Heat treatment — austempering transformation product During the austempering process (isothermal transformation of austenite at an intermediate temperature), austenite changes into which microstructure?

Introduction / Context:
Austempering is a heat treatment that improves toughness and reduces distortion compared with conventional quench and temper. Instead of forming brittle martensite immediately on quenching, the steel is held isothermally so that a different microstructure develops. Knowing the correct transformation product is essential for selecting processing routes.

Given Data / Assumptions:

• Austenitize the steel and then quench rapidly to a temperature above the martensite start temperature.
• Hold isothermally at this bainitic range until transformation is complete.
• Final properties target a good balance of strength and toughness.

Concept / Approach:
In austempering, the steel is quenched to a temperature where bainite, not martensite, forms isothermally. The process avoids the stresses and cracking associated with direct martensite formation and tempering. The resulting bainite can be upper or lower bainite depending on the transformation temperature, each with distinct toughness and strength characteristics.

Step-by-Step Solution:
Start with fully austenitized steel.Quench quickly to an intermediate bath (commonly salt bath) just above the martensite range.Hold isothermally so that austenite transforms to bainite.Cool to room temperature after completion; tempered martensite is not required because bainite already possesses tempered-like attributes.

Verification / Alternative check:
Time-temperature-transformation diagrams show a bainite region between pearlite and martensite; austempering uses this region to produce bainite without passing into the martensite start temperature.

Why Other Options Are Wrong:

• Martensite: forms with rapid quench below martensite start, not during austempering holds.
• Troostite and sorbite: historical terms for tempered structures, not the direct product of austempering.
• Tempered martensite: would require forming martensite first and then tempering, which is not the austempering route.

Common Pitfalls:
Confusing austempering with martempering; martempering aims to reduce thermal gradients before forming martensite, whereas austempering forms bainite isothermally.

Final Answer:
bainite