In my opinion, there is nothing more important to a good quality knife than the heat treating process. If the maker has not mastered the process for the steel being used, even good grinding or forging cannot make a good blade. I do a full quench, but I only heat the edge of the blade to critical. The spine, ricasso and handle of the knife are left dead soft. This makes for a tough, resilient knife. We will be using a #2 torch tip with an oxy/acetylene mix. I have drawn a line along the bevel to show the approximate area we will be heating.
A soft flame will be used to "paint" the area on both sides until critical temperature is reached, at which point the blade will be quenched in veterinary grade mineral oil. It takes patience to heat the blade correctly and not overheat the area around the tip, which will cause grain growth and possibly ruin the steel. I will be using a magnet to constantly check the steel for its critical temperature. Carbon steels have a very unique property. When they are heated to their critical temperature and are "in solution", they lose their magnetic attraction. First, I do what I call my "quick" normalize. The normalizing cycle is explained in Terry's post. However, in this case I do not let the blade completely cool to room temperature. I have never had a problem with O1 warping when doing the quick method. I dim the lights in the shop a bit to make it easier to see the color as the blade approaches red. I start heating along the spine, keeping the flame moving above the ricasso and staying about two inches back from the tip. When the blade starts to turn blue I start heating at the bottom of the ricasso making sure not to overheat the edge. Now I carefully, but quickly go back and forth from spine to ricasso, and then ricasso to tip on both sides. Experience tells me when it is time to start checking the steel with a magnet. As the steel gets close to critical I check it often, and when it becomes non-magnetic I shut off my torch and hang the blade in still air for a few minutes.
The quench oil needs to be heated before it can be used or it will not cool the blade quickly enough to achieve proper hardness. I heat the oil until it reaches 130 - 150 F. I have a couple of small pieces of mild steel that I heat to a red color then place them in my fairly small tank, which you will notice is a drywall mud pan. I let them sit for about 10 minutes and stir the oil occasionally until the temperature stabilizes. I relight the torch and start reheating the knife in the same manner as before. When I start to see a little color in the spine and ricasso I start painting the edge in the area shown in the first drawing. I take my time and go back and forth from one side to the other and along the edge with the knife held edge up. I check the magnetism frequently. When the steel becomes non-magnetic I give it a couple of extra passes with the flame to keep it just above critical as it is moved to the tank and quenched. This part can be very exciting when you get an oil fire! I use a pair of vice grips for this and hold the blade suspended in the oil for about a minute, and then set it on the bottom, edge down. The vice grips keep the blade vertical in the quenchant ensuring good coverage by the oil. I let it sit for five minutes and then remove the blade, wipe it down and let it cool until it is comfortable to hold. I do the file test described by Terry to make sure the steel has fully hardened. The knife is ready to be tempered.
I clean up the knife a little before tempering to check for cracks and to observe the color during the tempering cycles. The knife is very hard at this point and the edge very brittle. I use a very slow grinder speed and a 320-grit belt for the cleanup, making sure the steel does not heat up.
The Stock Removal Blade - Part 3 - continued
I temper three times, cooling for 30 minutes between cycles. The first cycle is for two hours at 350 F. The second and third are one hour at the same temperature. I use the kitchen oven and a high quality industrial thermometer. I turn on the oven before beginning the quench cycle and by the time I am ready to temper the oven temperature has stabilized. IMPORTANT NOTE: Failure to obtain permission from the spousal unit before tempering in the kitchen oven can have dramatic effects on your general health and well being! I usually do my tempering late in the afternoon and early evening then put the knife in the freezer after the last cycle. Our freezer holds a constant 0 F. Testing seems to indicate that I get a very slight improvement in edge retention over blades that have not been frozen. I am still testing the freeze process. When the knife comes out of the oven for the last time is has a nice light gold color.
I used to be of the opinion, because my results were very consistent, that I only needed to test every 10th or 12th blade. I decided that this was not right and my customers deserved better. Terry Primos and Ed Caffrey get a lot of credit here since I admired their dedication to quality and commitment to each customer on every blade. I clean up the blade on the leather platen then take 400-grit belt to put on a good convex edge and polish with an 800-grit belt. I lightly buff the edge with some white buffing compound then strop it on leather loaded with the same compound until it is hair shaving sharp. I like to vary my tests and am often guided by what I have around. Sometimes I will cut manila or sisal rope if available. I usually have a lot of cardboard to cut up, which is a great test. I sometimes will chop a few boards or limbs, especially for larger knifes like fighters or Bowies. However, one I always do is the brass-rod, edge-flex test. I have found that for my knives this test is a good indicator of the blade quality. Ed Caffrey gives an excellent description of the process here: Testing Knives. . .. I have included a photograph of my test using a copper rod since I ran out of brass. If you look carefully you can see a slight deflection in the edge.
If the edge does not pass it must either be tempered at a slightly higher temperature or rehardened and the whole thermal process repeated with the tempering cycle set to a lower temperature. I also do periodic destruction testing for two reasons: to see how much abuse a knife can take and to look at the grain of the steel. Again, so as not to be repetitive, please see Terry's post for additional photographs and testing descriptions. For this test I cut some newsprint, chopped some wood and then shaved the hair. The final picture shows a close-up of the blade edge after testing. There are no chips, deflections, flat spots, etc.
The blade passed and is ready for a hand finish. The first thing I do is gently stone or grind off the edge of the blade with a very fine belt, just enough to dull it. I never do finish or assembly with a sharp knife. Next, I hollow grind the handle. This lightens the knife considerably and leaves a nice cavity for the adhesive when I bond the handles. It also makes it much easier to do the final flat sanding due to the smaller surface area. I use the small-wheel attachment with a 1.25” wheel and a slightly worn 50-grit belt.
I will use the Bader and a new 400-grit belt and the leather-covered platen to do the final cleaning of the bevels, then switch to hand sanding. With all of the preparation done before heat treatment and the belt cleanup, I usually only have to use one or two grits to finish a knife. I finish my knives to 400-grit, with a few additional passes using 600-grit paper to make the lines look clean and smooth. When done with the Bader I put the knife in the knife vice for hand finishing. The knife vice is by Rick Leeson, owner of Custer Forge and Welding. He also built my forge. Custer Forge and Welding.
All of my finishes are longitudinal, from ricasso to tip. For the final finish I use oil or Windex. Don't laugh, Windex really works as a cutting fluid! Engnath has a great description on his site. Select the "Handsanding" choice for a detailed lesson on hand finishing. There are some great diagrams that show the process. I pay particular attention to the plunge area and the area of the ricasso that will be visible between the handle and the plunge, once the handle is in place. The blade is almost ready. Notice that there are still a few scratches in the blade. I will sand these out, make sure the handle is flat, and when all of the scratches are out and the blade looks right, I wipe it with WD-40, dry it off and wrap it with masking tape from ricasso to tip to protect the finish during handle assembly. Unfortunately, I have found that in my damp NW environment I would occasionally get rust on the blade between finishing and handle installation if I get delayed for any reason. The WD-40 seems to work well for preventing the rust.