The Iron and Steel
Iron, Carbon Steel and Stainless steel are brothers
Iron, carbon steel and stainless steel are the materials that include iron as their main ingredient. Actually pure iron does not exist in this world because iron unites with the other chemical substances very easily. Iron, itself, usually exists with other chemical substances in nature as we mentioned.
Iron ores or iron sands, raw material of iron, exists as chemical compounds(iron oxide) of iron(Fe) and oxygen(O), also includes a small amount of impurities such as silicon(Si), manganese(Mn), phosphorus(P) and sulfur(S).
Because it is hard but fragile, it cannot be used fro knives without processing and the impurities have to be removed. “Sweden ore”, which is well known as a good raw material originally has much less impurities.
In order to get rid of the impurities, it should be heated in 1,500 degrees centigrade with reducing agent, such as carbon(C) that easily unites with oxygen. Then pig iron is made in this process.
This is what we call smelting process to make "TAMAHAGANE" from iron sands. In the old time, we used to use "TATARA" pot but now blast furnace is used in modern ways.
To make it have more strength as a knife material or structural metal, extra carbon(C) mingled more than necessary in the smelting process and the other impurities have to be taken away after that. This is done in the oxidation refining process and molten steel is made. This is what we call metalmaking process, which used to be done by metalsmith in the old time but now converter and open hearth furnace is being used.
Carbon(C) and the other elements are adjusted properly in this process. Also sometimes Nickel(N), Chrome(Cr), Tungsten(W), Molybdenum(Mo), Vanadium(V) and the other elements are artificially added.
Iron and Carbon steel are classified by its content rate of carbon. Carbon steel usually used for knives includes 0.08〜2.0% of carbon and if the ratio is less than 0.08%, we call that forging steel or soft iron. They are softer and won’t be harder even if it is heated. If it is more than 2.0%, we call that pig iron or cast iron and they are used for material of casting because of the hardness and fragility.
Stainless steel is an alloy that has both corrosion resistance function and durability for abrasion. More than 12% of Chrome(C) is added into it to improve weakness of corrosion and sometimes Nickel(Ni) and Molybdenum(Mo) will also be added if necessary.
Stainless steel can also be roughly divided into two kinds by its original content ratio of carbon as we mentioned. One is materials can be harden by heating and the other is can NOT be harden just like 18-8 stainless steel used for silverwares.
Because stainless steel is a processed(ingredients-adjusted) product made of iron, it is not different substance but one of the iron that has more corrosion resistance. Also you have to be aware that stainless steels can be harden includes more Carbon(C) and it will be more likely to get rusty. This is one of the reasons why you need to take care of stainless kitchen knives as well.
