Iron-Carbon Alloys

Iron-carbon alloys constitute the cornerstone of the structural and machinery manufacturing industries owing to their high modulus of elasticity, adjustable hardness, and robust mechanical performance. Pure iron exhibits a soft structure with low strength; however, as the carbon content added to the alloy increases, hardness and tensile strength increase, whereas ductility and toughness decrease.

Classification of Iron-Carbon Alloys

Alloys are grouped into three main categories based on their percentage of carbon (C):

a) Irons: Alloys with a carbon content of less than 0.1% (C < 0.1%).

b) Steels: These are further divided into three subgroups based on carbon content:

Low carbon steels: 0.1% < C < 0.2%
Medium carbon steels: 0.2% < C < 0.5%
High carbon steels: 0.5% < C < 2%

c) Cast Irons: Alloys with a high carbon content. Depending on the production method and internal structure properties, they are primarily divided into two types: white cast iron and gray cast iron.

Production Process of Iron-Carbon Alloys

Iron ores, which are mostly found in nature as Fe2O3 and Fe3O4 compounds, are reduced in a blast furnace to pig iron (or cast iron). A layer of coke, a layer of iron ore, and some limestone are placed in the blast furnace. The pig iron obtained from the iron compounds reduced at high temperatures contains 3-4% carbon. This molten pig iron is placed in steel furnaces, and the excess carbon is burned off by passing oxygen through it, reducing it to the desired level. The liquid metal taken from the steel furnace is poured into molds called ingots to obtain raw steel. During casting, the FeO remaining in the molten mass is deoxidized by carbon, and the CO gas formed during this process continues to partially rise to the surface as bubbles and partially remain in the mass until solidification is complete. Due to its boiling-like appearance, this steel is called hot-cast or uncooled steel. The remaining gas voids are usually welded shut during rolling. The quality of these steels is relatively low. On the other hand, silicon, aluminum, or similar additives added to the ladle during casting directly deoxidize FeO, preventing the formation of gas bubbles and thus preventing boiling. Steel obtained by this method is called quenched steel. If this process is carried out carefully and a better result is obtained, complete quenching is achieved. Calmed steels are considered superior in terms of mechanical properties and weldability.

Steels contain a certain amount of phosphorus and sulfur during production. These elements embrittle the steel, reduce weldability, and lower quality; therefore, their quantity should be limited.

The production schedule shown below illustrates the stages iron-carbon alloys go through during production and provides some of the terms used.

                     IRON ORES
                   Fe₂O₃ – Fe₃O₄
                              │
                             ▼
┌──────────────────────┐
│                 Blast Furnace                 │
│                                                           │
│ Coke + Iron Ore +  Limestone    │
│                                                           │
│                                                           │
│ Reduction                                       │
└──────────────────────┘
                               │
                              ▼
                          Pig Iron
                          %3–4 C
                               │
                              ▼
┌──────────────────────┐
│               STEELMAKING                    │
│                                                           │
│Oxygen Blowing                             │
│Carbon Reduction                        │
└──────────────────────┘
                             │
                            ▼
                  MOLTEN STEEL
                             │
                            ▼
┌──────────────────────┐
│                     CASTING                      │
│                   (Ingot Mold)                 │
└──────────────────────┘
                             │
       ├───────────────│ 
       ▼                                      ▼
  DEOXIDATION                 FeO + C
       │                                         │
       │                                        ▼
       │                                       CO
       │                                         │
       │                               Gas Bubbles
       │                                         │
      ▼                                        ▼
KILLED STEEL                  RIMMED STEEL