I'm going to focus this post on the steel used to make an axe, but the principles apply to all steel products (like knives) because I'm just going to be talking about the properties of the steel itself.
Firstly, I think it's prudent to define what is steel and what is not (i.e. variations, like stainless steel). Steel is a mixture of iron, which is soft yet tough, and carbon, which is very brittle and stiff in most of its many forms (allotropes as scientist like to call them - diamonds, graphite, graphene - they're all pure carbon, but have different molecular structures) and when a small amount of carbon is mixed into iron (~0.5-2%) the resulting mixture is called steel. Stainless steel is very different - it will not corrode as easily, but still corrodes in harsh environments like acidic and basic solutions, it is significantly softer than steel, and it has 10-20% of nickel and/or chromium added to the iron/carbon mixture, which makes it more expensive that regular steel. There are many other things that can be added to steel to change its properties, including molybdenum, vanadium, and silicon.
So, back to good ol' iron-carbon steel. Even with just these two components, the steel can take on a variety of phases (atomic crystal structures) and those phase can take a variety of microstructures (a distribution of phases that can't be seen by the naked eye). Steel takes on a variety of phases and microstructures, and luckily, a lot of scientist and engineers have put significant effort into creating and verifying the phase diagram that holds a bevy of information. There's quite a lot going on in this diagram. First, this is an EQUILIBRIUM phase diagram, meaning that at a specific temperature and composition and after a long period of time when changes to the atomic structure have stopped changing, these are the phases of the steel. Other phases are possible, but are not stable at these compositions and temperatures.
Ferrite - The solid solution of carbon in iron. At 0% C this is pure iron. Body Centered Cubic (BCC) crystal structure, which is a cube with iron atoms on each of the corners and another iron atom in the center of the cube. The maximum solubility of carbon in iron is 0.02% at 723oC. At 0 oC temperature the solubility falls to 0.008%. The carbon atoms are located in the crystal interstices, meaning the empty space between the iron atoms, since carbon atoms are smaller than iron atoms.
Austenite -The solid solution of carbon in γ iron is called austenite . This has a Face Centered Cubic (FCC) crystal structure with a high solubility for carbon compared with α ferrite. The solubility reaches a maximum of 2.08% at 1148oC . The solubility decreases to 0.8% at 723 oC. The carbon atoms are dissolved interstitially, as with ferrite. The difference in solubility of carbon in iron between the austenite and ferrite is the basis for the hardening of steels.
Cementite -This is an intermetallic compound which contains 6.67% C and 93.3% Fe. Cementite is a hard brittle compound with and orthorhombic crystal structure each unit cell has 12 Fe atoms and 4 C atoms.
So, that's really the basic materials science that goes into making steel. It's pretty easy to see that you want Austenite steel for your axe, but wait, there's more! In part 3 I'll talk a bit about heat treatments, and I'll move on from the atomic structure to the microstructure of the steel, which plays jut as important of a role in the material properties.
Firstly, I think it's prudent to define what is steel and what is not (i.e. variations, like stainless steel). Steel is a mixture of iron, which is soft yet tough, and carbon, which is very brittle and stiff in most of its many forms (allotropes as scientist like to call them - diamonds, graphite, graphene - they're all pure carbon, but have different molecular structures) and when a small amount of carbon is mixed into iron (~0.5-2%) the resulting mixture is called steel. Stainless steel is very different - it will not corrode as easily, but still corrodes in harsh environments like acidic and basic solutions, it is significantly softer than steel, and it has 10-20% of nickel and/or chromium added to the iron/carbon mixture, which makes it more expensive that regular steel. There are many other things that can be added to steel to change its properties, including molybdenum, vanadium, and silicon.
So, back to good ol' iron-carbon steel. Even with just these two components, the steel can take on a variety of phases (atomic crystal structures) and those phase can take a variety of microstructures (a distribution of phases that can't be seen by the naked eye). Steel takes on a variety of phases and microstructures, and luckily, a lot of scientist and engineers have put significant effort into creating and verifying the phase diagram that holds a bevy of information. There's quite a lot going on in this diagram. First, this is an EQUILIBRIUM phase diagram, meaning that at a specific temperature and composition and after a long period of time when changes to the atomic structure have stopped changing, these are the phases of the steel. Other phases are possible, but are not stable at these compositions and temperatures.
Ferrite - The solid solution of carbon in iron. At 0% C this is pure iron. Body Centered Cubic (BCC) crystal structure, which is a cube with iron atoms on each of the corners and another iron atom in the center of the cube. The maximum solubility of carbon in iron is 0.02% at 723oC. At 0 oC temperature the solubility falls to 0.008%. The carbon atoms are located in the crystal interstices, meaning the empty space between the iron atoms, since carbon atoms are smaller than iron atoms.
Austenite -The solid solution of carbon in γ iron is called austenite . This has a Face Centered Cubic (FCC) crystal structure with a high solubility for carbon compared with α ferrite. The solubility reaches a maximum of 2.08% at 1148oC . The solubility decreases to 0.8% at 723 oC. The carbon atoms are dissolved interstitially, as with ferrite. The difference in solubility of carbon in iron between the austenite and ferrite is the basis for the hardening of steels.
Cementite -This is an intermetallic compound which contains 6.67% C and 93.3% Fe. Cementite is a hard brittle compound with and orthorhombic crystal structure each unit cell has 12 Fe atoms and 4 C atoms.
So, that's really the basic materials science that goes into making steel. It's pretty easy to see that you want Austenite steel for your axe, but wait, there's more! In part 3 I'll talk a bit about heat treatments, and I'll move on from the atomic structure to the microstructure of the steel, which plays jut as important of a role in the material properties.
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