RC Hardness & a heat treat question

D

David Metzger

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Oct 8, 2010
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I don't have much experience in choosing RC hardness for blades. I realize some knife makers use personal preferences or steel manufacturers recommendations. Some study charts and try to determine where they start loosing too much toughness. Others have just done their own testing.

From what I have read there is a very significant difference in blade hardness in that 56 - 62 range. I am thinking I would almost always prefer 60-62RC because I am not chopping logs. I will be working in O1 steel for the next few months and would like you opinions. I plan on trying a large range of knives from Chef - Hunters - Whittlers - Fillet -Camp - Paring - Military etc. Usually flat or convex ground 1/10 to 2/10 steel. 5/32 the mostly.

Any way, I would appreciate your opinion or any good research you know about or any particular type of knifes that would benefit from low or high RC.

Secondly, will I be OK with a smaller - regular top loading kiln (at least with O1) that also handles pottery for my wife? How many knives can I practically heat treat at one time - and any important factors like - Do I need to get the temp up again in the kiln between blades? With timing and temps so critical how is heat treat done in a commercial shop?

Thanks for your help in advance. Your input on just what you find interesting is great!

David
 
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Most folks who are starting out in knifemaking tend to believe that "harder is better". With more than a couple of decades of knifemaking behind me, I have learned that is not always the best approach.

High hardness might seem best, but it produces other undesirable issues...including brittleness, and difficulty in re-sharpening. The logic often is that if a blade is super hard, it will hold an edge longer...not necessarily true. If edge geometry is too thin for a given level of hardness, the edge will easily chip, producing a dull blade more quickly than an edge of lower hardness that does not chip. If the edge geometry is thickened up to the point where edge chipping is not an issue, it's often overly obtuse, which makes cutting with the blade more difficult. The point I'm trying to make is that there is MUCH more involved in a blade that a given steel at a given hardness level. Each blade type/style requires it's own specific geometry for the given steel AND the given hardness in order to produce a superior cutting tool.

You asked how many blades can be heat treated practically? My opinion is that in order to produce the best blade(s) you can, they should be done one at a time, particularly for a steel like O1.
What you have to understand is that production knife facilities do not necessarily pick their blade steels based on what will make "the best" knife. In a production situation, steels are often chosen for their ability to be worked/ground/heat treated within their facility, with that facility's equipment, in the most efficient manner. Quality of the finished product often takes second place to the ease and efficiency of producing the product.

Knife factories generally operate off the premise of...How many knives can we produce in a day, to an ACCEPTABLE level of quality that the public will purchase at the price point we set?
This differs from a Sole proprietor custom knife maker in that most all who are/have been successful will produce the very best knife they can, no matter how much time, effort, or material is required.
 
Thanks Ed, I have read many of your post and am honored. I do want to produce something better than mass producers. I think I may have gone overboard on hardness because I really hate my kitchen knives that I would guess are in the low 50's. I will try to take a critical look at each blade I make.

Thanks Again
 
I think a good range to shoot for is Rc 59-60. Ed is spot on about the wrong impression many have on high hardness equaling great blades. There is a place for all hardnesses from Rc54 to Rc 64. The edge geometry and the hardness go hand in hand and should match the function of the knife.

You can use a top loading kiln, but will need a long pair of tongs, and some kiln gloves, to reach in without incinerating your hands. As far as multiple blades go, after the soak time, the kiln can drop in temperature a bit without any problems for the next blade(s). I would think 3-6 blades would be a good maximum per batch, depending on your skill level.

