1. Buy or borrow from the library this book.
http://www.amazon.com/gp/aw/d/1566375436/ref=aw_d_detail?pd=1
.....Over 2% carbon and you get wrought iron.............
+1 on getting a good metallurgy book.
Slight correction on the carbon statement...Above 2% it is called CAST iron. Wrought iron is 0% carbon.
I'll try and explain it in a simplified way, but we have to understand what any analogy is about:
If we take simple steel and think of it as a mix of carbon and iron, we find out that .84% carbon is just enough to bind up with all the iron and make a perfect arrangement. Metallurgists call it the eutectoid. Knifemakers call it 1084 steel.
The iron and carbon can make several matrix shapes, called structures - the structure we want for hard steel blades is called Martensite. In this structure, one carbon atom is trapped inside eight iron atoms. This is one GRAIN. The grains are arrange in layers like bricks. The mortar would be called the Grain Boundary.
If allowed to sit at a high enough temperature, this mortar melts, and the grains combine into larger bricks, and then big blocks, and if left long enough and hot enough, into giant boulders. If the grains are small and tightly packed, the steel is very strong - like a good masonry brick wall. If they are large, it is easy for a crack to form along them - Like a crack on a cinder block wall. If they are gigantic, they may fall completely out of the matrix - like a boulder falling off a granite outcropping.
If we add extra carbon, it is hyper-eutectiod. The extra carbon gets tied up as hard iron carbides. If we add other elements that easily bind with carbon ( carbide formers), these ALLOY ingredients also make hard carbides. These alloys may also affect the grains by depositing themselves along the grain boundaries. Since these elements and their carbides don't melt as easily as the
mortar holding the steel together, it prevents the grains from melting together into larger grains. This is called grain refinement.
The basic things that matter in a knife blade are called hardness, toughness,edge retention, and wear resistance. These are a combination of the steel choice, the heat treatment, and the blade geometry.
If we go back to an analogy again, we can think of a bar of steel as a type of cement wall. It has to have sand ( iron ), and Portland cement ( grain boundaries). We can also add pebbles (small carbides), small stones (large carbides). We could, but shouldn't, use big rocks.
If we mix the right amount of sand and Portland, we get what is normally called cement. It is moderately hard, has a very fine grain, and moderate strength. Because of its nature, it can wear away from erosion (lower toughness). If we add some pebbles, we get concrete, which is harder because of the harder pebbles. It has a slightly larger grain structure. If the amount of pebbles is right, this is very strong. The pebbles need to have just the right amount of room for the cement to fill all the voids. Too few pebbles, and there are large sections of cement with no pebbles, too many pebbles, and there is not enough cement to make all the joints strong. If we use small stones,instead of pebbles, the hardness will be more, but there will be longer and larger joints between them for the cement to try and hold together. This makes the wall easier to break ( more brittle), but also much harder and longer lasting. If the rocks are too big ( huge carbides or very large grains), the wall will fall apart easily under stress.
Now, back to metallurgy:
The hardness of the steel is controlled by the alloy ingredients.....and the HT.
The grain size is controled by the alloy ingredients....and the HT.
The toughness of the steel is controlled by the alloy ingredients.....and the HT.
The wear resistance is controlled by the alloy ingredients....and the HT.
The edge retention is controlled by the blade geometry....and the HT.
See any patterns here????? It is the choice of the steel type ( alloy) and the heat treatment that can make or break ( pun intended) a knife blade.
Hardness - the harder the carbides formed by the alloy ingredients, the harder the steel. This has tradeoffs, as pointed out in the cement wall analogy. Too hard and it breaks because it is brittle. Too large and it cracks between the hard places. Just right and you get hard and strong. The HT makes a very hard steel at first. This is so hard and brittle that it will break easily. It is tempered to make it softer. The amount of temper is controlled by the temperature. Just enough to make it still hard, but less brittle is what we want.
Grain Size - Grain size is mainly controlled by holding the steel at the right temperature as the carbon diffuses into the iron. At temperatures just a bit above the point where the steel becomes austenite, the grain remains small. This is between 1450°F and 1550°F. Go higher and the grains will start to melt together and grow rapidly into larger and larger grains.
Toughness - The type of alloy elements can make the grain boundaries stronger and more plastic. This allows some stretching and bending before they separate. This makes the steel tougher. The HT can be selected to make the grain size small and the steel a bit more plastic, so it is tougher. The higher tempering ranges for stainless steel give tougher blades.
Wear Resistance - The ability of the steel to resist eroding/abrading away is wear resistance. It determines how fast a blade needs to be re-sharpened, and how easily it is to re-sharpen the blade. The harder the steel the more it resists wear. The alloys that have harder carbides make for longer wearing edges. Lower tempering temperatures make for harder blades, which resist wear.
Edge Retention - All this hardness, and grain size, and toughness, and wear resistance sounds really good, but there is a catch. If it is too thin, the edge may just have minute pieces fall off as it cuts. This is called micro-chipping. At extreme hardness, the grain boundary fails, and the whole grain (or carbide) pops right off. Also, if too hard, it resists any attempt to bend, and fails rapidly ( brittleness).
Adjusting the thickness and angle of the edge (and blade) is the first way to control edge retention. The other way is picking a steel and HT regimen that fits the type of use the blade will get.
Going back to the wall analogy, if the cement is made so it has a proper ratio of sand and pebbles, it will resist a little flex in an earthquake. If earthquakes are expected, you might add some very flexible things, like metal wire, vinyl, and long fibers. This will allow some movement without catastrophic failure. If the rocks are too large, the cracks will run rapidly through the wall. If the wall is make from big stones and boulders, it will collapse and fall down.
If you just need a little wall to keep the rabbits out of your turnip patch, make it thin and hard.
If you need a wall to last a century, make it less hard, a bit tougher, and thicker.
If you need a wall to withstand a massive bombardment in an attack, make it from the biggest and hardest stones you can....and make it thick and round.
If the wall will be shaken occasionally, make it a bit more flexible by using smaller stones and good mortar.
If the wall will have to withstand massive earthquakes regularly, make it with straw in the mortar, and stones set so they can move from side to side without falling out. Slightly softer stones will survive better than harder ones.
What we get from this ramble is that understanding the part each one of these things plays in blade metallurgy and knife construction is how we get a blade that will survive the needed function.
A disposable scalpel blade that is used for one cut has a vastly different metal choice, blade geometry, and HT requirements than a machete that will need to hack brush in the jungle for years.
