The purpose of hardening steel is to create an edge that will stay sharp thru hard use, along with creating a microstructure in the blade with the properties you want in that particular knife.

Very good answer Clay.

I understand that hardening the blade makes it hold a good edge but doesn't tempering bring it up to the same hardness as hardening and tempering.

Daniel

After hardening, the purpose of tempering is to take some of the brittleness out of the blade and make it more tough. As an example, we harden a blade and it is 64 on the rockwell C scale. This blade is hard but very brittle and prone to chipping on the edge. By tempering the steel, we can bring the hardness down to say 58-59 on the Rockwell C scale which will still hold a good edge and it will hold up to heavy use better than the fully hardened blade. We do not want to temper too much to and make the blade too soft. That is the key to the process -- a balance between hardness and toughness. Tempering alone cannot achieve this. If the hardness of the blade starts at 45 on the C scale, tempering it will not raise the hardness-- just keep it the same or make it softer ( if the temp is high enough)

There are volumes on this subject on the internet and in metalurgy books and I certainly canot do justice to what is already in print. I suggest "Steel Metallurgy for the Non-Metallurgist" by J. D. Verhoeven. This book can probably be gotten from your local library via an inter library loan.

I also noticed your location and I would suggest attending Gary's hammer in in Branson in September. Attending Brion's part on the JS knife will answer many of your questions. See this link for more info: Gary's hammer in

Brian

"Hardening" and "Tempering" are two distinct and different actions associated with steel...this is how I often explain it to students:

Hardening: Placing the steel in a state of it's highest possible hardness. While this may sound good in relationship to a knife blade, the problem arises that along with hardness come brittleness.

Tempering: This is a "controlled softening" of the steel. The objective being to find that "sweet spot" in the hardness (coupled with the type of grind applied to the specific knife) range of a specific steel where the steel is hard enough to hold a long lasting edge without chipping, yet not be so soft as to deform or "bend" at the edge during use. This "sweet spot" within the hardness range does not only depend on the specific steel type, but also on the blade/edge geometry.

This is the "tightrope" that ever knifemaker walks, and is the reason that most of us are so intense about testing the blades we make. What I seek to achieve is what I term the "proper working hardness" for a given blade. The "proper working hardness" does not necessarily correspond to a given number on the rockwell or brinell scales, but involves numerous aspects, including the heat treat (which includes both hardening and tempering), the specific steel, and the geometry of both the blade and the edge. I not only seek a blade that holds and edge and is durable, but it also must be relatively easy for the owner to re-sharpen. (this is something that I personally feel that a lot of knifemakers who place too much emphasis on Rc numbers overlook)

There is a lot more to a knife that just steel and handle material. It's a blending of many different areas to achieve a desired out come. Although I cannot remember where it came from, a quote that I heard many years ago, and have always remembered is...."Steel is the heart of the knife...Heat Treating is the spirit."

Thank you Ed for posting the definitions, it can’t be done enough since the English language has totally botched up the difference between hardening and tempering and this has lead to more confusion for the majority of the population. Since the Victorian period forward literature mistakenly refers to the entire heat treating process as “tempering”. But yet when somebody reaches their limit and snaps, they have lost their “temper” so somebody must have gotten it at one time in our distant past.

Something else that is definitely worth discussing on this topic is the idea, and we all have encountered it, that instead of toughening a blade in the temper, why not just slow down the quench so it doesn’t get so hard in the first place? I shudder….!

For a truly sharp knife to continue to cut it needs to hold its shape against pressure, abrasion and even impact on a cross section less than 1 micron. When you do the math this concentrates those forces to incredible levels and give you a whole new appreciation for a good edge holding blade. The strongest and most abrasion resistant phase in steel, hands down, is martensite* (let’s leave carbides for another discussion). If you want the best edge you can get, you want it to contain as much martensite as possible.

You get the most martensite by careful heating and thorough quench hardening. If you nailed the quench the blade could now Rockwell at 65 or better. If you then draw that blade back in the temper you will have perhaps 61 Rockwell martensitic steel that will hold an excellent edge. By slowing down the quench you will only ever achieve 61 Rockwell. Yes, same hardness, but for different reasons. The slower quenched blade will come out of the hardening at 61 because it is a mix of martensite and soft, poor edge holding, pearlite.

And to make matters worse you haven’t even tempered it yet, so if you temper the slower quenched blade in the oven next to the fully hardened one it will lose even more hardness and you may have a 55 Rockwell edge instead.

@Daniel - we are spoiled to have folks like Ed Caffrey & Kevin Cashen sharing their insights and knowledge. They have both posted extensively on heat treating and I try to absorb every post. I may not be ready for salt baths but I relish Caffrey's postings. And Cashen's postings? Mind warping. I usually have to re-read them.

You could do far worse than go to the search box at the top of the website (the one under your log-in) - type in Kevin Cashen - and start reading old threads related to heat treat.

Thanks guys that helps a lot.Yes Michael we are spoiled up until a week or so ago I would always google any questions I had, but then my friend Kyle Royer suggested I join ABS and now that I have it's amassing how much more I can learn by asking on this forum rather than using google.

Daniel

Now that is what we want to hear. Welcome to the American Bladesmith Society!

Daniel, We are glad this site and the members could be of assistance. There is some really good knowledge here. We are indeed fortunate to have Kevin and Ed and the rest of the members sharing what they have learned.

Welcome again, tell Kyle hello, and I hope to see you in Branson.

Brion