# Oxy-fuel cutting

Process principle of flame cutting
Flame cutting of rails
Flame cutting machine 1959

The autogenous flame cutting or gas cutting is a separation method with which among other things, metal sheets (in particular mild steel are separated) by a flame on the surface of the material to ignition temperature and heated by the supply of oxygen burns. The heat of combustion released heats the material layers underneath to the ignition temperature, so that the process continues automatically ( autogenously ) downwards . The resulting liquid slag is blown out of the joint by the cutting oxygen. The tool is a flame cutter , the machine tool is a flame cutting machine ; The procedure is standardized in DIN 2310-6.

Oxy-fuel cutting, together with plasma fusion cutting and laser cutting , counts to thermal cutting , which in turn counts to ablation . The additional designation autogenous serves to distinguish it from laser flame cutting . Among flame cutting or flame cutting means all cutting processes in general, in which material is burned; however, it usually refers to oxy-fuel cutting.

In terms of tools and process principles, it is related to gas fusion welding , which is rarely used . There is an important difference between the tools: The torch cutter consists of a central nozzle through which the cutting oxygen is passed and another nozzle that is in a ring around the oxygen nozzle and through which an oxygen- fuel gas mixture flows, which is ignited at the nozzle outlet . Acetylene is usually used as the fuel gas . The oxygen nozzle is missing in gas welding; In addition, the kinetic energy of the gases is lower there, so as not to drive the melt out of the joint to be welded, while this is absolutely necessary for flame cutting. Analogously, plasma fusion cutting is related to plasma welding , and laser cutting is related to laser welding .

Oxy-fuel cutting is used for unalloyed and low-alloy steels for medium to large sheet metal thicknesses, but also, for example, railroad tracks . Larger amounts of alloying elements prevent flame cutting. It is relatively economical compared to other processes, but the cutting edges are of poor quality. In addition to plasma and laser cutting, important alternatives are water jet cutting , electrical discharge machining and electron beam machining .

## History of flame cutting

Oxy-fuel cutting worker (1948)

As early as 1887, a coal gas- oxygen fan burner for melting sheet metal was developed in England , which was used to break into a bank in Hanover in 1890. In 1902 Ernst Menne developed a hydrogen burner with a large excess of oxygen to melt blast furnace tap openings . In 1903 the first acetylene- based welding torch was patented in France , and in 1908 flame cuts were made under water for the first time. Also before the First World War , longitudinal and circular cutting machines with template control and electric drive were developed and investigations into cutting speed and gas consumption were made. This first development cycle ended in the 1920s, when the first tests with liquid fuels also took place.

Since the beginning of the 20th century it has been possible to produce pure oxygen by liquefying the air ( Linde process ) and rectification . Since 1930, liquid oxygen has been produced on a large scale in Germany using heat exchangers at low cost. This accelerated the spread of welding and flame cutting processes and stimulated research. At about the same time, the technology was used for the dismantling of ships under water. In the 1930s and 1940s, the first exact quantification attempts of the chemically effective amount of oxygen and the heat of combustion as well as experiments to optimize the nozzle shape were undertaken in Germany. The mechanisms of cutting grooves were also clarified with the help of streak optical slow-motion images. In the 1950s, the maximum cutting speed was determined based on the kinetic gas theory .

The first CNC-controlled flame cutting machine was built in 1964. Differentiated models of the chemical and physical processes involved in flame cutting have existed since the 1970s. The use of flame cutting in offshore technology has been optimized by Japanese scientists and technicians in particular since the 1970s, with liquid fuels often being used to prevent the formation of explosive mixtures of fuel gas and air or oxygen. CNC-controlled multi-head flame cutting machines were developed for mass production. Development since 1990 has concentrated on process automation, the development of flame cutting robots and the dismantling of sheet metal of great thickness at high cutting speed, for example when dismantling nuclear facilities .

The development of the competing methods of plasma cutting and laser cutting in the 1980s led to increased economic studies which, despite the advance of plasma and laser technology, generally confirmed the advantages of flame cutting in unalloyed steels with sheet thicknesses of over 50 to 60 millimeters. In any case, laser cutting comes up against technical limits in this area. However, the profitability of flame cutting technology can be further increased by using combined flame cutting / laser cutting machines.

## Significance, areas of application and range of materials

Comparison of the cutting speeds that can be achieved with oxy-fuel cutting, laser cutting and plasma fusion cutting

Flame cutting is used in particular with unalloyed and low-alloy steels . In addition, titanium can be cut with the standard variant. This is not possible for almost all other materials. For high-alloy Cr-Ni steels or aluminum , however, there are special processes that are less suitable than plasma cutting, but involve lower investments . With flame cutting, the investments in equipment and tools are relatively low. The costs for the operating materials (oxygen and fuel gas) are also low, but the labor costs are high in relation to them.

In addition to other profiles , the sheet metal thicknesses that can be cut are between 2 mm and up to 3 meters; However, thicknesses between 10 mm and 300 mm are common. In the area of ​​up to 5 mm, the temperature influence is particularly great and causes heat distortion in the sheets. Here other processes are more economical, faster and achieve better surface qualities. Thicknesses over 300 mm, on the other hand, can only be cut by flame cutting.

Vertical cuts and miter cuts are possible. A variety of joint shapes are possible thanks to special torch arrangements , which is why flame cutting is used to prepare the sheets for subsequent welding. It is estimated that 75% of all weld seams are created by flame cutting. In Germany, the cutting length of all cuts produced by flame cutting is around 750,000 km annually.

The achievable accuracies are always worse than with competing processes, regardless of the sheet thickness, the cutting speeds are just under 1 m / min for sheet thicknesses of around 10 mm and drop quickly above 100 mm. However, above a thickness of around 13 mm it is the fastest process, below it plasma cutting is faster.

## Procedural principle

Result of a cutting disc (center) and a flame cutter (front right) on railroad tracks

The heating flame, also known as the preheating flame, heats the surface of the workpiece to be cut to a locally limited ignition temperature, which for structural steel is between 1150 and 1250 ° C. Then the material burns with the blown oxygen. In the case of iron, around 54 kJ / cm³ of heat of combustion are released, which is sufficient to heat adjacent material layers to the ignition temperature. In steel, the iron reacts with the oxygen to form thin-bodied iron oxide, which is known as slag . The slag and about 20% of the molten iron are blown out by the kinetic energy of the cutting oxygen. It is therefore blown in at pressures of 7 to 9 bar; with high-performance nozzles, up to 20 bar are also possible. The task of the cutting oxygen jet is on the one hand to provide the oxygen required for the combustion and on the other hand to blow out the slag that is produced. The heat released during combustion also heats the material layers underneath to the ignition temperature and burns it with the oxygen jet. The process continues in depth without the aid of the heating flame, which is only necessary for the movement in the cutting direction in order to heat the top of the workpiece to the ignition temperature.

## requirements

In order to be able to cut a material by flame cutting, several requirements must be met.

1. The ignition temperature must be below the melting temperature . In the case of structural steel, the former is 1150 ° C and above. With 0.25% carbon it is 1250 ° C, while the melting temperature is around 1500 ° C. The first condition is thus met for structural steel. However, as the carbon content rises, the ignition temperature rises, while the melting temperature falls at the same time. With a carbon content of 0.85%, the ignition temperature is the solidus temperature . Pure flame cutting is possible up to this value. With a higher carbon content, there is partial fusion cutting. Steels with a carbon content of up to 1.6% can basically be flame-cut, but only with poor cutting quality, while tool steel and cast iron, both of which have a high carbon content, cannot be flame-cut. Most of the alloying elements in steel increase the ignition temperature.
2. The oxides that are formed during combustion must have a lower melting point than the material. An exception is titanium, which can be flame-cut, although the melting point of titanium oxide is around 1970 ° C, higher than that of titanium (1670 ° C). At 660 ° C, the melting point of aluminum is well below that of aluminum oxide (2050 ° C). Chromium oxides and nickel oxide also have melting points above that of steel, which is why neither aluminum nor Cr-Ni steels can be flame-cut.
3. The slag must be as thin as possible in order to be driven out by the cutting oxygen. In the case of aluminum, chrome and silicon, they are relatively tight and solid and cannot be blown out.
4. The material should have the lowest possible thermal conductivity and high heat of combustion. With materials like copper with high thermal conductivity, a lot of heat is conducted away from the kerf, so that the ignition temperature is not reached at depths that can no longer be reached by the heating flame.

## Operating gases

A purity of 99.995% is recommended for the required oxygen. With lower purities, only significantly reduced cut surface qualities are possible. A purity of 99.5% is considered economically necessary: ​​Even with a purity of 98.5%, the achievable cutting speed decreases by 15% and the amount of oxygen required increases by 25%.

As combustion gases are acetylene (ethyne chemically named), propane and natural gas used, and acetylene for the largest share. Important requirements for the combustion gases are the flame temperature , the ignition speed and the primary flame output , all of which should be as high as possible. All three properties also depend on the mixing ratio with oxygen.

The maximum of all three values ​​for acetylene is on the one hand above that of other fuel gases such as methane or ethene and on the other hand in the range of low mixing ratios of 1: 1 to 1: 2 of acetylene to oxygen. The disadvantage is that it tends to explode at pressures above 2 bar and temperatures above 300 ° C. The working pressure must therefore remain limited to around 1.5 bar. In addition, precautions must be taken to ensure that the temperature of the bottle remains below 300 ° C.

Propane, on the other hand, is significantly less sensitive to pressure and temperature and is stored in bottles in its liquid state so that larger quantities can be stored. For this, the characteristic values ​​are lower than for acetylene and about four times the amount of oxygen is required.

## Influence of the alloying elements in steel

### Flame cuttability

The most important criterion for assessing the flame-cuttability is the carbon equivalent , the so-called CEV value. It is also used to determine weldability .

${\ displaystyle CEV = C + {\ frac {Mn} {6}} + {\ frac {Cr + Mo + V} {5}} + {\ frac {Ni + Cu} {15}}}$

Values ​​less than 0.5 are considered to be easily flame-cut or weldable. Steels up to about 0.45% C can be flame-cut without preheating. Up to 1.6% C they can only be cut with preheating, as the heat requirement increases. The elements silicon, manganese, tungsten, molybdenum and copper increase the ignition temperature of steel. In addition, they usually form oxides with high melting temperatures and are therefore difficult to blow out. The effects of individual elements can be weakened or intensified when they occur together.

material Ignition temperature Melting temperature of the material Flame cuttability
Pure iron 1050 ° C 1536 ° C very good
Steel with less than 0.1% C 1050 ° C 1520 ° C very good
Steel (0.1-0.3% C) 1000-1200 ° C 1450-1500 ° C Well
Steel (0.3–2.0% C) approx. 1250 ° C approx. 1400 ° C satisfying
Cast iron (2.5-3.5% C) 1350-1450 ° C 1150-1200 ° C only with powder flame cutting
(add Fe powder in the cutting jet)
Alloy element upper limit content
without preheating
upper limit content
with preheating
carbon 0.45% 1.6%
Silicon 2.9% 4.0% at max. 0.2% C
manganese 13.0% at max. 1.3% C
chrome 1.5% 10.0% at max. 0.2% C
tungsten 10.0% at max. 5% Cr; 0.2% Ni; 0.8% C 17.0%
nickel 7.0% 34% at min. 0.3% C and max. 0.5% C
molybdenum 0.8%
copper 0.7%

### Change of composition in the peripheral zones

There are some changes in the composition of the materials in the areas at the cut surface. They result not only from the combustion itself, but also from the influence of heat.

