Oxy-Fuel Torch Cutting – Acetylene vs Propane

Oxy-fuel cutting involves a chemical reaction where a preheating flame raises the temperature of the metal to its ignition point, typically between 1600°F to 1800°F (870°C to 980°C). Subsequently, a high-pressure stream of pure oxygen is directed onto the heated area, causing rapid oxidation that generates additional heat and melts the metal. The molten metal, or slag, is then blown away by the oxygen jet, creating a narrow cut or “kerf.” The fuel gas’s primary role is to provide the necessary preheat to initiate this process.

Oxy-fuel setups consist of three main parts:

• A fuel gas (commonly acetylene or propane)
• A pure oxygen supply
• A cutting torch with tips/nozzles matched to the gas and task

But the fuel gas you choose—acetylene or propane—can significantly affect performance, safety, and cost.

The Science Behind Oxy-Fuel Cutting

Oxy-fuel cutting only works on ferrous metals like mild steel and wrought iron. That’s because it relies on oxidation, where metal reacts with pure oxygen to form iron oxide (slag), which is then blown away by the force of the torch’s oxygen jet.

The process works in three steps:

1. Preheat the steel to ignition temperature (~1,600°F)
2. Introduce pure oxygen through a cutting jet
3. Oxidation begins and a stream of molten slag is expelled

Acetylene: The Traditional Choice

Properties and Flame Characteristics

Acetylene (C₂H₂) is renowned for producing the highest flame temperature among common fuel gases when combined with oxygen, reaching approximately 5,800°F (3,200°C). This intense heat facilitates rapid preheating and is particularly advantageous for cutting thick or dense materials.

The acetylene flame comprises two distinct zones:

• Inner Cone: Where primary combustion occurs, producing carbon monoxide and hydrogen.

• Outer Envelope: Where secondary combustion takes place as the gases react with atmospheric oxygen, forming carbon dioxide and water vapor.

Adjusting the oxygen-to-acetylene ratio allows for different flame types:

• Neutral Flame: Equal oxygen and acetylene, ideal for most cutting operations.

• Carburizing Flame: Excess acetylene, useful for applications requiring a reducing atmosphere.

• Oxidizing Flame: Excess oxygen, suitable for cutting thick steel sections.

Equipment and Handling

Acetylene is stored in specialized cylinders filled with a porous material saturated with acetone to stabilize the gas, as it becomes unstable above 15 psi. These cylinders must always be kept upright to prevent acetone from entering the gas lines, which can lead to equipment damage or explosions. Additionally, acetylene has a wide flammability range (2.5% to 82% in air), necessitating stringent safety measures during storage and use.

Advantages of Acetylene:

• Faster preheat time
• Narrow, high-energy flame cone
• Ideal for thin to medium-thick materials
• Best suited for mobile work where speed and heat density matter

Propane: The Cost-Effective Alternative

Properties and Flame Characteristics

Propane (C₃H₈), a liquefied petroleum gas, produces a flame temperature of approximately 5,120°F (2,800°C) when combined with oxygen, which is lower than that of acetylene. However, propane’s flame distributes heat over a broader area, making it effective for cutting and heating applications, especially when using torches designed to swirl the gas for efficient combustion. Unlike acetylene, propane doesn’t store energy in its primary cone, but the wider flame transfers heat more uniformly.

While propane’s lower flame temperature results in longer preheat times, it offers a higher total heat output due to its greater calorific value, making it suitable for cutting thicker materials when equipped with appropriate torch tips.

Equipment and Handling

Propane is stored in pressurized cylinders and is heavier than air, posing a risk of gas accumulation in low-lying areas if leaks occur. Proper ventilation and leak detection are essential safety measures. Unlike acetylene, propane does not require special cylinder configurations, and its storage and handling are generally considered safer due to its narrower flammability range (2.1% to 9.5% in air).

Advantages of Propane:

• Inexpensive and widely available
• Burns cleaner (less carbon soot)
• Longer torch tip life (less carbon buildup)
• Better for cutting thick steel, heating, or gouging

Acetylene vs Propane – Technical Comparison

Cutting Thin vs Thick Steel

Thin Steel (<1″)

Acetylene is preferred when cutting thin sheet metal or plate steel under 1 inch thick. The high-temperature inner cone delivers a rapid preheat and a narrow kerf, making it ideal for precision cutting with minimal slag.

Thick Steel (1″ to 36″+)

Propane becomes more effective with thicker materials due to its broader flame and efficient heat soaking. In industrial settings, such as shipbreaking or steel mills, propane torches have been used to cut through solid steel blocks up to 3 feet (36″) thick. These are often cut using:

• High-capacity torch tips
• Extended-reach torches
• Multi-operator torch rigs

These deep cuts are slow but steady, taking advantage of propane’s ability to distribute heat into thick material volumes over time.

Summary Table: Steel Thickness vs Preferred Fuel

Choosing the Right Fuel for Your Application

Torch Tip and Nozzle Considerations

Not all torch tips work the same way with both gases. Propane tips have multiple preheat holes to compensate for the slower piercing rate, while acetylene tips often use a conical jet.

Always check tip sizing charts to match your fuel and metal thickness. Incorrect tips can lead to:

• Poor cuts
• Tip overheating
• Excessive slag

Maintenance and Operational Efficiency

• Clean tips regularly to prevent carbon buildup
• Use check valves and flashback arrestors
• Monitor oxygen and fuel pressures closely

Efficient operation depends on:

• Minimizing gas waste during preheat
• Using proper cutting speeds to reduce slag and rework

FAQs

Which gas is safer: acetylene or propane?

Propane is generally safer due to its stability at higher pressures and lower reactivity. Acetylene is more volatile and must be stored and handled carefully.

Can you switch between gases using the same torch?

Yes, but you’ll need to change the tips, regulators, and possibly hoses. Each gas requires specific tip designs and pressure settings.

What’s the thickest steel oxy-fuel can cut?

With the right setup, oxy-fuel cutting can slice through steel up to 3 feet (36″) thick, especially with propane or natural gas and multi-torch configurations.

Why doesn’t oxy-fuel work on stainless steel or aluminum?

These metals don’t oxidize the same way as mild steel. Instead of forming a slag, they resist oxidation or form refractory oxides, which block the cutting process.

Is propane really cheaper than acetylene?

Yes. Propane is significantly cheaper and burns more efficiently in long cuts. It also lasts longer per cylinder, making it ideal for large-scale operations.

Conclusion

The choice between acetylene and propane for oxy-fuel cutting depends on specific operational requirements, including material type, thickness, desired cut quality, and cost considerations. Acetylene offers higher flame temperatures and cleaner cuts, making it suitable for precision work and welding. Propane, while producing lower flame temperatures, provides cost advantages and is effective for large-scale cutting tasks. Understanding the properties, equipment needs, and safety measures associated with each fuel gas is essential for optimizing cutting performance and ensuring workplace safety.