Heat-treating for machinability — Machinability of hard alloys and tool steels is improved chiefly by which heat-treatment process?

Introduction:
Tool steels and high-carbon alloys can be difficult to machine in their as-rolled or hardened states. A targeted heat treatment that changes the morphology of carbides dramatically improves machinability. This question asks you to pick the process that best promotes easy cutting and good surface finish before final hardening.

Given Data / Assumptions:

• Material: high-carbon steels/tool steels with carbide networks.
• Goal: pre-machining condition with low cutting forces and tool wear.
• Subsequent operations may include hardening/tempering after machining.

Concept / Approach:
Spheroidising heat treatment holds steel just below the eutectoid temperature or cycles around it to transform lamellar cementite in pearlite into discrete, globular carbides dispersed in ferrite. This structure reduces hardness and increases ductility, lowering cutting forces and improving chip formation. It is the standard pre-machining treatment for tool steels before final hardening.

Step-by-Step Solution:
Identify need: break up lamellar/continuous carbide into globules.Select treatment that accomplishes this: spheroidising just below A₁ (or by thermal cycling).Result: softer, machinable microstructure with good surface finish potential.

Verification / Alternative check:
Metallography of spheroidised steels shows discrete carbides; machinability indices rise compared with normalized or as-quenched conditions. Standards recommend spheroidise annealing for high-carbon and alloy tool steels pre-machining.

Why Other Options Are Wrong:

• Annealing to full softness may still retain lamellar pearlite; less effective than spheroidising for tool steels.
• Tempering reduces brittleness after quench but does not create globular carbides from lamellae.
• Normalising refines grains but can increase hardness (pearlitic/bainitic) and is not optimal for machinability here.
• Age hardening applies to certain alloys and increases hardness, the opposite of the goal.

Common Pitfalls:
Assuming the softest hardness guarantees best machinability—carbide morphology matters as much as bulk hardness.

Final Answer:
Spheroidising (to form globular carbides in a ferritic matrix)