Critical Temperatures in Steel — Variation of the Upper Critical Point (A3) State whether the following statement is correct: “The upper critical temperature (A3) of steel varies with the carbon content.”

Introduction / Context:
The iron–carbon phase diagram defines several critical temperatures that govern transformations during heating and cooling. One of these is the upper critical point (A3) for hypo-eutectoid steels, above which the structure is fully austenitic. Understanding how A3 changes with carbon content is foundational for selecting heat-treatment temperatures such as normalising and hardening.

Given Data / Assumptions:

• A3 refers to the upper critical temperature for hypo-eutectoid steels (less than about 0.8% carbon).
• Steel composition varies by carbon content while other alloying elements are assumed minimal.
• Equilibrium (slow) heating conditions are considered for conceptual clarity.

Concept / Approach:
As the percent carbon increases from very low levels toward the eutectoid composition, the A3 line on the Fe–C diagram slopes downward. This means the required temperature to achieve complete austenitisation decreases as carbon increases (up to the eutectoid point). For hyper-eutectoid steels, the relevant line is Acm rather than A3; nonetheless, the statement that the upper critical “varies with carbon” is correct when read as a general observation that critical temperatures in steels depend on composition.

Step-by-Step Solution:
Recognise that A3 is composition-dependent within the hypo-eutectoid range.Note the trend: increasing carbon content → lower A3 temperature until the eutectoid point.For hyper-eutectoid steels, Acm applies; this also varies with carbon, reinforcing the general idea that critical temperatures are composition-dependent.Therefore, the statement “A3 varies with carbon content” is correct.

Verification / Alternative check:
Consult any standard Fe–C diagram: the A3 boundary is not horizontal; it is a sloped line dependent on carbon percentage.

Why Other Options Are Wrong:
False: contradicts the phase diagram which clearly shows a carbon dependence.“Only for hypo-” or “only for hyper-”: critical lines (A3 or Acm) change with carbon in their respective ranges; limiting it to one range is misleading.

Common Pitfalls:
Assuming fixed critical temperatures for all steels; alloying and carbon shift these temperatures.

Final Answer:
True