Iron and steel

Differences between iron and steel

There are many differences between iron and steel. Primarily, iron is a pure element while steel is an alloy comprising of iron and carbon. Steel = Iron + Carbon + X + Y +....

The different percentages and identities of elements X, Y, ... make for the different types of steel.

 However, in this alloy iron is present in a greater quantity. You can add various other metals to steel so as to produce alloys that have different properties. For example, if chromium is added to steel, stainless steel is the product. It is durable and doesn’t rust easily. In the construction industry steel is used on a large scale. This is because steel is stronger than iron and it has better tension and compression properties. The carbon makes the iron harder.

Galvanized steel

Galvanization is the process of applying a protective zinc coating to steel or iron, to prevent rusting. Galvanized steel is steel that has gone through a chemical process to keep it from corroding. The steel gets coated in layers of zinc oxide because this protective metal does not get rusty as easily. The coating also gives the steel a more durable, hard to scratch finish that many people find attractive. For countless outdoor, marine, or industrial applications, galvanized steel is an essential fabrication component.

Cast iron

Cast iron is a term used to describe a range of metals that include certain elemental compositions. It is primarily composed of iron (Fe), carbon (C) and silicon (Si), but may also contain traces of sulphur (S), manganese (Mn) and phosphorus (P).  It has a relatively high carbon content of 2% to 5%. Because it contains a mixture of elements, cast-iron metals are considered alloys, with all cast irons containing more than 2 percent carbon. However, carbon contents exceeding about six percent are too weak and brittle to be of much use. Cast iron has a low tensile strength and is quite brittle, but can be cheaply produced in intricate shapes. The additional carbon makes the molten iron more fluid and easier to cast in complex shapes. Highly suitable for casting large and heavy objects (such as engine blocks), it is widely used in making such things not subjected to high stress. Cast iron generally is brittle and cannot be bent easily. Cast iron breaks when it is loaded with significant pressure, although the carbon content in cast iron makes this metal good for casting.

 2% to 6.67% of carbon in iron is called as cast iron and below 2% carbon called as steels.
Composition of Cast Iron:
Carbon - 2.5 to 3.7%
Silicon - 1.0 to 3.0%
Manganese - 0.5 to 1.0%
Phophorus - 0.1 to 0.9%
Sulphur - 0.07 to 0.10%

Ductile iron

Ductile iron, also known as ductile cast iron, is a type of cast iron. While most varieties of cast iron are brittle, ductile iron has much more impact and fatigue resistance, due to its nodular graphite inclusions. A typical chemical analysis of this material:

• Carbon 3.2 to 3.6%
• Silicon 2.2 to 2.8%
• Manganese 0.1 to 0.5%
• Magnesium 0.03 to 0.05%
• Phosphorus 0.005 to 0.04%
• Sulfur 0.005 to 0.02%
• Copper <0.40%
• Iron balance

Other elements such as copper or tin may be added to increase tensile and yield strength while simultaneously reducing ductility. Improved corrosion resistance can be achieved by replacing 15% to 30% of the iron in the alloy with varying amounts of nickel, copper, or chromium. Much of ductile iron is in the form of ductile iron pipe, used for water and sewer lines.

Ductile iron vs Cast iron

Ductile iron can be bent without breaking, whereas cast iron is brittle and breaks when bent.

As ductile iron can be bent, it can be made into different shapes and forms. But one cannot make out different shapes and forms using cast iron. While ductile iron is flexible, cast iron is not.

When comparing the two, cast iron corrodes more quickly than ductile iron.

Ductile iron consists of iron, carbon, silicon, manganese, magnesium, phosphorous and sulphur. Tin and copper are also sometimes found. Ductile iron also consists of nodular graphite, which gives it flexibility. Cast iron mainly consists of carbon and silicon.
With regard to practical uses, ductile iron pipes are used in sewer and water lines. This is because they are much stronger than cast iron. Moreover, ductile iron is better than cast iron in difficult terrains.
Ductile iron is mainly used in the automobile industry such as trucks, tractors and oil pumps. Cast iron is mainly seen in the construction industry.

Mild steel

Mild steel, also known as low carbon steel, is made from iron, sulfur, phosphorus, manganese and carbon. Mild steel is the least expensive of all steel and the most common steel used. It is weldable, very hard and, although it easily rusts, very durable. Low-carbon steel contains approximately 0.05–0.15% carbon making it malleable and ductile. Mild steel has a relatively low tensile strength, but it is cheap and easy to form. Containing a maximum of 0.29% carbon, this type of steel is able to be magnetized and used in almost any project that requires a vast amount of metal. Its structural strength prevents it from being used to create load-bearing girders and structural beams.
Unlike high-carbon steel, mild steel is easily welded. The properties of the steel allow the electrical current to travel through the metal without distorting the makeup of the material. Some types of high-carbon steel, such as stainless steel, require special techniques in order to properly weld the material.

Chemical composition of Mild steel
Carbon	0.16-0.18%
Silicon	0.40% max
Manganese	0.70-0.90%
Sulphur	0.040% Max
Phosphorus	0.040% Max

Diference between Mild steel and Cast iron

Cast iron is generally brittle, while mild steel is not. Cast iron has a carbon content from about 2%-4%. Mild steel has less than one tenth as much carbon as cast iron- around 0.2% - 0.3%.
Cast iron also typically has from 1%-3% silicon. Most mild steel contains no more than 0.3% silicon. Steel melts at ~1450 degrees C. The addition of large amounts of carbon and silicon to C.I. lower the melting point to around 1200 C and make it more fluid than molten steel. Obviously this makes it easier to melt and to pour. The main advantage of cast iron is it's cheap to produce.

It also has some desirable properties despite it's brittleness. For example it's easy to cut and carve with machine tools. It's also very heat resistant and excellent at absorbing vibration. For this reason C.I. was commonly used for engine blocks and exhaust manifolds in cars and trucks in the past. Cast iron also rusts much more slowly than steel which is why it's still commonly used for pipes. C.I. pipes have been known to last 300 years.

Stainless steel

Stainless steel is an alloy of Iron with a minimum of 10.5% Chromium. Other alloying elements are added to enhance their structure and properties such as formability, strength and cryogenic toughness. These include metals such as:

• Nickel
• Molybdenum
• Titanium
• Copper

Non-metal additions are also made, the main ones being:

• Carbon
• Nitrogen

 Stainless steels contain sufficient chromium to form a thin layer of chromium oxide on the surface of the steel, which prevents further surface corrosion by blocking oxygen diffusion to the steel surface and blocks corrosion from spreading into the metal's internal structure. Increasing the amount of Chromium gives an increased resistance to corrosion. Stainless steel also contains varying amounts of Carbon, Silicon and Manganese. Other elements such as Nickel and Molybdenum may be added to impart other useful properties such as enhanced formability and increased corrosion resistance. Stainless steel is used where both the properties of steel and corrosion resistance are required.