Understanding Energisers in Pack Carburising

Carburising is a heat treatment process that adds carbon to the surface of low-carbon steel. This increases the surface hardness and wear resistance while keeping the core relatively soft and tough. One common method is pack carburising.

What is Pack Carburising?

Pack carburising involves packing the steel parts to be treated in a sealed container along with a carbonaceous material (like charcoal or coke) and a catalyst or 'energiser'. The container is then heated to a high temperature (typically 900-950°C) for several hours. At this temperature, carbon monoxide (CO) gas is formed from the carbonaceous material. The CO gas then reacts with the steel surface, transferring carbon into it.

The reaction is primarily:

\(2\text{CO} \rightarrow \text{CO}_2 + [\text{C}]_{\text{steel}}\)

where \( [\text{C}]_{\text{steel}} \) represents carbon dissolved in the steel.

The Role of Energisers

While the carbonaceous material provides the carbon, the reaction rate can be slow. Energisers are added to the pack to accelerate the formation of carbon monoxide gas and the subsequent carbon transfer to the steel surface. They do this by promoting the reaction between the carbon and oxygen in the container.

Common energisers are alkaline earth carbonates or alkali metal carbonates. These compounds decompose at carburising temperatures, releasing carbon dioxide (\(\text{CO}_2\)). The released \(\text{CO}_2\) then reacts with the carbonaceous material to form more carbon monoxide (\(\text{CO}\)).

The primary reactions involving an alkaline earth carbonate energiser (like Barium Carbonate, \(\text{BaCO}_3\)) are:

1. Decomposition of energiser: \(\text{BaCO}_3 \rightarrow \text{BaO} + \text{CO}_2\) (This happens at carburising temperatures)
2. Reaction of \(\text{CO}_2\) with carbon: \(\text{CO}_2 + \text{C} \rightarrow 2\text{CO}\) (This generates more carbon monoxide gas)

This increased concentration of CO gas enhances the carburising potential of the atmosphere within the pack, leading to faster carbon diffusion into the steel.

Analyzing the Options

Let's examine the provided options:

• Barium carbonate: Barium carbonate (\(\text{BaCO}_3\)) is a widely used and effective energiser in pack carburising. It decomposes at the required temperatures and significantly boosts CO generation.
• Sodium phosphate: Sodium phosphate is generally used in detergents, water treatment, or food additives. It is not known to function as an energiser in high-temperature pack carburising processes.
• Hydrogen peroxide: Hydrogen peroxide (\(\text{H}_2\text{O}_2\)) is a strong oxidiser often used for cleaning or bleaching. It is a liquid at room temperature and would decompose rapidly and potentially violently at high carburising temperatures, and it does not serve the purpose of promoting CO formation from solid carbon and air/existing CO atmosphere in the pack.
• Sodium carbonate: Sodium carbonate (\(\text{Na}_2\text{CO}_3\)) is an alkali metal carbonate and can also act as an energiser in carburising, similar to barium carbonate, by decomposing to form \(\text{CO}_2\). However, barium carbonate is often preferred or used in combination with other carbonates due to its stability at high temperatures and effectiveness. While sodium carbonate can act as an energiser, barium carbonate is a very common and effective one listed among standard practices for pack carburising energisers.

Based on the function of energisers in pack carburising and common industrial practices, barium carbonate is a standard and effective energiser.

Therefore, barium carbonate is used as an energiser in pack carburising.

Revision Table: Pack Carburising Energisers

Additional Information on Carburising

Besides pack carburising, other methods exist for adding carbon to the surface of steel:

• Gas Carburising: Uses a carbon-rich gas atmosphere (e.g., natural gas diluted with carrier gas) in a furnace. Offers better control over carbon potential and case depth compared to pack carburising.
• Liquid Carburising: Involves immersing the steel parts in a molten salt bath containing cyanides. Provides rapid case formation but involves hazardous cyanide salts.
• Vacuum Carburising: Performed in a vacuum furnace using hydrocarbon gases. Offers precise control, reduced distortion, and environmentally friendlier operation compared to salt baths.

The choice of carburising method depends on factors such as desired case depth, production volume, part geometry, control requirements, and environmental considerations.