Hello Everyone,
I just had the blade snap on a chef knife I have been working on(BUMMER!!) and I am hoping to get some help on looking at the steal to figure out what I did wrong. The steel is 1084 I bought from the Baron, I use a home built Propane been-can forge and a IR thermal meter w/a magnet as a backup to determine my different metal states/temps. For this blade I tried to follow the following process as closely as possible. Start forging at high heat to get to rough shape then reducing heat as I near the final forged shape. Let air cool, grind to pre-HT shape. Heat to high heat, let cool covered in Kaowool (need to order Vermiculite), do this 3 times for normalization. Since it’s been on average about 20 degree’s f in my shop lately even with the kaowool cover it doesn’t take more than an hour to cool. Once normalized I heat just the blade to high heat, them quench in vegetable oil @140 deg.(will be moving to Canola/peanut oil), plunging up and down for 40 sec. then check for straight, correct, then back in the oil for another minute or so. Check the hardness with a file, then 3 rounds of tempering @425 degrees for 1 hour each. After that I went to finish grinding, but I had to do a LOT of finish grinding because I was so worried about getting the blade to thin and having problems during the quench that I left it too thick. I was completely done with the grinding when I flexed the blade just a bit and it snapped. So I broke it down into chunks to see the grains. Unfortunately since I just started forging knives in the last 3 months I have no idea what the grains are supposed to look like, other than big grains bad small grains good?
I THINK my main problem was that at some point I over heated the blade which created a fracture, but I also think somewhere along the line I messed up the heat treating. I think maybe I need to extend my temper to 2 hours and only do 2 instead of three, but I think my problems started before that point anyhow. By looking at the pictures can anyone give me some hints/tips on what/where maybe I went off track? Is the grain structure in the blade too rough, indicating too large of crystals? I am also curious about something I notices in the tang. Where I didn’t have hammer strikes the grain underneath is very very fine, but where there are hammer strikes it is very course. Are these changes in the crystal structure caused by the energy imparted by the hammer strikes?
You've definately got some large grain there. There are numerous things that could have contributed but one that I would work on is the normalizing. There is no need for the Kaowool. Take the blade to just above critical & air cool (back to black) and then reduce the temp. approx. 25 degrees (each) for the next two times. (If you have an oven, soak time may also help as it appears that most of the large grain is in the center of the blade.)
You may have overheated the blade during you initial forging also which would add to grain growth. Hopefully one of the metalurgists will come in here with more.
Gary
Thanks Gary, I will take your advice on the normalizing and see if I get better results.
Chris,
In one way, this is a blessing in disguise. Your grain size is all over the place. You needed to know that. Small grain equals a tougher blade, all other things equal. You have a range of forging temperature to stay in for the best blade possible. Also, your descriptions are not exact as they should be. "High Heat" might be one thing to me and another to you. I suspect its a combination of a couple things.
While this stuff is not above understanding, it does require us to stay within some close tolerances in order to get good results. You might consider getting with an experienced bladesmith in your area as soon as you can and watch him during the heating/forging sequences. Is there a knife making meeting in your area? You are looking in the right place by being here. That's a good thing. You might do some reading of the various threads already in place to see if you can figure out what the troubles was.
Hello Chris. Yes that is some pretty big grain size. I am thinking that using that small a forge you did not get an even heat and got hot spots in areas. Like Lin states, what you consider high heat might be something different to another person. Also as Gary states the normalizing procedure could be suspect.
On 1084 I usually forge between 1600 and 1900, say an orange color for beginning forging. On thermal cycling I do it after forging and before grinding. That is just me. I will do the first at about 1600 then cool in air until black, then at 1500 and cool in air until black, then at 1200-1300 and into vermiculite. This leaves the 1084 very easy to grind and work. I then do the grinding and go to 120 grit for heat treating. I bring the blade up to 1475 to 1500 and quench in oil. Canola works fine for 1084, provided it is at 120 degrees or so. I will then temper at 425 for two hours, allow to cool, then do another 425 two hour cycle. I am getting 58-60 RC with this method and a pretty tough blade. On larger blades I will also draw the spine with the edge in a water bath and draw the tang.
One thing to do is write down the processes for each blade with a known steel. When you get a process that works well this will give you a recipe for each blade of that steel.
Let us know how it goes.
Brion
Brion Tomberlin
Anvil Top Custom Knives
ABS Mastersmith
