Question

Improvement of machinability in high carbon steel is obtained by

• A. Full annealing
• B. Spheroidizing annealing
• C. Process annealing
• D. Stress-relief annealing

Hint:

For exams, associate specific heat treatments with their primary purpose:

• Spheroidizing \(\rightarrow\) Maximum softness and machinability in high-C steels.

• Full Annealing \(\rightarrow\) General softening and grain refinement.

• Normalizing \(\rightarrow\) Uniform, fine-grained structure with higher strength than annealed steel.

• Hardening (Quenching) \(\rightarrow\) Maximum hardness (forms martensite).

Answer 1

The Correct Option is B

Step 1: Identify what makes high carbon steel hard to machine.
In the as rolled or normalized condition, high carbon steel contains pearlite made of alternating hard cementite plates and soft ferrite plates. It is the continuous, plate like cementite network that chews up cutting tools and produces poor surface finish.
Step 2: Think about changing the shape of the problem phase, not removing it.
Cementite cannot be eliminated, since it carries the carbon, but its shape can be changed. If the hard phase is broken up into small, widely spaced round particles instead of continuous plates, the cutting tool mostly travels through the soft ferrite matrix and only occasionally meets a small round cementite particle, which machines far more easily.
Step 3: The heat treatment that achieves this.
Spheroidizing annealing does exactly this: the steel is held for a long time at a temperature near the lower critical temperature, allowing the cementite lamellae to break up and coalesce into small spherical globules dispersed in soft ferrite. This gives the softest, most machinable condition of all the standard heat treatments.
Step 4: Ruling out the alternatives.
Full annealing only coarsens the pearlite but keeps it lamellar, process annealing is meant for cold worked low carbon sheet, and stress relieving barely changes hardness at all, so none of them can match spheroidizing for improving the machinability of high carbon steel.
\[ \boxed{\text{Spheroidizing annealing}} \]