§ 1. The essence and conditions of oxygen cutting. Basic cutting parameters for separation oxygen cutting. The influence of impurities in steel on the oxygen cutting process. .

The essence of the oxygen cutting process is based on the combustion of metal in a stream of oxygen and the removal of the resulting liquid oxides by this stream. Cutting is preceded by heating the starting point of the cut to a temperature of approximately 1300-1350 ° C (for steel). After this, the supply of cutting oxygen is turned on, which oxidizes the metal and cuts it. The burnt (oxidized) metal is blown out with a stream of oxygen. During the combustion (oxidation) of a metal, a large amount of heat is released, which heats the nearby (lying in front) layers of the metal to the ignition temperature. Thus, the process of metal combustion in oxygen spreads throughout the entire thickness of the metal being cut.

Only those metals that satisfy the following conditions can be used for conventional oxyfuel cutting:

1. The ignition temperature of a metal in oxygen must be lower than its melting point. Low-carbon steel melts at a temperature of approximately 1500 ° C, and ignites in oxygen at a temperature of 1300-1350 ° C; it lends itself well to oxygen cutting.

2. The melting point of the resulting oxides must be lower than the melting point of the base metal, since otherwise the refractory oxides will not be blown away by a stream of oxygen and the cutting process may stop.

Chromium steels form refractory chromium oxides with a melting point of about 2000° C. They can only be cut by oxygen-flux cutting.

3. The amount of heat released during the combustion of metal in a stream of oxygen must be sufficient to heat the subsequent underlying (lying in front) layers, i.e., to maintain a continuous cutting process.

5. The slags formed during the combustion of metal must be fluid, since refractory and viscous slags will not be blown out of the cut cavity by an oxygen stream. Therefore, cast irons that form refractory silicon oxides cannot be cut: only oxygen-flux cutting is possible for them.

With an increase in the carbon content in steel, as well as various impurities, the cutting process becomes more complicated. Their influence on the cutting process is given in table. 52.

52. The influence of impurities in steel on the cutting process

Approximately the ability of various steels to be cut is indicated by the equivalent carbon content in it, which is determined by the formula

C eq =C+0.155 (Cr+Mo)+0.14 (Mn+V)+0.11Si+0.045 (Ni+Cu)

In this formula, the numbers next to the symbols of the elements indicate their content in steel in weight percent.

When cutting steels with a high equivalent carbon content, preheating is required (Table 53).

53. Steel preheating temperature during cutting

Based on the direction of the cutting oxygen jet and the nature of the cuts formed, two main types of oxygen cutting are distinguished: 1. Separation, forming through cuts (cutting parts from a sheet, cutting metal into pieces, bevelling edges for welding, etc.).

2. Superficial, which produces depressions (grooves) of oval shape on the surface of the metal (removing defective seams, gouging the surface, melting grooves, etc.).

The main cutting parameters for separation oxygen cutting are shown in Fig. 127.

Rice. 127. Basic cutting parameters:

B B - width of the cut at the top, B H - width of the cut at the bottom, f - non-perpendicularity of the cut, l - depth of the grooves (roughness), Δ - lag, r - melting radius of the upper edge

The cutting speed is greatly influenced by the purity of oxygen (Table 54). As oxygen purity decreases, cutting speed decreases significantly. It is most advisable to use oxygen with a purity of 99.5% or more. It is not advisable to use oxygen with a purity of 95%, since in addition to the low cutting speed, the cut surface is unclean, with deep marks and difficult to separate burrs.

54.Dependence of cutting speed on oxygen purity

* The cutting speed for oxygen with a purity of 99.5% is taken as 100%.

TO category:

Metal cutting

The essence of the metal cutting process

Cutting, or cutting, is the separation of parts (blanks) from long or sheet metal. Cutting is performed both with and without removing chips. Methods of cutting with removal of chips: with a hand hacksaw, on hacksaws, circular saws, lathe-cutting machines, as well as gas, arc cutting and other methods.

Without removing chips, materials are cut with hand lever and mechanical scissors, wire cutters, pipe cutters, press shears, and stamps. Cutting also includes cutting metal.

Rice. 1. Flaring diagram: 1 - pipe end, 2 - flange, 3,4 - rollers, 5 - grooves, 6 - pipe before rolling, 7 - pipe after rolling

Rice. 2. Hand scissors for cutting metal: a - with straight blades, b - straight right, c - with curved blades

The essence of the scissor cutting process is to separate pieces of metal under the pressure of a pair of cutting knives. The sheet to be cut is placed between the upper and lower knives.

The upper knife, lowering, presses on the metal and cuts it.

The high pressure experienced by the blades when cutting requires a large sharpening angle p. The harder the metal being cut, the greater the sharpening angle of the blade; for soft metals (copper, etc.) it is 65°, for metals of medium hardness - 70 - 75° and for hard metals - 80 - 85°. In order to reduce the friction of the knife blades on the metal being cut, the blades are given a small back angle a (1.5-3°).

