Steel Materials: Properties, Treatments, and Applications
Production of Steel
Steel materials are derived from iron-containing ores. The most commonly used are oxides, which undergo mechanical treatments to reduce their size and classify them. Physical processes such as hydro-separation, flotation, and magnetic separation are used to separate the ore from the gangue. The resulting ore is agglomerated and fed into a blast furnace. The ore is introduced into a hopper along with a flux to lower the melting temperature of the scrap. Coking coal provides the heat needed to melt the ore and reduce it. The raw material is fed through the hopper and down a pipe lined with refractory material. 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 has no industrial application due to its high content of carbon, silicon, sulfur, phosphorus, and manganese. It is very hard and brittle and oxidizes rapidly. It is transported in special furnaces to converters, where it is refined into steel or used in foundries. There are several types of converters, depending on the process used to reduce oxides:
- Bessemer: Liquid pig iron is introduced into a converter lined with acidic materials. The converter is rotated vertically, 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 and cooled. It uses cast iron, scrap, and flux, and it allows for the elimination of sulfur and phosphorus impurities.
- 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 tilted to remove the slag, and then coking coal and ferro-alloys are added, resulting in high-quality steels.
Properties and Treatments of Steel
Steels corrode when exposed to air and moisture, so they are often protected with other materials, such as through galvanization. They may experience buckling and brittle fracture (loss of ductility with loads that cause fatigue at low temperatures). Once steel is produced, its properties can be modified by adding alloying elements or through mechanical, thermal, and thermochemical treatments.
Alloying Elements
The hardness and mechanical strength of steel increase with the percentage of carbon, but this also makes the steel more fragile and difficult to weld and machine. Ductility increases as the percentage of carbon decreases, but the steel becomes weaker. Mechanical strength can be increased by adding elements like chromium, nickel, and silicon, but this makes the steel more difficult to machine. Copper alloys provide very high corrosion resistance. Vanadium provides high surface hardness, fatigue resistance, and shock resistance. Manganese provides high hardness and wear resistance. The addition of other elements can change the phase diagram of the steel.
Mechanical Treatments
Mechanical treatments combine mechanical and thermal energy to produce plastic deformation and change properties such as elasticity, plasticity, hardness, and toughness. Cold treatments are performed at temperatures lower than the melting point, increasing hardness but decreasing resistance. Hot treatments are performed with molten metal, decreasing hardness but increasing ductility.
Heat Treatments
Heat treatments do not change the composition of the steel but do alter its structure, modifying properties such as increasing stiffness and strength, decreasing brittleness, and eliminating internal stresses. These treatments often involve metastable constituents like austenite. Martensite, a supersaturated solid solution of alpha iron and carbon, is obtained by rapidly cooling austenite. Bainite, a mixture of ferrite and cementite, is formed by the slow cooling of austenite.
Tempering
Tempering is a conventional heat treatment aimed at obtaining 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
TTT (Time-Temperature-Transformation) diagrams represent the time needed at any temperature to start and complete a phase change. 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 the end. The Bs line marks the start of the transformation to bainite, and the Bf line marks the end. The Ms line indicates the start of the transformation to martensite, and the Mf line indicates the end. Tempering requires a rapid cooling rate to enter the S area of the diagram.
Types of Tempering
- Austenitizing Tempering: Applied to all steels, involving heating above the austenitizing temperature and cooling in appropriate media.
- Martempering: Steel is heated above the austenitizing temperature, held for a period, and then cooled in a salt bath to a temperature slightly above the martensite transformation 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 cross the S curves and transform austenite into bainite. The resulting bainite remains unchanged.
- Surface Tempering: The surface of the piece is rapidly heated to the austenitizing temperature and then quickly cooled, resulting in a thin layer of martensite while the rest remains intact.
- Hardenability: Defined as the ability of steel to harden through the formation of martensite by heat treatment. It is determined using the Jominy test, which keeps all factors influencing hardening constant. The specimen is heated to the austenitizing temperature, held for a period, removed from the furnace, rapidly cooled, and then its hardness is measured.
Annealing
Annealing involves heating the part below the austenitizing temperature and cooling it slowly, often by coating it with hot ash or sand. The goal is to eliminate stresses and imperfections from tempering, obtain fine-grained homogeneous structures, remove brittleness, decrease hardness, and increase plasticity and toughness.
Tempering (Post-Quenching)
This heat treatment follows quenching and serves to eliminate brittleness and internal stresses. It reduces hardness but increases toughness. It involves heating below the austenitizing temperature to obtain more stable structures and then rapidly cooling to room temperature.
Normalizing
Normalizing involves heating ordinary steel to a temperature above the austenitizing temperature, holding it until complete transformation, and then cooling it in air. This refines the steel, eliminates stresses, and results in softer products.
Surface Treatments – Thermochemical
Thermochemical surface treatments alter the chemical composition of the steel surface by adding other elements to improve its properties.