Steel Materials: Properties, Treatments, and Applications
Steel materials result from processing iron-containing ores. The most commonly used ores are oxides, which undergo mechanical treatments to reduce their size and classify them. Physical processes like hydro-separation, flotation, and magnetic separation are employed to separate the ore from the gangue. The resulting ore is then agglomerated and fed into a blast furnace.
In the blast furnace, the ore is introduced along with a fluxing agent to lower the melting temperature of impurities. Coking coal provides the heat necessary to melt the ore and facilitate its reduction. The raw material descends through a refractory material-lined pipe. Reduction processes occur at high temperatures, leading to the formation of slag, which consists of oxides. This slag floats on the molten iron and is extracted.
The resulting iron, known as pig iron, has limited industrial applications due to its high content of carbon, silicon, sulfur, phosphorus, and manganese. It is also very hard, brittle, and prone to rapid oxidation. Therefore, it is transported in special furnaces to converters, where refining takes place to produce steel or to foundries.
Types of Steel Converters
- Bessemer: Liquid pig iron is introduced into a converter lined with acidic materials. The converter is rotated, and air is injected through a nozzle to oxidize impurities. It is then tilted to remove the slag that floats on the steel.
- Thomas: Similar to the Bessemer process, but the converter is lined with basic materials to remove basic impurities.
- LD Converter: Instead of injecting air, pure oxygen is injected through a lance when the converter is vertical. This process uses cast iron, scrap, and flux, allowing for the elimination of sulfur and phosphorus impurities.
- Open Hearth Furnace: Pig iron and scrap are introduced into a hearth. Coking coal and hot air are added to maintain high temperatures. This is a slow process that yields high-quality steels.
- Electric Arc Furnace: Cast iron and scrap are melted in the furnace. High temperatures are achieved by introducing electrodes that produce an electric arc, causing the material to melt. Oxygen is injected to oxidize impurities. The furnace is then tilted to remove the slag, and coking coal and ferro-alloys are added, resulting in high-quality steels.
Steels are susceptible to corrosion when exposed to air and moisture. Therefore, they are often protected with other materials, such as in galvanization. They may also experience buckling and brittle fracture, which is a loss of ductility under fatigue loads at low temperatures. Once steel is produced, its properties can be modified by adding alloying elements or through mechanical, thermal, and thermochemical treatments.
Modifying Steel Properties
Alloying Elements
The hardness and mechanical strength of steel increase with the percentage of carbon, but this also makes it more fragile, difficult to weld, and machine. Ductility increases with a lower carbon percentage, but the steel becomes weaker. The addition of elements like chromium, nickel, and silicon increases mechanical strength but can make machining more difficult. Copper alloys provide high corrosion resistance, while vanadium imparts high surface hardness, fatigue resistance, and impact resistance. Manganese contributes to high hardness and wear resistance. The addition of other elements can alter the phase diagram of the steel.
Mechanical Treatments
Mechanical treatments combine mechanical and thermal energy to induce plastic deformation and modify properties such as elasticity, plasticity, hardness, and toughness. Cold treatments are performed at temperatures below the melting point, increasing hardness but decreasing resistance. Hot treatments are carried out with molten metal, decreasing hardness but increasing ductility.
Heat Treatments
Heat treatments do not change the overall composition of the steel but modify its microstructure, thereby altering properties. These treatments can increase stiffness and strength, decrease brittleness, and eliminate internal stresses. They often involve the formation of metastable constituents from austenite. Martensite, a supersaturated solid solution of alpha iron and carbon, is obtained by rapidly cooling austenite. Bainite, a mixture of ferrite and cementite, forms during the slow cooling of austenite.
Specific Heat Treatments
Tempering
Conventional tempering aims to produce martensitic steels. Austenite is heated to a high temperature, maintained for a sufficient time, and then rapidly cooled to achieve a modified crystal structure. Factors influencing tempering include steel composition, heating temperature, heating time, and cooling rate.
Cooling Media
Water is the fastest and most commonly used cooling medium. Oil provides a slower, gentler temper and is used for alloys.
TTT Diagrams
Time-Temperature-Transformation (TTT) diagrams represent the time required at any temperature to initiate and complete phase transformations. They are obtained through specimen testing. Above the AC1 temperature, the structure is entirely austenite. The Ps line indicates the start of the transformation to pearlite, and the Pf line indicates its completion. The Bs line marks the start of bainite formation, and the Bf line marks its end. The Ms line indicates the start of martensite formation, and the Mf line indicates its end. Tempering requires a rapid cooling rate to enter the area of martensite formation.
Types of Tempering
- Austenitizing Tempering: Applied to all steels, involving heating above the austenitizing temperature and cooling in an appropriate medium.
- Martempering: Steel is heated above the austenitizing temperature, held, and then cooled in a salt bath to a temperature slightly above the martensite transformation start temperature. It is held long enough and then rapidly cooled.
- Austempering: Similar to martempering, but with a longer residence time in the salt bath to allow the austenite to transform into bainite. The resulting bainite remains unchanged upon further cooling.
- Surface Tempering: The surface of the workpiece is rapidly heated to the austenitizing temperature and then quickly cooled, resulting in a thin layer of martensite while the core remains unaffected.
- Hardenability: Defined as the ability of steel to harden through martensite formation during heat treatment. It is determined using the Jominy test, which involves keeping all factors influencing hardening constant. The specimen is heated to the austenitizing temperature, held, rapidly cooled, and then its hardness is measured.
Annealing
Annealing involves heating the part below the austenitizing temperature and cooling it slowly, often by covering it with hot ash or sand. The objectives are to eliminate stresses and imperfections from tempering, obtain a fine-grained homogeneous structure, reduce brittleness, decrease hardness, and increase plasticity and toughness.
Normalizing
Normalizing involves heating the steel above the austenitizing temperature, holding it until complete transformation, and then cooling it in air. This process refines the steel, eliminates stresses, and produces softer products.
Surface Treatments (Thermochemical)
Surface treatments alter the chemical composition of the steel surface by adding other elements to improve its properties.