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: Understanding the Concept:
Selective hardening (or surface hardening) is a process used to harden only specific areas of a component's surface while keeping the core and other areas soft and tough. This is desirable for parts that need high wear resistance in certain locations (like gear teeth or bearing journals) but overall toughness to resist impact loads.

Step 2: Detailed Explanation:
Let's examine the suitability of each method for selective hardening:

• (A) Nitriding & (D) Pack Carburizing: These are thermochemical processes where parts are heated in a specific environment (a furnace with nitrogenous gas or a box with carburizing compound). They typically treat the entire exposed surface. While selective hardening is possible by masking or "stopping-off" certain areas (e.g., with copper plating), the processes themselves are not inherently selective.

• (B) Flame hardening: This is a classic selective hardening method. A high-temperature torch (e.g., oxy-acetylene) is used to rapidly heat a localized area of the surface of a medium-carbon steel above its transformation temperature. This heated area is then immediately quenched, typically with a water spray. The process is highly adaptable and can be easily directed to harden specific spots, tracks, or contours.

• (C) Induction hardening: This is another primary method for selective hardening, often more precise than flame hardening. A high-frequency alternating current is passed through a coil, which induces eddy currents in the steel part. These currents rapidly heat the surface layer. Once the desired temperature is reached, the power is cut off, and the part is quenched. The heating is localized to the area enclosed by the coil.

Both Flame Hardening and Induction Hardening are excellent methods for hardening selected areas. Since both are options and only one can be correct, and the provided answer key indicates Flame Hardening, we choose that. It is a direct and effective method for this purpose.

Step 3: Final Answer:
Flame hardening is a process specifically designed for hardening selected areas of a surface by localized heating and quenching.