Bending strength and hardness

M

Mr.Wizard

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Feb 28, 2015
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The charts in the Hitachi YSS Tools Steels flyer show a negative correlation between hardness and and bending strength:

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However the charts in Correlation of Yield Strength and Tensile Strength with Hardness for Steels—E.J. Pavlina and C.J. Van Tyne show a positive correlation between hardness and yield strength. Why this seeming contradiction? Are not bending strength and yield strength closely related?

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Regarding yield strength, I am citing text from the "Iron, Steel and Swords" glossary. https://www.tf.uni-kiel.de/matwis/amat/iss/index.html

Yield Strength/stress:
The particular stress (or the strain caused by it) that marks the onset of plastic deformation. A rule of thumb is: Stress below the yield stress produces only elastic deformation.
For metals the yield stress is the same as hardness, just given in different units.
(end quote)

So the last sentence explains why yield strength is roughly equivalent and correlated to hardness in the graph.
The toughness would be the yield strength plus the energy absorbed by inelastic deformation (ie bending).
It is presumed (by me) that harder steels absorb less energy inelastically; so much so that the overall toughness is usually lower.

It is possible that for tensile strength, they are not using the ultimate tensile strength, which is complete breakage. Instead, using the definition of "Tensile strength is a measurement of the maximum deformation a certain material can take without necking."
Necking would be the beginning of inelastic deformation, so has the same conclusion as yield strength above.
 
The amount of energy to flex steel is unaffected by the hardness. The point at which is takes a set it breaks is affected by hardness. You might not be able to flex as far with the hardened steel, as it might break, the the force to move it that far is unchanged. This is almost always misunderstood by those new to metallurgy.
 
Bend tests with brittle materials will have lower strength because they fracture prematurely. With tool steels the stress to initiate a crack at a carbide can be lower than the strength of the matrix, for example. Beyond a certain hardness the steel will have lower strength because the “fracture strength” is lower than the “ultimate strength.” Bend tests are used as toughness tests with tool steels.
 
Willie71 Willie71 Is this a correct restatement? Modulus of elasticity is similar across steel types and hardness therefore yield strength in steel is congruent to the magnitude of deflection at the point plastic deformation begins.

Larrin Larrin What steel has the highest bending strength, and why? I guess a kind of spring steel and temper (that's sort of what makes a spring a spring I think?), but what micro-structure makes this possible?
 
The highest bending strength would be a steel that can achieve relatively high hardness with a tough matrix and little or no carbide/impurities. Similar to what controls toughness in an impact test but hardness is also a factor.
 
Mr.Wizard said:
Willie71 Willie71 Is this a correct restatement? Modulus of elasticity is similar across steel types and hardness therefore yield strength in steel is congruent to the magnitude of deflection at the point plastic deformation begins.

Larrin Larrin What steel has the highest bending strength, and why? I guess a kind of spring steel and temper (that's sort of what makes a spring a spring I think?), but what micro-structure makes this possible?
Click to expand...

I don’t know if it’s across all steel types, as carbide volume and structure can be quite different, but comparing apples to apples, your statement is true. Larrin can better answer if the statement is true across steel types.
 
Larrin said:
The bend test is a toughness test, so yes. But if you mean would it also do well in an impact test the answer is also yes. For example you can compare to Erasteel PM M4 (2004) which tested at 70J at 61 Rc while the 2017 steel we are discussing tested at 120J. https://www.erasteel.com/wp-content/uploads/2019/11/GB_Grade_ASP-2004.pdf
Click to expand...

Using those numbers you gave me, we can extrapolate that Erasteel 2017 has a theoretical Charpy toughness value equal to Zwear or Vanadis 4 extra, I think.
 
hugofeynman said:
Using those numbers you gave me, we can extrapolate that Erasteel 2017 has a theoretical Charpy toughness value equal to Zwear or Vanadis 4 extra, I think.
Click to expand...
Most likely better than those steels. Unfortunately we can’t compare directly to V4E because Uddeholm tests their izod specimens in the transverse direction. However we can compare 4-point bend tests where 2017 does better. Bohler has K490 which is in the same class as V4E but the Izod values can be directly compared where 2017 is higher. K890 is more similar to 2017 and those two are closer to each other though Erasteel’s numbers are a bit better. Though comparing datasheet values between different manufacturers is always a bit tricky since they are basically marketing numbers.
 
ASP 2012 is their toughest pm steel, correct? Should be in the same toughness class as cpm-1v or Z-Tuff or maybe a bit higher.
 
hugofeynman said:
ASP 2012 is their toughest pm steel, correct? Should be in the same toughness class as cpm-1v or Z-Tuff or maybe a bit higher.
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Yes I believe so. ASP2012 is basically a "matrix steel" which is produced with powder metallurgy. Not sure how it would compare directly with 1V and Z-Tuff, though they are all in a similar class of steel. It appears max hardness is a bit limited but I have never heat treated any so I don't know what it can do.
 
hugofeynman said:
Using those numbers you gave me, we can extrapolate that Erasteel 2017 has a theoretical Charpy toughness value equal to Zwear or Vanadis 4 extra, I think.
Click to expand...

The carbon level puts it more in line with 3v, I think, but higher alloying otherwise.
 
hugofeynman said:
ASP 2012 is their toughest pm steel, correct? Should be in the same toughness class as cpm-1v or Z-Tuff or maybe a bit higher.
Click to expand...


Hehe, CPM-1v.

That's like when you see someone driving a Chrysler 200. :D
 
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