1. ESSENCE OF THE OXYGEN CUTTING PROCESS

The basis of the process of oxygen cutting of steel is the property of iron to burn intensively in a stream of technically pure oxygen, being heated to a temperature of the order of 1300-1400 ° C, close to the melting point of steel.

When cutting, the metal is heated with an oxygen gas flame. Acetylene, propane-butane, pyrolysis, natural, coke and city gases, and kerosene vapor are used as fuels.

The metal is heated in a narrow area at the beginning of the cutting line, and then a stream of cutting oxygen is directed to the heated area and the cutter begins to move along the intended cutting line. The metal burns across the entire thickness of the sheet, forming a narrow gap (cut) in it. Intense oxidation (combustion) of iron occurs only in the layers bordering the surface of the cutting stream of oxygen, which penetrates (diffuses) into the metal to a very small depth.

For the combustion of 1 kg of iron, theoretically, from 0.29 to 0.38 m 3 of oxygen is required, depending on what oxide is produced during combustion - FeO or Fe 3 0 4. The practical consumption of oxygen can differ greatly from the theoretical one, since both oxides are present in the slag in different proportions, part of the metal is removed from the cut in a molten state, part of the oxygen is spent on blowing out liquid metal and slag, and is also lost into the environment. For cutting, technical oxygen with a purity of 98.8–99.7% is used. With a decrease in oxygen purity by 1%, its consumption per 1 m of cutting length increases by 25-35%, and cutting time - by 10-15%. This is especially noticeable when cutting thick steel. It is not advisable to use oxygen with a purity lower than 98% for cutting, since the cut surface is not clean enough, with deep grooves and difficult-to-remove slag.

There is also a method called pulsed oxygen cutting. This method developed by VNIIAvtogenmash and consists in the fact that after initial heating, cutting oxygen is released along the entire length of the cutting line. The cutting process takes only a few tens of seconds. For example, a pipe with a diameter of 219 mm and a wall thickness of 15 mm is cut in 77 seconds. For cutting, sectioned cutters with intra-nozzle gas mixing are used (see Fig. 90, i).

2. BASIC CUTTING CONDITIONS. INFLUENCE OF STEEL COMPOSITION ON CUTTING

Basic cutting conditions. To cut metal with oxygen, the following conditions are necessary:

a) the combustion temperature of the metal in oxygen must be below the melting point, otherwise the metal will melt and turn into a liquid state before its combustion in oxygen begins;

b) the resulting metal oxides must melt at a temperature lower than the combustion temperature of the metal and not be too viscous; if the metal does not satisfy this requirement, then oxygen cutting it without the use of special fluxes is impossible, since the resulting oxides will not be able to be blown out of the cut site;

c) the amount of heat released during the combustion of metal in oxygen must be large enough to maintain the cutting process. When cutting steel, about 70% of the heat used for heating is released when the metal burns in oxygen and only 30% is supplied from the preheating flame;

d) the thermal conductivity of the metal should not be too high, otherwise, due to intense heat removal, the cutting process may be interrupted.

The influence of steel composition on cutting. The above conditions are most fully met by pure iron and steels with low carbon content. Pure iron has a ignition temperature in oxygen of 1050 ° C, and a melting point of 1528 ° C. When the steel contains 0.7% carbon, its ignition temperature in oxygen rises to 1300° C, which is equal to the melting temperature of steel of this composition. According to A.N. Shashkov, selective oxidation of iron in oxygen when cutting steel begins at a temperature of about 1130°C, and at 1300°C and above, intense carbon burnout begins.

In addition to the composition of the metal, the ignition temperature is also influenced by the state of the metal surface, the size of its pieces, pressure and oxygen flow rate. The rough surface makes it easier for the metal to ignite in oxygen. Iron powder can ignite in pure oxygen at a temperature of 315 ° C, i.e., significantly lower than rolled metal. Metal on the surface of a large piece of steel ignites at a temperature of 1200-1300°C. At a pressure of 25 kgf/cm2 and an oxygen flow rate of 180 m/sec, the ignition temperature of carbon steel in oxygen is reduced to 700-750° C.

The essence of oxygen cutting. Oxygen cutting is a method of separating metal, based on the use of heating it to the ignition temperature - the heat of a gas flame and an exothermic (with the release of heat) oxidation reaction of the metal, and to remove oxides - the kinetic energy of cutting oxygen.

Based on the nature and direction of the oxygen jet, three main types of cutting are distinguished: separation, in which through cuts are formed; surface, in which the surface layer of metal is removed; oxygen lance, which involves burning deep holes in metal.

In Fig. Figure 6 shows a diagram of separation cutting. Metal 3 is heated at the initial point of the cut to the ignition temperature (in oxygen for steel up to 1000-1200°C) by a heating acetylene-oxygen flame 2, then a stream of cutting oxygen 1 is directed, and the heated metal begins to burn, releasing a significant amount of heat according to the reaction 2Fe + 2O 2 =Fe 3 O 4 +Q.

