Several Heat Treating Questions

Ban

Ban

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Is the critical temperature for any given steel necessarily the point where the steel has reached a non-magnetic state? I am asking because I want to find out if this applies to all steel. i.e. Simple High Carbon Steel, CPM High Alloy Steel, Air Hardening Tool Steel, etc…

The other question pertains to D-2. I have read that it should be preheated at 1450-1550 deg F and then the temp should be raised to 1850-1900 to harden. From what I understand the preheat is to reduce some stress to allow the steel to adjust to the higher hardening temperatures. My question is… can you overdo the preheat? The reason for the concern is because I notice that it takes quite some time for a furnace to go from 1500 degrees to 1900 degrees.
 
I am very interested in seeing the responses to the D2 question. D2 is my favorite steel. I use it at work for tools and make lots of knives out of it! When I heat treat it at work, I have the luxury of more than one oven. I go right from a preheat oven to the hardening temp. oven. At home, I preheat first then the oven "ramps" up to high temp in about a half hour. I get identical Rc numbers. I cryo treat then double temper the knife blades. After all that I just figure I have done all I can do to releive the stresses in the blade. Again, I will be following this topic to see what other people have to say!!!
 
Ban,
The non-magnetic holds true for simple carbon steels and those with a lower levels of alloying elements. Not so for the the higher alloy content steels. The reason for the soak times is to allow the elements to go into "solution", I guess the best way to word it would be to say that the extra time is necessary to allow all the elements to "mix" within the overall matrix. If the "soak" time is ignored, the full potential of a particular steel will never be realized. As for the D-2 question, it doesn't fit in with what I do, so all I would be giving you is metallurgy theory,instead of experience.
http://www.mtn-webtech.com/~caffrey

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Ed Caffrey
"The Montana Bladesmith"


 
I dont think you can overdo the preheat, assuming your blade is wrapped in heat treating foil, in a salt bath or controlled atmosphere furnace. Soak time at critical temp in very important also.

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www.simonichknives.com
 
Concerning D2
1. Normally when you come to the point were get into the autenitic area of a alloy every time you spent in it counts. Because you start the transformation apha/gamma, the grain starts to grow and the carbide starts to get into sollution. But D2 is a high alloy steel, so that the grain grows fairly slow below 1850°. When you come into the area between 1850°+ this starts to gain speed so that soaktime is reduced half every 75° higher.
Now, that means the rest on such a ramp at fist depends at the diameter of the treated pice and the amuont of HT goods in the furnace. For those small pices in an average athmospheric furnace, I suggest 5-8 minutes to equalize heat in the treatet body for preheating.
When you furnace is slow in getting up from the last preheatstage to the 1900 range, you should try to get another one that makes your preheating. Why? Although sollution an grain groce is not so segificant below 1850° compared to the range above, it happens. The heating up in the furnace is not a thing that is very exact in high temperaturs even by using digital control. It works average. the more accurate way is to devide this process into two seperate parts (preheating and austenizing). you will get better control upon the grain size because you really want to have fine grain, for superior performance. The prediscribed 30 minutes in reaching the austemp seem to belong to a good furnace.
HT for D2 suggestions :
Preheating equalizingtime 5-8 minutes
1.Stage 850°F
2.Stage 1250°F
3.Stage 1800°F
austenizing
1940°F soak 20 minutes
Quench Oil for vacuumfurnace maximum pressure on H2 or He
cryo within 2hours!!! soak 1hour
temper 390° soak 1/2 hour
cryo again soak 1/2 hour
temper 390° 1hour (lead you to ~62 HRC)
adapt temper if needed

have lots of funn


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I agree with most all of the theory/experiences of all the other participants on these questions. Remember that most, if not all, heat treat info from the steel suppliers is compiled from tests run on 1" cubed steel, not on much thinner knife sized pieces which have been ground/forged. Bearclaw

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Bearclaw,
My hats off to you!!! I was beginning to think that nobody was getting the picture on the heat treat thing. A lot of people are out there "thumping" their heat treating guides, not realizing that there is a difference between the specs/methods for a 1" cross section,(which is the standard that the heat treat guides are written on) and something the thickness/dimensions of a blade. Now thats "Thinking in the Fourth dimension!"

http://www.mtn-webtech.com/~caffrey

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Ed Caffrey
"The Montana Bladesmith"


 
Well shoot, why didnt I think of that! Ed and Bearclaw, you guys are absolutely right. I do most of my own heat treating due to the control I have, and I have some 420-V to try but my furnace wont do it so I went looking for a KNIFEMAKER to do the treat for me and not a commercial shop. Knife blades and blanking punches are 2 totally different tools. Thanks for bringing up that aspect Bearclaw, the bells went ding ding ding!

Hey Ed, how was the Montana show, I was too busy getting married to attend! ha ha

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www.simonichknives.com
 
Hi folks,
seems I got the fire burning again.
Austemp:
Well grain growth is a funtion of temperatur, time and the alloy itself and its beginning structure. Note the 20 minutes are just right for fine grain, I know because I did metallograpic analysis on it.
By the way i had a industrial vacuumfurnace that went pritty quick to where i wanted. This fact needs to be respected.
But annyhow i really don't care about what datasheets say, because they are designed by the steel manufacturer as a suggestion for the heattreater and user on a standard platform 1"square, etc....
Neither D2 was designed to be a bladesteel nor the ht suggestions are made therefor.
So I need to think different. If i am really tinking of applying someting like D2 on a blade, where I will have a very coarse structure for knife specs, because of the huge ledeburitic carbides(Primary carbides), I need to make this brittle thing as tough for the needed hardness as possile. The way to do so, is to make fine ausgrain that transforms into fine martensite by cooling.
I was doing intensive analysis upon that and more, in my work.
So the way I discribed to heattreat was designed to get the most out of that steel concerning knife application and respecting the heatingmethode (furnace) and diameter of the crossection.
Because of the fine grain that I got and a higher solution of the carbides, I got more cuttingedge stability and more corrosion resistance.
Personally I almost don't use the high alloy stuff annymore, ecept for the need of corrosionresistance and raw agressive translatoric cutting action.
Well hope that helped you, try out and tell me what you think.

[This message has been edited by Roman Landes (edited 01 September 1999).]
 
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