Metallurgy Monday #2: Stop Cracking Blades In The Quench

[@patricknowak8292]

Since i don't comment on your videos often and I am giving a technical response to this one, let me introduce myself by saying that I am a professional metallurgist who was introduced to the trade through knife making more than 30 years ago. Professionally I work in a very large forge shop making some of the largest forgings used in North America. As a hobby, I still have a blacksmith shop along with some fabrication and machining capabilities. I do some occasional knife work but mostly other kinds of forging projects these days. However, technical metallurgy stuff is my bread and butter. Nice job introducing TTT diagrams. They were a real revolution to the steel industry when they were first introduced in 1930. Edgar Bain and E.S. Davenport did the initial work to develop the first ones and it was this work that was the reason Bainite is named after Edgar Bain. To add a bit more detail to why blades and other steel parts crack during quench: there are two primary sources of stress during quenching. The first is contraction of the object during initial cooling. This is fairly intuitive as most of us understand that metal expands when heated and contracts when cooled. HOWEVER, when martensite forms there is an increase in volume. Martensite actually has a greater volume than austenite so even though the blade is fairly cool when that transformation happens, there is a significant increase in volume and that increases the stress in the blade enormously. Your advice to cool rapidly only at the very beginning of the quench works because the stress from contraction is minimized. Additionally, the difference in temperature between the hottest and coldest part of the blade prior to the start of martensite transformation is also minimized. This means that when martensite begins to form it will largely form at about the same time everywhere (assuming no insulating clay or similar techniques are used). When uniform transformation to martensite does not happen, as in the cased of very large cross sections or when quenched rapidly all the way to room temperature, the outside of the blade transforms to martensite first. A moment later, the inside also transforms to martensite, but because the outside is already very hard and brittle, the volumetric expansion of the interior sometimes is enough stress to cause cracks.

[@FireCreekForge]

Thanks for filling in some more detail, and thanks for watching the videos! Just goes to show how detailed and in depth the subject of metallurgy really is.

[@GlennQueener]

Thank you! As a relatively new blade smith, I find these helpful.

[@matthewplayer8337]

This finally connected the dots! Thank you

[@T_B]

I've never made a knife. I may never make a knife. However, this was cool to see! Very informative and explains why/when you do things (rapidly) pre/post quench. Great video! --Houston, Tx.

[@christoskettenis880]

Very informative tutorials

[@chaspruitt2610]

Good video, good info, thank you!!! 👍🏼

[@krissteel4074]

My biggest cause when I was starting out was just simply grinding too thin and not leaving enough stock to absorb the trauma. This will also show itself in some pretty wild banana bends too if your grind is a bit off-kilter by design or carries some extra mass on one side. Generally though, if you follow through a thermocycle regime of normalisation, grain refinement, annealing and stress relief (in that order) you probably won't run into too much trouble on high carbon steels- it doesn't necessarily apply to high alloy steels and your mileage can vary quite a bit with those.

[@FireCreekForge]

For sure. I don't use regimen of multiple cycles though. (any more) because I don't want alloy banding with the steels I'm using. (not all steels are prone to this). The grain can be corrected (so far as is necessary) in a couple operations. Thanks for watching

[@krissteel4074]

@@FireCreekForge I've been forging a lot of Apex Ultra recently and its something which seems to require a lot of thermocycle processing to get the most out of it, particularly the grain reduction needing it three times. Like the homogenity is there for the alloy + carbides, but they tend to be very large and need some work to get them smaller. Pretty strange steel, compared to my workhorses in 1084 and 26C3 that don't need a lot of work in that department. More akin to O1 than a carbon steel with all its alloying components I guess.

[@williammorrin7933]

Got quickly question my friend. (If you use kerosene for Damascus) Does it matter were you get your kerosene for Damascus? I asked one other yt blacksmith\bladesmith and they didn't know cause they don't use it for Damascus.

[@FireCreekForge]

I dip my billets in diesel. WD-40 works great also. I've never used kerosene but as I understand that's basically the same as WD-40 plus some additives.

[@williammorrin7933]

@@FireCreekForge thanks my friend