- How thick is your steel? The thinner the piece, the less time needed to soak. 80CrV2 has a little extra stuff in the recipe so it requires a longer soak at austenitizing temp than 1084 to allow all the ingredients to spread evenly throughout the steel. I am using 1/4". I normalize the blade first (see below). I bring the blade up to 1485˚f; and let it soak there for 10 minutes before quenching in Parks 50 quench oil. Then three 2 hour temper cycles @ 350˚f;.
Here are a few pearls of wisdom from Stacy as he explained a few things for me (come to think of it, I'm going to re-read this a few more times myself)
:
Normalizing is a process to return the grain structure to "normal" after a thermal process like forging, and once again before final hardening. It repairs any damage to the grain structure and re-distributes the alloy ingredients.
In stock removal, it probably isn't really necessary, but since you don't know what condition the steel was in when you got it, normalization is a good idea before the final quench.
Lets look at what normalizing does to understand how to do it:
In the first heat, you need to put all the alloy ingredients into solution and re-set the grains. This is done at a temperature about 100-200F above the austenitization point. For most steels it would be between 1550-1650F. 1600F is a good average.
Heat to this temperature and hold for long enough to make sure things are in solution, five minutes is good.. Let cool to black heat, and then quench in water to cool to ambient. The steel now has evenly distributed alloy ingredients and an even grain size. The actual grain size may be a bit large, but all that maters at this step is evenness.
The second heat is slightly above critical (50F above the curie point. is close enough). Most folks just use 1350-1400F for this step. Hold for a few minutes, cool to black, and water quench.
The steel now has well distributed alloys and a fine grain size.
The third heat is sub-critical, about 1250F. Hold for about 5 minutes, allow to air cool to black, and then water quench.
The steel now has even alloy distribution, fine grain, and a fine pearlite structure.
It is ready for the austenitization and final hardening quench.
Hope this helps.
Stacy
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Pearlite Nose
Lets just describe it as it happens:
You place a blade in the kiln (Kiln is already per-heated to 1500°F)
The blade rises to 1500°F. On the way up, it crosses 1414°F and becomes non-magnetic. This happened because the structure of the steel changed to austenite. The time spent above this point is called "austenitization".
The steel needs to sit at the target temp for a while to let all the alloy ingredients dissolve. This time period is called the "soak".
When ready to quench, upon removal from the kiln the blade will start to cool in the air. As long as it stays above 1350°F, it will still remain austenite. Because air is a fairly slow heat exchanger, the blade usually has at least two seconds before it cools to 1350°F. The larger the blade, the more the mass, so the cooling in air is slower on larger and thicker blades. Thin blades will loose temperature much faster.Once it crosses that point ( called the Ac1) it wants to change its structure. You have a certain amount of time to get it below 900°F or it will automatically convert to pearlite. This time window is called the pearlite nose ( because it looks like a noise on a TTT graph). The amount of carbon and the alloy contents determine how fast or slow the time across the pearlite nose is. On some steels, like 1095, it is less than one second. On steels with high alloy content and lots of manganese it can be minutes. It is best to get past the pearlite nose in a quick and sudden drop. This is the "quench" part of HT. Selection of the proper quenchant is critical to get past the nose fast enough. If the steel does not "pass" the pearlite nose, it will become soft pearlite and the HT will have to be redone.
Once past this nose, the steel continues to cool down between 900°F and 400°F. In this range the cooling rate can be slower, but should be more or less continuous. The quenchant selected and methods use determines the time spent in this range. You have from 30 seconds to several minutes for most knife steels to make the drop between 900° and 400°F. During this time the steel is still austenite. That structure is very soft and easily bent by hand ( with proper gloves on, of course ). Straightening of minor warps and curves is easily done during this period of time.
The structure in the steel now is called "super-saturated austenite". That means the austenite should not still be there, but cooled in such a way that it didn't convert....yet. It wants to change structure, but was cooled too fast for that to happen. If it continues its cooling rate too slowly, it will start forming pearlite and bainite, but that takes very, very slow cooling in a programmed oven.
Once the steel reaches 400°F under normal cooling rates, the super-cooled austenite starts to convert to martensite. This point is called the martensitic start, or Ms. Below this point the steel continues to gradually change into martensite until all the austenite that can change is converted. Cooling in this range should be slower and gentle. This is where cracks form due to the stresses created as structures change. The different structures have different sizes. When a larger structure is created, it has to move the adjacent structures to get some room. This can cause warp and cracks if it does not happen evenly and smoothly. This conversion to martensite stops at what is called the martensitic finish point - Mf. Any austenite that was not converted is called "retained austenite" - RA. The Mf varies depending on the alloy content and type. It is around 200°F for simple carbon steels, and around -100°F for stainless and high alloy steels.
Once the austenite has converted, the new martensite is called "brittle martensite", and will break easily if not tempered because the structure is under great internal stress. Once tempering is done, it is called "tempered martensite". Tempering twice and reaching the full Mf point at the end of the quench will take care of most all the RA in a knife steel. If the tempering is delayed too long, the steel can spontaneously crack due to the internal stress, and the retained austenite can become stable, and will not convert in tempering. This is why the temper should be fairly son after the quench. On the most crack prone steels, it should be tempered within 10 minutes or so.
