Understanding Carburising Flames in Welding

Welding processes often use different types of flames depending on the metal being joined. An oxy-acetylene flame is one common method, and it can be adjusted to be neutral, oxidizing, or carburising. Each type of flame has distinct properties and is suitable for specific applications and materials.

A carburising flame, also known as a reducing flame, is created when there is an excess amount of acetylene compared to oxygen in the gas mixture. This flame is characterized by three distinct zones: an inner cone, an intermediate cone (feather), and an outer envelope. The excess acetylene tends to add carbon to the molten metal pool, which is sometimes desired or necessary for certain metals.

The question asks which metals are typically welded using a carburising flame.

• Neutral Flame: This flame has a balanced ratio of oxygen and acetylene and is used for welding most common metals like mild steel, cast iron, stainless steel, copper, and aluminium (though specific techniques might vary).
• Oxidizing Flame: This flame has an excess of oxygen and is hotter than the neutral flame. It is typically used for welding metals like brass and bronze, where the excess oxygen helps prevent zinc fuming in brass and can help with silicon bronze welding.
• Carburising Flame: This flame has an excess of acetylene. It is often used for welding metals that are susceptible to oxidation or require the addition of carbon. It's particularly useful for high-carbon steel (though not "carburised steel" in the sense of just surface hardening), and also for certain non-ferrous metals and alloys.

Let's look at the options provided:

1. steel: Mild steel is typically welded with a neutral flame. High-carbon steel might sometimes use a slightly carburising flame.
2. copper and brass: Copper can be welded with a neutral flame, while brass is usually welded with an oxidizing flame to control zinc fuming.
3. aluminium, nickel, monel etc.: These metals and alloys are often welded using a carburising flame. For aluminium, it can help reduce oxidation. For nickel and nickel alloys like Monel, the carburising flame protects the molten pool from oxidation and nitrogen absorption, which can cause porosity and embrittlement.
4. carburised steel: Welding previously carburised steel surfaces can be complex and might involve specific techniques, but the term "carburised steel" itself doesn't directly dictate the *flame* type for the weld itself; rather, the base metal's carbon content and desired outcome do. High-carbon steels (which include carburised surfaces) may use a slightly carburising flame.

Based on typical welding practices for these materials, a carburising flame is well-suited for welding metals such as aluminium, nickel, and alloys like Monel. The reducing nature of the flame helps in preventing detrimental oxidation during the welding process for these specific materials.

Therefore, the metals typically welded with a carburising flame from the given options are aluminium, nickel, monel etc.