Plain Carbon Steels: Classification and Limitations | Metallurgy

In this article we will discuss about:- 1. Introduction to Plain Carbon Steels 2. Classification of Plain Carbon Steels 3. Limitations.

Introduction to Plain Carbon Steels:

Plain carbon steels are iron-carbon alloys in which the properties are primarily derived from the presence of carbon. Some incidental elements like manganese, silicon, sulphur and phosphorus are present in small amounts due to the method of making steels and, not to modify the mechanical properties.

Alloy steels are those steels when, one, or more of the alloying elements are intentionally added to plain carbon steels to enhance, or induce some property, or properties. It is a bit difficult to make a clear cut distinction between plain carbon and alloy steel.

However, AISI (American Iron and Steel Institute) adopted the following definition. ‘Carbon steels are regarded as steels-containing not more than 1.65% manganese, 0.60% silicon and 0.60% copper, all other steels being regarded as alloy steels. Common alloying elements are nickel, chromium, vanadium, silicon, manganese, etc.

Classification of Plain Carbon Steels:

Plain carbon steels can be classified based on the carbon content of the steels in two different ways as:

I. According to the micro-structure in the annealed state:

i. Hypo-Eutectoid Steels:

Microstructures of these steels contain varying proportions of proeutectoid ferrite (also called free ferrite) and pearlite, i.e., the amount of pearlite increases from 0% upto 100% as the carbon content of the steel increases to 0.77%.

ii. Eutectoid Steel:

This steel with 0.77% carbon has 100% pearlite in its microstructure.

iii. Hyper-Eutectoid Steels:

The microstructure of these steels contain pro-eutectoid cementite (or free cementite) and pearlite. The amount of free cementite increases up to a maximum of 22.11% as the carbon content of the steel increases from above 0.77% to 2.11%. Commercial steels have carbon, normally, up to 1.5% maximum as the higher carbon content makes the steel extremely brittle.

II. Though based on the carbon content, but classified according to the level of main mechanical properties of practical importance.

This is the most commonly used commercial classification:

i. Low Carbon Steels:

These are steels having carbon up to 0.25%.

ii. Medium Carbon Steels:

These steels have carbon between 0.25% to 0.55%.

iii. High Carbon Steels:

These steels have carbon from 0.55% to ideally a maximum of 2.11% but commonly up to 1.5% max. in commercial steels.

Carbon is the single most important element profoundly effecting the mechanical properties of the steels. Figures 1.34 and 1.36 illustrate that as the carbon content of the annealed steels increases, hardness, yield strength, tensile strength, (hardenability) increase, whereas the ductility, malleability, toughness, machinability, (weldability), impact strength decrease.

i. Low Carbon Steels:

As the carbon content is low (up to 0.25%) in these steels, they combine fair strength with high ductility with excellent fabrication properties (for rolling, drawing, pressing, welding etc.). These steels are not hardened as the hardenability is low to produce martensite. The hardness of martensite (if produced) is low.

These steels are classified in two classes:

(a) Conventional Low Carbon Steels:

These steels contain about 0.1% carbon with 0.3-0.4% manganese and are cold worked low carbon steels. These steels have yield strength of 200-300 MPa, tensile strength of 350-370 MPa and percentage elongation of 28-40%. Because of high ductility, these steels find applications in the form of cold-rolled sheets.

Their excellent formability suits for cold deformed shapes such as stampings of automobile bodies, the refrigerator bodies, tin cans, corrugated sheets, and solid drawn tubes. Rimmed variety finds applications as rods and wires for nails, rivets, fencing, binding, cable armouring, ferro-concrete bars, mattress wires, etc.

Low carbon steels suffer from strain ageing, which does act as a method of strengthening, but as it raises the transition temperature of the steel, steels become brittle at room temperature. Low carbon steels also suffer from Luder’s band formation, which causes the surface to be rough and unpleasing, and which is undesirable. This defect appears to be minimal if the ferrite grain size (ASTM No between 7 and 9) is tine.

If a small amount of about 1% reduction by cold working is done just before the stamping, or drawing process, Luder bands can be avoided. This is called ‘temper rolling.’ The microstructure of low carbon sheet, or strip consists essentially of ferrite and some carbide, the latter being in the form of pearlite, or individual carbide particles.

