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)?
-
Alower critical point
-
Bupper critical point
-
Cpoint of recalescence
-
Dpoint of decalescence
-
ECurie point
Answer
Correct Answer: point of decalescence
Explanation
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:
Identify the process direction: heating.Recognize phenomenon: heat absorption at transformation → decalescence.Name the point where change starts: point of decalescence (corresponding to Ac1).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:
point of decalescence