accidental differential heat treat??

[beyondmyken]

First knife attempt...so the knife tutorials that I have read all have the whole blade in the charcoal. I didn't have enough for this and was impatient so I got it going and put the 1/8 in thick with 4 1/2 blade full tang 1095 edge first in the charcoal (had some old wood too). After about 20 minutes the edge lost the magnetic signature but the back still had it. Placed it edge first in 10w-30 and have since played with it without using an oven to draw back the temper. I dragged a file across it, drew the edge across a brass rod and have used it to baton wood. Was still able to cut newspaper afterwards and general cutting chores at the same feel of my known quality bought knives. I assume that most of the back of the blade and pretty much all of the tang are still at the original hardness but the edge seems to be OK for general cutting and not too brittle.
Any comments appreciated. Sorry about the long post.

 

[mete]

If you have hardened the blade [or part of it ] you must temper it otherwise you might get a surprise - PING !! Temper it at 400F for 2hours .

 

[Ripper]

If you have hardened the blade [or part of it ] you must temper it otherwise you might get a surprise - PING !! Temper it at 400F for 2hours .

A question here: What exactly (or even closely) is the minimum time needed at tempering heat to make a usable knife? Is an hour or two hour temper cycle absolutely necessary? The reason I ask is: wouldn't it be possible for heat from a still hot spine to creep down and hit the edge at around 400 degrees? From what I understand this would be similar to how the kamis of Nepal heat treat khukuries (edge quench by pouring water on the blade; continuing to pour while watching the color change as the heat from the spine tempers the hardened area; reference ). Surely they are not maintaining 400 degrees (or whatever temp they need) for any long amount of time, but they make excellent cutting tools.
Along this same line; if the consensus is that a long soak is optimum for a superior product, how constant does the temperature of the temper cycle need to remain? How harmful is it to dip below (in this case) 400 degrees for a short amount of time? The reason I ask this is because of a thread about toaster ovens being used as a heat source for tempering (I use one also).
Lots of toaster ovens use a bi-metal thermostat to control heat; so they will be cycling below the set temperature while the thermostat cools enough to make contact again. Any insight would be appreciated.
Regards,
Greg

 

[mete]

While there are many shortcuts that may be taken ,especially with a 'forgiving' steel such as 5160 ,I can only give you the proper metallurgical procedures. As quenched the martensite is brittle and highly stressed.These stresses may cause microcracks which latter cause a broken blade or they may break immediately. The more complex the alloy the more the risk.Therefore the steel should be tempered immediately after quenching.The 400F temperature is used because this is the temperature where the martensite becomes very stable.Two hours @400F will be fine for a blade. If it maintains + or - 25F that's ok if you get wider temperature swings get a better toaster. You should of course check with a thermometer to see what you actually get. You could also reduce cycling with a heat sink - when I make bread I have a baking stone [pizza stone] in the oven which, even if the bread is in a pan ,minimizes temperature fluctuations. Use a brick, that will work fine.

 

[Ripper]

Thanks Mete.
Regards,
Greg

 

[Sweany]

Sounds to me like you didn't get maximum hardness from your quench if you are still beating on it. Try flexing it sideways a bit. That should tell you if you need to temper it or not. If it breaks, yep shoulda tempered that one.




You can do a forge temper and yeah the blade will reach the correct temp kinda,maybe it will maybe it won't. Will it be even the lentgh of the blade? Depends a lot on the operator. BUT

It's sure gonna be much more even at time, in an oven than a couple of passes through the forge.

I've made small blades with the procedures the Nepalese use. The ovens will be much better IMO.

Blade oven cycling below the tempereing temp shouldn't be a problem when you are cycling a blade through an oven for a couple hours. Spikes above your temp is another matter though.

My .02