I will expand on the multi-blade/temperature thing:
When you austenitize the steel there is a certain temperature that you want( target temp) . This temp is calculated to allow the carbon and alloy ingredients to be distributed, but not to allow for the steel to grow oversize grains. The hold time ( soak) at the temperature gives the ingredients time to diffuse.
Once the soak time is up, the diffusion is complete ( for our purposes), and we quench the steel to lock in the configuration of the structure. The steel is cooled in the quench at a speed that prevents the formation of pearlite, and ends up as martensite. The critical spot on this cooling curve is around 900-1000F. We call this the "pearlite nose". If we cool fast enough to supercool the austenite, we miss the nose and go to martensite....too slow and we get soft pearlite and a useless blade.
Many worry that the blade needs to be plunged in less than one second from 1500F to below 900F ( specs for 1095). This is not so. Once in the quench tank, the quenchant needs to be capable of that drop, but before the blade enters the quenchant, the steel only needs to stay above 1350-1400F, at which point it begins to convert to pearlite if allowed a slow cooling.
The steel will not cool down in air or an oven that has dropped 50-100F to any degree that will be a problem.

So, the Cliff Notes version of the above is:
Just close the oven/kiln door and wait a few minutes between blades to allow a bit of rebound. It is not necessary to return to the exact target temp and re-hold for the soak time once it has been reached the first time.
 
Thanks Stacy, That really helps. I can read the heat treat steps all day long, but it's the timing and requirements in those in between steps that could really mess up an otherwise good heat treatment. I understand a lot more now. Excellent post.
David
 
Gary, could you please comment on multiple blades in air-hardening stainless such as s30v? Since a soak time of 20-30 minutes is recommended at 1950° do you have a ten minute window to plate quench them? As well, would you wrap the blades separately in foil or put multiple blades in each packet?
 
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Ed Caffrey said:
Most folks who are starting out in knifemaking tend to believe that "harder is better". With more than a couple of decades of knifemaking behind me, I have learned that is not always the best approach..
Click to expand...

I wish I can explain it to every customer :(
 
As I understand it, many pottery kilns aren't meant to be opened at their peak temperatures. The sudden drop in temperature from opening the lid could damage parts of the kiln not meant for that sort of thing.

I don't know how true this statement is, it's only what I've heard. Hopefully someone more knowledgeable on the matter will pitch in.
 
zaph1 said:
Ed, could you please comment on multiple blades in air-hardening stainless such as s30v? Since a soak time of 20-30 minutes is recommended at 1950° do you have a ten minute window to plate quench them? As well, would you wrap the blades separately in foil or put multiple blades in each packet?
Click to expand...

I know that I may be jumping into a direct question and hope both of you will forgive the intrusion but I may be able to answer your question. To address your question on S30V, I used it exclusively for the first year it was available and it took me a large number of experiments and several phone chats with Ed Severson (it's primary engineer) to finally get a H/T that I was satisfied with. I guess what I am getting at is that there are other air quench steels that I would recommend more unless you have a lot of experience with heat treating. S30V can be more that a little finnicky with the H/T. (If you want the exact method of H/t that I used for it send me a PM and I'll be glad to discuss it with you.)

You definately want each blade in it's own packet. With a more forgiving air quench steels like A2/D2, I will usually just snip off the end of the packet straight from the kiln, remove the blade, and place in a cooling rack in still air. I get a little bit of scale this way but not enough to cause any problems and it is easily removed producing a very consistant hardness.

Most air quench steels aren't quite as fussy about the time from critical to below the nose but I wouldn't waste any more time than necessary.

Hope this helps a little.

Gary
 
Good enough answer, Gary. Thanks. Do you recommend the plate quench or just air quench for s30v? It is definitely a good use of the programmable oven. I'd hate to try to HT s30v using a forge, hehe.

PM Sent.
 
On Ed Severson's advice, I ended up doing a plate quench. I had a problem with getting the blades out of the packets fast enough for optimal results however and press quenching the enclosed packet straight from the oven didn't work that well either. What I ended up doing was to quench the enclosed packet very quickly in water to pull the heat out of the foil and then press quenching the enclosed packet between two 2" thick pieces of steel. YOU MUST MAKE SURE THAT THE SEAMS ON THE PACKET ARE WATER TIGHT! This gave me the most consistant results.

Gary
 
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