(b) Conventional Mild Steels:

These steels have carbon content in between 0.15% to 0.25%, i.e., higher carbon content than conventional low carbon steels and thus, have higher strengths but lower ductility, and thus are hot worked steels. As these steels too are not hardened, these are air cooled (normalised) after hot rolling, or forging, etc. Thus, these steels develop mechanical properties as a result of control of composition and grain size (controlled by the deoxidation practice).

Although, these steels have moderate yield and tensile strengths, but have proper combination of strength, ductility, toughness and weldability to perform satisfactorily in structural applications. Little more manganese (0.6 to 1.25%) increases the yield and tensile strengths without reducing the ductility. Copper (0.2%) is often added for improving the corrosion resistance.

Steels have composition:

C = 0.18 – 0.25%; P = .035%; S = 0.04%;

Mn = 0.60 – 1.25%; Si = 0.15 – 0.35%; Cu = 0.2%

The microstructure of these steels contains about 25% fine pearlite and 75% ferrite, which results in:

Yield strength = 300 – 350 MPa; % Elongation = 26 – 30%;

Tensile strength = 400 – 450 MPa

These steels find applications for structural components, which are welded in situ, for example, for heavy plates for tanks, ship-hulls, pressure vessels, boilers; bridges and building construction items as I-beams, channels, angles, wide-range beams, girders (plate and box), H-beams; oil pipe lines, low temperature applications, etc.

Most of above applications require primarily good weldability with adequate strength and ductility. Conventional mild steels as well as low carbon steels have excellent weldability, but mild steels are stronger too. During welding, the heat-affected zone (HAZ) does attain temperatures above A1, and thus becomes austenite. After welding, this region gets cooled faster due to surrounding cold metal (due to its good thermal conductivity).

Because of low hardenability of these steels, even if carbon is about 0.25% with a bit higher manganese content, the HAZ region gets non-martensite products (Some martensite if it forms, has a maximum hardness of HRC 45, and is not brittle), and thus, is not harmful. As with the plates and structural shapes, there is an increasing interest in oil-pipelines to use now, micro-alloyed steels.

Anisotropy in Properties of Mild Steels:

Mild steels in hot rolled state have generally anisotropic properties, i.e., ductility and toughness are lower in directions normal to the rolling direction.

This is due to:

(a) Bands in steels

(b) Non-metallic inclusions

(c) Deformation and annealing texture- These have relatively minor impacts unless the controlled rolling is continued in the ferritic range.

Banding in mild steels is seen in microstructure as alternate bands (layers) of ferrite and pearlite (Fig. 1.41), specially when ferrite and pearlite are in almost equal proportions, such as in a steel containing 0.25% C and 1.5% Mn. Such cast steels invariably have interdendritic segregation of manganese caused during solidification. Manganese reduces the activity of carbon in austenite, and thus, makes carbon to segre­gates along with itself.

After hot rolling, in regions of segregation of manganese and carbon, austenite changes to pearlite during cooling. Banding removal by homogenisation is a diffusion-based problem. It takes very long time to remove it (~ 200 days at 1200°C). The spacing between the segregated regions may be reduced first, say by rolling, before homogenising annealing is planned.

Banding is not harmful in most of the applications of the plate steels. But it is easier to propagate a crack parallel to bands than normal to them. Banded steels should not be used in applications where lamellar tearing (crack propagating in ferrite parallel to bands) can occur, particularly in heavy welded structures, such as in very large ships and off-shore platforms.

Grange illustrated that anisotropic properties of hot rolled steels are not so much due to banding as are due to elongated non-metallic inclusions. These inclusions get elongated during rolling in its direction and cause anisotropy. The amount, type, distribution, shape and size of inclusions are important. Manganese sulphide inclusions being plastic at hot working temperatures get deformed.

Sulphides are seen generally as long ‘stringers’. Alumina particles, though are least deformed in hot rolling, but commonly are not less damaging, the way these occur in steels as these form dendrites during solidification (i.e., are elongated). The composition of the silicate inclusions controls their deformation behaviour.

To obtain uniform mechanical properties in all directions, particularly in HSLA, or expensive alloy steels, the sulphur and oxygen content of the steel should be reduced as much as possible to prevent the formation of sulphides and oxides. To obtain this, the steel is first deoxidised with aluminium and then treated by injecting lime, powdered Ca-Si alloy, calcium carbide, or iron coated Ca or Mg wire in ladle, when the oxides and sulphides join the slag leaving behind the steel having less than 0.005% S and 0.002% oxygen.

