Heat treatment — best description of the spheroidising process in steel Which option most closely describes the common procedure used to spheroidise cementite in medium/high-carbon steels to improve machinability and toughness?

Introduction / Context:
Spheroidising converts lamellar cementite in pearlite into rounded or globular particles within a ferrite matrix. This reduces cutting forces and improves ductility prior to cold forming. It is especially important for tool steels and high-carbon steels before machining or cold drawing.

Given Data / Assumptions:

• Objective: transform plate-like cementite into spheroids.
• Material: medium/high-carbon steel containing significant pearlite or carbides.
• Process aims for subcritical or near-eutectoid temperatures for extended times.

Concept / Approach:
The standard spheroidising anneal holds the steel just below the eutectoid temperature (below A1, roughly 700°C) for many hours followed by very slow cooling. Cycling slightly above and below A1 is also practiced to accelerate spheroidisation, but the canonical textbook definition focuses on prolonged holding below A1 to allow cementite plates to coarsen and spheroidise by diffusion without re-forming lamellae.

Step-by-Step Solution:

Verification / Alternative check:
Metallographs after subcritical holds show discrete cementite spheres instead of lamellae; hardness drops and machinability rises significantly compared with normalised condition.

Why Other Options Are Wrong:

• Up to A1 then still air: may be too brief and too cool to produce full spheroidisation.
• Slightly above A1 then cool to 600°C: can be used in practice, but the most widely cited definition is prolonged subcritical holding, making option (a) the best textbook answer.
• Quenching to ice water: produces martensite, not spheroidised carbides.
• None of the above: incorrect because (a) is a standard method.

Common Pitfalls:
Overheating well above A1 which recreates lamellar pearlite on cooling; insufficient time leading to incomplete spheroidisation.

Final Answer: