As you know, steel is the most widely used engineering material due to the wide range of applications it can perform. Because of its low cost and high tensile strength, it is primarily used in tools, automobiles, buildings, infrastructure, machines, ships, trains, and other appliances, among other things. Steel is primarily an alloy of iron and carbon, with a few other elements thrown in for good measure. Iron is the most important component of steel. Iron is commonly found in the Earth's crust in the form of ore, which is typically an iron oxide, such as magnetite or hematite. Iron is also found in the atmosphere in the form of sulfide minerals. Oxygen is removed from the ore during the extraction process, and then the oxygen is reacted with carbon to form carbon dioxide. Smelting is the term used to describe this process. If the operating temperature is high enough, iron can exist in two crystalline forms: face-centered cubic (FCC) and body-centered cubic (BCC), depending on the operating temperature. Additionally, the Fe-C mixture is combined with other elements to produce steel with improved properties. Manganese and nickel (Ni) are added to steel to increase its tensile strength and promote the formation of a stable austenite phase in Fe-C solution. Chromium (Cr) is added to steel to increase hardness and melting temperature, and titanium (Ti), vanadium (V), and niobium (Nb) are also added to steel to increase hardness. There are two types of steels based on the alloying elements used. The term "high-alloy steel" refers to steel that contains alloying elements in excess of 10%, while the term "medium-alloy steel" refers to steel that contains alloying elements in the range of 5–10%. Low-alloy steel is defined as steel that contains less than 5% of the alloying element chromium. Steel has a density that ranges from 7.1 to 8.05 g/cm3 depending on the alloying constituents used in its production.

 

Classification of Steel - Alloying Elements, Heat Treatments

Stainless steels are used in a variety of applications. The alloy Fe-18Cr-8Ni-1Mn-0.1C has the characteristic of being a -alloy. It helps to stabilize austenite for its increasing temperature range, which is where austenite can be found. It raises the temperature at which austenite crystallizes (A1) and lowers the temperature at which austenite crystallizes (A3). The majority of these steels were subjected to the solution annealing type of heat treatment, which is typically reserved for austenitic stainless steels. One of the most important requirements for this treatment is the complete dissolution of all precipitated phases, which are predominantly chromium-rich carbides, in which the precipitate of M23C6 occurs in the range of temperatures from 673 to 1173 K. The solution annealing temperature for other stainless steels should be maintained in the range 1273–1393 K, according to the manufacturers' recommendations.

Tool steel is a type of steel that provides the necessary hardness with a simpler heat treatment and retains its hardness even at high temperatures. The primary alloying elements are molybdenum, tungsten, and chromium. These elements have high wear resistance, high strength, and toughness, but they have a low ductility due to their low ductility. Tempering is one of the most common heat treatments available for tool steel, and it necessitates meticulous preparation before it can be used. The conversion of austenite to martensite in various complex tool steels, such as high-speed steel, requires twice the amount of tempering as the first. Because of secondary hardening, high-speed steel (18 weight percent W, 4 weight percent Cr, 1 weight percent V, 0.7 weight percent C, 5–8 weight percent Co, rest Fe) is the best choice for high-speed machining. Furthermore, high-temperature annealing is carried out on the material with a predominantly ferritic structure in order to achieve a maximum bending strength of 4700 MPa. First tempering of these types of steels results in the highest possible hardness, which is then followed by a second tempering process that lowers the hardness to the desired working level. In some cases, the third temper is required for secondary hardening of steels in order to ensure that any new martensite formed as a result of austenite conversion during tempering is properly tempered. This is a matter of personal preference and comes at the lowest possible additional cost.

High-entropy alloy steel: The essential elements of high-entropy steels are iron, cobalt, nickel, chromium, copper, aluminum, and manganese. In conjunction with the increase in Al content, the cast microstructure expands from the FCC to the BCC phase. The hardness of the BCC phase is higher than that of the FCC phase, and the corrosion resistance of the BCC phase is also higher than that of the FCC phase. Some high-entropy alloy steels, such as Al-Fe-Cr-Co-Ni-Ti alloy coating, were equipped with laser cladding, and the effects of annealing temperatures (873, 1073, and 1473 K) on the structure and properties of the alloy steels were investigated in detail. By using laser cladding and fast solidification, we can demonstrate that the intermetallic precipitation compounds in the coating are effectively suppressed. Additionally, we demonstrate that the microstructure of the coating forms a dendrite structure of BCC, which has a superior hardness (698 HV). Because of this, the coating's grain size increases slightly and its microhardness decreases slightly after being subjected to a range of annealing temperatures between 1073 and 1373 degrees Celsius. Furthermore, the structure's thermal stability and microhardness are superior at elevated temperatures as indicated by this specification. In the dendritic boundary, aluminum and iron are improved, whereas cobalt, nickel, titanium, and chromium are improved in the interdendritic boundary. In addition, the degree of segregation increases as the annealing temperature is raised higher and higher.

Mild steel, also known as plain carbon steel, is currently the most widely used type of steel because it is both cost-effective and offers material properties that are suitable for a wide range of applications. It has a carbon weight percentage in the range of 0.05–0.30, which makes it more malleable and ductile. Beyond the fact that it is contemptible and simple to manufacture, it has an extremely low tensile strength; however, surface hardness can be increased by carburizing. Because of its ductile nature, failure due to yielding is less risky, making it the best material for certain applications (for example, structural steel). Low-carbon steels contain a lower percentage of carbon than other steels and are easier to work with because they are more deformable than other steels.

Higher-carbon steels: Carbon steels that have successfully undergone heat treatment contain carbon in concentrations ranging from 0.30 to 1.70 weight percent. The presence of impurities in various elements has a significant impact on the superiority of the steel that is produced as a result. Steel becomes brittle and crumbles when exposed to high temperatures, even if the sulfur content is minimal. Manganese is used to increase the hardenability of steels, which is beneficial. While the term "carbon steel" is typically associated with steel that is not stainless steel, the term "carbon steel" may also refer to carbon steel that is incorporated into alloy steels. Currently available modern steels are prepared with a variety of alloying elements to allow them to be used in a variety of applications. It is necessary to alloy the steel with additional elements, typically manganese, molybdenum (Mo), nickel, or chromium up to 10% by weight, in order to increase the hardenability of the steel. It is made stronger by adding small amounts (less than 2 weight percent) of added elements, typically 1.5 weight percent manganese to provide additional strength.