tough vs hard

Joined
Aug 21, 2002
Messages
30
I hear the words tough and hard thrown around when describing knives. For instance, when you quench in water you get a hard blade but when you quench in oil you get a tough one or something like that. I am open to the possibility that I might be a little "touched" but i'm having a hard time understanding or seeing the differance between the two.
 
Smack the spine of your blade into the edge of your vice.
If it dents the vice and stays intact you have a tough blade, if it cracks it is a hard blade.
 
I almost cracked a vise once :)
Hard and tough is sometimes hard to explain but Robert pretty well has it down.
 
What Robert and Raker said. A blade will be very hard if quenched
in water or oil, if it has been heated past a critical temperature.
The specifics will depend on steel type and size and thickness of
blade. Many types may crack when quenched in water. Once the blade is hard, it has to be drawn, tempered, ( heated to a much lower temperature ) to make it less hard and more tough ( less likely
to break ). For each steel type there is a sweet spot ( not too hard
and not too soft. We have custom knives made, or make them ourselves, so that they can be JUST RIGHT, to match the task it is designed for.
 
I'd say Dan about covered it. And the intended usage is very important. If I'm making a neck knife(as I will be doing a few tomorrow), I generally will go harder(though more brittle) than I will with the big chopper that I have waiitng around for heat treatment. With the chopper, due to it's intended usage, I will temper it down more, so ti's not as hard, but will be tougher and able to stand up to more abuse.
 
twynn5586

Metallurgically speaking, there is a "scale" or a "spectrum", it sounds like this...

Hard = Strong = Brittle = Unstable

Soft = Weak = Tough = Stable

Its becomes a balance to play and there is always measure in between. The words and characteristics are almost directly associated with each other and knowing compromise is the key to making a good steel tool.
 
It is no wonder you are confused on the subject of hardness and toughness twynn5586. This confusion is the natural result of talking to people who are themselves confused ie. the blind leading the blind.

Since I am a sighted person in the land of the blind ;) I will try to properly explain both hardness and toughness. As an added bonus I will also try to explain how they are related in the world of knife steel.

First you must know your terms:

Hardness is defined to be "the ability of a material to withstand indentation by another hard object".

Toughness is defined to be "the amount of impact energy the material is able to absorb before failure".

Now that we know what hardness and toughness are how do we go about measuring them?

Hardness is most commonly measured using the Rockwell Hardness Test. In this test an indenter is pressed into the surface of the material being tested. The test compares the difference in penetration between a minor initial load (98N) and a major additional load. The result of the test is read directly from machine. The Rockwell Hardness Test provides for different hardness scales for different materials.

Code:
Rockwell Scales

Scale 	Indenter 	    Additional Force(kN) 	Application
A 	120&#176 Diamond Cone 	0.59     	Sheet steel ; shallow case hardened
B 	Ball dia 1.588mm 	0.98     	Copper, Aluminum alloys, Low Carbon Steel
C 	120&#176 Diamond Cone 	1.47     	Most Widely Used -Hardened Steels, Cast irons etc
D 	120&#176 Diamond Cone 	0.98     	Thin but Hard steels

The C scale is used for hardened knife steels. So if you see something like "58 HRc" or "58 Rc" or "58 Rockwell" or "hardness of 58" etc. the author probably means that the knife blade has (or claims to have; many makers lie about this) a hardness of 58 on the Rockwell C Scale. Higher numbers indicate harder materials.

In practical terms the hardness of a blade determines the blade's resistance to scratching and edge roll. Increased hardness also strongly correlates with increased abrasion resistance in a given material.

Toughness is usually measured with a Charpy Impact Testing machine (see link for a more complete explanation). This machine measures the amount of energy absorbed when a standard sample is broken by impact. This energy is usually given in either foot-pounds (ft-lb) or newtons (N). Due to end user ignorance charpy results are rarely advertised. Instead meaningless marketing terms are used; examples include "tough", "strong", "resilient", "durable", "robust" etc. Higher charpy numbers indicate tougher materials.

In practical terms the toughness of a blade determines both the amount of abuse (prying, impact loading etc.) the blade will withstand and the blades resistance to chipping.

Now that we know what hardness and toughness are how do they relate to knife steels?

First you should know that in a given steel as you raise the hardness the toughness decreases. This relationship is due to the physics of the material and the mechanics of crack propagation and is not related in any meaningful way to the type of heat treatment, quenching agent, tempering method, forging technique, mystical incantations etc. the maker used.

So to get an improved combination of toughness and hardness you must change materials, nothing else will help.

If you need more explanation/clarification let me know.
 
Back
Top