How Carbon Affects The Mechanical Properties Of Steel: An In-depth Look

Steel Manufacturing Process

Discuss about the Effect of Carbon Content on the Mechanical Properties of Steel.

In this ever-changing world of ours, metals are one of the essentials tools that are required in this revolutionizing and up keeping the fast-paced environment. With metals used in our daily lives for numerous of things, it is essential to know more about how carbon affects the type of metals we used (Murata et al, 2008).  The selection of the proper type of steel is pretty much vital in determining the right steel composite before manufacturing the steel.

As mentioned earlier, on the different types of applications for the steel, we are required to know that the different types of steel produced are dependent on the mechanical and physical properties of the steel required for their respective application.

Going in depth with this report, we will understand how metal is manufactured and the different types of steels on how carbon effects on the mechanical properties of steel.

Steel is a component made of alloys of carbon and iron plus other elements. It is mostly used in infrastructures, buildings, automobiles, and machines due to its high strength of tensile and low cost. Iron forms the main metal in steel (Li et al, 2010). Steel is normally made up of crystals known as grains. In addition, steel can be categorized into 4 categories depending on its chemical composition. These categories are:

Carbon steels- this kind of steel can be sub-grouped into another four subcategories namely:

• Low carbon steel is usually known as mild steel. This is one of the most common and largest carbon steel group. It entails too many shapes i.e., flat sheet and structural beam. Other elements can be added with an aim of obtaining the desired quality and properties such that the level of carbon maintained low as aluminium is added with an aim of drawing quality while the level of carbon in structural steel is kept high as the content of manganese is increased.
• Medium carbon steel. it is a type of steel with slightly higher manganese content making it stronger when compared to other steel of low carbon hence very hard to form, cut and weld. They are frequently tampered and hardened through exposure to treatment by heat.
• High carbon steel. it is widely referred to as carbon tool steel it possesses a higher quantity of carbon than mild steel and medium carbon steel as the name suggests. It is almost impossible to cut, weld and bend. When treated by heat, it develops the high intensity of brittleness and hardness.
• Ultra-high carbon steel. It is the highest carbon containing a type of carbon steel making it the strongest and hardest. It entails similar features as high carbon steel apart from it being the strongest and hardest.

• Stainless steels- this is also known as inox steel. It is an alloy of steel with at least 10.5% mass content. They are commonly applied where there are intentions of resisting corrosion (Taneiken et al, 2012). This is achieved through increasing the content of chromium addition of molybdenum facilitates resistance to corrosion through acid reduction and preventing pitting attacks in solutions of chloride. The following features make stainless steel the most preferred material for many applications: its resistance to staining and corrosion, reduced or less expensive cost maintenance and a known lustre. Stainless steels are normally rolled and used as sheets, plates, bars, wires and tubing used in cookware, cutlery, instruments for surgery and most appliances (Esawi et al, 2010).
• Tool steels- this is commonly carbon and alloy steels variety that are target fully developed to be made part of working tools. They developed with a particular suitability that originates from its hardness properties, abrasion resistance and its limited possibilities to deformation. In addition to this, it can sustain an edge of cutting even at increased or high temperatures. In relation to the above features at properties of tool steels, they are at sometimes utilized in the shaping of other distinct materials. Carbides presence performs a key role in the development process and quality of tool steels. The following are the key elements of an alloy that form carbides: tungsten, chromium, vanadium, and molybdenum (Gao et al, 2013). The high-temperature performance of steel is determined by the dissolution rate of varying types of carbides into the iron which is in form of austenite. The slower rate is usually considered for heat-resistant steel. To enhance reliability and performance adequacy, adoption of the proper heat treatment is required. With an aim of minimizing cracking chances in water quenching, the content or amount of manganese is usually kept low. There exist 6 groups of tool steels namely: cold work, water hardening, shock resistant, high speed, special purpose and hot work. Selection of choice group solely depends on the temperature for working, working cost, the hardness desired and resistance to shock, strength and the desired toughness. Tool steel is majorly applied in pressing, cutting, metal coining and extradition.
• Alloy steels- this is steel comprising of many alloys totalling to 1% and 50% in weight. This is to improve its properties relating to mechanical. Generally, every steel is an alloy despite all steels not referred as alloy steels. The commonly used alloys are manganese being the widely used, nickel, chromium, vanadium, silicon, and boron. The less frequently used alloys are aluminium, cobalt, copper zinc and many other more.

Steel is produced from scrap and iron ore. During the process of making steel, impurities such as excess carbon, nitrogen, silicon, phosphorous and sulphur are eliminated from the raw material, iron (Wray, 2013). Alloying materials or elements which includes nickel, manganese, chromium, and vanadium can also be added with an aim of generating varieties of steel grades. Quality of products cast from the liquid steel is also maximized through limiting dissolved gases including nitrogen, oxygen and other related impurities.

