Steel

[breakdown52]

When people talk about the toughness and hardness of a steel what do they mean by the two. When I've heard of them they are referred to as two different things.

 

[Ankerson]

When people talk about the toughness and hardness of a steel what do they mean by the two. When I've heard of them they are referred to as two different things.

Hardness is technically the actual HRC hardness of the steel when people talk about it if they actually know what they are talking about in the 1st place. I have seen the term used wrong so many times I can't even count all of them....

Toughness is how tough the steel is, mostly referring to the edge deforming or hard to break.

 

[bdmicarta]

Hardness is basically another way of talking about the strength. A steel with higher strength is harder, IOW takes more force to make a small dent into it which is basically how hardness is tested. Toughness is a measure of how much the steel can deform without fracturing. A steel with low toughness will fracture easily, or chip or tear depending on configuration.

The way steel works, these 2 properties are at odds with each other and there is usually a tradeoff between them. Make the steel harder, it is less tough, make the steel tougher, it can't be as hard. Different alloys will move this point of tradeoff one way or the other and for instance allow you to have better toughness at the same level of hardness, or better hardness at the same level of toughness.

 

[Wanimator]

I agree with Ankerson.

 

[marthinus]

When people talk about the toughness and hardness of a steel what do they mean by the two. When I've heard of them they are referred to as two different things.

If possible see if you can obtain the following book.

Arthur C Reardon-Metallurgy for the non-metallurgist, second edition-Materials Park, Ohio ASM International (2011).

On page 50 it reads:

"A short list of the important types of mechanical properties includes:
• Hardness, as a measure of resistance to indentation
• Linear elastic constants that relate to strain
under tensile, compressive, and shear loads
• Yield strength (under tensile, compressive,
and shear loads), indicating the stress level
required for the onset of permanent (plastic)
deformation
• Ultimate strength (under tensile, compressive, and shear loads), indicating the maximum engineering stress that the material can
withstand without fracture. Ultimate tensile
strength (UTS) typically is associated with
the onset of necking of tension-test specimens (Fig 3.2).
• Fatigue strength, indicating the levels of cyclic stresses that cause fracture due to metal
fatigue over time
• Impact toughness, indicating energy absorption from loads that cause very high strain
rates (i.e., impact)
• Fracture toughness, indicating resistance to
fracture with preexisting flaws or stress raisers in the geometry of a part
• High-temperature creep deformation and
stress rupture, where high temperatures
cause metals to permanently deform as a
function of time
• Damping properties
• Wear-resistance properties (due to wear
mechanisms such as galling, abrasion, and
erosion)" (Arthur C Reardon-Metallurgy for the non-metallurgist, second edition-Materials Park, Ohio ASM International (2011))

Hardness can also be measured differently.

Brinell hardness - Entire area is analysed.
Knoop hardness
Rockwell “B” hardness
Rockwell “C” hardness - Most commonly used, small indentation on small area for knives.
Vickers hardness (diamond pyramid
hardness) (Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel - By John D. Verhoeven (2005) and Arthur C Reardon-Metallurgy for the non-metallurgist, second edition-Materials Park, Ohio ASM International (2011)).

Here is also another good read and it is free.

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

 

[RamZar]

Crucible put out an informative piece of literature which may be helpful to you as well: Knife Steel Basics. It's only four pages long and you can just read the "Properties of Knife Steel" section.

 

[RBid]

Great links, sirs. Thank you for answering the OP's question with great info.