Heat treatment aim — relieving internal stresses and improving machinability: Which heat treatment is chiefly used to relieve prior cold-work stresses in steel and to improve machinability without full phase refinement?

Introduction / Context:
Cold working introduces high dislocation density and residual stresses, reducing ductility and complicating machining. Choosing the right subcritical heat treatment can restore workability economically without the longer cycles of full annealing.

Given Data / Assumptions:

• Material: low to medium carbon steel after cold forming.
• Objective: relieve internal stresses, restore ductility, and improve machinability.
• Avoid unnecessary grain growth or long furnace times.

Concept / Approach:
Process annealing heats steel below the lower critical temperature (below Ac1) to allow recovery and partial recrystallisation, lowering hardness and relieving stresses caused by cold work. It is quicker and less transformative than full annealing, which heats above Ac1 to form austenite and then furnace cools for maximum softness and grain refinement. Normalising heats above Ac3 and air cools, usually increasing hardness relative to full anneal. Spheroidising targets high-carbon/tool steels to form spheroidised carbides for machining, not generally for low-carbon cold-worked parts.

Step-by-Step Solution:
Identify need: stress relief + better machinability for cold-worked steel.Match process: subcritical heating below Ac1 = process annealing.Eliminate others: full anneal is slower; normalising increases hardness; spheroidising is specialised for high-carbon steels.Choose process annealing.

Verification / Alternative check:
Shop practice: sheet/strip steels are routinely process-annealed between forming passes to restore ductility and machinability.

Why Other Options Are Wrong:
Normalising refines grains but typically raises hardness versus full anneal.

Full annealing does increase machinability but is more time- and energy-intensive than needed for simple stress relief.

Spheroidising is ideal for high-carbon steels to machine before hardening, not the general low-carbon case here.

Common Pitfalls:
Using full anneal when a faster, cheaper process anneal would suffice; confusing stress relief temperatures with austenitising ranges.

Final Answer:
process annealing