Question

Selected areas of the surface can be hardened by

• A. Nitriding
• B. Flame hardening
• C. Induction hardening
• D. Pack carburizing

Hint:

For selective hardening, think of processes that apply localized energy. Flame hardening (localized heat from a torch) and induction hardening (localized heat from induced electric currents) are the two prime examples. Carburizing and nitriding are furnace processes that treat the whole part unless specific areas are masked.

Answer 1

The Correct Option is B

Step 1: Understand the goal of selective hardening.
Many components need a hard, wear resistant patch, say a gear tooth flank or a shaft journal, while the rest of the part stays soft and tough to absorb shock. This requires heating and quenching only that specific patch, not the whole component.
Step 2: Rule out the whole surface treatments.
Nitriding and pack carburizing work by exposing the entire surface of the part to a reactive furnace atmosphere or pack, they are not inherently local, and confining them to just one area needs extra masking steps that are not part of the basic process.
Step 3: See how flame hardening naturally localizes heat.
Flame hardening uses a hand held or mechanically traversed oxy-acetylene torch to rapidly heat only the exact contour the operator directs it at, above the transformation temperature, followed immediately by a local water spray quench. Because the torch itself defines the heated zone, the process is selective almost by definition, and it needs no special tooling like an induction coil shaped to the part.
Step 4: Conclusion.
Since the torch can simply be aimed and traced along whichever surface needs hardening, flame hardening is the classic answer for hardening selected areas of a component.
\[ \boxed{\text{Flame hardening}} \]