**1. Introduction**

Steel, because of its numerous applications, is the most important material among any engineering materials. It is mostly used in tools, automobiles, buildings, infrastructure, machines, ships, trains, appliances, etc., due to its low cost and high tensile strength. Primarily, steel is an alloy of iron and carbon, along with some other elements. The prime material of steel is iron. Iron is commonly found in the Earth's crust in the form of ore, generally an iron oxide, i.e., magnetite or hematite. The extraction of iron from iron ore is done by removing oxygen and then reacting it with carbon to form carbon dioxide. This process is called smelting. Iron has the ability to have two crystalline forms, i.e., face-centered cubic (FCC) and body-centered cubic (BCC), depending on the operation temperature. Fe-C mixture is also added with other elements to produce steel with enhanced properties. Manganese and nickel (Ni) in steel are added to increase its tensile strength and promote stable

austenite phase in Fe-C solution, chromium (Cr) increases hardness and melting temperature, and titanium (Ti), vanadium (V), and niobium (Nb) also increase the hardness. There are two types of steel depending on the alloying elements. If the alloying elements are above 10%, it is referred to as high-alloy steel, and in case of alloying element with 5–10%, it is referred to as medium-alloy steel. If the alloying element in the steel is below 5%, it is called low-alloy steel. The density of steel varies from 7.1 to 8.05 g/cm3 according to the alloying constituents.

When 0.8% of carbon-contained steels (identified as a eutectoid steel) are cooled, austenitic phase (FCC) of the combination tries to revert to the ferrite phase (BCC). The carbon is no longer contained in the FCC austenite structure, which causes excess of carbon. The alternative method to remove carbon from austenite is the precipitation of the solution like cementite and parting behind a neighboring phase of ferrite BCC iron with small quantity of carbon. A layered structure called pearlite is produced when the two, ferrite and cementite, precipitate at the same time. In case of hypereutectoid composition (>0.8% carbon), the carbon will predominantly precipitate out in the form of large inclusions of cementite on the austenite grain boundaries until the amount of carbon in the grains has reduced to the eutectoid composition (0.8% carbon), at which stage the pearlite formation takes place. For steels that have less than 0.8% carbon (called hypoeutectoid), it results in ferrite formation initially in the grains unless the residual content reaches 0.8%, at which stage pearlite formation takes place. No bulky cementite inclusion occurs in the boundaries in hypoeutectoid steel. The cooling process is assumed to be very slow due to the above reasons, hence letting adequate time for the transmission of carbon. Increased rate of cooling does not allow the carbon to migrate for the formation of carbide in the grain boundaries. Rather it will form large amount of finer structure pearlite; hence the carbide is further extensively dispersed and performs to prevent slip of defects inside those grains, ensuing in hardening of the steel. At very high rate of cooling, the carbon has no time to transfer; as a result it is confined inside the austenite and transforms to martensite. The martensite phase is the supersaturated type of carbon, the most strained as well as stressed phase which is exceptionally hard although brittle. Considering the carbon content, the martensite phase obtains various forms. Carbon below 0.2% obtains a ferrite (BCC) form, whereas at higher level of carbon, it acquires a body-centered tetragonal (BCT) structure. Thermal activation energy is not acquired for the conversion from austenite into martensite.

Martensite has a lesser density (as it expands at the time of cooling) than austenite does. As a result the conversion among them consequences a variation in amount. During the above process, growth occurs. Internal stresses as of this growth usually acquire the compressed crystal form of martensite and elongated form on the left over ferrite, along with a significant quantity of shear on the constituents. When quenching is not appropriately done, it can cause crack on cooling due to the internal stresses in a part. They cause interior work hardening and other microscopic imperfections. It is ordinary for quench cracks to appear when steel is water quenched, even though they may not always be visible.
