Mild steel or so-called carbon steel was the most common types of steel being used nowadays in almost all forms of industrial applications and industrial manufacturing. Basically, it refers to a group of low carbon steel with relatively low tensile strength, malleable and cheap which consist the maximum composition of Carbon between 0.15% to 0.3%. Furthermore its surface hardness can be increased through carburizing. Apart from the cheapest types of steel, it was widely used in every type of product created from steel because of its weldability, good strength and hardness and although it has the tendency to rust, it is very durable to build materials. Plus, it had the ability to be be magnetized and used in almost any project that requires a lot number of metal.
2.1.2 Types of Mild Steel
There were many types of Mild Steel that had been discovered until recently, among them were A36, High Strength Low Alloy (HSLA) Steel, Abrasion Resistant (AR), Pressure Vessel Quality (PVQ), 4140, 8620 ,1045,1018 and Free Machine Steel (FM 45). The most popular mild steel specification for carbon steel is A36 which normally comes in shapes of plates and bars for bolted and welded construction. High Strength Low Alloy (HSLA) Steel usually contain more than 15% composition of carbon and it is stronger than any ordinary plain carbon steels. HSLA normally comes in the appearance of Corten and Exten material. They are widely used in cars, cranes, bridges and other structures to handle stress at low temperatures. Abrasion Resistant (AR) is commonly used in truck and hopper bodies, shoots, and wear as it is a very hard mild steel that is abrasion or wear resistant. While for the Pressure Vessel Quality (PVQ), because of its properties which are high strength and low weight mild steel it was commonly used in earth moving, transport equipment such as booms, buckets, and pressure vessels. Among the example of PVQ are A514 and T1. 4140 and 8620 perform well under high heat, heavy load wear-resistant material and was used for dyes and molds. 1045 is a standard mild steel which is more durable than A36 though it is harder to machine and weld. It is commonly used for axles, bolts, connecting rods, hydraulic rams and etc. 1018 is the most frequently available of the cold-rolled steels and chemically similar to A36. It come generally in round rod, square bar and rectangle bar. It have good edges such as strength, ductility and ease of machining. Last but not least, Free Machine Steel (FM 45) is a mild steel that has average durability in strength and easy to machine.
2.1.3 Mechanical Properties of Mild Steel
In materials science, the ‘strength’ phrase can be assumed as a high resistance to breakage. Mild steel was well known because of its high resistance to breakage and therefore its mechanical properties can be considered as very strong due to the low amount of carbon it contains. Mild Steel also was recognized as having a high tensile and impact strength compared to the High Carbon Steel due to the fact that it can be easily malleable even when in cold. Another distinguished differences of mechanical properties between high carbon steel and Mild Steel was mild steel bends or deforms if being subjected under stress while a high carbon steels usually shatter or crack. This cause Mild Steel was widely preferred in the construction area in the interest of its weldability, machiniability, malleablility and high strength. As a result of its high strength and easy to malleable, the former is actually quite soft which cause its easy to machine and welding both to itself or to other types of steel. Nevertheless, the former cannot be hardened through heat treatment processes unlike the latter.
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Mild steel usually contains few other alloying elements other than Carbon to give them certain desirable mechanical properties. For example, lets take a look at 1018, a common type of mild steel. 1018 contains an approximately 0.6%-0.9% Manganese (Mn), 0.04% Phosphorus and 0.05% Sulphur. Varying these chemicals will affects certain properties such as corrosion resistance and strength. For instance, Phosphorus, Sulphur and Silicon are undesirable and can be considered as trace elements as they have negative impacts on the steel due to their bad effects on the steel and its properties. The reason why Phosphorus is consider as a trace element because it affects primarily the ductility and the toughness of steel mostly when the steel is in the quenched and tempered conditions. In fact, the Phosphorus has a tendency to react with the iron to form a compound known as ironphosphide (Fe3P) which has the particularity of being brittle. Hence, phosphorus renders steel less tough and ductile while it increases brittleness.
While for the Sulphur which also was a trace element, it has a great tendency to segregate (that is to isolate itself in the structure). It also reacts with iron to form iron sulphide which produces red or hot-shortness, since the low melting eutectic forms a network around the grains so that these hold but loosely together, and the grain boundaries may easily break up during hotforming. So, from here we can see that Sulphur plays a great role in the decline ability of steel such as weldability, impact, toughness and the ductility of the steel.
Carbon is an element whose presence is imperative in all steel. Indeed, carbon is the principle hardening element of steel. That is, this alloying element determines the level of hardness or strength that can be attained by quenching. Furthermore, carbon is essential for the formation of cementite (as well as other carbides) and of pearlite, spheridite, bainite, and iron-carbon martensite, with martensite being the hardest of the microstructures. Carbon is also responsible for increase in tensile strength, hardness, resistance to wear and abrasion. However, when present in high quantities it affects the ductility, the toughness and the machinability of steel.
Whereas element like Carbon and Manganese are desireable as it can enhanced the properties of steel. First, lets observe Carbon element. Carbon is an element whose presence is imperative in all steel. Indeed, carbon is the principle hardening element of steel. That means that the alloying element determines the level of hardness or strength that can be attained by quenching. Furthermore, carbon is essential for the formation of cementite (as well as other carbides) and not to mention pearlite, spheridite,bainite, and iron-carbon martensite, with martensite being the hardest of the microstructures. Carbon is also responsible for the increase in tensile strength, hardness,resistance to wear and abrasion. However, when present in high quantities it will affects the ductility, the toughness and the machinability of steel.
Same as Carbon, Manganese also contributes greatly towards increasing strength and hardness, but to a less extent than carbon. To be more precise, the degree to which manganese increases hardness and strength is dependent upon the Carbon content of the steel. In fact, manganese contributes to the increasing the strength of the ferrite, and also toward increasing the hardness of penetration of steel in the quench by decreasing the critical quenching speed. Moreover, still consisting of a considerable amount of manganese can be quenched in oil rather than in water, and are therefore less susceptible to cracking because of reduction in the shock of the quenching. In addition, the latter enhance the tensile strength, the hardness, the harden ability, the resistance to wear and it also increase the rate of carbon penetrating in the coefficient of thermal expansion of steel whereas it is detrimental to both thermal and electrical conductivity.
2.1.4 Application of Mild Steel
Nowadays, every objects that are fabricated of steel are using mild steel type of material which includes automobile chassis, motorcycle frames and most cookware product. Due to its poor corrosion-resistance, it must be painted or protected and sealed in order to prevent rust from damaging it. A light coat of oil or grease is able to seal this steel and aid in rust control.Unlike high-carbon steel, mild steel is easily welded. The properties of the steel allow the electrical current to travel through the metal without distorting the makeup of the material. Some types of high-carbon steel such as stainless steel, require special techniques in order to properly weld the material. Being less brittle than high-carbon steels, the mild variant is able to flex and give in construction projects where a higher-carbon version could simply break.
Most of the pipeline in the world is created using mild steel. This allows the pipe to not only be easily welded into place, but also lets the pipeline flex and avoid cracking and breaking under pressure. The corrosive properties of the steel pipeline mean that it must be properly sealed through painting or a process often used on pipelines that involves wrapping the pipe with a corrosive-resistant material.Often in very cold climates, a warming type of insulating material is wrapped around the pipeline. This material helps keep the cold inside the pipe running smoothly. The wrap also prevents the soft mild pipe steel from becoming brittle and cracking. The constant expansion and shrinkage due to cold and warmth cycling in the pipe could create structural integrity problems, but these are held in check by the insulating wrap. On a much smaller scale, household pipes can be prevented from becoming cold and breaking by the use of electrical heating tape.
2.2 Theory Test
2.2.1 Tensile Test
A tensile test, also known as tension test is the most fundamental type of mechanical test one can perform on the material. This tests are simple, relatively inexpensive, and fully standardized. By pulling on something, one will determine how the material will react to forces being applied in tension. As the material is being pulled, one will find its strength along with how much it will elongate. The objective of conducting Tensile test until the material breaks or rupture is to obtain a complete tensile profile. A curve will result showing how it reacted to the forces being applied. The point of failure is of much interest and is typically called the Ultimate Strength or UTS on the Stress vs Strain diagram. The total of 4 samples of mild steel plate were prepared for this test purposed where all the samples were machined following the standardized dimension given.
Stress vs Strain Diagram
2.2.2 Hardness Test
Rockwell hardness testing is a general method for measuring the bulk hardness of metallic
and polymer materials. Although hardness testing does not give a direct measurement of any
performance properties, hardness correlates with strength, wear resistance, and other properties. Hardness testing is widely used for material evaluation due to its simplicity and
low cost relative to direct measurement of many properties. This method consists of indenting the test material with a diamond cone or hardened steel ball indenter. Only 1 samples of mild steel material plate was prepared for this type of test as this test only required 1 samples indent 10 data on that 1 samples.
2.2.3 Impact Test
The impact properties of samples were measured using the standardized dimension from the strength lab that had been acquired from the lab technician. Then the samples that had complete the machining process will have to undergo two test which are Charpy test and Izod test. These two test were meant to measure the the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material’s notch toughness and acts as a tool to study temperature-dependent ductile-brittle transition. The only distinctive differences between the charpy and izod lies in the way that the specimen are supported in the apparatus machine.
2.2.4 Microstructure Observation
In Microstructure observation, the samples of mild steel plate will be observed under the microscope in order to monitor the microstructure of the specimen. Before the observation start, the sample will have to be prepared by first grinding the sample with grit sand paper starting from 240, 320, 400, 600 and lastly 1200. After that the sample will be polished by using either powder or paste depend on which is more suitable. Only after all these process were done we can start observe the microstructure under the microscope starting from 5x, 10x, 20x, 40x, 50x and etc.
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