Cutting to Prepare for Welding or for Demolition
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Cutting-torch is the general name of a useful tool designed to perform metal cutting.
Its various forms are based on a number of physical principles.
Three main types, addressed in this presentation are available.
The first is known as Oxygen Cutting, that is cutting using oxyfuel flame, the second is Arc Cutting and Gauging, the third is Laser Cutting.
Cutting-torch and other Severing Tools
What is in here for me?
Why is a Cutting torch a useful tool for metal construction preparation or for demolition? Because its principles of operation are simple, the techniques can be learned quickly, and its implementation is inexpensive when compared to mechanical cutting.
If cutting is well performed there is no need for further preparation of cut surfaces before welding, a remarkable benefit of the application of this technology.
Why some metals do not lend themselves to be severed by Cutting torch? Because they are rich in elements that prevent ready oxidation.
Several techniques, derived from those developed for welding, permit using a Cutting-torch for cutting metals, even of considerable thickness, as needed in preparation for further construction or, at the other end, for demolition.
In a sense it could be said that the process of cutting is the opposite of welding: while welding consists in uniting together two separate metal pieces by progressive local melting, cutting is the progressive severing (in two or more sections) of one piece, by local heating and burning away the metal.
|A different cutting process with definite advantages in certain circumstances is not based on heating and is not discussed here. Its presentation can be found in the page on Abrasive Water Jet Cutting.|
An Article on Abrasive Water Jet Cutting was published (7) in Issue No. 47 of Practical Welding Letter for July 2007. Click on PWL#047 to read it.
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Oxygen cutting or oxyfuel gas cutting is performed by the high temperature exothermic reaction of oxygen with the base metal.
A Gas Cutting-torch for oxyfuel gas cutting processes, is fed on combustible gases like acetylene, propane, hydrogen, other gases or by the combustible gases originating from gasoline, without electrical energy supply.
Metal cutting by Cutting-torch is made possible by the tendency of certain metals to become readily oxidized under local conditions and burnt away by a powerful stream of oxygen gas.
The Cutting-torch using the oxyacetylene flame is a special piece of equipment designed to implement a process whereby the chemical reaction of a pure oxygen stream with steel at elevated temperature produces local separation along the line of advancement.
The principle underlying the operation is the capability of high purity oxygen to react with iron.
A small area must first be heated to the temperature of self sustained ignition (870 °C = 1600 °F) (less than iron melting point), liberating heat that sustains the reactions producing various types of iron oxides.
This burning heat also melts some of the iron, and the drops are swept away with the molten iron oxides, removed by the force of the impinging oxygen stream, leaving in its wake a cut called kerf.
|For a selection of modern OXY-Gas Welding and Cutting Equipment, Torches and Kits, you may wish to have a look at My Store.|
Tip!: For best quality cutting effects it is most important that the oxygen used be of high purity, at least 99.5%.
Advantages of oxyfuel gas cutting are:
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It should be noted that not all materials are easily sectioned by these methods: outstanding in this group are aluminum and copper which cannot be cut.
However, for most of the constructional steels, these may be the most economical ways to perform manual or automated cutting according to lines or patterns as needed.
Specfic variants can be used by adding special powders for stainless steels or other difficult to cut materials.
The Cutting-torch used is different from the customary welding torch (see Gas Welding Equipment).
In one design the tip presents a crown of ports that supply the gas mixture employed to obtain the preheating flame, and a central channel through which the powerful pure oxygen stream is made to impinge on the heated spot to provide rapid oxidation and removal of oxides and metal drops.
Once started the exothermic chemical reaction is self sustaining along the thickness of the metal (if the advancement speed of the Cutting-torch is not excessive), while the preheating flame itself is needed to bring new surfaces to ignition temperature.
Critical to the success of the operation is the fact that iron oxides melt at lower temperature than the metal itself.
The Cutting-torch severed surface presents striations whose appearance indicates the quality of the cutting process.
Ideally the surface should be as smooth as possible, with square edge and limited drag (curving in the direction opposite to advancement), absence of molten metal drops and minimum adhering slag.
Accurate examination permits to improve the aspect through appropriate selection of parameters.
The use of hydrogen instead of acetylene as a flammable gas in the Cutting-torch has found natural application in underwater cutting, where hydrogen can be compressed without danger to balance the hydrostatic pressure (which cannot be said of acetylene).
Maximum depth for safe use of acetylene is about 6 m (20 ft) because its maximum safe operating pressure is 100 kPa (kilo Pascal or Newton per square meter)(15 psig).
The equipment is designed to provide an additional air bubble around the cutting tip, by supplying a continuous flow of compressed air.
Besides Hydrogen, other fuel gases used at any depth reached by divers in underwater cutting are Methylacetylene-propadiene (stabilized) and Propylene.
Safety issues should be addressed by anyone using Cutting-torch in the practice of metal cutting with these and other techniques.
In another page on SAFETY we provided some useful links to information and codes pertaining to this important subject.
The importance of thorough study and application of safe practices cannot be overemphasized.
Tip!: A relatively new development, commercialized as a self contained unit, uses regular gasoline as fuel, with a special Cutting-torch and is claimed to have economic advantages over normal oxyacetylene cutting torch, within its range of applicability.
Recommended Practices for Safe Oxyfuel Gas Cutting Torch Operation
American Welding Society / 04-May-2009 / 50 pages
Safe Practices for the Preparation of Containers and Piping for Welding and Cutting
American Welding Society / 24-Aug-2007 / 20 pages
Fire Safety in Welding and Cutting - Set of 25 Pamphlets
American Welding Society, 01-Jan-1992
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Plasma Arc Cutting
The successful development of Plasma Arc Cutting (PAC) passed through different stages where modified torches based on GMAW or GTAW were used. These stages may have historic interest but no practical applications.
