Heat Treatment — Identifying Austempering A steel is austenitised at about 875° C (fully austenitic) and then rapidly brought to an isothermal bath at about 250° C to 525° C and held until transformation occurs. What is this process called?

Introduction / Context:
Austempering and martempering are isothermal heat-treatment processes that tailor microstructure to achieve specific combinations of strength and toughness. Distinguishing them is vital for selecting heat treatments that minimise distortion and cracking while delivering desired properties.

Given Data / Assumptions:

• Austenitising at ~875° C ensures a fully austenitic structure.
• Rapid transfer to a bath at 250–525° C.
• Isothermal hold until transformation completes.

Concept / Approach:
In austempering, the steel is quenched to a temperature in the bainitic range (typically 250–400° C for lower to upper bainite, sometimes up to ~525° C depending on alloy), held isothermally until bainite forms, and then cooled to room temperature. The result is a bainitic structure with good toughness and reduced distortion. In contrast, martempering uses an isothermal hold just above the martensite start temperature to equalise temperature, followed by air cooling so martensite forms uniformly during the final cool.

Step-by-Step Solution:
Identify bath temperature range: 250–525° C → bainitic region.Isothermal hold until transformation → bainite formation.Conclude the process is austempering.Exclude normalising/annealing which involve air/furnace cooling from austenite, not isothermal bainitic holds.

Verification / Alternative check:
TTT/CCT diagrams show bainite noses within ~250–525° C; the described sequence matches austempering practice.

Why Other Options Are Wrong:
Normalising: air cool yields pearlite/ferrite.Annealing: furnace cool for softness, not isothermal bainitic transformation.Martempering: hold just above Ms, then form martensite on final cool, not bainite.

Common Pitfalls:
Confusing austempering with marquenching; check whether bainite or martensite is the goal.

Final Answer:
austempering