Steel is iron alloyed with carbon between 0.05 and 0.3%. In addition, it contains smaller properties of phosphorus, sulphur, silicon, and manganese. These steels are known as plain carbon steels. In some cases certain other alloying elements are added, such steel is known as alloy steel.
Types of Steels
Steel metal is of different types like plain carbon steel, medium carbon steel, high carbon steel, alloy steel etc.
Plain Carbon Steels
According to the percentage of carbon, these steels are types as low or mild, medium and high carbon steels.
Low Carbon Steel or Mild Steel
It has 0.05 - 0.3% carbon. It is undoubtedly one of the most versatile materials available. It has good strength and ductility. It can be easily rolled, forged and drawn. Fabrication by welding and machining is also easily carried out. For chemical equipment, it is therefore used on a large scale. Some of the applications are in pressure vessels, pipes and fittings, machine components and structural sections.
The corrosion resistance of mild steel is low. If the rate of corrosion is slow it is certainly economical to use such steel. In cases where contamination of the product with products of corrosion cannot be accepted, the surface of steel may be protected by paint, metal spraying or other protective lining. This type of protection is not satisfactory for severe corrosion conditions, because it is usually not possible to obtain a perfect coating. Note that allowable stress remains fairly constant at 1230kg/cm2 over a wide temperature range of 30 to 340 ⁰C.
Medium Carbon Steel
It has a carbon content between 0.3 and 0.5%, which helps to improve its hardness, strength and fatigue sections. The material is used for the fabrication of shafts, springs, gears, bolts and certain structural sections. This is difficult to weld.
High Carbon Steel
It has a carbon content of more than 0.5%, up to a maximum of 2.0%. It is much harder and stronger than medium and low carbon steels, but much less ductile. The material is primarily used for cutting tools and dies.
Alloy Steels
Several elements, such as nickel, chromium, silicon, manganese, molybdenum, tungsten, beryllium, vanadium, cobalt, and titanium are used as alloying elements. Generally, certain properties are associated with certain elements.
Nickel and Chromium:- Corrosion and high-temperature resistance.
Manganese:- Abrasion resistance and toughness.
Chromium, tungsten, vanadium, molybdenum, cobalt:- Cutting Action.
Nickel, chromium, titanium:- Creep resistance.
Silicon, manganese:- Elasticity.
A few of the important alloys used in the chemical industry are mentioned here.
Low Alloy Steels
These contain small percentages (below 10%) of nickel, chromium, molybdenum, manganese, etc. in different combinations. These are used primarily for machined components and to a limited extent for pressure vessels. In these steels, the carbon content is usually of the order of 0.15%, manganese 1%, and silicon 0.3%. Steels with 0.5%, 1%, 2.25%, 3% and 5% chromium have a better corrosion resistance than ordinary mild steel and are used for high-temperature service under mild corrosive conditions.
High Alloy Steels
There are two types:- straight chromium steels, with a chromium content is 13 to 17% and chromium-nickel steels or stainles steels with a chromium content between 18 to 25% and nickel content between 8 to 20%. The carbon content varies between 0.03 to .25%. According to the carbon content, straight chromium steels with 0.08% carbon are used for plates, 0.1% for forgings, 0.2% for casting, and 0.15% for bolting. Only those varieties with a carbon content higher than 0.1% are hardenable to heat treatments.
Special mention must be made of stainless steel. These are essentially high-chromium or high nickel-chromium alloys of iron containing small amounts of other essential elements. They have excellent corrosion resistance and heat resistance properties. The most common variety of stainless steel contains nearly about 18% chromium and 8% nickel. The addition of molybdenum to the alloy increases the corrosion resistance and high-temperature strength.
Other Types of Steels
There are more than a hundred different types of stainless steel, which can be classified according to the alloy content, microstructure and major characteristics.
a) Austenitic Stainless Steel
These are the most important types of steel, containing mainly iron, nickel 6 to 22% and chromium 16 to 26%.
b) Martensitic Stainless Steel
These have generally the lowest alloy content. Chromium content ranges from 11% upwards, nickel 1.25 to 2.5% or a higher carbon content. The higher carbon steels are generally poor in corrosion service.
c) Ferrite Stainless Steel
These are alloys containing 13 to 30% chromium as a major alloying element with low carbon. Due to its good oxidation resistance, it finds many uses at high temperatures. These are identified by the AISI 400 series. They are suitable for very mildly corrosive service.
d) Duplex Steel
As the name suggests it is a two-phase mixture of approximately 35 - 55% ferrite and 65-45% austenitic steel. The two-phase microstructure has a very fine grain size imparting high strength. The corrosion due to organic acid is substantially reduced with duplex steel.
e) Nitrogen added to Stainless Steel
When nitrogen of the order 0.15 to 0.20% is added to stainless steels the yield strength is raised substantially. These are called high-proof stainless steels and are most commonly used in moderately high-temperature applications. These are produced in 304, 316 variations.