The process of making steel is divided into two categories namely:

• Primary and
• Secondary

The primary process of making steel entails conversion of liquid iron from a blast furnace and a scrap of steel into either steel or direct reduced iron in a furnace of an electric arc. In this process, steel is made from molten pig iron-rich in carbon. Oxygen is then blown through the molten pig in order to lower the content of carbon of the alloy and converts it into steel. The chemical nature of the refractories makes the process be referred to as basic. The vessels are lined with compounds of calcium oxide and magnesium oxide with an aim of withstanding the increased temperatures and corrosive properties of the molten metal and slag present in the vessel (Wolf & Kurz, 2011). The chemistry of the slag process is controlled aimed at ensuring impurities of phosphorous and silicon are eliminated from the metal. 

Chemical Compounds of Steel

Whereas, the secondary process entails crude steel refining prior to casting. The involved operations are conducted in ladles. During this process, agents of alloying are introduced as lowering of the dissolved gases is conducted. Removal of impurities and inclusions is to a priority and occasionally their chemical properties are altered with an aim of facilitating better steel quality.

The following are common elements present in steel and there positive or negative impacts on steel generally:

Carbon- this element is normally added to iron in order to make steel. Iron in its pure form is usually soft and a 2% addition of carbon introduces toughness and strength to it. Steel structure plates entail less than 0.3% of carbon. The strength of steel normally increases while ductility of the steel decreases when the carbon amounts are progressively being increased. This forms the reason as to why iron with high carbon quantity added to it are always very brittle in nature and rarely elastically respond to dynamic loading (Thomas & Ramaswamy, 2013).

Silicon­- this element is usually added to carbon steels to facilitate the de-oxidation process. Silicon aid in removal of oxygen bubbles from molten steel. In addition, it is sometimes used to increase the strength and hardness of steel though not as effective as manganese.

Manganese-it is considered the second most important alloying element of steel after carbon. Similar to carbon which plays a key role in ductility, strength, and hardening, manganese performs the role of reducing oxides content and counteracting the availability iron sulphide. It is a very essential for makers of steel to ensure that carbon and manganese contents do not exceed the desired amount in order to prevent the steel from being too brittle as it decreases weldability.

Phosphorous- in the structure of steel, this element is considered to be the least or unneedful residual element. This is since all steel applications deserve very little requirements from phosphorous. Despite its limited use, phosphorous improves the embrittlement of steel leading to a reduction in ductility and toughness of steel. Presence of excess phosphorous in a steel usually results in HIC cracking in H2S environments that are wet (Kwon, 2012).

Sulphur- it’s also a residual element entailed in pressure and structural steel vessels. It aids in reducing the effects witnessed as a result of toughness, ductility, and weldability. It normally appears as inclusions of sulphide decreasing the strength of the steel.

Types of Carbon in Steel

Chemical compounds of steel

The chemical compound of steel constitutes of the following:

Carbide. In relation to chemistry, this is a compound composing of carbon and limited electronegative elements.

Cementite. This is a compound of intermetallic carbon and iron. To be more exact, it is an intermediate carbide transition metal possessing the chemical formula Fe₃C. In relation to weight, carbon constitutes 6.67% and 93.3% iron with an orthorhombic structure of the crystal.

Corrosion inhibitor. This is a chemical compound that decreases the material rate of corrosion when added to a liquid or gas.

Non-metallic inclusions. These are non-metallic chemical compounds that are present in steel and its alloys (Esaka et al, 2016). They originate from chemical reactions, physical effects and melting and pouring process emerging contamination.

There are four types of carbon in steel depending on the amounts of carbon present. They include the following:

Low and mild carbon steels- it is also known as mild steel. It contains 0.16% to 0.29% of carbon. It is tough malleable, ductile and can be easily welded and joined. It can be easily corroded hence used as a general purpose material, nails, screws, and bodies of cars. In addition, it is also used as reinforcement bars and covering of the roof.

Medium carbon steels.- its carbon content ranges between 0.3 to 0.6% and can be heat treated. Possess low hardening properties since alloys that can be heat treated are usually stronger though have lower ductility properties (Isfahany et al, 2011). They are commonly utilized on the railway wheels, tracks, gears, and crankshafts.

High carbon steel. Its carbon content ranges between 0.6-0.99percent. It is the best type of steel for making cutting or masonry nails since its carbon makes the steel hard and of more strength while being cheaper in comparison to other hard substances. Manufacturers commonly prefer steels of high carbon for tools meant for cutting metals or even machinery that is bent to form parts of metals. Steel with high carbon content is normally hard, besides the extra carbon content improves its brittleness than other steel types (Zackrisson & Andren, 2014). It is the most type of steel that fractures when exposed to stress.

