Question

The advantage of pack carburising is

• A. Close control of surface carbon is possible
• B. Suited for production of thin carburized cases
• C. Prepared atmosphere is not required
• D. Components can be directly quenched from carburizing temperature

Hint:

Compare surface hardening methods:

• Pack Carburizing: Simple, low-cost, good for thick cases, poor process control.

• Gas Carburizing: Excellent control, good for mass production, requires complex furnaces.

• Liquid Carburizing (Cyaniding): Fast, uniform, but uses toxic salts.

• Nitriding: Produces very hard case, low distortion, but slow process.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Pack carburizing is a case-hardening process where low-carbon steel components are heated in a sealed box while packed in a solid, carbon-rich compound (e.g., charcoal mixed with an energizer like barium carbonate). At high temperatures, carbon monoxide is generated, which then provides the carbon that diffuses into the steel surface, creating a hard, high-carbon case over a tough, low-carbon core.

Step 2: Detailed Explanation:
Let's analyze the advantages and disadvantages based on the given options:

• (A) Close control of surface carbon is possible: This is a disadvantage of pack carburizing. It is difficult to precisely control the surface carbon concentration and the case depth compared to more modern methods like gas or vacuum carburizing.

• (B) Suited for production of thin carburized cases: This is incorrect. Pack carburizing is a relatively slow process and is generally more economical for producing thick cases (e.g., $\gt $ 1 mm). Gas carburizing offers better control for thin cases.

• (C) Prepared atmosphere is not required: This is a key advantage. The process is self-contained. The necessary carburizing atmosphere (carbon monoxide) is generated within the sealed box from the packing material itself. This makes the process simple, low-cost, and does not require expensive and complex atmosphere-controlled furnaces.

• (D) Components can be directly quenched from carburizing temperature: This is generally not recommended. Direct quenching from the high carburizing temperature can lead to a coarse grain structure in both the case and the core, resulting in brittleness. The standard practice is to slow cool, then reheat to a lower temperature to refine the grain structure before quenching.


Step 3: Final Answer:
The main advantage of pack carburizing is its simplicity and the fact that it does not require an externally prepared and controlled furnace atmosphere.