**2.2 Detailed study of low- and high-carbon steel**

## *2.2.1 Mild- or low-carbon steel*

*Welding - Modern Topics*

varies from 7.1 to 8.05 g/cm3

austenite into martensite.

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

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

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

The carbon steels are composed of carbon and iron by means of carbon up to 2.1 wt%. At the same time, when the carbon content increases, steel has the capability to become harder as well as stronger by heat treating, though it undergoes less

water quenched, even though they may not always be visible.

**2. Role of major alloying element in steel**

according to the alloying constituents.

**188**

**2.1 Carbon**

Mild steel, well known as plain carbon, is at present the common variety of steel as it is cost-effective and offers material properties for a lot of applications. It contains carbon wt% in the range of 0.05–0.30, building it more malleable and ductile. It has comparatively low tensile strength, other than being contemptible and simple to produce; surface hardness can be improved by carburizing. Due to its ductile nature, the failure from yielding is less risky, so it is best applicable (e.g., structural steel). The density of mild or low steel is ~7.85 g/cm3 [5] and Young's modulus is ~200 GPa [6]. Low-carbon steels include a smaller amount of carbon than other steels and are easy to handle as it is more deformable.

## *2.2.2 Higher-carbon steels*

Carbon steels that successfully experience heat treatment contain carbon in between 0.30 and 1.70 wt%. The impurities of different elements also have a considerable consequence on the superiority of the ensuing steel. Small amount of sulfur content makes steel brittle and crumble on operational temperatures. Manganese is added to enhance the hardenability of the steels. The name "carbon steel" can be employed in terms of the steel that is not stainless steel; in addition to it, carbon steel can be involved in alloy steels. Current modern steels are prepared with various mixtures of alloying elements to execute in various applications. The steel is alloyed along with additional elements, typically manganese, molybdenum (Mo), nickel, or chromium up to 10 wt%, in order to develop the hardenability. High-strength low-alloy steel has small additions (<2 wt%) of added elements, usually 1.5 wt% manganese, to offer extra strength.
