Heat treatment practice — definition of process annealing for hypo-eutectoid steels In process annealing of hypo-eutectoid steel, which temperature path is followed and how is the cooling carried out to restore ductility without full austenitisation?

Introduction / Context:
Process annealing is an intermediate heat treatment used in manufacturing to restore ductility and reduce hardness after cold working, enabling further forming operations. For hypo-eutectoid steels, it is distinct from full annealing and normalising, both of which involve full austenitisation above the upper critical temperature (A3).

Given Data / Assumptions:

• Steel composition is hypo-eutectoid (carbon less than eutectoid).
• Goal is to soften and relieve work-hardening without major grain growth.
• No need for complete transformation to austenite.

Concept / Approach:
Process annealing heats the steel to a temperature below or close to the lower critical temperature (A1). At this subcritical range, recovery and partial recrystallisation occur in the cold-worked ferrite, reducing dislocation density and restoring ductility. Slow cooling avoids introducing new stresses and maintains the softened structure.

Step-by-Step Solution:
Select subcritical temperature: near or below A1 to target recovery/recrystallisation of ferrite.Avoid full austenitisation (above A3) which would be full annealing/normalising.Cool slowly (often in furnace or still air) to minimize residual stresses and hardness.Hence choose “heated below or close to the lower critical temperature and then cooled slowly”.

Verification / Alternative check:
Textbook process charts list process annealing for low-carbon steels at approximately 550–700° C (below A1), aligning with the selected option.

Why Other Options Are Wrong:
Options (a)–(c) describe full annealing/normalising regimes above A3; (e) introduces quenching which hardens instead of softening.

Common Pitfalls:
Confusing process annealing with spheroidising or full annealing; the purposes and temperature ranges differ.

Final Answer:
heated below or close to the lower critical temperature and then cooled slowly