Heat Treating Air Hardening Steels

grease-man

grease-man

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Apr 9, 2006
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My father told me a while back that he wouldn't mind a knife made from S-7. I thought I'd like to arrange that for him sometime this year. My question is about the air quench. Would I just remove the piece at temperature and let it hang in the air? Or would cooling it faster be better? (like in front of a fan) I know some people like to use quenching plates, but I don't have the time or interest in building a rig for that. Mainly because I mostly work with steels of the oil hardening variety, and shop time is a precious commodity. Any and all replies are appreciated. Thanks. -Matt-
 
2 steel blocks with smooth sides and the knife sandwiched between them is a cheap quench plate set up. May not stop a heavy warp if it was going to happen anyways, but acts as a heat sink. Havent used S-7 so I'll let other answer about its HT.
 
The problem you may encounter with air hardening steels is the controlled soak time and temperature in the HT.
Stacy
 
I guess my first question would be "Why S7"?

It's a great steel for chisels, but rated as low to medium wear resistance - and at most desireable hardnesses, the toughness isn't there.

You'll have to hold it soaking at 1750 degrees for 30 to 45 minutes - and that's not something you can do by color or with a magnet. Being 50 - 75 degrees out in your tempering will result in a too soft blade so accurate temps are important there too..

It can be quenched interrupted oil quench - but that isn't for the faint of heart either.

Then again - if it's something Dad wants, it's almost certainly worth the old college try. :)

Rob!
 
I did forget to mention that I have a digitally programmable kiln, so soak time will not be a problem. The problem may be that the kiln has a high temp rating of 1700F. I thought maybe the extra 25 or 50 degrees might not make that big of a difference, but it sounds like it might. I could always try it, and if it doesn't work, get it to someone who can do it. Dear ol' dad is worth the extra money. :D My other question is the air quench. Do I just hang it up by a wire and let it come down slow or try to cool it faster somehow? Thanks again. -Matt-
 
For the air quench you can plate quench it. Remove it from the oven and take out of the foil. Quickly place it between two large metal plates (aluminum is the usual metal) and allow it to cool while clamped between them. You can skip the plates (they help avoid warping, though) and just hang in front of a fan or blow compressed air down the blade. Your kiln might reach the extra 50 degrees with a long warm-up time.The problem is that at full power it may shorten its life a bit. I would say make the knife and try it in the kiln ( it will harden at 1700, just not perfectly. Give it a little extra soak time). If it does not get hard enough, send it to one of the HT guys who does stainless steels. They are all set up for those temperatures.
Stacy
 
I did as RJ Martin sugested with my first batch of D2 and had good success, Tempered out to RC 61
 
The specs I have for S7 (from Crucible and Timken) say that max hardness for S7 is HRC57. This is about like a Schrade pocket knife. I have never liked the edge holding capabilities of those knives. I have made some knives from S7 when I wanted something that would hold up to chopping and being hit with rocks to cut bone etc., but for a hunter I think A2 or D2 are better choices.

S7 is about as expensive as D2-higher than A2. I recycled some percussion bits after the carbide inserts broke and had pretty good luck with them.
 
Based upon the info given here, I think I'll just stick to the steels and HT processes that I know. I'll pick a better steel for the application, and explain to him why. Since I'm his kid, he's gonna love the knife no matter what it's made of anyway! :D Thanks again for the info guys! -Matt
 
S5 is tougher than S7 and can be quenched in oil (unless you are in California, where environmental law prohibits oil quenching).
 
Rob! said:
I guess my first question would be "Why S7"?

It's a great steel for chisels, but rated as low to medium wear resistance - and at most desireable hardnesses, the toughness isn't there.
Click to expand...

I prefer S5 as it is stronger and harder and tougher than S7, but anyway S7 does have some useful properties as a cutting tool and is in fact used in cutting tool applications.

How can you claim the toughness isnt there at high hardness values mate? Have you actually looked at the hard numbers for yourself between shock resisting tool steels and other types of tool steels at similar hardness? If you did, you'd realise the mis-truth in your claim.

As for wear resistance, people need to accept that knife tribology isnt understood in scientific terms. People look at abrasive wear but how can we be sure thats entirely relevant, or what percentage of relevance it has overall in comparison to say adhesive wear? Other studies have shown that fine grain and carbide size can be questionably more important than high abrasive wear resistance. Whats for certain is that shock resisting tool steels dont have great wear resistance in comparison to say ceramic cermet knives, but the fact they will resist shattering and their very high yield strength in comparison to other materials makes them inherently more reliable. I'd rather resharpen a blade than be stuck with a broken blade. And, it isnt just "knife abuse" where these properties come into play - the ASM have done studies where microchipping has occured in cutting tools that lack toughness in real world cutting tool applications.
 
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nullack said:
How can you claim the toughness isnt there at high hardness values mate? Have you actually looked at the hard numbers for yourself between shock resisting tool steels and other types of tool steels at similar hardness? If you did, you'd realise the mis-truth in your claim.
Click to expand...

Actually, yes I have - at least in this instance. http://www.crucibleservice.com/datash/dsS7v6.pdf?CFID=605463&CFTOKEN=28682308

The steel is described as low to moderate wear resistance and at any useful hardness for knives, it has barely begun it's toughness curve. Perhaps we have different opinions on the interpretation of useful hardness.

IMHO, there are simply better choices - CPM3V coming to mind if toughness is a significant requirement.

http://www.crucibleservice.com/datash/ds3Vv5b.pdf?CFID=605463&CFTOKEN=28682308

Those were my reasons for questioning the choice of S7.

Rob!
 
Rob what do you think that data sheet shows bud? :)

You say the toughness isnt there at the higher hardness levels. I suggest you go and compare the toughness of S7 at HRC 58 against L6, A8, A2, D2, M4 and a bunch of other tool steels. In fact, in many comparisons to other tool steels it is more tough by a factor of over 300%+ at the same HRC values. Shock resisting tool steels are specifically designed to have high toughness in tool making at relatively hard tooling levels.

CPM3V does not meet the toughness levels of shock resisting tool steels. Ive previously provided tensile and yield strength data on some shock resisting tool steels - what is the tensile and yield strength of cpm3v at different hardness levels mate? The yield strength of S5 in particular is exceptionally high.

As for wear resistance, specifically abrasive wear resistance, ASTM tests have shown it to have more resistance than L6 as I previously shared. If wear resistance is the primary issue cutting tools should be sintered carbide or ceramic cermets not ferrous alloys.

There is good reason why shock resisting tool steels are used in the tooling industry for cutting tools - it has desirable mechanical properties for the application.
 
+ 1! on the S-5, besides your dad may like a Better more obscure versioin of S-7 really your call either one should cut o.k. your dad probably wont be able to tell the difference, and if S-7 or S-5 is what will make him happy, then whatever. ask him if its alright to swich steels before you make the knife.
 
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What was the reason for bringing this thread up to the top? It's more than two years old. It doesn't matter how many threads you guys bump and troll together, people just aren't drinking the koolaid.
 
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