Carbon accumulates at the cut edge, which comes from the material and is held back by an oxide layer. At high cooling speeds, the hardness of the material can therefore increase more than the originally present content would allow. However, the temperatures are not sufficient to allow carbon to diffuse from the interior of the material into the edge zones. On the upper edge of the workpiece, the carbon can react with the oxygen in the cutting oxygen jet, which is why its content decreases there. The increase in hardness and carburization at the cut surface varies depending on the alloy content and can lead to hardening cracks at higher contents of 0.45% C or more. This also includes materials such as S355Jo (St 52-3), 13CrMo4-5 or C 60, where the hardness can be between 600 and 700  HV  0.5.

The behavior of other alloying elements depends on their affinity for oxygen and iron. Elements such as copper, nickel or molybdenum have a lower affinity for oxygen than iron and therefore accumulate in the surface layers, while chromium, manganese or silicon oxidize.

## Edge properties

Because of the high heating and cooling speeds, the material properties change in the so-called heat - affected zone on both sides of the kerf. With steels that can form martensite , hardening occurs, with all others also internal stresses due to distortion and possibly cracks as a result of the hardening and distortion.

Since carbon accumulates in the edge zones and there are high cooling speeds here after the cut, martensite formation occurs. The associated increase in volume leads to internal pressure stresses . After cooling, residual tensile stresses remain at the edge of the heat-affected zone, which result from the plastic compression. These internal stresses fundamentally improve the ( long-term ) strength of the workpiece. The hardness is around 700 HV 1 and drops quickly after around 0.5 to 1 mm inside the material. By preheating the material, the hardness of the edge zone can be reduced to around 400 HV 1. The hardened zone then extends deeper into the material.

However, grooves form on the cut surface, which develop a weak notch effect, which reduces the fatigue strength. However, their effect is reduced by the internal stresses, so that relatively high fatigue strengths are available. If these grooves are removed, this reduces the notch effect, but also relieves the internal stresses, so that only minor improvements in fatigue strength are possible with a comparatively large amount of rework. A heat treatment does not affect the notch effect, but worsens the effect of residual stresses, so that overall device characteristics are deteriorated.

In the case of thin sheet metal, a relatively large proportion of the heat migrates into the material and leads to large warpage due to thermal expansion. Sheet thicknesses of at least 5 mm are therefore required for the standard variant of oxy-fuel cutting. With laser flame cutting, the laser provides concentrated heating of a localized, narrowly delimited point, which leads to a significantly lower heat-affected zone, so that significantly smaller sheet metal thicknesses can also be cut with lasers. There are special processes with water cooling for oxy-fuel cutting. This on the one hand cools the workpiece and on the other hand binds the combustion gases.

## Process parameters

The most important influences on the cutting quality have the operating gases, the cutting nozzle, the machine used and the material.

The pressure, quantity, temperature, purity, mixing ratio and flow properties of the operating gases play a role. In addition to age and condition, the exact construction and distance from the sheet metal are important for the cutting nozzles. The machines' design, age and condition also influence the result. In addition, the feed, i.e. the cutting speed, is important. When it comes to the material, the sheet thickness, composition and temperature play a role. Together with the condition of the surface and possible defects inside, they influence the reaction to form iron oxide.

The precise influences and possible errors are listed in DVS leaflets 2102 and 2103. The most common mistake is too high or too low a cutting speed.

## Accuracies, qualities and tolerances

Accuracies that can be achieved with different sheet thicknesses with oxy-fuel cutting (yellow), plasma fusion cutting (blue), water jet cutting (gray), laser cutting (red) and spark erosion (EDM, green)
Cross-sections of the kerfs in plasma fusion cutting, laser cutting and oxy-fuel cutting. The heat affected zone (HAZ) is highlighted in red. The extended HAZ on the top is typical for flame cutting.

