Useful HT metrics other than RC?

[vanadium]

This one's a question for the steel nerds out there.

It seems that although there's a lot of variation in heat treat techniques and results, the differences in performance aren't necessarily captured by the Rockwell C number. Negative examples that come to mind are CRK's "old" heat treat of S35VN, and the "bad" ZT ELMAX that existed for a short while. Positive examples include the HT work of Paul Bos, Paul Farner, and Renzo Fantoni.

In an ideal world, is there another standard measure of heat treat results, besides RC, that makers should be putting on their spec sheets?

 

[91bravo]

I used to work in a heat treating company and we used Brinell testing instead of Rockwell. For plate and bar stock. Both methods are accurate and the Brinell dimple has to be measured through a scope. I'm not sure how they measure the width and depth of the dimple on the RC scale.

 

[Twindog]

Hardness doesn't give you the whole picture. You really need to know the microstructure of the steel -- the grain size and size and distribution of carbides. Any steel allow of a given Rc could have wildly different strengths.

It would be nice if makers had a representative sample of their blades tested for microstructure.

 

[me2]

Short answer is no. There are few enough knife buyers that understand hardness. Any other single test would be worse. Combining several tests would be good, but if the average knife buyer doesn't understand one test, it only gets worse the more complicated it gets.

 

[Twindog]

Short answer is no. There are few enough knife buyers that understand hardness. Any other single test would be worse. Combining several tests would be good, but if the average knife buyer doesn't understand one test, it only gets worse the more complicated it gets.

Rc hardness is certainly a common and useful measure of knife blades, but its usefulness can be limited -- even misleading. If I remember correctly, Sal at Spyderco doesn't like to advertise hardness numbers because they are often misleading.

Say you have two makers who are selling a knife in 52100 steel. Both measure out at 62 Rc. One maker may have been aiming for that number, the other may have been shooting for an Rc of 60, but missed the tempering process. One knife is strong and tough, with good edge wear resistance. The other is brittle, with poor edge stability.

One of my favorite charts was posted by Juha. It shows how a simple Rc number can be misleading. The chart below shows the toughness of the steel varying wildly, even though the hardness and steel alloy remains the same. Same steel. Same Rc. But toughness varies from almost 80 J/cm2 to about 15 J/cm2. The difference was in the microstructure of the steel, specifically grain size.

 

[Rich S]

While hardness on any scale can be misleading and/or wrong. I feel that blade configuration plays just as big, maybe bigger role in blade performance than does it's RC rating. Just MHO.
Rich

 

[Tommy-Chi]

How to verify blade steel? (I take a lot of notes on Blade Forums so not all these ideas are mine.)

- Ask the maker/supplier to provide material certs or do a SEM chemical analysis (one is less expensive than the other). And if you're going to go through all that effort, might as well send the blade under a Rockwell hardness testing machine too
- HRC although this may be the least helpful
- Cut tests
- Submerge part of the knife in vinegar and see if it (allegedly D2) tarnishes. D2 will tarnish as will others, but you need to know which.
- Price is no guarantee of quality.