Heat treatment choice for castings — most common stress-relief and structure refinement step For as-cast components that need reduced residual stress and improved machinability, which heat treatment is most commonly applied immediately after casting?

Introduction / Context:
Castings often contain residual stresses, chemical segregation, and relatively coarse microstructures. A post-casting heat treatment is typically used to improve dimensional stability, machinability, and in some alloys, ductility, before any subsequent hardening steps.

Given Data / Assumptions:

• Generic “castings” refers to common ferrous and non-ferrous cast components.
• Goal: relieve internal stress, homogenize structure, and make machining easier.
• No case hardening or high-strength final properties are requested at this stage.

Concept / Approach:
Annealing is the standard post-casting heat treatment: heat to an appropriate subcritical or intercritical temperature (depending on alloy), hold for homogenization and stress relief, then cool slowly. This reduces hardness, evens out microsegregation, and lowers residual stress. Normalising is more associated with wrought products or steel castings where refinement by air cooling from above the critical range is desired; tempering follows a prior quench; carburising adds carbon at the surface and is not a generic casting treatment.

Step-by-Step Solution:

Verification / Alternative check:
Foundry practice routinely specifies stress-relief/anneal for cast irons and steel castings prior to machining; specifications define soak times and furnace cools.

Why Other Options Are Wrong:

• Carburising: changes surface carbon; not a general casting requirement.
• Normalising: used for some steel castings to refine grains, but the most universal first step for castings is annealing for stress relief.
• Tempering: requires a prior quench; not applicable directly to as-cast parts.

Common Pitfalls:
Assuming a single heat treatment fits all; skipping anneal leads to machining distortion and tool wear.

Final Answer: