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Is hardening equivalent with reducing the ductility of a metal and improving the yield strength of it?

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  • $\begingroup$ Hi, I listed a short answer below, but no offence meant, Wikipedia seems to cover the subject pretty well in a general sense. $\endgroup$
    – user81619
    Commented Jun 21, 2015 at 18:08

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From Wikipedia:

Hardening is a metallurgical and metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. A harder metal will have a higher resistance to plastic deformation than a less hard metal.

Link to original reference: Hardening Metal

Ductility is reduced after hardening, but you don't say which metal or alloy you have in mind and, because of the varying crystal structure, it's a property that varies with the metal.

Again from Wiki:

There are several ways in which crystalline and amorphous materials can be engineered to increase their yield strength. By altering dislocation density, impurity levels, grain size (in crystalline materials), the yield strength of the material can be fine tuned. This occurs typically by introducing defects such as impurities dislocations in the material. To move this defect (plastically deforming or yielding the material), a larger stress must be applied. This thus causes a higher yield stress in the material. While many material properties depend only on the composition of the bulk material, yield strength is extremely sensitive to the materials processing as well for this reason.

Yield Strength Factors From Wikipedia

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  • $\begingroup$ Thank you! The reason why I posted this question is that I didn't found a site that combined hardening and his influence on ductility and yield strength. So if hardness is directly proportional to yield strength, is yield strength, unlike ductility, then independent on the crystal structure? $\endgroup$ Commented Jun 21, 2015 at 18:53
  • $\begingroup$ I have edited my answer as it seems other factors, including introducing other elements, can improve yield strength. If you think about it, bronze for example is improved by the addition of tin into pure (weak strength) copper. This is not really my field, could I suggest, just to make sure you get the correct answer, that you read the wiki links in my post and then you submit the question about yield strength again after looking through the tags (there are lots of them) to see is there one more specific to your question. $\endgroup$
    – user81619
    Commented Jun 21, 2015 at 20:05
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Stress-strain curve for a structural steel

The image shows the stress-strain curve for a structural steel (not to scale). The steel has a ductility, i.e it has a failure strain, of just over $\epsilon_y$, after which it breaks.

Start loading from the origin and you initially go up the straight line of slope $E$. Up to the initial yield stress of $^0\sigma_y$, the material is elastic and, if unloaded, will return down the same line to the origin.

If you reload from the origin, you go up back up the line until $^0\sigma_y$ then, if loading continues, you follow the curved line. If you unload from $\sigma_y$ say, you come back down the second straight line with slope (approximately) $E$ to $(\epsilon_p,0)$. Now you have induced a permanent plastic strain of $\epsilon_p$ in the material and you recover an elastic strain of $\epsilon_e$.

If you now reload from $(\epsilon_p,0)$, you will go back up the second straight line but further yielding will not occur until the current yield stress of $\sigma_y$. You have then raised the yield stress and, in this sense, hardened the material.

However, the original failure strain of just over $\epsilon_y$ doesn't change so, on reaching the point $(\sigma_y,\epsilon_y)$, there is very little strain left until the material breaks. In this sense, the material is less ductile than it was at the start.

Whether this work hardening is an improvement or not depends on what you want the steel to do. If you want high yield stress and ductility, you have to alloy the steel.

Just to confuse matters, work hardening raises the yield stress but it isn't the same thing as what is commonly called the hardness of the material. The latter is more complicated, is usually measured by an indenter test, and depends on a number of factors such as yield stress, work hardening rate, etc.

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