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Posted: 9/10/2002 10:41:22 PM EDT
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For part 2 I decided to start a separate thread. This is not to clutter up the forums, but rather as a courteousy to those with slower connections. Having one long thread loaded with images can be irratating when you have to wait 5 minutes for everything to load. At the end of part one we had refined the profile of the forged blade at the grinder and had brought the blade to a 120 grit finish. Sometimes I take the blade to a 220 grit finish at the grinder. My method of heat-treating differs from the norm. Most of the smiths I know will heat-treat at the 220 grit finish, with the blade slighty over the final dimensions. This is because with there will be some scale and pitting to deal with. The blade has to go back to the grinder after hardening and tempering. I instead have adopted a method using a non-scaling compound called PBC, which is available from Brownells. It allows me to heat-treat with the blade very near the final dimensions, and I don't have to return to the grinder to clean it up. For anyone interested in the whole story on the compound and how to use it, you can check out the Scale Prevention During Heat Treating article on my website. But first I will take the blade to a 400 grit hand-rubbed finish. There is nothing particularly magic about 400 grit. It's just that the compound seems to adhere better at that finish than a finer grit. Here's a shot of a blade being prepped for heat-treating. http://www.primosknives.com/articles/scale_prevention/pre-finish.jpg Next the blade is brought to 580 degrees F. and covered with the compound. The compound melts over the surface of the blade and blocks out the oxygen in the heat-treating furnace. High temperatures and oxygen are what cause scale. Here's a shot of the compound being melted over the blade. http://www.primosknives.com/articles/scale_prevention/ns-prep.jpg In case there are any stock removers who use high alloy / high chromium steels, I should mention that this is not for your steels. You will need to stick with either a heat-treating service that provides a controlled atmosphere where the oxygen can be purged, or use the stainless steel heat-treat foil wrap if you're doing it yourself. The high alloy steels require temperatures that are too high for this compound. This compound is good up pto about 1650 degrees F. It works great for carbon steels since the critical temperatures for them range anywhere from 1400 - 1525 degrees F. The blade is placed in the heat-treating furnace and brought to critical temperature. I can see more questions popping up about terminology, so let me just briefly throw a couple of more things in here. I don't want to get too technical in this thread, but to explain critical temperature, I have to mention something about a couple of the various micro-structures that occur in steel. To save time, let me just copy another exeprt over from an article on my website. When steel is heated to prescribed temperatures, then cooled at a specific rate, it undergoes physical internal changes which manifest themselves in the form of various micro-structures such as pearlite, bainite, and martensite. These micro-structures (and others) provide a wide range of mechanical properties, making steel an extremely versatile metal. There are two crystalline micro-structures that we will be dealing with in this part of the discussion. Those two are Austenite and Martensite. Let's assume that in the annealed (softened) state, our steel is probably primarily composed of fine Pearlite. As we reach temperatures within the critical range, this Pearlite transforms into Austenite. If you've ever heard a maker talk about taking the steel to a non-magnetic state, then quenching, here's what that's all about. One of the interesting things about Austenite is that it is non-magnetic. Once you have begun to enter the critical range the blade will not attract a magnet. The internal structure is transforming to Austenite. If cooled quickly enough, this Austenite shears off into yet another micro-structure called Martensite. This is the hardest form of the steel, and is exactly what we'll be shooting for. In case you're wondering, when the structure shifts to Martensite, the steel will once again be attracted to a magnet. |
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Once we have achieved critical temperature, the blade is removed and placed in a quenchant. The temperaure rapidly drops and the Austenite transforms into Martensite. In other words the blade is hard -- REAL hard. http://www.primosknives.com/articles/scale_prevention/edge-quench.jpg This is a mockup of the quenching process. This blade has already been quenched and is down to about 200 - 300 degrees F. I couldn't take a shot of the actual quenching, which was done at apprximately 1475 degrees F. It's much more exciting. he http://www.primosknives.com/articles/scale_prevention/file-test.jpg The black junk you see on the blade is some of the compound used to block out the oxygen and prevent scale. See how clean the blade is? Let's take a closer look at the blade and see if the file did any damage. http://www.primosknives.com/articles/scale_prevention/file-test2.jpg Nope. No damage. If there were just a few little scrape marks that would be no big deal. The main thing to make sure the file can't bite into the steel. In this case the file did no damage to the blade. That sucker is hard! So hard that it is extremely unstable and can be easily broken. The next step will be to temper the blade. This process is also referred to as the draw, because we will be "drawing" out some of the hardness. We will also be relieving the tremendous stresses the steel is under. Tempering is done with yet another series of heats, but this time much lower heats. For this particular steel I will be tempering at about 400 degrees F. I do 3 cycles of 2 hours at 400 degrees F. with a 20 minute cool down period in between. So we're looking at another 7 seven hours before we can test the blade. I test every blade after tempering. To test a knife after assembly just doesn't make any sense to me at all. If it fails the tests, then you screwed because the knife is finished. I also don't believe in things like testing every 10th knife. Those things have my name on them and I want to be confident about each one. |
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Here's a shot of a blade after tempering. http://home.sport.rr.com/primos/knifemaking/temper-colors.jpg These strange colors you see on the blade are oxides that build up on the surface during the tempering process. These colors tell a story of what's happening with the steel. The colors vary from steel to steel, and the colors I watch for vary as well. In the tempering process, we are doing a couple of things as mentioned before. We are drawing out some of the brittleness and relieving stresses in the steel. There is one more thing happening that some folks don't know about. Remember that at the hardening phase, we have tried to achieve maximum hardness by reaching maximum Martensite transformation. The untempered Martensite is very unstable. The tempering process is creating tempered Martensite, and also can help transform any retained Austenite into Martensite. BUT, the newly formed Martensite is now untempered Martensite, so we have a combination of tempered and untempered Martensite. The second tempering resolves that. The third one? Well, that one's really just for good measure in my opinion. It's like wearing both a belt and suspenders. have On some steels the peacock would indicate that we've drawn the steel back a bit too far. On 1084 however, thats the colors I like to see. We'll have a tough blade with good edge-holding ability. This will be eveident when I get around to posting the tests. |
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