11. Steel heat treating

To understand the benefits of heat treating processes first requires an awareness of metal and alloy structures.

When a molten metal solidifies the atoms arrange themselves into definite patterns called crystal structures.

The two most common crystal structures in metals are body-centered cubic and face-centered cubic.

These crystal structures grow uniformely in all directions within each developing crystal.

As the metal cools these crystals are confined by the adjacent developing crystals forming grains.

The line of intersection between grains is called a grain boundary.

Because the grains form independently their crystal structures develop tilted in various directions.

All atoms in this crystalline structures are held in place by electro-magnetic attraction to neighbouring atoms.

If a force or a load is applied to a metal these electromagnetic bonds stretch allowing the atoms to move slightly.

When the load is removed the bonds pull the atoms back into position.

If the applied force exceeds the metal yield strength those electro-magnetic bonds will break causing permanent stretching or deformation.

To make metals stronger and more resistant to deformation it’s necessary to strengthen their crystal structures.

This is done by adding alloying elements which are other metals or non-metal elements like carbon.

The addition of an alloy introduces foreign atoms within crystal structure of the base metal disrupting the structural uniformity.

This disruption results in an increased strength.

12. Ductility of metals

Iron is malleable at high temperatures. Therefore, it must be heated up before forging. When it is red hot it can be pounded with a hammer into a desired shape. Likewise, two pieces of red-hot iron can be linked together to form part of an ornamental trellis for example. Copper and aluminium are malleable at room temperature. Metals can be drawn into rods and wires by heating and pulling apart. Copper can be drawn into the very thin wires used in electric cables.

13. Brittle and ductile materials.

I am a geologist who studies how rock deform.

So what am I doing in my kitchen ? I am in my kitchen because rocks deform in the same way a lot of other things deform.

When something changes its shape, it responses to stress. It’s being deformed. Deformation can be brittle or ductile. Take a bread stick. Bend it. It breaks. It’s brittle deformation. Take a caramel bar. Bend it. It changes shape but it stays a one piece. That’s ductile deformation.

What controls whether something is brittle or ductile ? Three things: temperature, composition and something called “strain rate”.

Let’s consider how temperature affects whether something is brittle or ductile.

Take a frozen stick of butter. Bend it. It breaks. That’s brittle deformation.

Now take butter at room temperature. It flows like the caramel bar. That’s ductile deformation.

Now if we take a candle at room temperature , bend it – it breaks. That’s brittle. But what happens if we warm it up a bit ? If the candle is warmed up a bit, it flows like the caramel bar. Butter at room temperature and wax at room temperature deform differently because they have different compositions. But at lower temperatures both are brittle. And at higher temperatures both are ductile.

So what do butter and wax have to do with rocks ? Rocks are cold near the earth surface and they get hotter and hotter with depth. Cold rocks near the surface tend to break if they are stressed. They are brittle. At about 15 kilometers within the earth most most rocks are hot enough to flow if they are stressed. They are ductile.

Let’s talk about strain rate (which is how fast deformation occurs). Different strain rates can cause the rocks to be either ductile or brittle at the same temperature. Here’s how.

Take Silly Putty (*). If I pull it slowly it flows in a ductile fashion. I pull it fast. It breaks. It’s brittle. Layers of sedimentary rock are just like these layers of play-doh (**). If I push on them slowly, they fold. Some mountains made of folded rocks look just like this.

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* Silly Putty (also known as Thinking Putty) is a toy based on silicon polymers. It bounces, but breaks when given a sharp blow; it can also flow like a liquid and will form a puddle given enough time

** Play-Dohis a modeling compound used by young children for art and craft projects at home and in school. Composed of flour, water, salt, boric acid, and mineral oil.