Ultra-high carbon steel-it is a low alloyed plain carbon steel with 1-2% of carbon. Its hardness is achieved through compression toughness and strong with improved ductility of tensile and ambient temperatures.

It possesses similar characteristics as high carbon steel thought it contains higher strength and hardness.

Impacts of Carbon on Steel

Addition of carbon to iron in the process of making steel makes the steel stronger and toughen up to the maximum point. From this point, the material will become stronger but with reduced toughness just like cast iron sheet. Iron is strengthened by carbon through its crystal lattice being destroyed. The distortion process is same to work hardening through a complex effect which depends on heat treating process of steel and the exact percentage of added carbon which is not normally uniform or standard to all metals.

The intensity of effectiveness achieved by the addition of carbon with an aim of strengthening steel largely rely on the lattice spacing, structure of the crystals and possible chemical effects of steel and carbon.

In addition, carbon also increases the hardness of the steel (Jacques et al, 2008). While the hardness and strength of steel are being increased, carbon too decreases the weldability and ductility of the steel material and improves its machinability which relates to demand for little power and strength in cutting, quick cutting and eventual obtaining of a better end product.

Transformations in phases that involves microstructure changes may occur through:

• Diffusion

Steel is comprised of metals and other chemical elements manufactured at increased temperatures in molten state. Carbon forms the most added common element in steel. Despite addition of other elements such as manganese and chromium to iron in making steel, carbon is the abundant composition and less expensive.

 The iron that is normally utilised in making steel originates from either iron ore or recycled steels. This component of steel usually makes it versatile and useful in relation to varying properties of engineering due to its allotropic elements (Aragon, 2013).

. Iron crystals possess a body centred cubic structure at low temperatures of below 723 degrees whereas at temperatures above 910 degrees, the structure of iron crystal shifts to face centred cubic leaving the cell centre open as the particles of the crystal are situated at cell corner and centre of each face of the cube.

The frequent FCC structure openness accommodates increased number of carbon atoms. The atoms of carbon infiltrates the existing spaces between atoms of iron within the crystal enhancing the carbon percentage in the solution which simultaneously increases steel hardness. Carbon presence in steel gives steel a wide and different properties (Trillo & Murr, 2008).

 Carbon as a predominant alloy in steel makes steel exhibit the following properties:

• Improved tensile strength. This entails the load quantity or amount that steel can withstand before breaking. This is significantly enhanced by the high carbon amounts in steel.
• Enhanced ductility. This involves the ability of steel to change in shape or deform during exposure to stress. This property of steel is witnessed by its capability to stretch to form a wire (Aguilera et al, 2012). This is made easily possible through the carbon element dominancy.
• Improved thermal conductivity. This is the ability of steel to conduct heat. Carbon plays a major role in the heat conduction of steel.
• Corrosion resistance. This is generally the ability of steel to resist chemical reactions involving elements capable of corroding or degrading it.

Transformations in Phases

Documents required for steel selection

This forms a type of document that is necessary before selection of steel

Face diagram of steel

The iron-carbon phase diagram is generally complex hence steel part consideration up to 7% carbon

Fe-Fe₃C Phases in Phase Diagram

α-ferrite- solution of solid of C in BCC Fe

• The form of iron is stable at room temperature
• Solubility of C is at maximum at 0.022 wt%
• At 912 degrees Celsius , it transforms to FCC γ-austenite

γ-austenite- solution of solid of C in BCC Fe

• Solubility of C is at maximum at 2.14 wt% (Hwang et al, 2013).
• At 1395 degrees Celsius , it transforms to δ-ferrite
• Below temperatures of 727 degrees, it is not stable unless rapidly cooled.

δ-ferrite- solution of solid of C in BCC Fe

• It is a similar structure as α-ferrite
• Always stable at temperatures higher than 1394 degrees Celsius
• At 1538 degrees Celsius, it melts

Fe₃C

It is a metastable intermetallic compound which indefinitely remains a compound at room temperature but at 650-700 degrees Celsius slowly decomposes into α-Fe and C

Fe-C solution of liquid

Conclusion

Carbon greatly affects the ever changing working tools in the modern world which are mainly made of steel metal in the ever-changing mechanical environment. The desired type of steel to be used mainly depend on the temperature of the concerned working environment, working cost, the hardness desired and resistance to shock, strength and the desired toughness (Kulka & Pertek, 2013). The prevailing working environment determines the relevant type of selected type of steel depending on the steels mechanical and physical properties.

It is of great essentiality deciding on the most suitable type of steel for a given type of activity after proper consideration of the desired mechanical and physical properties to be achieved by the selected type of steel. Effective measures and care of the selected material must form a great concern as steel and its alloying elements are very costly to obtain and maintain.

References

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