Oxy-fuel Cutting 101
How the Oxy-fuel Cutting Process Works

Cuts What?

The oxy-fuel cutting process is capable of cutting steel whose composition is less than .3% carbon. Mild steel is comprised of 98% iron, 1% manganese, carbon up to .3% and various other elements in small amounts. Oxy-fuel cutting cannot cut non-ferrous metals such as aluminum, stainless, brass or copper. Elements such as chromium, nickel, molybdenum inhibit the ability to cut steel with the oxy-fuel process. Manganese, silicon, phosphorus, and sulfur in normal amounts have little effect on the cutting process.

Heat and Oxidation

It’s a simple matter of heat and added oxygen resulting in rapid oxidation. Oxy-fuel cutting torches use a copper tip with a series of holes arranged in a circle to deliver a mixture of a fuel gas and oxygen to create a pre-heat flame. Preheat flame temperatures will range from 4,440° to 6,000° F depending upon the fuel gas used and the adjusted ratio of oxygen to fuel gas.

How Thick!?

Mechanized oxy-fuel machine torches, which are normally either 10” or 18” in length, are typically rated for cutting up to 12” to 15”. Some specialty machine torches are rated for cutting up to 30” and more. Cutting thick plate does not require high oxygen pressures. However, cutting thick plate does require a much higher volume of oxygen. Also, the hoses and gas manifold must be designed to deliver the required flows. For example, a cutting machine designed for multiple torch thick plate cutting will typically be configured with 1” I.D. oxygen supply hoses.

Fuel Gases

Natural gas (if available with enough pressure) will deliver excellent performance and the lowest cost per foot of cut. Alternatively, propane will produce excellent results as well. If cutting low alloy steel, propylene or propylene mixes should be considered.

Acetylene outperforms the other gases on thin plate (less than ¼”). However, this segment of the market was taken over in the 1980s by plasma and then by laser. Additionally, acetylene is far and away the most expensive fuel gas per cubic foot. It also requires numerous cylinders to be manifolded together to provide the required flow for multi torch operations.

Oxygen Purity

The AWS Handbook states:

  • The oxygen used for cutting must have a purity of 99.5% or higher
  • Lower purity reduces the efficiency of the cutting operation
  • A 0.1% decrease in oxygen purity will reduce cutting speed by 10%
  • Low purity oxygen also increases the amount and tenacity of the slag to adhere
  • Oxygen purity below 95% results essentially in an unacceptable melt-and-wash action

Single and Two-Piece Tips

Single piece cutting tips are normally designed for cutting with acetylene. Single piece tips are copper with a drilled and swaged hole for the center cutting oxygen flow and four to six drilled holes for the preheat gas flow.

Two-piece tips are designed with an outer copper shell and an inner insert with multiple machined splines to deliver the preheat gases. Two-piece cutting tips are normally designed for cutting with alternative fuels such as natural gas and propane.

Victor-Equipment-one-and-two-piece-tip.jpg
The image shows an assembled Victor Equipment two-piece tip on the left as well as the copper shell and brass insert. A one-piece copper tip is shown on the right.

The Right Tip

Each torch tip manufacturer provides charts specifying the proper size tip based on material thickness. Below is a chart from Harris for their style 98-6 torch when using either propane or natural gas as the fuel. Premium results require that the cut chart be carefully followed.

Harris-Propane-Natural-Gas-Chart.jpg

 

Straight and Divergent Bore

Straight bore tips typically use between 40 - 60 PSI of cutting oxygen. Machine cutting tips are typically divergent bore tips with a tapered or flared outlet hole. Divergent bore tips use 70 - 100 PSI of cutting oxygen and offer 25% increase in cutting speed over straight-bore tips.

Cutting-Oxygen-Bore-Designs_ESAB.gif

 

Preheat Flames

Once the preheat gases are ignited, the preheat flame is adjusted to a neutral flame (equal amount of fuel and oxygen), an oxidizing flame (excess amount of oxygen) or a carburizing flame (excess amount of fuel). Oxy-fuel cutting is normally done with a neutral flame.

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Neutral pre-heat flame

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Oxidizing pre-heat flame

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Carburizing pre-heat flame

 

"Walking Up" the Flame

The preheat flame is adjusted to produce a softer or more aggressive flame by increasing the amount of both fuel and oxygen. Some refer to this technique as “walking up” the flame. Aggressive preheat flames more quickly bring the metal to its kindling temperature.

