2 Noob Questions...

DonL

Well-Known Member
Okay, I've searched for the answer for these two questions and keep getting mixed results. Figured I might get mixed answers here, but I'm gonna ask anyways!

First off, what is a double/triple quench? Is it an interrupted quench or do you take the blade back to the hardening temp after you quench it the first time?

I see some makers stating that they double/triple quench and I was just wondering what that actually meant and the benefits of doing it.

The other question I have is about blade geometry. I read a lot where the maker says they ground the blade to a convex shape. Are they talking about grinding an actual convex cutting edge or is the entire width of the blade convex? If it's the entire width, how does one go about grinding a convex shape on the blade? I've always been under the impression that a flat grind was the toughest edge you could put on a blade.

Thanks for your help. I promise to have more of these noob questions in the future :what!:
 
Double/tripple quenching is just that; it is austinizing the steel and quenching two or three times. It is done to refine grain so no soak time, other than to assure that the steel is heated evenly all the way through, is needed because we are not concerned with carbon going back into solution if refinement of grain size is all that is desired. Of course if one would want to go from this step to hardening the last cycle will have a soak time, or not, depending on the requirements of the steel in use. Austinization temperatures will also need to be kept low to prevent growing the grain between quenches. One would also need to temper or normalize the steel immediately following this process if they intended to do the final hardening later.

The down side is that quenching can cause microscopic cracks in the steel along the crystal boundries, especially with a carbon content of 1% or more, though it can occure with the lower carbon steels that are used for making blades. To my way of thinking, three quenches results in three times the microscopic cracks. Tripple normalizations will do the same thing without these microscopic stress fractures.

As far as the geometry question goes, the maker may be refering to a convex grind across the whole width of the blade or they may be refering to just the secondary bevel at the edge. Either way the best way of doing this would be with a slack belt on a grinder. Some have argues that if a flat file is used to cut the bevels in the blade a slightly convex profile is formed because of the flexing of the file while draw filing.

Doug
 
Normalizing blades as part of the forging process is standard practice these days. As the blade is being thinned down during forging, the steel should be worked at cooler temperatures. As Doug states the temperature needs to be kept at the low end of austintizing temps if proper normalizing, the reduction of grain size, is to take place. This is when a digital gas or electric kiln comes in handy. Steel is all about temperature control.

Fred
 
Thanks guys. I guess what I'm having a problem with is the actual sequence. I also don't understand the need to double/triple quench if you normalize 2 or 3 times as is suggested by some. I'm strictly talking about stock removal. Is this even necessary with stock removal knife making or is it a forging thing?

As for the sequence of double/triple quenching, is the sequence as follows? Take the steel to whatever temp is needed for hardening, quench in quenchant, then take back to hardening temp again, quench in quenchant and repeat the process once more?

OR do you take to hardening temp, quench for a certain amount of time (like, maybe a 5 count), remove momentarily - dip again - wait - then dip again until the steel is cool?

Just trying to get my head around this one!
 
Ok, now you're getting into an interupted quench. That is often used with a shallow hardening steel in a water or brine quench. If you are quenching in oil I don't see a need for it. If your steel is hardening in oil you don't need to do a water or brine quench. I know that there are those who have even quenched steels like 5160 or even 52100 in water but I don't see the need for it.

If you do a triple normalization you there is no reason to do a multiple quench. It doesn't hurt to do some form of grain reduction with stock removal even though you do stock removel. If nothing else it could relieve any uneven stresses that built up in the billet during shaping. If you don't buy annealed steel bars you could also do normalizations to soften the steel for shaping. To check on the grain size of the steel that you get from your supplier take a short piece austinize it then quench it. Then strike it with a hammer to break it. The inside should look about like 1000 grit sandpaper in roughness. If it looks courser, like 400 grit or larger, then you will need to do grain size reduction.

Doug
 
I have a friend who does convex grinds exclusively, and he starts them on the platen, rocking the blade from edge to spine, starting at the tip and working back. Doing it this way he can control the amount of convex. He uses the slack belt only to blend everything in. I am not a convex guy, but his grinds are very nice, and I like them better than ones that I have seen done on a slack belt.

Dale
 
Dale, convex grinds can be good for their strength at the edge, either full convex or, what I will do, a flat primary grind with a convex secondary grind. However if I want to make a blade for slicing, such as in a kitchen knife, I will do a flat primary and secondary grind with a more accute angle on the secondary grind. That doesn't mean that a convex blade can't be as sharp. I can put a hair popping edge on my heavier camp knives. The convex edge tends to make a thicker wedge in cross section that it has to move through while slicing.

Doug
 
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