?HRC/ Rockwell Scale Question?

[Joe Duder]

Okay just so I'm getting this right, the HRC of a blade is the rating of the blades hardness. The harder the blade the longer it will hold an edge, but is more prone to chipping and damage because it is more brittle. The lower the HRC, the softer the blade, but the more durable the blade becomes. At a softer HRC the blade can then handle more impact and punishment.

Question time:

I see different makers using different techniques to construct there blades and different heat treat methods to achieve their desired HRC rating. Do these different techniques and methods make these blades perform any better than the others if they tested at the same HRC?

Example:

Knife maker "A" uses a brute de forge technique to construct his blades and uses the most basic heat treat method to achieve HRC 59 on his 01 blade.
Knife maker "B" machines his blades and heat treats with Cryo to achieve HRC 59 on his 01 blade.
Knife makers A and B's blades are made to the exact same specs, the finish products are identical. Will one perform any better?

 

[marthinus]

Okay just so I'm getting this right, the HRC of a blade is the rating of the blades hardness. The harder the blade the longer it will hold an edge, but is more prone to chipping and damage because it is more brittle. The lower the HRC, the softer the blade, but the more durable the blade becomes. At a softer HRC the blade can then handle more impact and punishment.

To an extent this is true, the chemical composition comes into play as well. There are some that use N690 at HRC of 60 and AEB-L at 62 in choppers with little to no issues and those are considered to be brittle HRC readings. Same with simple carbon steels such as CPM-3V, O-1, 1075, however, they are heat treated appropriately (therefore onto your next question)

Question time:

I see different makers using different techniques to construct there blades and different heat treat methods to achieve their desired HRC rating. Do these different techniques and methods make these blades perform any better than the others if they tested at the same HRC?

Example:

Knife maker "A" uses a brute de forge technique to construct his blades and uses the most basic heat treat method to achieve HRC 59 on his 01 blade.
Knife maker "B" machines his blades and heat treats with Cryo to achieve HRC 59 on his 01 blade.
Knife makers A and B's blades are made to the exact same specs, the finish products are identical. Will one perform any better?

Yes, the procedure done during heat treatment is very important.

The advancements of cryogenic treatment of steel when done right has been known for some time to improve steels wear resistance and hardness. A bit of googling can find some papers:

Comparison of Cryo on different steels
metals-cryogenic-quenching-of-steel-revisited
key_engineering_materials CRYO treatment on 3 tool steels

There are many other aspects in the heat treat process that can contribute to how a steel will perform for the end line user. Holding time, time between austenisation and tempering, method of quench, quenching medium, maybe even secondary hardening to name but a few. This is why a maker spends time talking to the maker of the steel, Bohler, Crucible, Erasteel, Damasteel and follow their recommendations, they have the metallurgists in house. Some makers claim "secret" heat treat with an obscure amount of tempers, secret heat treating ovens etc, IMO things like that, unless proven with scientific metallurgical tests are false, there is a reason why there is recommended hardening and tempering program with steels from a company.

Some steels do not even like deep cryo treatment. H13 in one of the above studies did not give any respond to cryo treatment.

Not only that, but HRC measurements can be severely flawed if not calibrated, monitored and kept in spec. There was quite a big discussion on HRC measurements on the Spyderco forum and why Spyderco do not publish their HRC data. Simple conclusion is that too many people get focused on those numbers without taking into account other aspects such as the process followed during heat treat.

Here is a good book about heat treating:

Thank you to John D. Verhoeven for making this book available to the public.

Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel - By John D. Verhoeven (2005)

Here was a good read from Kevin R. Cashen (American Bladesmith Society Board Member and has done a lot of work regarding analysing steels under the microscope) regarding some Cryo treatment.

"The skinny is that conversion of retained austenite is the one absolutely proven and uncontroversial claim of the cryo folks, it just works. However... if you have enough retained austenite to notice a difference in 10XX, W's, O-1, L6, 52100, 5160 or the other low alloy steels the better solution is to fix your initial heat treatment (overheating before the quench being the most likely culprit). With these simple steels, any treatment to deal with R.A. after the hardening operation is simply putting a band-aide on the symptom without addressing the real problem, whether folks like it or not the old harsh saying is true, that it is just propping up a poor heat treatment. This is aside from all the other quirky or theoretical claims of cryo and just dealing with retained austenite. If a person is gaining 2 points HRC in their 1095 from freezing, the are blowing it badly in the hardening operation, and need to deal with it, instead of announcing to the world how poor of a heat treater they are by extolling the virtues of keeping knives in the kitchen freezer."

 

[Joe Duder]

Thanks for your input Marthinus.

So, if done correctly, knife maker B's blade will out perform knife maker A's product?

Will it be a night and day performance difference (knife B can cut through 40 more passes through 3/8 hemp rope at 10lbs of pressure, than knife A) or a more subtle difference (knife B can cut through 5 more passes than knife A)?

 

[Joe Duder]

Does a knifemaker care to add his $.02?

 

[me2]

Tothe first point, hardness testing on the Rockwell C scale measures the steel's ability to resist indentation by a diamond tipped indenter at a specified load.

