Critical transformations on heating steel — name of the starting point During heating of plain carbon steel, what is the conventional name for the temperature point at which the allotropic/metallurgical change first begins (i.e., the start of the critical transformation on heating)?

Introduction / Context:
On heating and cooling steels, transformations occur over a critical temperature range. Recognizing the terms used for these phenomena is vital for heat treatment: hardening, normalizing, and annealing depend on accurate identification of transformation starts and finishes.

Given Data / Assumptions:

• Plain carbon steel under equilibrium or near-equilibrium heating.
• Observation of temperature plateaus or changes due to latent heat of transformation.
• Terminology distinguishes heating versus cooling behaviors.

Concept / Approach:
When steel is heated, the transformation from ferrite/pearlite toward austenite absorbs heat. This causes an apparent retardation or halt in temperature rise, termed decalescence. Conversely, on cooling, release of latent heat causes a momentary temperature arrest or rise known as recalescence. The “lower critical point” (Ac1) is the temperature where austenite formation begins on heating; the phenomenon observed at that start is decalescence. Hence, the named temperature point associated with the start on heating is called the point of decalescence.

Step-by-Step Solution:

Verification / Alternative check:
Classic heat-treating texts illustrate temperature–time curves showing arrests on heating (decalescence) and on cooling (recalescence) aligned with Ac1 and Ar1 respectively.

Why Other Options Are Wrong:

• Lower/upper critical “points” are temperature values (Ac1/Ac3) but the phenomenon name for the start on heating is decalescence; the option explicitly asks for this name.
• Recalescence occurs on cooling, not heating.
• Curie point is the magnetic transition, not the structural phase change.

Common Pitfalls:
Interchanging decalescence and recalescence; assuming the Curie transition controls hardening.

Final Answer: