heat treating question

[Herbey De Hoyos]

Ok maybe somebody can answer this one for me. Im fairly new at knife making and just started to try to heat treat my own blades.The problem is that after i remove the scale off of my blades there is a random pattern that stays on my blade kind of looks like water spots when you dont dry off your car after you washed it but some of them are slightly higher than the rest of the surface. Is this due to mabye heating the blade a little to much before the quench or is it because my quench oil is too cold at time of quenching, or mabye some other reason? Either way if some one could help me figure this out it will be greatly appriciated. thank you in advance.

[RJ Martin]

Herbey: We will be glad to help if we can. Please let us know:
The type of steel?
Heat treating in a forge, kiln or other method?
Is the blade protected in foil?
If we have all the details, it makes troubleshooting a lot easier.

Almost 30 years ago, I heat treated some blades in my dad's charcoal barbecue. Everywhere a briquett touched the blade, a raised spot was produced. That was extra carbon (and other junk!) being absorbed into the steel.

[Herbey De Hoyos]

RJ , I think this is what happend to mine , you see I buit a brake drum forge but dont have any coal so i thought i could use charcoal. The steel is 1018. is there any way that i can protect the blade or should i just go out and buy some coal .

[SteveS]

Herbey are you saying the knife is 1018 or the brake drum? That won't harden. You won't get a knife.

Steve

[Herbey De Hoyos]

Originally Posted by SteveS

Herbey are you saying the knife is 1018 or the brake drum? That won't harden. You won't get a knife.

Steve

OOPS i meant 1095 im sorry for the typo. I am aware that 1018 is a mild steel or soft steel , and is not a good knife steel .

[Quenchcrack]

I did not see an answer to your question so I will try to give one. I think you are seeing decarburization of the surface. This happens when hot steel is exposed to the air and the oxygen combines with the carbon on the steel surface to form CO and CO2. You can either wrap the blade in stainless wrap when you heat treat it or grind the surface to remove the pattern. Most makers wrap the blade.

[Fox Creek]

Yup, that "marbelized" pattern is decarurization. You can grind it off, etc. No harm done. You CAN harden a blade with out getting this, but its tricky. You have to bring it up to heat very slowly and evenly and quench from a lower heat.

[RICK LOWE]

Herbey, I had the same problem using charcoal in my monemade forced air forge. Really a pain to sand out! I just started using a one brick propane set up and have done about 7 blades in it and find almost no scale and none of those bothersome "water spots". I'm using 1095 and 1084 and the one brick works real well. Pretty soon I plan to make a little bigger steel forge, but this works for now. Bottom line, the charcoal does let more air into the mis and I feel there is a problem with lots of impurities in store bought charcoal.
Rick

[Herbey De Hoyos]

thanks guys for the info. i did a searching around and found a local store that sells lump coal instead of briquettes and so far this seems to have helped alot plus i added an air shield to my forge and that seems to have reduced the amount of scale some times a little trial and error goes a long way .

[Quenchcrack]

My forge is a gasser and I have been promising myself to get some short pieces of 2" pipe to use as a muffle. Put the pipe into the forge, let it heat up, put the blade into the pipe (open on the ends of course) and let it heat up by radiation. Put real charcoal on the bottom of the pipe and consume most of the O2 as it heats up. I think this should improve the surface quality a bit. Now to try it...... :confused: