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Larrin was interested in what I may have on spheroidal annealing and similar treatments so I thought I would risk inflicting even more metallurgical yakity yak on anyone interested instead of an e-mail or other venue. So here is what I have on the subject in excerpts from talks and Power Point presentations I have made.
Annealing becomes necessary whenever we need to the steel to be a soft or stress free as possible for the purpose of machining, forming or to avoid distortion in subsequent heat treatments. With steel there are typically two types of annealing available- Full or lamellar annealing and spheroidizing.
Full annealing is also referred as lamellar annealing because it results in pearlite which is a lamellar segregation of iron and carbon (ferrite and cementite for those more particular). The treatment involves heating entirely above the point where all carbon will be into solution and forms austenite, and total recrystalization will occur, then the steel is slow cooled to below 900F. from around 1200F to 1000F. In the cooling process the steel will form pearlite. The slower the cooling the coarser the pearlite will be due to the wider spacing of the iron and carbide lamellae, as longer times allow for greater distance of travel for the carbon.
This type of anneal works best for simple steels that have .8% carbon or less. If the steel has in excess of .8% then this anneal is note best move because not only will it have a more difficult time dealing with the carbide, the slow cooling can allow the carbide to segregate into the grain boundaries.
For more complex steels, and those of higher carbon content, a spheroidal anneal is often a better method. In this treatment the current crystals/grains are left alone since it is sub-critical and no recrystalization occurs. Instead the carbon balls up into round globs in a softer carbon depleted ferrite. With course pearlite machining can be touchy because it is like pushing a bar though a random pile of plates, while machining spheroidal cementite is like pushing a bar through a pile of ball bearings. But the carbon does stay out of the grain boundaries. Basically on the way up to austenite carbon tends to ball up, on the way down it tends to make sheets. Steels at or below .8% C can be spheroidized but they just wont form big obvious spheres, and it can be more trouble than it is worth.
Spheroidizing has some down sides. If you are not holding soak times now, you wont like your steel spheroidized as it takes much longer for the carbon to go into solution, spheres tend to be very stable structures and the carbon has farther to travel, while the lamellae of pearlite dissolves rather quickly unless it is quite coarse.
Spheroidizing can be done in one of several ways. The steel can be heated around 20-40 degrees above the lower critical (say 1335F for this conversation, so from 1350f to 1375F) and held there for around an hour before cooling no more than 50F per hour until it is below 1000F. Or it can be soaked for at least an hour just below lower critical. Or it can also be cycled just above and just below lower critical several times. At any rate if you heat above non-magnetic you will have lost control of your spheroidizing.
Here are some more of my annoying micrographs to show what I am talking about:
This is full annealed 1095. You can see that the reason why pearlite gets its name is how it resembles mother of pearl, but you can also see the pure white carbide in the grain boundaries, very brittle stuff and rather hard to get rid of in just one or two low temperature heats.
This is spheroidized steel, you can see a huge difference from the pearlite above and there will be no heavy carbide in the grain boundaries, they are some there but you cant see them; as it should be.
Here are two samples of 52100 taken from the same bar just one was full annealed and the other was spheroidized. Both were soaked and quenched from 1550F and formed martensite, in other words hardened (the tannish brown stuff in the background is martensite). But you can see that both have their issues. The lamellar sample still has grain boundary cementite which will leave it more brittle, but the spheroidal sample has much less of its carbide dissolved into the martensite. However the spheroidal sample would be in better shape with just a little more soak time, while the lamellar sample really needs some normalizing to break up that cementite and even things out.
Annealing becomes necessary whenever we need to the steel to be a soft or stress free as possible for the purpose of machining, forming or to avoid distortion in subsequent heat treatments. With steel there are typically two types of annealing available- Full or lamellar annealing and spheroidizing.
Full annealing is also referred as lamellar annealing because it results in pearlite which is a lamellar segregation of iron and carbon (ferrite and cementite for those more particular). The treatment involves heating entirely above the point where all carbon will be into solution and forms austenite, and total recrystalization will occur, then the steel is slow cooled to below 900F. from around 1200F to 1000F. In the cooling process the steel will form pearlite. The slower the cooling the coarser the pearlite will be due to the wider spacing of the iron and carbide lamellae, as longer times allow for greater distance of travel for the carbon.
This type of anneal works best for simple steels that have .8% carbon or less. If the steel has in excess of .8% then this anneal is note best move because not only will it have a more difficult time dealing with the carbide, the slow cooling can allow the carbide to segregate into the grain boundaries.
For more complex steels, and those of higher carbon content, a spheroidal anneal is often a better method. In this treatment the current crystals/grains are left alone since it is sub-critical and no recrystalization occurs. Instead the carbon balls up into round globs in a softer carbon depleted ferrite. With course pearlite machining can be touchy because it is like pushing a bar though a random pile of plates, while machining spheroidal cementite is like pushing a bar through a pile of ball bearings. But the carbon does stay out of the grain boundaries. Basically on the way up to austenite carbon tends to ball up, on the way down it tends to make sheets. Steels at or below .8% C can be spheroidized but they just wont form big obvious spheres, and it can be more trouble than it is worth.
Spheroidizing has some down sides. If you are not holding soak times now, you wont like your steel spheroidized as it takes much longer for the carbon to go into solution, spheres tend to be very stable structures and the carbon has farther to travel, while the lamellae of pearlite dissolves rather quickly unless it is quite coarse.
Spheroidizing can be done in one of several ways. The steel can be heated around 20-40 degrees above the lower critical (say 1335F for this conversation, so from 1350f to 1375F) and held there for around an hour before cooling no more than 50F per hour until it is below 1000F. Or it can be soaked for at least an hour just below lower critical. Or it can also be cycled just above and just below lower critical several times. At any rate if you heat above non-magnetic you will have lost control of your spheroidizing.
Here are some more of my annoying micrographs to show what I am talking about:
This is full annealed 1095. You can see that the reason why pearlite gets its name is how it resembles mother of pearl, but you can also see the pure white carbide in the grain boundaries, very brittle stuff and rather hard to get rid of in just one or two low temperature heats.
This is spheroidized steel, you can see a huge difference from the pearlite above and there will be no heavy carbide in the grain boundaries, they are some there but you cant see them; as it should be.
Here are two samples of 52100 taken from the same bar just one was full annealed and the other was spheroidized. Both were soaked and quenched from 1550F and formed martensite, in other words hardened (the tannish brown stuff in the background is martensite). But you can see that both have their issues. The lamellar sample still has grain boundary cementite which will leave it more brittle, but the spheroidal sample has much less of its carbide dissolved into the martensite. However the spheroidal sample would be in better shape with just a little more soak time, while the lamellar sample really needs some normalizing to break up that cementite and even things out.
? I plan to begin with stock removal and HT. I just want to avoid making poor choices that will produce an inferior blade.
