Carbon Steels | Carbon Steel Composition | Carbon Steel Density | Carbon Steel Properties

Carbon Steels | Carbon Steel Composition | Carbon Steel Density | Carbon Steel Properties

What is Carbon Steel?

Carbon steels are so widely used engineering materials, compared to the other ferrous and non-ferrous materials, that often one overlooks the fact that there are many types, each designed by the metallurgist, to have particulars properties required by the designer.

Carbon Steel Composition

It is basically an alloy of iron and carbon-containing manganese (1.0% max.); silicon (0.5% max.); sulphur (0.04% max) and phosphorus (0.5% max). The last two elements are introduced as impurities by the raw materials used during the manufacture of steel. These are kept as minimum as possible. Silicon is residue from the steelmaking process. Carbon and manganese have a significant influence on the mechanical properties and uses of steel.

Advantages of Carbon Steel

Reasons for which carbon steel is widely used are:

a) It has a wide range of mechanical properties, which can be further improved, by the addition of alloying elements and suitably control ling carbon content.

b) It is comparatively cheap.

c) It is readily available in different sizes and shapes.

d) Its properties can be improved further by hear treatment.

e) It can be easily shaped into any form by bending, drawing, or forging.

f) It can be machined and welded with ease.

g) It can be re-melted and recycled again. About 40% of today’s production of steel is by recycling.

h) It consumes less energy to produce carbon steel.

Carbon Steels Contents

Carbon steels are alloys of iron and carbon-containing between 0.1% and 1.5% of carbon. In addition to carbon, it also contains the following materials either as residual elements from the process or by design. The residual elements affect the properties of carbon steel, as described below.

(a)   Silicon

Silicon comes as an impurity from the ore. It varies between 0.1% and 0.5%. It improves resistance to corrosion and oxidation. Excess silicon results in the breakdown of cementite. It is a powerful graphitiser and is therefore never added in large quantities in high carbon steel. It increases hardenability.

(b)   Sulphur: 

Sulphur comes as an impurity from coke used in the blast furnace. It combines with iron and forms ferrous sulfide (FeS) which causes embrittlement. However, sometimes sulfur is added to low carbon steels to improve their machinability, where strength is not essential e.g., free-cutting steels.

(c)    Phosphorus:

 Phosphorus is also an impurity from ore. It makes the steel brittle. It is therefore limited to 0.05%. It improves machinability. In higher quantities, it can improve the fluidity of casting steels where maximum strength and toughness is not the prime requirement.

(d)   Manganese: 

Manganese is an essential constituent of carbon steel. It combines with residual sulfur and forms sulfide stringers which increase the machinability of steel and reduces brittleness. It increases the yield point strength and toughness of steel by forming stable carbide. It is kept between 0.5% and 1.0% in medium and high carbon steels.

(e)   Nitrogen: 

It increases the strength, hardness, and machinability of steel but decreases ductility and toughness. Nitrogen can reduce the effect of boron on the hardenability of steel. In aluminum killed steel, nitrogen forms aluminum nitride particles that control the grain size of the steel, thereby improving both toughness and strength.

(f)     Oxygen: 

It can slightly enhance the strength but seriously reduces toughness. It is mostly observed in rimmed steels.

(g)   Hydrogen: 

Hydrogen dissolved in steel during manufacture can brittle it, which can cause flaking during cooling from hot rolling temperatures. Dissolved hydrogen rarely affects finished mills products as it backs out of the hydrogen during reheating the steel before hot forming.

(h)   Arsenic and antimony: 

These increase susceptibility of steel to temper embrittlement.

(i)     Tin:

 It can make steel susceptible to temper embrittlement and hot shortness.

 Types of Carbon Steels and Properties of Carbon Steel

Carbon steel is an alloy of iron and carbon. Its properties depend upon the structure i.e., the iron-carbon system as described above. Ferrite is relatively a soft and ductile material. Pearlite is harder and less ductile. Carbon steel having these two substances will have therefore different properties.

The hardness and tensile strength increase with an increase in carbon content. However, toughness, machinability, and ductility decrease with an increase in carbon content.

In case it contains other alloying elements, its properties will change further. Therefore, it can be classified into three groups namely: low, medium, and high carbon steel depending upon the percentage of carbon.

Types of Carbon Steel

Their properties and uses are described as under:

(i) Low Carbon Steel (Mild Steel)

It contains carbon up to 0.25%. It is soft, tough, ductile, and low tensile strength steel. It can be easily worked and welded. Mild steel is general-purpose steel and is used where hardness and tensile strength are not the most important requirements. Typical applications are bodywork for cars and ships, screws, nails, wires, structural steel etc.,

(ii) Medium Carbon Steel

It contains 0.3 to 0.55% carbon. Strength and hardness are improved while ductility is reduced. IT can be forged, rolled, and machined. It is used for agricultural tools, fasteners, dynamo and motor shafts, crankshaft, connecting rods, gears etc.,

(iii) High Carbon Steel (Tool steel): 

It contains 0.6 to 1.5% carbon. It is the hardest of plain carbon steels, possesses good tensile strength. It is therefore used for withstanding wear, where hardness is a more requirement than ductility. It is used for machine tools, saws, hammers, cold chisels, punches, axes, dies, taps, drills, razors. The main use of high carbon steel is thus as tool steel. Hence it is also known as tool steel. These are always used in the hardened and tempered condition.

Carbon Steel Density

Most commonly used value for density of standard Carbon Steel is 7.85 g/cc  and is used  in  calculating the weight of steel product like pipes and Steel Plates  by multiplying the carbon steel density  by volume of the product. The density of steel varies depending on the alloying elements , however usually ranges between 7,750 and 8,050 kg/m3 (484 and 503 lb/cu ft), or 7.75 and 8.05 g/cm3 (4.48 and 4.65 oz/cu in).

 Carbon Steel Grades and  Uses of Carbon Steel

The following table gives the typical applications of carbon steels with varying carbon content.