Knife Test

D

Doug Lester

Well-Known Member
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Above is a destructive test of the knife pictured on the right. It was a spear point, single edged blade with an integral bolster forged from 3/4" 52100 rod and austempered at 430 degrees for four hours. I kind of screwed up the design so it got volunteered for testing.

It passed the wire cutting test without indenting or chipping. I cut 3/8" hemp rope about 25-30 times before it started to drag. I didn't do any chopping with it due to it's small size. A larger knife that I did the same way from the same rod did much better at the slicing test and stood up to some chopping without loosing its edge.

Anyway, this knife bent 90 degrees easily with the aid of a 12" piece of black pipe. As you see from the picture of the bent blade it kept the bend but there was no crack in the edge of the blade or anywhere else. I then bent to blade back and forth through an ~180 degree arc five times before it broke. I would give it high marks for toughness but low on strength. I guess it boils down as to whether a bent blade or a broken blade is worse

I'm not sure if I want to stick with this technique or not. I may just use it for big choppers to give it added toughness over pearlite. What I could try, after over riding the controls on the roaster is to quench to 350 degrees in a tank to increase the percentage of martensite and then up quench to 500-510 degrees in the roaster for an hour to convert the rest of the austinite to bainite. The middle picture doesn't show it well but it had a very fine grain.
 
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did you use your normal thermal treatments before ht?

I have zero idea on what your talking about getting more martenzite out of the steel...I thought you more or less got that once you got it into solution and soaked properly...
 
I think I'm getting a little fancy on you here, Dave. You would have to be familiar with IT diagrams to follow but basically if you heat the quenchant to between Ms and Mf (usually actually listed as M90%) you will partially convert the austinite to martensite, unlike converting basically all of it by going below the Mf point.

To start out with I think that you are confusing austinite with martensite. That you get by heating the steel until it changes phases and the carbon goes into solution within the austinite crystals. With austempering you do not cool the steel to below the Mf point to form 100% martensite, or at least as much as that alloy will form. You can quench to between Ms and Mf and hold it there until the remaining austinite converts to bainite. Or you can do what I was talking about to hold it between those two limits just long enough to form a percentage of martensite and then taking the temperature to just above the Ms point in another bath. This allows the bainite to form faster.

A third way is to quench the steel to just above Ms and hold it there until all the austinite converts to bainite without forming any martensite. How well this works will depend on the alloy. With 52100 the third method will give an HRc of around 58. With 9260, another steel that I work with it will only give an HRc of around 55. If I wanted to get the steel harder than that then I would have to add martensite to the mix by one of the first two methods. The advantage of the first method is that you only have to have one bath of oil or molten salts. The down side is that at lower temperatures between Ms and Mf it could take many hours, possibly over a day, to convert the remaining austinite to bainite, depending on the alloy. That's where the second method comes into play by speeding the conversion up by up quenching to above Ms. The down side of doing that is possibly offsetting the hardness gained with the additional martensite by over tempering it. It's just something that has to be played with.

As far as normalizations before heat treating, yes that is done. That's a separate process to refine the steel in the blade that has to be done before quenching, whether you do a straight quench (to below Mf) and tempering or if you get fancy with things like austempering or marquenching, sometimes refered to as martempering.

Doug
 
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