Potential Structural And Functional Materials: Impact Of Carbon Content On Physical Properties Of Steel

Common mechanical properties of steel properties

Discuss about the Potential Structural And Functional Materials.

In the fast growing industry, steel remains to increase its uses as a potential structural and functional materials due to its high strength, soft-magnetic properties, wear resistance and good corrosion together with comparatively low material cost. Steel has a carbon content approximately 0.29 and 0.60 %.

Steel is well known to balance strength and ductility and has a good resistance to wear. It is applied in large parts, automotive components and forging. Hence it’s not shocking that much of modern research has been focused on the impacts carbon on the mechanical properties of steel. Even though there has been a lot of research which has been carried out to examine the impact of distortion limits such as strain, strain rate and temperature on the plastic characteristic of steel there has been very narrow studies on the effects of carbon content on the mechanical properties of steel.in general as the amount of carbon content increases as the hardness of steel increases at the same time elongation decreases (Kawai, 2016, p. 483).

From the previous researches which have been conducted it has been that carbon content in steel produces a small change in the softening characteristic at the deformation temperatures which are usually above no recrystallization temperature. With the increase in amount of carbon content in steel the stress ratio parameters such as oeffective are found to be higher as compared to the pure steel without carbon content.

With the carbon content increase in steel, the microstructure is altered into the martensite and the reserved austenite, plus the solid solution solidification of the carbon element. The main aim of this research is to evaluate and analysis the impacts of carbon content on the Physical properties of steel.

The amount of carbon content has a great influence on the physical properties of steel such as; steel toughness which is inversely proportional to the amount of carbon content existing in steel. Increasing the carbon content in steel increases the hardenability of steel so as the weld of steel cools it can successfully be quenched, leading to a hard, brittle material in the heat affected areas (Davis, 2011, p. 876). Increased amount of carbon content reduces malleability and ductility in steel.

The mechanical strength and hardness of steel increases with the increase in the amount of carbon content present in steel, But the impact toughness decreases as the hardness and strength increases.

Classification of Steel Depending on the Carbon Content

Steel strength refers to the capacity of steel to endure loads inclining to elongate as conflicting to compressive strength that endures loads inclining to decrease size. In other words tensile strength opposes tension i.e. being pulled apart. While compressive strength of steel opposes compression i.e. being pulled together (Mukoyama, 2012, p. 76). The strength of steel is measured by the highest amount of stress that it can withstand when being pulled together or stretched before breaking.

 Ductility refers to the extent of the degree through a steel can elongate or strain amid the start of yield and ultimate break under tensile loading as shown below. Many designers who uses steel depends on  ductility  for a number of phases of design such as reorganization of stress at the final limit state, fabrication process ,condensed risk of exhaustion and bolt group design (Demeri, 2013, p. 250). The figure below shows the stress strain for steel.

Fig 1: stress-strain behavior for steel.

Hardness refers to the measure of the resistance confined plastic distortion that is introduced by either mechanical abrasion or indentation. Some materials are tougher than others .Macroscopic hardness is usually described by the tough intermolecular bonds. Hardness of steel is dependent on many aspects such as elastic stiffness, strain, toughness, ductility and viscosity (Sherby, 2013, p. 117).

Most of the steel types are weldable. Nevertheless, welding includes locally melting the steel, that later cools. The process of cooling can either be slow or fast depending on the surrounding material. The susceptibility to embrittlement also relies on the alloying elements mainly, but not entirely, the carbon content. The susceptibility can be conveyed as the Carbon Equivalent Value and the different product standards for steel standards given expressions for establishing this value.

Fracture toughness refers to the mechanical property of steel which describes the ability of steel to resist fracture, it is considered to be one of the most essential property of steel regarding to design specification of various components. The fracture toughness of steel is established from stress intensity factor at which cracks starts to develop in steel. In most cases the fracture toughness is considered as a qualitative method of showing the resistance of steel to brittle.

Steel is classified into three categories as shown below.

1. Plain carbon steel
2. Mild carbon steel

• High carbon steel

This category of steel are usually iron which contains less than 1 percent carbon content, together with small amount of phosphorus, Silicon, Manganese and sulphur. The weldability and other mechanical properties of this steel are major products of carbon content, even if residual elements and alloying do have a minor influence (The Minerals, 2015, p. 43).

The plain carbon steel is fabricated into a wide arrange of products which include: car bodies, structural beams, cans and kitchen appliances.

This type of steel contains up to 0.25% carbon respondent to heat treatment as an improvement in the ductility but the carbon content that is contained in this type of steel has no effect in relation to its strength. Its strength is generally low due to the low amount of carbon content (Woolman, 2014, p. 371). It is easy to bend and work with but it is not encouraged to be used as structural steel due to its low strength.

