(a) Steel Properties and Carbon Content:
The carbon content in steel significantly influences its properties in several ways:
- Strength and Hardness: As the carbon content increases, the strength and hardness of steel also increase. This is because carbon atoms form strong bonds with iron atoms, hindering dislocation movement and making the material more resistant to deformation.
- Ductility and Malleability: Conversely, higher carbon content reduces the ductility and malleability of steel. The tangled crystal structure caused by carbon atoms makes it more difficult to bend or shape the material without cracking.
- Weldability: High-carbon steel becomes less weldable due to increased susceptibility to cracking around the weld zone. Careful control of heat and filler materials is necessary during welding.
- Machinability: Low-carbon steel is easier to machine due to its softer nature. As carbon content increases, machining becomes more challenging and requires specialized tools.
- Corrosion Resistance: Generally, higher carbon content reduces the corrosion resistance of steel. However, certain high-chromium stainless steels with moderate carbon content offer excellent corrosion resistance.
Therefore, the optimal carbon content for steel depends on the desired properties for a specific application. A balance between strength, ductility, weldability, and other characteristics is often sought based on the intended use.
(b) Explaining Heat Treatment Terms:
(i) Annealing:
Annealing is a heat treatment process that softens a work-hardened or quenched steel by relieving internal stresses and promoting grain growth. This increases ductility and malleability while reducing strength and hardness. It typically involves heating the steel to a specific temperature above its recrystallization temperature and then slowly cooling it. Annealing is used to improve formability, relieve welding stresses, and prepare steel for further processing.
(ii) Normalising:
Normalising is a heat treatment process similar to annealing but involves heating the steel to a higher temperature (above its critical temperature) and then cooling it in air. This refines the grain structure, resulting in a balance between strength and ductility compared to annealing. Normalising is often used for forging and casting processes to improve mechanical properties and machinability.
(iii) Hardening:
Hardening is a heat treatment process that involves austenitizing (heating above the critical temperature) a steel followed by rapid quenching (cooling). This rapid cooling traps carbon atoms in the austenitic lattice structure, forming a metastable phase called martensite. Martensite is very hard and brittle, significantly increasing the steel’s strength and hardness. However, it also becomes more brittle and susceptible to cracking. Hardening is often followed by tempering to improve toughness and reduce internal stresses without significantly sacrificing strength.