Lower hardness but same YS

[seep]

Can someone explain me the possible reasons why after homogenization of an austenitic steel, I obtain smaller hardness (about 100HV smaller) but the same yield strength? For me it's not logical... Lower hardness is probably conencted with recrystallization and grain growth, but shouldn't it be connected with lower YS?

 

[metengr]

First, there is not enough background information because more information helps to narrow or eliminate possible factors. Second, was this a solution heat treatment performed on the same heat of austenitic stainless steel? Was the material worked or what was the material condition before heat treatment? How did you measure yield strength and what about tensile strength values in addition to hardness testing?

 

[seep]

Samples were first as-forged (higher hardness) and then annealed in 1000C and quenched in water. Yield strength was measured by compression test.

If not in this case, what can be a general explanation of this phenomenon?

 

[EdStainless]

Are you measuring true stress/strain in compression?
Of course they don't agree, they are in very different conditions.
Strain hardening, especially if it was from warm work can result in interesting (seemingly inconsistent) properties.
Now if you have multiple annealed samples that differ from each other that would be different.


I have generated many samples of the same SS alloy that have

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Plymouth Tube

 

[seep]

What I meant is that YS after annealing should get lower (as hardness), but in my case is the same for as-forged and annealed alloy (even if hardness decreases).

 

[mrfailure]

Note that hardness correlates in some manner to tensile strength, but not necessarily yield strength.

 

[OGMetEngr]

Hardness correlates roughly to both, but the correlation is not as good between yield and hardness. There's a good paper "Correlation of Yield Strength and Tensile Strength with Hardness for Steels" that attempts this for various microstructures and steel types. This info was pulled from data from the Colorado School of Mines, and provides information for this correlation or lack-thereof.

 

[EdStainless]

In a cold worked material I can change the engineering yield (0.2% offset) very easily with no impact on the grain size or tensile strength.
In wire it is common to add just a little more work in the form of flexing the wire by running it looped around a roller.
The slight bending back and forth causes redistribution of residual stresses and can greatly lower the measured yield strength.

The should be no correlation between the properties of cold or warm worked part and the same material after annealing other than the reduction of UTS. Everything else depends on conditions.

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Plymouth Tube

 

[OGMetEngr]

Just ran across this nugget in Corrosion of Austenitic Stainless Steels that may provide an explantion:

The twin spacing does not affect the proof stress because the stacking fault energy, which controls the work-hardening rate, has little or no effect at the
low strains at which the proof stress is measured. The twin spacing is much more important than the grain size in controlling the tensile strength because the effect of stacking fault energy on the work-hardening rate, and hence on the tensile strength, is quite significant. However, in high stacking fault energy austenites, in which there are relatively few twins, the tensile strength will depend on the grain size, following a Hall-Petch type of relationship.


See my previous post regarding how hardness correlates better to ultimate tensile strength. You could have more closely spaced twins in the higher hardness/UTS material and more widely spaced twins in the low hardness/UTS material. Just one possibility for your case.