This type of steel contains carbon content ranging from 0.25 to 0.705 which improves in the machinability by heat treatment. This type of steel is mainly adapted in forging or machining where surface hardening is required. As the name suggest this type of steel is a low-cost steel which is easy to shape. The strength and hardness of this type of steel can be improved by the additional of carbon content (Pharma, 2015, p. 169). The mild carbon steel is used for production of gears, bolts, axles, studs and other machine parts due its strength.

Usually the high carbon steel contains carbon content of o.6 to 1.0% together with manganese contents ranging from 0.30 to 0.9%.The high carbon content present makes the steel to be very hard unfortunately the high amount of carbon content also makes steel to be brittle and the level of ductility reduces as compared to the mild steel. The high and medium carbon steel are widely used in many common applications. Adding the amount of carbon as the major alloy for higher hardness and strength is the best approach to enhance the performance of steel. Nevertheless, the elevated amount of carbon content has negative impacts to steel such as reduced ductility and weldability (Mukoyama, 2012, p. 77).

The high carbon steel has wide applications such as; rail steels, forging grades, spring steels, pre-stressed concrete, wear resistance, wire rope and the high strength bars.

To enhance the functionality of steel in this uses it is advisable to maximise hardness and strength by increasing the amount of carbon content present. The limiting factor to addition of carbon content will vary depending on the nature of application. For bar products and forging steel, it may be weldability and toughness. For the case of high strength wire the limiting factor to the addition of carbon content can be eutectoic carbon level, more than which the presence of grain edge carbides will greatly reduce draw ability (Morral, 2013, p. 67).

The material which was used for assessment was a medium carbon steel and the mechanical properties that were investigated include; hardness and tensile strength. It is very easy to achieve this properties of steel during its manufacturing. Many parameters can be controlled with the main aim of achieving a desired level of hardness or tensile strength.

Standard test procedures were used, standard test specimen were prepared for all the test which were conducted.

The data that was used for this study was obtained from the secondary sources such as; books, journal and other publication. The data contained in this report is directly from the secondary sources.

Some of the practical sessions that had been carried out in the previous studies are clearly reviewed below.

The properties and methods which were used are shown in the table below.

The tensile testing that was carried out was a standard test conducted by use of the UTM testing machine. After the test was concluded the percentage elongation, tensile strength and the percentage reduction in the area were determined. The tensile strength of steel is given by:

UTS= P _max / A _o

Where by P _max is the maximum; load applied

A_O is the original cross section area.

The percentage elongation after fracture is given by;

Where

The level of hardness of the specimen which was tested was determined by the level of penetration of the indenter on the steel specimen. This was determined directly from the calibrated dial gauge of the machine.

The data which was collected from the experiment that was carried out was further analysed to determine the effects of carbon on other mechanical property of steel as discussed below.

Hardness refers to the measure of resistance of a material to the applied forces. Other steel properties such as yield strength, plasticity, toughness are directly related to the strength of steel (Kawai, 2016, p. 272).

The mechanical strength and hardness of steel increases with the increase in the amount of carbon content present in steel, But the impact toughness decreases as the hardness and strength increases. From that perspective lower amount of carbon content are required for the increase of the impact toughness. The figure below shows the influence of the amount of carbon content on the hardness of steel.

Fig 2:  influence of the of carbon content on steel hardness.

Even if the addition of the amount of carbon can offer better yield strengths and greater toughness. The amount of steel carbon content does not always relate to its strength. The precedence of carbon in steel makes it more hardenable.

The role of carbon as an interstitial impeding dislocation movement factors into strength.

Carbon content in steel increases brittleness and at the same time reduces weldability due to it affinity to form martensite (Honeycombe, 2015, p. 190). Due to that carbon can be both a blessing and at the time a curse when dealing with commercial welding.

There are exceptions of steel that have up to 2% of carbon content but most of them comprises 0f less than 0.35 % carbon. Any type of steel that contains carbon content ranging from 0.35 to 0.86 % can be hardened through the heat-quench-temper cycle.

The steel toughness is inversely proportional to the amount of carbon content present in steel. The increase in amount of Carbon content can result to the decrease in the steel toughness and a reduction in the amount of carbon content present in steel can result to the increase in steel toughness.

If the amount of carbon is low automatically steel will exhibit more elastic behaviour due to the higher plastic deformation with respect to the applied load. The level of plastic deformation is higher due to the lower level of hardness of steel due to less amount of carbon content (Demeri, 2013, p. 549).

Initially an increase in the amount of carbon content resulted to the increase in steel hardenability by quench heat treatment. But as the concentration of carbon increases past a set value it makes steel brittle and then reduces the toughness. That phenomenon is usually observed in the cast iron like nodular or grey.

Increased amount of carbon content reduces malleability and ductility in steel. This is due to the fact that an increase in the carbon content results to steel becoming harder by the heat threating but the ductility reduces. The more the material becomes harder its ability to elongate decreases (Davis, 2011, p. 56). For the case of steel when the Carbon content is high it becomes very hard and stiff in that it cannot be able to elongate any more.

