Axles are made by heating rods of iron embedded in charcoal powder. The process is known as :

Options

A. tempering
B. annealing
C. nitriding
D. case hardening

Answer

To solve the question regarding the process used to make axles by heating rods of iron embedded in charcoal powder, we can follow these steps:

### Step 1: Identify the Process
The question states that iron rods are heated in charcoal powder. This indicates a specific metallurgical process.

### Step 2: Understand the Role of Charcoal
Charcoal is a source of carbon. When iron is heated in the presence of carbon (from charcoal), it can absorb carbon atoms, which alters its properties.

### Step 3: Define the Process
The process of heating iron in the presence of carbon to harden its surface is known as **case hardening**. This technique creates a hard outer layer while maintaining a softer inner core.

### Step 4: Explain the Purpose
Case hardening is used to increase the hardness and wear resistance of iron or steel components, such as axles, which are subject to friction and stress.

### Final Answer
The process used to make axles by heating rods of iron embedded in charcoal powder is known as **case hardening**.

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