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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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Old 11-15-2017, 07:09 PM
blitt214 blitt214 is offline
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Knife flex

I broke a small knife I forged to see the grain structure which turned out good, but was wondering when I try to bend it how much flex should I look for
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Old 11-15-2017, 09:08 PM
jimmontg jimmontg is offline
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Don't worry too much about it

Blitt, how much flex do you want or I should say, expect? It depends on the structure of the blade too. What is the steel, 1080? How much it bends isn't as important as how good was the grain and how did it cut? The bend test for passing the professional BladeSmith requirements is about how good are you at doing a differential heat treatment. The spine of the blade is softer than the edge and thus bends to 90 degrees. Also the actual cross-section of the knife plays a big part. I made an O1 tool steel filet knife that had a decent amount of flex for how thick the blade was, 1/8" (not much) and that knife I traded for $300 worth of free fishing guide service in FL because it held an edge through a lot of fish scales and bones. The charter skipper had never had a knife that could go through about 80 fish without sharpening. I made sure he knew how to take care of it as it wasn't stainless, I even gave him a bottle of Birchwood Casey Super Blue as I never expected to sell that knife for anywhere near $300 and he gave me and my son a lot of fun and fish for supper.

To be honest I do not care much about flexibility except for some filet knives. I care about edge holding and edge chipping during the brass rod test. When you broke the blade did the grain look like grey velvet? If yes then good. Did the cutting test go well and the edge hold up? If yes then good. Did the edge not roll over or chip during the brass rod test? If it did well there then do not worry about how flexible it is, you did a good job. Seriously, you did well?

I might also add that all these tests you can do by hand is as good as it gets unless you want to buy a Hardness Tester. I never felt the need to have a hardness tester except for D2 and Stainless alloys. Also I personally never had my own RC tester, I used the ones where I worked.

The only thing I might add is the edge a short bevel with a lot of meat behind it? Because if it has an ax edge, or short bevel then the brass rod test isn't helpful because you don't deflect the edge at all. I sharpen my blades at about a 15-20 degree angle. I have found by the way that cutting cardboard is a good test and also chopping through a 1" thick piece of hardwood will tell if the edge is chippy, I actually use Dymondwood for chopping.

So I hope I haven't overloaded you with too much information Blitt. I hope some others will chime in with their experience because my view about flexibility may differ from those more informed about the matter. I also want to add an anecdote about one of my S30V knives I had HT by Peters a pro HT company. They HT the blades to my specs and I had them all sold pretty fast in FL.

One of the knives broke after 3 months or so of use and luckily I kept back a couple for just such an occurrence. Well it turned out the deckhand who broke the knife was in the habit of sitting there and flexing the blade on the wooden cutting table. He would just stand there and push down on the blade . He work hardened the knife and it finally broke. I still replaced it, but I told the skipper to tell his employee not to stand there and flex the blade over and over. The knives were .090 thick and rather stiff at RC 60 and they weren't very flexible. I went with edge holding over flexibility. When I make a thinner filet for panfish or trout I go a little less on the hardness like 58-59. I use a flat grind usually and at that hardness the S30V is fairly decent on flexibility, but it won't withstand being flexed to the point of being work hardened.

So now you know more than you wanted too. Pardon me, I am in the quarantine ward of a hospital for chicken pox and am bored to tears. I am hoping to get out this week.
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Old 11-16-2017, 05:43 AM
blitt214 blitt214 is offline
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I appreciate the info, best of luck getting free. The grain looked really good I was super happy. I didn't do any other testing I really was just interested in the grain since this was the first knife I ever quenched and tempered. I just tried to bend it for the heck of it. It was a small knife maybe a 3/32 and a full flat grind. It bent maybe 5 degrees then snapped
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Old 11-17-2017, 06:09 AM
WBE WBE is offline
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Temper is of some concern, but flex is more dependent on blade thickness. An example would be a spring. A springs travel range and power is not adjusted by temper, but by thickness.
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angle, back, bevel, blade, brass, edge, file, fishing, fishing knife, flat, flat grind, forge, forged, hand, how to, knife, knives, made, make, rod, scales, sharpening, small, steel


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