The non-metallic inclusions, when present, must be small, equiaxed, or globular and non-deformable, specially in high strength steels. The shape is modified by changing the composition of oxides or sulphides by adding in steel, Ca, Ce, rare-earth metals, Zr or Ti, so that these inclusions become globular with lower plasticity at hot working temperatures, or even during cold working. Minimum amount of Zr added is [6(%N) + S%] as it combines with both nitrogen and sulphur. Zirconium dissolves in MnS and decreases its plasticity, but often replaces it by virtually non-deformable ZrS.

If the steel owes it properties to nitrogen, then Zr cannot be used, and then Ce is added. Ce is very expensive and a minimum Ce/S of 1.5 is needed for complete sulphide modification. Calcium can globurise both oxides and sulphides. For example, with alumina type oxides, it forms calcium aluminate globules. But calcium has high vapour pressure, low solubility and great reactivity, and its effect is difficult to control in steel making.

Ce and rare-earth metals form almost non-deformable sulphides, or oxysulphides. As the particles are not elongated during rolling, these appear similar in transverse as well as in longitudinal directions, causing increase of transverse ductility and toughness similar to that obtained in longitudinal direction.

ii. Medium Carbon Steels (0.25% to 0.55% C):

These steels have higher strength but lower ductility than low carbon steels. These steels are often used in normalised condition for a great variety of components. In major industries, these steels are used in hardened and tempered state such as for making camshafts, connecting rods, gears, spindles, friction discs, piston rods, cross pieces, plungers. These steels form base of components of machines and are often called ‘machinery steels’.

Some important applications are:

(i) Drop forgings (0.25 – 0.5% C) for general engineering purposes, boiler drums, for agricultural tools such as hoes, spades, forks, etc.

(ii) 0.3 – 0.4% C steels are used for shafts, high tensile tubes, wire, fish plates.

(iii) 0.4 – 0.5% C steels are used for turbo-electric discs, shafts, rotors, die-blocks, gears, and tyres.

iii. High Carbon Steels (Carbon more than 0.55%):

These are heat treated steels to obtain high hardness, wear resistance, cutting properties, and have least ductility. These are mainly tool steels.

Limitations of Plain Carbon Steels:

The largest tonnages of metallic materials produced are plain carbon steels, signifying their extensive applications. Moreover, carbon steels are cheap and available in large quantities, in quite a large variation of shapes and sizes. Their heat treatments are simple. An engineer should try to use as far as possible the carbon steels. They have moderate strength and can resist satisfactorily, ordinary temperatures and atmospheres.

As it is difficult to harden carbon steel-parts thicker than 1.5 cm up to centre (due to low hardenability), higher uniform strength by hardening and tempering cannot be obtained in thicker parts. Even in thinner sections, in heat treated conditions, carbon is increased to get a maximum strength of 700 N/mm2, above which, a rapid decrease in ductility and impact strength takes place.

Modern applications demand much higher strengths with adequate ductility, corrosion resistance in more severe atmospheres and exposure to higher temperatures. Hardened carbon steels show sharp drop of hardness (almost linear) even at stress relieving conditions.

The most important limitations of carbon steels are:

1. Low hardenability.

2. Low corrosion and oxidation resistance.

3. Major loss of hardness on stress-relieving tempering treatment.

4. Poor high temperature properties.

The limitations of carbon steels are overcome by the use of alloy steels. The presence of alloying elements, not only enhances the outstanding characteristics of plain carbon steels, but improves some other properties, or even induces specific properties.

The alloying elements not only improve the hardenability, improve corrosion and oxidation resistance, increase resistance to softening on tempering, increase high temperature properties, but also increase resistance to abrasion, and increase strength of the parts that cannot be subjected to quenching due to physical limitation of parts or the structure in which it is employed.

Alloy steels are expensive and may require more elaborate processing, handling and even heat treatment cycles. Every effort should thus, be made to use carbon steels. Care should be taken that many inherent properties of plain carbon steels cannot be improved by adding alloying elements in it.

Stiffness is one such property which is measured by relationship between stress and strain within elastic range i.e. by modulus of elasticity. Alloying elements enhance the elastic limit but the modulus of elasticity is the same of both the plain carbon and the alloy steels. The design of the structure can change the stiffness. Judicious and cautious use of expensive alloying elements has to be made.