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What makes different steel require different quenches?


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In other words, what determines if a steel should be quenched in either water, brine, oil, or air? I've tried looking, but google doesn't like this question and asked for a CAPTCHA when I type it...

 

I would like to know some of the science behind it for my own curiosity.

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The goal is generally to quench things as slowly as possible and still achieve the desired hardening. Things that get hard in oil will get hard in water too, but it may be too fast and therefore crack the steel. What you want to see for any given steel is a TTT diagram. That will show you how much time you have to go from "hot" to "cold". In your searching, you may want to look up any specific element's effect of the TTT diagram in steel. Generally speaking, C, Mn, and Cr are going to be your biggest players (in terms of added to steel for that purpose). They all give you more time, though some more than others.

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Think of those elements that Jerrod mention as the equivalent of adding salt to water and then freezing it. The salt doesn't prevent the phase change in the water, it just slows it down and lowers the temperature needed to produce ice. Those elements do the same thing with the iron matrix in the steel. They slow the phase change as the steel cools and it gives more time to trap the carbon in the iron matrix.

 

Doug

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The elemental side of it was what I was looking for, so thanks! Is it safe to assume, to an extent, the more complex the alloy, the slower the quench? I.E. 1060 being very simple does well in water, where D2, being very complex, is an air-hardening steel. If this is a gross oversimplification I apologize ahead of time.

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That is not a good assumption. Partially because "complex" is a very nebulous term in these regards. Generally speaking, blade alloys with more alloying elements will harden with a slower quench, yes. But add enough of the right/wrong element and it won't harden at all (like 304 stainless). Notice I said "blade alloys" above. If you stick with materials generally considered to fall in that category then you can think of that as a very rough rule of thumb. But that doesn't really do you any good. You should know what to do in as precise a manner as possible for every alloy. And if you are going with commercial alloys, the information you need is out there to find.

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I really see no need, personally, for me to even try forging anything that complex at the moment. It was more of a question to satisfy my curiosity. The metallurgic aspect of this craft interest me, even though I'm a long way from understanding it. Thanks for the info!

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From my personal point of view, I don't think that there is a need to forge air hardening steel at all. They come with their own set of headaches that just don't justify their use to me. There is no substitute for knowing the requirements of the alloy that you are using.

 

Doug

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Carbon aside (we are talking knife steel here anyway, so anything .6% and above), Manganese will be the biggest alloy contributor to how fast a quench needs to be. Chromium comes next. Generally, a steel with Manganese .3% and lower will require a fast oil. Get to .2% and lower, fast oil may not be fast enough. For example, W2 and 1095 are essentially the same steel, of course W2 has some vanadium for grain boundary pinning and a little bitty touch of Chromium....but 1095 has .4% Manganese and W2 has .2% Mn. This small variation is big when it comes to quench oils. Just ask "Jeff the Millwright" on Bladeforums how he figured that out. What may harden 1095 may not harden W2.

 

A simplification.... alloy with .9% C/.4%Mn needs a fast oil to harden (1095). Alloy with .9% carbon/1% Mn will harden in a medium speed oil (O1).

 

52100 is another good example to look at. .3% Mn (less than 1095!). But why does it harden quite well in medium speed oil, and a fast oil is not needed? Because of the Chromium count of 1.5%.

 

for more in depth explanation, search the term "hardenability".

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Another thing to consider with the alloyed elements is how they are dissolved in the lattice. A lot of the major contributors are substitutionally dissolved, meaning they replace the Iron in the lattice. However, Carbon is interstitially dissolved, meaning that it falls between the iron. Interstitial alloying allows for them to move around and become more homogeneous at lower energy states, but the substitutional are much harder. Speaking in very general terms, the more of those substitutionally alloyed elements you have, the harder it is for things to change phase and become homogeneous (which also goes into required soak times for higher alloy steels). But like Jerrod said, it is a very rough estimation and not a very good one to use the gross percentage of alloyed elements as an approximation for determining quench speed which is highly dependant on the ratio and composition.

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