The development of a Cutting-torch using the principles applied for Plasma Arc welding, includes a constricting nozzle to force a localized plasma stream to melt locally the base metal, while a gas jet removes the molten metal.
Plasma Arc Cutting-torch employs a very high-temperature, 10,000 °C to 14,000 °C, (18,000 °F to 25,000 °F), (compared to 3000 °C, or 5500 °F, for oxyfuel cutting), high-speed, constricted arc between an electrode placed within a torch and the material to cut. The nozzle concentrates the plasma on a small spot and provides a high speed gas jet that blows away the molten metal.
The electrode, usually of tungsten, is connected to the negative polarity of a dc constant current, drooping volt ampere curve, and is generally water cooled, for operation with current of more than 100 A.
Transferred arc cutting mode, where the arc is struck between electrode and work, can be used on almost any material that conducts electricity, including those that that do not burn with oxyfuel gas cutting.
With non transferred arc method, where the arc occurs within the torch, and the non conducting work is excluded from the electric circuit, nonmetallic material such as rubber, plastic, wood etc. can be cut with good surface quality.
Originally inert gas was employed to protect from contamination the newly formed surfaces. Currently mostly compressed air is used as an economic solution, except that a dual stream of gas is sometimes used, when it is important to prevent contamination.
Tip!: This process permits cutting of materials that cannot be readily cut by other processes and, by virtue of its high speed attainable, has the added gain of a thin heat affected zone (very shallow influence on the properties).
|A selection of these outfits can be found in the page on Plasma Cutters by clicking on the following link for WA Store.|
Recommended Practices for Plasma Arc Cutting and Gouging
American Welding Society / 04-May-2001 / 54 pages
Recommended Practices for Shielding Gases for Welding and Plasma Arc Cutting
American Welding Society, 01-Jan-2003
Specification for Tungsten and Oxide Dispersed Tungsten Electrodes for Arc Welding and Cutting (ISO 6848:2004 MOD)
American Welding Society / 17-Apr-2009 / 38 pages
Air Carbon Arc Gouging and Cutting
American Welding Society, 01-Jan-2000
A detailed Article on How to select your Plasma Cutter was published in the November 2003 issue #03 of Practical Welding Letter. To reach it, just click on PWL#003.
An Article on Thermal Cutting Processes with secondary Water was published in the Issue 25 of September 2005 of Practical Welding Letter. To read the article Click on PWL#025.
An Article on Air-Carbon Arc Cutting was published in the November 2005 Issue No. 27 of Practical Welding Letter. Click on PWL#027.
An Article on Automated Plasma Cutting Retrofit was published (11) in Issue 103 of Practical Welding Letter for March 2012.
Click on PWL#103 to read it.
An Article on Traceability in Automated Cutting was published (2) in Issue 146 of Practical Welding Letter for October 2015.
Click on PWL#146.
An Article on Smart Plasma Arc Cutter was published (3) in Issue 165 of Practical Welding Letter for May 2017.
Click on PWL#165.
A new page was recently added to our website.
Click on Plasma-arc-cutting to see it.
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Read the detailed page on Laser Beam Cutting.
Here we are going to introduce a few details on applications developed to cut through diverse materials, even nonmetals, using the powerful light beam called laser although not a Cutting-torch.
This technique has known a large spread in various industries because of its natural adaptation to computer controlled automated systems.
The extremely high power intensity at the focal spot can produce localized melting, vaporization and material removal in a very short time.
By providing relative movement between the laser beam focus and the workpiece, the method permits rapid cutting of sheet metal in various shapes and dimensions and also drilling of precise holes.
The method may make use of accessory gas (either inert or oxidizing) blowing activity to further speed up the removal of molten material.
Most of the common metals, usually in sheet form, can be processed by laser cutting, except that the most reactive ones should be protected with a shielding atmosphere while locally at elevated temperature.
Tip!: The heat affected zone near the laser cut is minimal in depth and usually of no consequence, except that for highly sophisticated applications even a very thin "recast layer" might be considered unacceptable.
An Article on Underwater Laser Cutting for Nuclear Decommissioning was published (7) in Issue 138 of Practical Welding Letter for February 2015.
Click on PWL#138 to see it.
For a description of laser beam equipment and procedures, process basics, parameters and applications, see:
Recommended Practices for Laser Beam Welding, Cutting, and Allied Processes
American Welding Society / 24-Jun-2010 / 144 pages
See also our page on Laser Drilling.
We should add for completeness that holes drilling can be performed also using the electron beam, usually on dedicated machines: obviously these operate in vacuum, without assistance from gas streams.
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A relatively new process, called Plasmatron, whose practical applications should be closely watched, results from an invention by Russian scientists. A special Cutting-torch, containing water, generates plasma through water steaming. An electric arc struck inside the torch, vaporizes the water and then ionizes the steam under self produced pressure to provide a plasma of 8000 °C that is used for cutting and related processes.
The relatively simple equipment includes a special power supply of 2.5 kW and a water filled torch. There is no need for auxiliary gases or compressed air. It is claimed to cut steel up to about 10 mm and also any non combustible materials including stainless steel, high alloy steel, aluminum, titanium, brick, concrete and ceramic. Quality of cuts is said to be comparable to that of laser devices. No noxious gases are generated so that the process is ecologically safe even for operation in an enclosed space.
|A Special Issue, No. 73, of the Mid Month Bulletin for May 2012 was published on Metal Cutting. Click on PWL#105B to see it.|
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Cutting-torch performance for weld preparation or for demolition. Gasoline-, arc-, plasma- and laser-cutting. And more...