Critical transformations in steel: On heating plain carbon steel, the temperature at which the austenitic transformation is completed (change ends) is called the __________.

Introduction / Context:
Heat treatment relies on the precise control of transformation temperatures. For steels, the critical points mark where austenite starts to form and where it is fully formed during heating. Recognising these temperatures ensures proper normalising, hardening, and annealing cycles.

Given Data / Assumptions:

• Plain carbon steel behaviour is considered.
• Heating is slow enough to observe equilibrium critical points (Ac1, Ac3/Acm).
• Terminology: “change ends” refers to completion of the austenitisation during heating.

Concept / Approach:
The lower critical point (Ac1) is where austenite begins to form on heating. The upper critical point (Ac3 for hypoeutectoid or Acm for hypereutectoid) marks the completion of transformation to austenite—i.e., the end of change. Decalescence is the apparent temperature arrest during heating as transformation absorbs heat, while recalescence refers to heat evolution arrests on cooling. Thus, the correct term for completion on heating is the upper critical point.

Step-by-Step Solution:
Identify transformation start: Ac1 (lower critical point).Identify transformation end on heating: Ac3 or Acm (upper critical point).Relate terminology: “change ends” → upper critical point.Select “upper critical point”.

Verification / Alternative check:
TTT/phase diagrams and dilatometry show completion of austenitisation at Ac3/Acm, confirming the chosen term.

Why Other Options Are Wrong:
Lower critical is the start, not the end, on heating.

Recalescence pertains to cooling; decalescence describes the phenomenon of heat absorption but is not the nomenclature for the end point.

Martensite start is a cooling/quench transformation, unrelated to heating completion.

Common Pitfalls:
Interchanging decalescence/recalescence; forgetting that Ac1 is start and Ac3/Acm is completion on heating.

Final Answer:
upper critical point