To the second, it is not really true that softer steel is always tougher and harder steel isnt always more fragile. When heat treating, certain tempering temperature ranges will make the steel both softer and less tough at the same time. Naturally, its different for each steel and some dont show it.

For your question, you are correct. There are many ways to reach the same hardness and they are not equal.

To use your example, lets say maker A uses 1095 steel and quenches in used motor oil. After quenching, he/she tempers at 350 for a final hardness of 58. Maker B quenches in a fast quenching oil or water, but has to temper to say 450 or higher to get the same hardness of 58. Same steel, same hardness, but those knives will behave quite differently. Hardness is a quality control check that you got the microstructure you wanted from the HT procedur used. Comparing hardnesses from drastically different HT procedures is risky.

 

[Joe Duder]

Tothe first point, hardness testing on the Rockwell C scale measures the steel's ability to resist indentation by a diamond tipped indenter at a specified load.

To the second, it is not really true that softer steel is always tougher and harder steel isnt always more fragile. When heat treating, certain tempering temperature ranges will make the steel both softer and less tough at the same time. Naturally, its different for each steel and some dont show it.

For your question, you are correct. There are many ways to reach the same hardness and they are not equal.

To use your example, lets say maker A uses 1095 steel and quenches in used motor oil. After quenching, he/she tempers at 350 for a final hardness of 58. Maker B quenches in a fast quenching oil or water, but has to temper to say 450 or higher to get the same hardness of 58. Same steel, same hardness, but those knives will behave quite differently. Hardness is a quality control check that you got the microstructure you wanted from the HT procedur used. Comparing hardnesses from drastically different HT procedures is risky.

So basically the Rockwell C rating is significant, but not as significant as the process used to get it to that Rockwell C rating?

 

[me2]

I feel terrible now. You've just summed up in only one thread what took me years to figure out. Ok, I'm done crying now. Yes, that's it exactly. You can take a piece of 1095 at about 25-30 Rockwell C and just ruin it's toughness if you try. You can also fix it and harden it up to 60+ and have no toughness issues. Something to keep in mind if you're a steel head. Tempered martensite (the microstructure we're after in blades) is tougher than pearlite at the same hardness. The trick is you can't get pearlite as hard as martensite without really knowing what you're doing and having some special processes. Pearlite is one of the soft microstructures of steel. It's what you get if you heat up something like 1095 and cool it from 1450 F to room temperature in air.

Rockwell testing is one tool to make sure you're getting the microstructure you think you're getting. As an example, say you have a nice heat treating procedure you want to try. You try it a few times and have your blades hardness tested. What do you know, they're all within a point or 2 of each other. Now you know your procedure is consistent and reliable (2 points isn't that much of a difference). However, if your procedure had a hardness spread of 7-8 points, you don't really have control of it. Hardness testing cannot tell you the microstructure, or put another way, it can't tell you why you get the hardness reading you get. Put yet another way, hardness is a result of the microstructure of the steel, not an end unto itself.

 

[Joe Duder]

You'd think more consumers would be more interested in the heat treat process than the actual Rockwell C rating. But it seems that quite a few people who purchase blades (including I) and knife makers as well as salesmen, find that the name of the steel and the HRC is ample information when purchasing or selling a knife. At $150 or less, I think that info is good enough, but above $150, there should be a bit more info on the heat treat process IMO.

 

[coli]

I learned a lot reading this thread, thanks for the info

 

[me2]

The heat treating process can get really complicated. Most people just want a knife to cut. They don't usually care to know why. It's interesting to know, but I don't really think it's practical for all makers and especially manufacturers to put out there what their procedure is. You'd have everyone who's watched a youtube video on making a knife telling you the procedure was wrong. Of course we're not far from that now, with just hardness and steel info available.

With a goofy HT procedure, you can get a blade that both chips and rolls, won't hold an edge, and cracks when you try to baton with it. And that blade will measure the same hardness as another that holds an edge like diamond and takes a beating like an anvil.

 

[Fanglekai]

You'd think more consumers would be more interested in the heat treat process than the actual Rockwell C rating. But it seems that quite a few people who purchase blades (including I) and knife makers as well as salesmen, find that the name of the steel and the HRC is ample information when purchasing or selling a knife. At $150 or less, I think that info is good enough, but above $150, there should be a bit more info on the heat treat process IMO.

Few people want to learn enough about metallurgy to understand what's going on. Various companies use HRC as a marketing tool to sell their product, even though the HRC reading is only meant as a quick quality control check to determine if something went wrong in heat treatment. Two blades each reading 62 HRC could have vastly different microstructures and behave very differently. The microstructure is what's important.

 

[mattmanyam]

Few people want to learn enough about metallurgy to understand what's going on. Various companies use HRC as a marketing tool to sell their product, even though the HRC reading is only meant as a quick quality control check to determine if something went wrong in heat treatment. Two blades each reading 62 HRC could have vastly different microstructures and behave very differently. The microstructure is what's important.

This.

X 10000.

(Nice explanation, btw.)