Wednesday, June 04, 2008

Basic Chemistry of Steel Making

A little clarification on Steel and Wootz, becasue there is considerable confusion about what is Wootz, Damask, Bulaut, Damascus steel.

Noone knows how to make Wootz. It is a secret that has been lost for several centuries. Noone has made steel to the quality of Wootz since. Wootz is like 3-5 times stronger than any know modern steel. making Wootz has nothing to do with laminating, forge welding, pattern welding, layering, etc, etc. Japan did use lamination to produce their blades. But pattern welding was how knockoff sword makers tried to imitate Wootz. Pattern welding actually makes weak spots and if you buy ammunition some boxes say that it is not to be used in gun with Damsk or Wootz steel barrels. There have been several cases of the these cheaply made barrels blowing apart.

True Wootz has to do with creating the correct conditions to grow carbon nano-tubes matrix inside the molten iron. Some theorize this had to do with vanadium nucleation sites, and repetative heating and cooling cycles. But, like I said, noone knows. Pattern welding can fake the swirl pattern on Damascus Sabers, but noone knows how to make real Wootz. Wootz steel had extrememly high carbon content over 2% which should result in worthless Cast Iron. But instead of the carbon making the steel brittle, the carbon formed into a carbon nanotube matrix. This nano-steel was extrememly hard, yet flexible and elastic. (Ancient Nanotechology = Cool)

About regular modern and ancient steel. Cooled wrought Iron (ferrite-Faced Centered Cubic structure) cannot disolve signifanct amounts of carbon. If you heat it up to almost molten (910oC), it will phase transition to austenite (Body Centered Cubic) and readily dissolve carbon up to 2%. When the iron is allowed to cool, the carbon will precipitate as cementite. Mixtures of cementite and iron are called pearlite and bainite and are found in steel and cast iron.

To make steel you need to disolve the carbon in molten wrought Iron and then cool it rapidly. Instead of the carbon precipitating as cementite, the carbon will precipiate as martensite. Martensite is the key to non-wootz, non-stainless, ancient and modern steels. The martensite itself is not what is important. What is important is that when martensite forms it changes the configuration of austenite from a Body Centered Cubic configuration to a body-centered tetragonal structure. BCT is what leads to steel.

However, BCT iron full of martensite is extremely hard hard but still brittle. The large martensite crystals needs to be broken down. That is where tempering comes in. BCT iron is then reheated which transforms some of the brittle martensite to transform back into bainite or pearlite or a combination of ferrite and cementite. This is done until the right balance of hardness, brittleness and ductility is achieved. Steel has a mixture of martensite, bainite, ferrite and cementite. But if you see martensite, then you know the iron has been austenized and quenched. tempering only requires gentle reheating.

3 steps for making steel 1.austenizing. 2. quenching 3. tempering.

Bronze was supperior to wrought iron and cast iron in every characteristic. But steel beats Bronze hands down. So, some believe what drove history from the bronze age to the Iron age was a shortage of Tin traded by the phoenecians which produced pressure on countries to invest in steel research. When steel was discovered (think Manhattan Project), and able to be produced in large amounts, the Assyrians used steel chariots and swords to conquer Northern Israel and Egypt. Steel making didnt make your country dependent on foreign tin. Therefore, steel could be made locally, and became the the material of choice.

Steel also has elastic and memory properties that ancient bronze did not. So, steel could be used as a material to make springs and tension bars to store potential energy. Steel spring technology was used later in this way to make seige weapons and cross bows. Steel spring technology was the secret of sucess of the Romans and was involved in the fall of Carthage (end of the Phoenecians).