Heat treatment temperatures — full annealing of hyper-eutectoid steel For full annealing of a hyper-eutectoid steel (carbon greater than eutectoid), the appropriate furnace temperature relative to the critical points is typically:

Introduction / Context:
Hyper-eutectoid steels contain excess cementite at equilibrium. Full annealing aims to refine structure and soften the alloy for machining or shaping. The chosen temperature window must transform the pearlitic matrix while avoiding excessive dissolution and coarsening of proeutectoid cementite along grain boundaries.

Given Data / Assumptions:

• Hyper-eutectoid composition (C > eutectoid).
• Goal: produce a coarse pearlitic/ferrite-carbide structure after slow cooling.
• Critical points: lower critical (A1) and Acm (upper limit for cementite dissolution).

Concept / Approach:
For hyper-eutectoid steel, full annealing is commonly performed by heating above A1 but below Acm, typically around 30–50° C above A1. This austenitises the pearlite but keeps most proeutectoid cementite undissolved, preventing grain-boundary cementite networks from dissolving and later coarsening unfavorably. Very high temperatures risk grain growth and undesirable carbide morphology.

Step-by-Step Solution:
Select temperature: A1 + 30–50° C (i.e., slightly above the lower critical).Hold for homogenization; then cool slowly in the furnace to promote coarse pearlite.Avoid heating above Acm (option a/e), which can over-dissolve carbides and coarsen grains.Hence, choose “30° C to 50° C above lower critical temperature”.

Verification / Alternative check:
Heat-treatment charts specify sub-Acm anneals for hyper-eutectoid steels to control carbide distribution and machinability.

Why Other Options Are Wrong:
Above upper critical (a) applies to hypo-eutectoid full anneal; (b) and (d) lie below transformation range; (e) suggests quenching, not annealing.

Common Pitfalls:
Using the same rule as hypo-eutectoid steels (above A3) for hyper-eutectoid compositions; they require a different window relative to Acm.

Final Answer:
30° C to 50° C above lower critical temperature