Temperature required for full annealing in hyper-eutectoid steel

Full annealing is a heat treatment process used to soften the steel and improve its machinability, ductility, and toughness. It involves heating the steel to a specific temperature, holding it at that temperature for a specific time, and then slowly cooling it down. In the case of hyper-eutectoid steel, which has a carbon content greater than the eutectoid composition (0.76% carbon), the temperature required for full annealing is 50°C above the lower critical temperature (AC1).

1. Understanding hyper-eutectoid steel
- Hyper-eutectoid steel is a type of steel that contains more than 0.76% carbon.
- Due to the higher carbon content, hyper-eutectoid steel is prone to brittleness and has lower ductility.
- Full annealing is necessary to reduce the brittleness and improve the mechanical properties of hyper-eutectoid steel.

2. The critical temperatures of hyper-eutectoid steel
- Hyper-eutectoid steel has two critical temperatures: the upper critical temperature (AC3) and the lower critical temperature (AC1).
- AC3 is the temperature at which the steel transforms from austenite to a mixture of austenite and cementite.
- AC1 is the temperature at which the steel transforms from austenite to pearlite or ferrite and cementite.

3. The role of critical temperatures in annealing
- Annealing involves heating the steel above its critical temperature to transform the microstructure.
- In the case of hyper-eutectoid steel, the aim of annealing is to transform the excess carbon into a more desirable form and reduce brittleness.
- The lower critical temperature (AC1) is more relevant in annealing hyper-eutectoid steel as it determines the transformation of austenite into pearlite or ferrite and cementite.

4. Temperature required for full annealing
- To achieve full annealing in hyper-eutectoid steel, the temperature should be 50°C above the lower critical temperature (AC1).
- This temperature is necessary to ensure that the steel reaches the desired transformation stage, where the excess carbon is transformed into a more desirable form.
- Holding the steel at this temperature for a specific time allows for the diffusion of carbon and the formation of a more uniform microstructure.

In conclusion, the correct answer is option 'C' - 50°C above the lower critical temperature (AC1). This temperature is required for full annealing in hyper-eutectoid steel to achieve the desired transformation and improve its mechanical properties.