Good explanation of HT and testing terms

[Stacy E. Apelt - Bladesmith]

I found this list on the web, and thought it was very well explained:

Terms such as "hardness," "impact resistance," "toughness," and "strength" can carry many different connotations, making it sometimes difficult to discern the specific meaning. Some of the terms encountered, and their specific definitions are:

• Strength: also called rigidity, this is resistance to permanent deformation and tearing. Strength, in metallurgy, is still a rather vague term, so is usually divided into yield strength (strength beyond which deformation becomes permanent), tensile strength (the ultimate tearing strength), shear strength (resistance to transverse, or cutting forces), and compressive strength (resistance to elastic shortening under a load).
• Toughness: Resistance to fracture, as measured by the Charpy test. Toughness often increases as strength decreases, because a material that bends is less likely to break.
• Hardness: Hardness is often used to describe strength or rigidity but, in metallurgy, the term is usually used to describe a surface's resistance to scratching, abrasion, or indentation. In conventional metal alloys, there is a linear relation between indentation hardnessand tensile strength, which eases the measurement of the latter.[7]
• Brittleness: Brittleness describes a material's tendency to break before bending or deforming either elastically or plastically. Brittleness increases with decreased toughness, but is greatly affected by internal stresses as well.
• Plasticity: The ability to mold, bend or deform in a manner that does not spontaneously return to its original shape. This is proportional to the ductility or malleability of the substance.
• Elasticity: Also called flexibility, this is the ability to deform, bend, compress, or stretch and return to the original shape once the external stress is removed. Elasticity is inversely related to the Young's modulus of the material.
• Impact resistance: Usually synonymous with high-strength toughness, it is the ability resist shock-loading with minimal deformation.
• Wear resistance: Usually synonymous with hardness, this is resistance to erosion, ablation, spalling, or galling.
• Structural integrity: The ability to withstand a maximum-rated load while resisting fracture, resisting fatigue, and producing a minimal amount of flexing or deflection, to provide a maximum service life.

 

[Nathan the Machinist]

Something that is counter intuitive is ^elasticity (stiffness, Youngs modulus, modulus of elasticity) is basically the same for all steel. A bit under 30,000,000 PSI. Within the elastic limits, all steel is essentially the same stiffness. The flexibility of a blade is a function of it's geometry, not its heat treat or type of steel. Until you bend it (takes a permanent set) it's all the same.

People have trouble wrapping their head around that one.

 

[DevinT]

Good stuff guys.

Hoss

 

[running bird]

This is really helpful. When I first got interested in steels I had trouble understanding exactly what people mean when they use terms like "strong" or "tough" when describing steels. This would definitely help a lot for others who are becoming interested in knife making, steels, or just metals in general.

 

[BluntCut MetalWorks]

http://www.uddeholm.com/files/PB_Uddeholm_vanadis_4_extra_english.pdf

 

[AlaskanHunter]

Mr. Apelt, would you consider adding this to the HT sticky?

 

[Richard338]

Something that is counter intuitive is ^elasticity (stiffness, Youngs modulus, modulus of elasticity) is basically the same for all steel. A bit under 30,000,000 PSI. Within the elastic limits, all steel is essentially the same stiffness. The flexibility of a blade is a function of it's geometry, not its heat treat or type of steel. Until you bend it (takes a permanent set) it's all the same.

People have trouble wrapping their head around that one.

This is interesting.
I made 4 subhilts as similar as possible (given my capabilities).
Getting them back from Peters, one of them is much more flexible than the other three.
I wondered if it was some aspect of the heat treat (there were some spots like torch marks on two of them possibly due to straightening), but I'll take some measurements.
(I know that the stiffness depends strongly on the thickness).

 

[Nathan the Machinist]

(I know that the stiffness depends strongly on the thickness).

No, stiffness depends entirely on the thickness. Unless you're bending it so hard it's actually yielding (taking a permanent set) heat treat has nothing to do with how stiff a piece of steel is. That's a very common misconception and a lot of people have trouble believing it, but I assure you it is true. If one of the knives is more flexible, it has a thinner geometry (lower area moment of inertia, or sectional modulus )

the effect of thickness in non-linear. Doubling the thickness makes it 8 times stiffer. This is why adding a fuller to a design and adding the removed material to the thickness of the spine makes a meaningful difference in the structure of the blade, a small increase in thickness give a disproportionately large increase in rigidity.

 

[Richard338]

Yeah, I remember some of this coming up before in discussions of fullers.
I said "depends strongly" because I thought I remembered it going as the cube of the thickness as you've indicated, but I didn't have time to confirm.

 

[Willie71]

The only effect heat treat has on stiffness is the point at which the steel takes a set (permanent bend) or cracks. Within those limits, the same amount of force results in the same amount of deflection regardless of how hard the steel is.

 

[R.C.Reichert]

No, stiffness depends entirely on the thickness. Unless you're bending it so hard it's actually yielding (taking a permanent set) heat treat has nothing to do with how stiff a piece of steel is. That's a very common misconception and a lot of people have trouble believing it, but I assure you it is true. If one of the knives is more flexible, it has a thinner geometry (lower area moment of inertia, or sectional modulus )

the effect of thickness in non-linear. Doubling the thickness makes it 8 times stiffer. This is why adding a fuller to a design and adding the removed material to the thickness of the spine makes a meaningful difference in the structure of the blade, a small increase in thickness give a disproportionately large increase in rigidity.

So say we have 2 equal bars of 1/8" high carbon steel. One bar is annealed and the other bar is fully hardened... I always felt that the harder blade was also a lot stiffer because of it. For example a fully hardened file is extremely stiff and almost impossible to bend whereas a file that's been annealed can be bent in a vise. That's wrong? I'm probably just misunderstanding stiffness.

 

[Nathan the Machinist]

So say we have 2 equal bars of 1/8" high carbon steel, one dead soft, the other full hard. Same thickness, same width, same length. Put both in a vise and hang the same amount of weight on both of them, they will deflect the same amount...


...until you add so much weight that you reach proportional limit which is where you're bumping into the plastic range and the steel is starting to take a permanent set. At that point the strength (hardness) of the steel comes into play. But at that point you're not flexing stuff, you're bending it. <---permanent set...

Edit to add: Steel is a linear elastic material, meaning a certain amount of stress will cause a certain amount of strain until you get near the yield point. The yield point is what changes when you harden steel. A harder steel has a higher yield point. So, the perceived difference in stiffness in hard steel vs soft steel is actually a difference in yield point. In normal use you're not reaching the yield point on a knife. If you can feel a difference in stiffness in two knives it is due to geometry, not heat treat.


picture from Wiki

That straight area at the start is linear elastic range. Young's modulus = rise/run. That value is pretty much the same for all steel, just under 30,000 KSI. It's a little tiny bit higher if there is a lot of carbide or certain other alloy in the steel because carbide is stiffer than steel, but you'd never be able to feel it. And it has nothing to do with the hardness or heat treat. That angle is set in steel.

If two knives feel different, it is the geometry.

 

[R.C.Reichert]

So say we have 2 equal bars of 1/8" high carbon steel, one dead soft, the other full hard. Same thickness, same width, same length. Put both in a vise and hang the same amount of weight on both of them, they will deflect the same amount...


...until you add so much weight that you reach proportional limit which is where you're bumping into the plastic range and the steel is taking a permanent set. At that point the strength (hardness) of the steel comes into play. But at that point you're not flexing stuff, you're bending it. <---permanent set...

Thanks! That's interesting. Physics is so dang intriguing.