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The Newbies Arena Are you new to knife making? Here is all the help you will need.

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  #1  
Old 08-28-2016, 10:04 AM
dtec1 dtec1 is offline
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liquid nitrogen in HT

hey guys so I don't know how many of you use liquid nitrogen in your HT ing but for those that do...so after quenching the knife goes into liquid nitrogen and from what I understand it only takes a few hours but I generally leave it over night. so also from what I understand while the knife is in the liquid nitrogen it may be "protected" a little but as soon as it comes out it needs to be tempered to remove stress or you can get stress fractures (same as not tempering after a quench if you don't use liquid nitrogen) so 2 questions, am I right about the knife being "protected" while its in the liquid nitrogen??? and also can the knife be put in the liquid nitrogen for too long? I usually only keep it over night but for sake of conversation can I keep in in there for 2 days, 4 days, a week? I just want to know if there is a point where keeping it in liquid nitrogen (still un tempered) where it would actually damage the HT rather than help it?
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Old 08-28-2016, 10:35 AM
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Ray Rogers Ray Rogers is offline
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As far as I can tell, the blade is 'protected' while it's in the LN but if you are worried about it you can do a temper cycle before the LN. Once out of the LN the blade must be tempered even if you tempered before it went in - tempering is always the last step.

The benefit of cryo happens in the first hour or two but there is no harm in leaving it in there for much longer periods of time ...


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Old 08-28-2016, 10:47 AM
dtec1 dtec1 is offline
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exactly what I needed to know thanks ray...since I have the dewar now I wont have to do large batches all at once, I don't know why the last batch didn't harden (well it did just no where near as hard as it should be) but I annealed all of them the other day and re hardened 2 yesterday LN overnight and I got one tempering now (I tempered it low again) so I can grind it and test since I tempered low it SHOULD be hard on the brass rod test if not I don't know maybe something is wrong with the steel itself or somebody suggested that maybe they mis labeled it and it is a different steel, but lets hope I just stupidly overlooked something last time and its hard now...ill know in a hour or 2
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Old 08-28-2016, 11:12 AM
jmccustomknives jmccustomknives is offline
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Just out of curiosity, would you be willing to break one of those "soft" blades to read the grain. If the metal is correct something else is going on. Either it's getting to hot or not hot enough. Reading the grain may tell you.
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Old 08-28-2016, 11:38 AM
dtec1 dtec1 is offline
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it should be right on temp wise its all done in a even heat oven never had a problem before, yeh I would break one deffinitly but what would I look for in the grain? I know you want the grain to be small so if it isn't then the temp might be off? I wish there was a way to tell if the type of steel in right because if it doesn't harden then there is really no way to be sure if the steel is wrong....I just took one out of the oven from temper I have to run to store but as soon as I get home I will grind it and do a brass rod test then ill know if its hard or soft.
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Old 08-28-2016, 01:13 PM
jimmontg jimmontg is offline
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You're using 440c right? That steel is pretty much straightforward. Best is a double temper @ 300 with LN in-between. How long are you tempering and you do an oil quench which is permissible with 440c? Dtech the issue may, may be the thin parks-50 oil you use. What is your quench temperature? It should be around 1800 and Parks may form a vapor barrier like water at that temperature. I oil quenched 440c at work, but we used an 80 viscosity oil. The quenchant is the only thing that could make a difference if the material is marked correctly.

What you are looking for is a fine grey velvet looking grain on a break test. I do not see the point if the part passes the brass rod test though. I do not do a brass rod test on stainless. I use files of a known hardness. You need to find a machine shop with a Rockwell tester and have them test small files that you temper at differing temps, plus what you run through your oven, use 1095 as the hardness and tempers are a known quantity.

Those Rc files they sell at some knife suppliers are in 5 point graduations and not much use to you. They are more expensive than paying a shop to RC test for you, $125 btw. 55 to 60 is a big jump. 58 is my bottom line and with 440c 62-63 is your as quenched. Perfect is 59-60 with it. Call Alex at Texas Knifemakers Supply and ask him. He can RC those files as well and can give you a quote for them.

Last edited by jimmontg; 08-28-2016 at 01:15 PM.
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Old 08-28-2016, 01:46 PM
jimmontg jimmontg is offline
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In addition.

I'm looking at tempering data for 440c and it requires a low temperature quench, 300 without the cryo would seem to leave you in the Rc-57 range and cryo would add about 1 point maybe. I made a few knives with 440c, but not many and they were around 58 with a double temper at 300 degrees if my memory serves me right, but they were tempered only for 1 hour each with dry ice in-between and contrary to some HT opinion dry ice does make the steel tougher. Two hours btw is all you really need for LN from what I gather from my HT. Doesn't hurt or help to leave it in longer, leastways that's all my HT guy does and I have no complaints on the quality I get in return.