The quality of the cuts can be determined by numerous parameters. Three are standardized in DIN EN ISO 9013: the perpendicularity or inclination tolerance  , the mean roughness depth  and the dimensional deviations. Further influencing variables are the groove wake, the melting of  the upper edge, oxide residues and the formation of beard or melt droplets on the lower edge, as well as scouring , i.e. washouts that run in the direction of the cutting thickness. The groove lag is characteristic of flame cuts and cannot be avoided. Since it is easy to see with the naked eye, it is often used as a measure of the quality of the cut, although it is of little importance if the groove depth is small. When specifying the tolerances according to DIN, the standard is specified followed by three digits, which in turn specify the perpendicularity or inclination tolerance, the roughness depth and the tolerance class. ISO9013-342 therefore means squareness according to area 3, a surface roughness according to area 4 and tolerance class 2. They all depend on the sheet metal thickness  . ${\ displaystyle u}$${\ displaystyle R_ {z}}$${\ displaystyle n}$${\ displaystyle a}$

Area Perpendicularity or inclination tolerance in mm ${\ displaystyle u}$
1 0.05 + 0.003a
2 0.15 + 0.007a
3 0.4 + 0.01a
4th 0.8 + 0.02a
5 1.2 + 0.035a
Area Average surface roughness Rz5 in μm
1 10+ ${\ displaystyle 0.6 {\ tfrac {a} {mm}}}$
2 40 + ${\ displaystyle 0.8 {\ tfrac {a} {mm}}}$
3 70 + ${\ displaystyle 1,2 {\ tfrac {a} {mm}}}$
4th 110 + ${\ displaystyle 1,8 {\ tfrac {a} {mm}}}$

## Special procedure

With underwater flame cutting, ignition is carried out using spark plugs . The oxygen taken from the air during normal flame cutting must also be supplied. There is also a special variant for plasma fusion cutting , which takes place under water in order to avoid the creation of toxic fumes.

In powder flame cutting , low-carbon iron powder is blown into the heating flame. This releases more heat, which also enables the cutting of cast iron or high-alloy chrome-nickel steels. In fact, however, there is no flame cutting, but fusion cutting. The iron powder is only burned to reach the melting temperature. In addition, the metal powder has an abrasive effect, similar to water jet abrasive cutting .

The arc-melting-cutting, also oxy-arc process called, is a mixture of the art, when arc welding is used, with the flame cutting. Oxygen is blown through a hollow electrode. The arc that burns between the electrode and the material is used as a replacement for the heating flame, melts the material and burns the electrode. Together with the iron powder used, an alloy with a low carbon content is created that is at least partially flame-cut.

In the case of arc fusion cutting or the arc air process , a copper-plated graphite electrode is used to melt the material with an arc and blow it out with the oxygen. The process is mostly used for grooving or chamfering .

Commons : Oxy-fuel cutting  - collection of images, videos and audio files

## Individual evidence

1. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 390.
2. a b Ulrich Dilthey: Welding Manufacturing Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 231.
3. See the following: Ralf Versemann: Oxy- fuel cutting: More than 100 years of research and development. German Association for Welding and Allied Processes V., Committee for Technology, conference contribution 2006.
4. Traugott Gutermann: Autogenous flame cutting: It began 75 years ago. In: The Practitioner. H. 9, 1979, pp. 44-47.
5. Ivan Boschnakow: Flame cutting: New knowledge and technologies. Technical and scientific treatises of the Central Institute for Welding Technology of the GDR, Halle / Saale 1974.
6. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 401.
7. Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 255.
8. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 390 f.
9. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 244 f.
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11. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 245 f.
12. ^ A b Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 246.
13. ^ A b Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 247.
14. O'Brien (Ed.): Whelding Handbook. (Volume 2), 8th edition, 1991, p. 453.
15. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 247 f.
16. Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 235.
17. Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 236.
18. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 391 f.
19. Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 236 f.
20. ^ A b Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 251 f.
21. Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 250 f.
22. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer, 2015, p. 399 f.
23. ^ A b Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 250.
24. a b c Hans J. Fahrenwaldt, Volkmar Schuler, Jürgen Twrdek: Practical knowledge of welding technology - materials, processes, production. 5th edition. Springer, 2014, p. 251.
25. Ulrich Dilthey: Welding Production Process 1 - Welding and Cutting Technologies. 3. Edition. Springer, p. 237.
 This version was added to the list of articles worth reading on July 11, 2016 .