Reduction in the amount of carbon content in steel results to the steel becoming weaker and its ability to elongate increases thus its ductility increases.

Weldability is a general term which is used to describe the capability to obtain comprehensive weld from the welding process. This depends on many factors. Nevertheless when Carbon Equivalent is the defining factor for weldability, Then the automatically we are referring to the strength of the welded joint, in particular hardness of the joint. Higher amount of Carbon Content in the welded joint of steel results to the higher levels of hardness and thus makes steel brittle. At the same time the high levels of hardness also increases the chances of cracking after welding, mostly in the heat affected areas (Clinton, 2015, p. 152).

Increasing the carbon content in steel increases the hardenability of steel so as the weld of steel cools it can successfully be quenched, leading to a hard, brittle material in the heat affected areas. Steel with high levels of hardness has a considerably lower density than the one with lower levels of hardness which can cause residual stress, distortion and cracking.

Steel with high carbon content can also form carbide grains when it is cooled from the molten which can act as a crack propagation sites.

These impacts of the carbon content on the weldability of steel depends on  moderately fast cooling of the weld and in most cases can be controlled by the pre-heating and post-heating  of the welded area so as to lower the rate cooling (Bhadeshia, 2012, p. 45). The other options include welding with a high strength filler, low carbon to dilute the carbon content in the steel.

Conclusion

In conclusion, Steel is well known to stabilize strength and ductility and has a good resistance to wear. It is used for large parts, forging and automotive components. Hence it is not shocking that much of modern research has been focused on the impacts carbon on the mechanical properties of steel.

There are many mechanical properties of steel which greatly influences its application. They include; the steel strength, ductility, hardness, weldability and fracture toughness.

The amount of carbon content has a great impact on the mechanical properties of steel such as; increased amount of carbon content reduces malleability and ductility in steel. Increasing the carbon content in steel increases the hardenability of steel so as the weld of steel cools it can successfully be quenched, leading to a hard, brittle material in the heat affected areas.

The steel toughness is inversely proportional to the amount of carbon content present in steel.

Steel is classified into three Plain carbon steel, Low-alloy steel and High carbon steel. Many designers who uses steel depends on  ductility  for a number of phases of design such as redistribution of stress at the ultimate limit state, fabrication process ,reduced risk of fatigue and bolt group design. The susceptibility can be expressed as the Carbon Equivalent Value and the different product standards for steel standards given expressions for establishing this value.

From the study carried out it is very important for the amount of carbon content in steel to be controlled during the manufacturing process. The major mechanical properties of steels which plays a very essential role in its application might be affected with the increase or decrease in the amount of carbon content.

References

Bhadeshia, H., 2012. Volume 2 of Mechanical and physical Properties of the British Standard En Steels : B.S. 970-1955 / Comp. by J. Woolman and R.A. Mottram. 2nd ed. Sydney: Pergamon Press.

Clinton, R., 2015. Principles of heat treatment of steels. 4th ed. Chicago: New Age International.

Davis, J. R., 2011. Alloying: Understanding the Basics. 7th ed. Paris: ASM International, .

Demeri, M. Y., 2013. Advanced High-Strength Steels: Science, Technology, and Applications. 3rd ed. Auckland: ASM International.

Honeycombe, R., 2015. Steels: Microstructure and Properties. 1st ed. Chicago: Elsevier.

Kawai, J., 2016. Strength of Metals and Alloys (ICSMA 7): Proceedings of the 7th International Conference on the Strength of Metals and Alloys, Montreal, Canada, 12–16 August 1985, Volume 3. 4th ed. Paris: Elsevier.

Morral, J. E., 2013. Boron in steel: proceedings of the International Symposium on Boron Steels. 4th ed. Texas: The Metallurgical Society of AIME.

Mukoyama, T., 2012. Hartree-Fock-Slater Method for Materials Science: The DV-X Alpha Method for Design and Characterization of Materials. 4th ed. Berlin: Springer Science & Business Media.

Pharma, S., 2015. ENGINEERING MATERIALS: PROPERTIES AND APPLICATIONS OF METALS AND ALLOYS. 6th ed. London: PHI Learning Pvt. Ltd.

Sherby, D., 2013. Thermomechanical Processing and Mechanical Properties of Hypereutectoid Steels and Cast Irons. 5th ed. Berlin: Minerals, Metals & Materials Society.

The Minerals, M. &. M. S., 2015. HSLA Steels 2015, Microalloying 2015 & Offshore Engineering Steels 2015: Conference Proceedings. 4th ed. London: Springer.

Woolman, J., 2014. The Mechanical and Physical Properties of the British Standard EN Steels (B.S. 970 - 1955): EN 40 to EN 363. 3rd ed. Texas: Elsevier Science.