Question

Transformation of eutectoid steels to austenite is known as

• A. Austenitizing
• B. Carburizing
• C. Decarburizing
• D. Nitriding

Hint:

Austenitizing always refers to heating steel into the austenite phase region.

Answer 1

The Correct Option is A

Step 1: Understanding eutectoid steel transformation.
Eutectoid steel at room temperature mainly consists of pearlite. When it is heated above the eutectoid temperature (around 727$^\circ$C), it transforms into austenite.
Step 2: Definition of austenitizing.
Austenitizing is the heat treatment process in which steel is heated to a temperature where its structure becomes fully austenitic.
Step 3: Elimination of other options.
(B) Carburizing: Involves addition of carbon to the surface.
(C) Decarburizing: Removal of carbon from the surface.
(D) Nitriding: Addition of nitrogen to improve surface hardness.
Step 4: Conclusion.
Therefore, the transformation of eutectoid steel to austenite is called austenitizing.

Answer 2

The key distinction among these four processes is whether the treatment changes the phase (crystal structure) of the steel or instead changes its chemical composition at the surface. Let's sort the options that way.

1. Austenitizing: When eutectoid steel (pearlite at room temperature) is heated above the eutectoid temperature of about \( 727^\circ\text{C} \), the crystal structure itself transforms from the ferrite-and-cementite mixture of pearlite into a single austenite phase. No new element is added or removed, only the phase changes, and this phase-change process is called austenitizing.
2. Carburizing: This is a surface-hardening process where carbon is diffused into the surface of low-carbon steel from an external carbon-rich source, increasing the surface carbon content. It changes composition, not simply the phase of the existing steel.
3. Decarburizing: This is essentially the opposite of carburizing, where carbon is lost from the surface of the steel, usually as an unwanted side effect of heating in an oxidizing atmosphere. Again, this changes composition rather than describing a phase transformation on heating.
4. Nitriding: This process diffuses nitrogen into the steel's surface to form hard nitride compounds, improving surface hardness and wear resistance. Like carburizing, it changes the chemistry at the surface rather than describing the pearlite-to-austenite phase change.

Since only austenitizing describes a pure phase transformation of the existing steel (pearlite to austenite) without adding or removing any element, it is the correct term.

Therefore, the correct answer is Austenitizing.