440c is a high carbon, high chromium steel with less than one per cent of Moly and no Vanadium so no V carbides, but there is more than enough Chromium carbide formation though. The moly just adds some toughness along with the manganese as they prevent grain growth. Moly also forms carbides and helps with the chrome too. Far as I can tell all air quench steels have moly in them and all knife steels have manganese.
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Old 08-28-2016, 02:37 PM
shiny shiny is offline
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I am by no means an expert on the subject. On the contrary. But why not buy a rockwell tester instead of testing files that you temper and let them test for quite a bit of money. You could maybe spend a little bit more and you will know a lot more. I am not trying to end the discussion btw. Just curious why not buying a tester. I think they start from $400,-? You could test files for other ppl when you have it in your possesion.
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Old 08-28-2016, 02:57 PM
jimmontg jimmontg is offline
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I am not an expert on the machines, but a Rockwell C scale tester isn't that cheap. Maybe somebody else knows more about them. Machine we had was about $3000. It had C and B scale as well as Brinell conversion too.

Last edited by jimmontg; 08-28-2016 at 03:03 PM.
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Old 08-28-2016, 03:12 PM
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Ray Rogers Ray Rogers is offline
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The first tester I had retailed for $4500. After that, I bought one from Grizzly for $600 new and it worked just as well. Now Grizzly has them for $750 but they can be found on the used market in the $250 to $400 range.

All that being the case, hardness is just one aspect of a proper HT. The brass rod and occasional breaking of a blade will serve you just as well until that glorious day when you have a pile of extra cash that isn't needed for anything else ...


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Old 08-28-2016, 03:13 PM
jimmontg jimmontg is offline
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Looking at Ebay and they have some old machines for sale in that price range. I do not know enough about the various machines to be able to even guess which to buy.
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Old 08-28-2016, 03:21 PM
Doug Lester Doug Lester is offline
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From what I've read a good Rockwell tester can be had for around $1000. The problem with them is that they need to be in a clean environment with consistent temperature and humidity. I've also read to beware of the used units that you can find out there, especially if the site says no returns, because they may be so old that the manufacturer may not support them anymore.

You also have to learn to use them right. They must have parallel surfaces and you have to be aware that shallow hardening steels can lead to erroneous readings if the piece is too thick and the grain is fine.

Doug


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Old 08-28-2016, 04:23 PM
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Ray Rogers Ray Rogers is offline
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Doug is correct. Using one of these testers is yet another acquired skill set. A few months ago I consulted with an outfit in Nepal that had bought a tester and couldn't get consistent results with it.

An example: these testers need a solid steady floor. The floor in my shop has a very slight flex to it so when I use the tester I am careful to always put my feet in the same spot and never move them or shift my weight until the test is done ...


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Old 08-28-2016, 06:17 PM
jimmontg jimmontg is offline
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You're right Ray. Our's was mounted in the inspection dept. at a constant 76 degrees on a plate on cement as being a machine shop they had tolerances to 0.0001 or one ten thousandth of an inch. Even sheet metal where they had to hold +-0.001. Temperature changes would change the dimensions of a part. Our RC machine was hydraulic and very picky indeed now that you bring the memory back to me. Each part at a minimum had at least 3 measurements and on the big paper cutting blades many more. Always D2 for those. A very tough steel.

I've been looking at Alpha supply's A-11 steel as it seems to be almost perfect for a knife steel. Anybody have experience with it?
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Old 08-29-2016, 10:11 AM
dtec1 dtec1 is offline
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hey guys I didn't have time yesterday to grind out and do a brass rod test had a few family things going on but I will deffinitly have time today we will see how it works out.

jim yes I am using 440c, I have seen a couple different "instruction" sheets and spec sheets for 440c, however the steel njsteelbaron uses for 440c is from Latrobe and according to the Latrobe spec sheet liquid nitrogen is done immediately after the oil quench, and the temper afterward. when I first got the oven and started using 440c I had a bunch of different coupons tested for hardness and grain size they all had a slightly different HT some were also plate quenched from what I found out a plate quench will leave the steel slightly harder than a oil quench BUT the plate quenched steel had consistently larger grain, looking at the temper graph on the spec sheet (wich it showed different hardness compared to temper temp for steel that was oil quenched at 1875 deg F) the graph was very accurate compared to the results from the coupons and the temper is only 1 hour and NO second temper.

As far as a RC tester for me it is not a option right now just got a dewar for liquid nitrogen and it taped out my funds (I didn't even have enough for the dewar my grandmother paid for the rest for me grandma's awesome!) but I deffinitly don't have the money for a hardness tester right now but I think ray is right the brass rod test wont tell you exactly how hard it is but it will tell you if it is to soft or to hard and breaking a blade once in a while will tell you the grain size and I do think that it gives you a sufficient idea where the knife is.
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