that is about how much 1200-1300 Celsius ? And maybe ......again , maybe he pool out wrong blade to early and maybe again that blade was close to coil , so ..........................
I try to find simple answer what happens ...It is hard to believe that steel is wrong in this case , can t see how that would work ? But what I know about steel

 

[Larrin]

With a thermocouple in the center of a furnace I can't measure the difference between the coils being on or off.

 

[Mecha]

With a thermocouple in the center of a furnace I can't measure the difference between the coils being on or off.

The oven is maintaining a stable ambient temperature, but the coils must cycle on and off because when on, a coil is much hotter than the oven's temp. The inverse square law applies here, and I could see how a blade being near the coils, perhaps the outermost blade on a rack of them, might absorb a localized spike in heat every time the coils cycle on, as Natlek suggests. Whether or not that affects a blade, I'd imagine that has a lot to do with alloy, thickness, etc.

Inverse square law for our readers:

http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/isq.html

Good stuff.

I wonder if the pattern on JT's blade matches the uprights on a rack, or the coils themselves.

 

[Natlek]

With a thermocouple in the center of a furnace I can't measure the difference between the coils being on or off.

I have access to extremely expensive and precise thermal camera , I know one place where is one HT oven build like yours in USA and I will take pictures /camera can store them/ You'll be surprised what you'll see .............rainbow is inside . I think that your next article you write should be about HT ovens.....

 

[Natlek]

The insulation holds too much heat and those heating coils are not on continuously.

Hoss

Oven you build in USA are poor quality . How much temp . drops when you open door ? How long after that HT elements are ON ? For the god sake you have not seal on the door for reason only you there know ... difference between front and back on that kind build oven is huge ...............

 

[Larrin]

My oven is lower by 20F about 2 inches from the door. A perfectly reasonable thermal gradient if you ask me. I've used very expensive laboratory furnaces and they have similar thermal gradients. Oftentimes worse for the large furnaces. The "problem" of radiant heat from coils is way overblown in my opinion. Using the method recommended on this forum of setting the temp too high to avoid coils coming on just led to all kinds of temperature inconsistencies. Letting the furnace stabilize at the appropriate temperature before opening the door worked much better. I dont put anything right next to the coils so I can't comment on that.

 

[Willie71]

My oven is lower by 20F about 2 inches from the door. A perfectly reasonable thermal gradient if you ask me. I've used very expensive laboratory furnaces and they have similar thermal gradients. Oftentimes worse for the large furnaces. The "problem" of radiant heat from coils is way overblown in my opinion. Using the method recommended on this forum of setting the temp too high to avoid coils coming on just led to all kinds of temperature inconsistencies. Letting the furnace stabilize at the appropriate temperature before opening the door worked much better. I dont put anything right next to the coils so I can't comment on that.

I’ve checked my over with several thermocouples, and it’s hotter at the front by 10f, and lower at the door by 25f.

if I open the door within 100f of target temp, I get all sorts of temp variations, but if I let it equalize for 10 min at temp, it stays consistent. My recommendation is learn your equipment, and use it appropriately.

 

[Cushing H.]

The oven is maintaining a stable ambient temperature, but the coils must cycle on and off because when on, a coil is much hotter than the oven's temp. The inverse square law applies here, and I could see how a blade being near the coils, perhaps the outermost blade on a rack of them, might absorb a localized spike in heat every time the coils cycle on, as Natlek suggests. Whether or not that affects a blade, I'd imagine that has a lot to do with alloy, thickness, etc.

Inverse square law for our readers:

http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/isq.html

Good stuff.

I wonder if the pattern on JT's blade matches the uprights on a rack, or the coils themselves.

Ummm, for radiative heat transfer, the coefficient is T^4 for line of sight. I tried to get at this earlier, bu nobody bit. Again, i am not sure if this is the case but think it is worth considering. I also think it is worth considering the experience of that ABS master

 

[DevinT]

We need to send that blade to someone with a salt pot and see if there is still a variation in hardness.

Hoss

 

[JTknives]

So just to clear up a few things, we let the oven equalize for a minimum of an hr after it hits temp. We don’t use a rack in the oven, we use stainless angle iron and put a blade on each side and it slightly leans on it. I have data graphs on every heat treat I have done showing the soak and load times. The oven is remarkably stable. If I open the door and close it the temp hardly drops. If I put a few blades in it takes less then a min for the TC to stabilize and a few more minutes for the blades to hit temp. These 1084 blades got a 1600° soak and air cool. Then a 1500° soak for what I would guess was 15min and quenched into 60° parks 50. Here is the graph data on the heat treat process.

normalizing soak, all blades at once (in stainless foil pouch)

Here is the actual heat treat section. The real large dips are loading blades.

 

[DevinT]

So just to clear up a few things, we let the oven equalize for a minimum of an hr after it hits temp. We don’t use a rack in the oven, we use stainless angle iron and put a blade on each side and it slightly leans on it. I have data graphs on every heat treat I have done showing the soak and load times. The oven is remarkably stable. If I open the door and close it the temp hardly drops. If I put a few blades in it takes less then a min for the TC to stabilize and a few more minutes for the blades to hit temp. These 1084 blades got a 1600° soak and air cool. Then a 1500° soak for what I would guess was 15min and quenched into 60° parks 50. Here is the graph data on the heat treat process.

normalizing soak, all blades at once (in stainless foil pouch)

Here is the actual heat treat section. The real large dips are loading blades.

Good stuff.
Hoss

 

[Cushing H.]

We need to send that blade to someone with a salt pot and see if there is still a variation in hardness.

Hoss

I would propose this is a good idea, if a salt pot can be had. JT, i do not doubt your data ... but i have been fooled before. I am thinking that running that blade through a salt pot with its different heating process could be a pretty defnitive way to distinguish between material versus process. We might all be surprised by the results...

 

[Mecha]

I'm just sayin', you put a pizza in the oven and it's good, but if you put the rack too close to the roof, then the pizza gets burned along where the elements are, because it's way hotter right near the element. Not saying that's what happened here, though. I still think the steel is haunted.

 

[JTknives]

I'm just sayin', you put a pizza in the oven and it's good, but if you put the rack too close to the roof, then the pizza gets burned along where the elements are, because it's way hotter right near the element. Not saying that's what happened here, though. I still think the steel is haunted.

ok, then riddle me this?

How come just these blades? The number of blades that go through that oven is rather impressive. All the blades we heat treat are hardness tested and only these have done this? Like I said I have had this problem before and it was the EXZACT same material (alloy) from the same claimed source. It was from the beginning of the year if I remember correctly. And thy showed up during surface grinding. I will do some digging and see if I can find the data on that. We have had a handful of other heat treat failures but that was due to customers buying on eBay from a seller that was known to ship mild steel. We just did a batch of 1075 that I cut out from a sheet supplied by AKS and thy hardened beautifully.

Just did a quick search and found an email dated 2-28-18 from a customer that bought 1084 and could not get it to harden. Says he was getting upper 40s Rockwell.