Preheat Distance

Basic systems offer manual up/down adjustment of torch lifter by the operator. Advanced systems automatically control the torch lifter via an electronic circuit measuring capacitance or inductance between torch and metal being cut. The torch tip is positioned above the plate at the proper height for the fuel gas in use.

Acetylene preheat flames are set just above the plate surface for maximum heat input. Natural gas and propane preheat flames are set approximately ¾” above the plate for maximum heat input.

Preheat Time

If the high preheat flame is very soft, time to reach the kindling temperature may take up to 2 minutes. If the high preheat flame is sufficiently aggressive, preheat time on ½” to 6” material can be as little as 10 seconds.

The pre-heat flames should make a “star” pattern on the plate during pre-heating the plate before the cutting oxygen is turned on - as shown below.

Star-Pattern-on-the-Plate-During-Pre-Heating.jpg

Inside the circle of preheat flames is a hole that delivers a stream of high purity oxygen. The cut oxygen stream is turned on when the metal reaches an orange color which is its kindling temperature (1,600 – 1,800° F).

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Neutral pre-heat flame with cutting oxygen stream on

The high purity oxygen stream causes the steel to rapidly oxidize and this reaction is highly exothermic. An exothermic reaction is a chemical reaction that releases energy by light or heat. The oxygen stream causes the steel to actually catch fire just like paper or wood. Oxygen flow to cut a given thickness of steel is the same regardless which fuel gas is used. Too little oxygen causes a slow and ragged cut. Too much oxygen causes a wide concave cut.

Process Parameters

The operator must select the proper fuel gas for the task and the correct size cutting tip. The source of gases and hoses (both fuel gas and oxygen) must be capable of delivering the required flow at the prescribed pressure. Oxygen and fuel gas pressure must be set correctly. The preheat flames (low and high) must adjusted to the proper ratio of oxygen to the fuel gas. The cutting speed must be set according to the cut chart. The pierce height, preheat time and pierce time must be set. The correct torch tip to work distance must be maintained during cutting.

CNC Process Control

Basic systems require the machine operator to manually adjust pressures, pierce rate control and process event timing for each job. However, increasingly today’s CNC controls include embedded cut charts to prompt the machine operator how to perform the cutting. Once the machine operator selects the torch manufacturer, material thickness and fuel gas to be used, the cut chart typically displays suggested cutting speed, gas pressures, the timing of process events (preheat time, pierce time) and the correct size torch tip.

Gas Management

The gas management system should provide the ability to individually control pressures for low preheat fuel, low preheat oxygen, high preheat fuel, high preheat oxygen and cutting oxygen. It should also control the rate of rise of the cutting oxygen during piercing to minimize excessive spatter during the pierce. Pierce rate control is especially important when piercing steel thicker than 2”.

Gas Pressures Set by the CNC

Hypertherm CNCs provide analog outputs that can control proportional gas regulators as shown in the diagram below. Using this strategy allows you to reduce the number of regulators needed for the gas delivery system from the normal six to three as shown below. Analog outputs are available for three channels: fuel gas, preheat oxygen, and pierce/cut oxygen. 

Setting-Proportional-Regulators-Diagram.png

When adjusting the gas pressures or timers, use the Apply soft key to send the pressures to the cutting system. As you are fine-tuning the system for your requirements, you can select Apply, change the pressures on the screen, and select Apply again without exiting the screen.

Setting-Pressures-Via-CNC.jpg
The image is taken from a Hypertherm Phoenix CNC control software manual. It displays the gas pressures and timing the CNC can control. 

Auto Height Control and Internal Ignition

The IHT FIT + Three torch and control system offers ability to automatically maintain torch to plate height as well as ignition of the gases upon command from the CNC. The system controls preheat height, pierce height, cutting height and retract height after the cut.

IHT-FIT-Three-Machine-Torch.png
IHT FIT + Three Machine Torch

 

 

References

http://www.harrisproductsgroup.com/en/Expert-Advice/tech-tips/theory-of-oxy-fuel-gas-cutting.aspx

http://www.esabna.com/us/en/education/blog/oxy-fuel-torch-preheat-and-its-proper-adjustment.cfm

https://www.thefabricator.com/article/consumables/identifying-the-right-cutting-and-welding-tips

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