The heat from the combustion of iron Q, together with the heating flame, heats the underlying layers and spreads over the entire thickness of the metal. The smaller the thickness of the metal being cut, the greater the role of the preheating flame (with a thickness of 5 mm - up to 80% of the total amount of heat released during cutting, with a thickness of more than 50 mm - only 10%). The resulting oxides 5, as well as partially molten metal, are removed from the cutting zone 4 under the influence of the kinetic energy of the oxygen stream. Continuous supply of heat and cutting oxygen ensures process continuity.

Cutting conditions and cutability. To ensure a normal cutting process, the following conditions must be met:

1. The heat source must have the necessary power to ensure heating of the metal to the required metal combustion temperature, and the amount of heat released during the combustion of the metal in an oxygen stream must be sufficient to maintain a continuous cutting process,

2. The melting point of the metal must be higher than the temperature of its oxidation (combustion) in oxygen, otherwise the metal will melt when heated and be forcibly removed from the cut without the oxidation characteristic of the cutting process, which is the main source of heat.

3. The melting point of the metal must be higher than the melting point of the oxides formed during the cutting process, otherwise refractory oxides will isolate the metal from contact with oxygen and complicate the cutting process.

4. The resulting oxides and slag must be fluid and easily blown out by a jet of cutting oxygen, otherwise the contact of oxygen with the liquid metal will be slowed down or completely impossible.

Carbon steel satisfies all of the above conditions, so it can be cut with oxygen.

Copper does not satisfy the first condition during gas cutting due to its high thermal conductivity, which greatly complicates the start of the cutting process, and low heat release during oxidation. Therefore, the power of gas cutters is not enough to cut copper, and copper can be cut using a more powerful heat source - an electric arc.

The second and fourth conditions are not satisfied by cast iron. As the carbon content of iron increases, the cutting process deteriorates significantly due to a decrease in the melting point and an increase in the ignition temperature. Cast iron containing more than 1.7% carbon is not processed by oxygen cutting. In addition, the viscosity of the slag increases significantly with increasing silicon content, which is necessarily contained in cast iron, which is also one of the reasons for the impossibility of oxygen cutting of cast iron. The third condition is not satisfied when cutting aluminum, magnesium and their alloys, as well as steels with a high content of chromium and nickel. When these alloys are heated during the cutting process, a film of refractory oxide forms on their surface, preventing the flow of oxygen to the unoxidized metal.

Basic parameters of oxygen separation cutting:

characteristics of the preheating flame - power, combustible gas, mixture ratio of combustible gas and oxygen;

characteristics of the cutting oxygen jet - pressure, flow rate, shape, purity, cutting speed.

The heating flame has a neutral character during cutting (β=1.1 for acetylene, β=3.5 for propane-butane mixture). The power of the heating flame increases with increasing thickness of the metal being cut.

Oxygen cutting quality. Cutting quality is characterized by the accuracy of the trajectory and the quality of the cut surface. The smallest deviations of the cutting trajectory (line) from the specified one are obtained when cutting on machines with software, photoelectronic and electromagnetic control, the largest - when manual cutting without guides. The magnitude of the deviations depends on the length, thickness, condition of the sheet surface, the shape of the workpiece being cut, and the qualifications of the cutter.

The quality of the cut is characterized by the non-perpendicularity and roughness of its surface, the uniformity of the width of the cut, the presence of melting of the upper edge and burr on the lower edge (Fig. 7, a).

Non-perpendicularity of the cut surface is formed when the angle of inclination of the cutter to the sheet surface changes, as well as from the expansion of the cutting oxygen jet as it exits the cut. The roughness of the cut surface is determined by the number and depth of grooves left by the cutting oxygen stream (Fig. 7, c). The grooves usually have a curved outline due to the lag Δ of the cutting oxygen jet from the axis of the mouthpiece (Fig. 7, b). The greater the thickness of the metal, the less pure the oxygen, the greater the lag. Typically, the gap is from 1 to 15 mm when cutting sheets with a thickness of 5 to 200 mm in a straight line. The depth of the grooves depends on the oxygen pressure, cutting speed, uniform movement of the cutter and fuel composition. The amount of edge melting is directly dependent on the power of the heating flame and, inversely, on the cutting speed. GOST 14792 establishes three quality classes for machine cutting: 1st class - highest, 2nd class - increased, 3rd class - ordinary. For each class, maximum tolerances are established for surface non-perpendicularity, roughness and deviations from the cut line.

To increase productivity and cut quality, a number of types of oxygen separation cutting are used.

High-speed oxygen cutting is achieved by tilting the torch 45° in the direction opposite to the direction of movement. The cutting speed of sheet steel with a thickness of 3-20 mm increases 2-3 times, but the quality of the cut deteriorates.

High-quality high-speed oxygen cutting (flush-process) allows you to increase both the speed (1.5-2.5 times) and the quality of cutting. The first is achieved due to the sharp angle of inclination of the cutter - 25°, the second - by using special mouthpieces with three holes for cutting oxygen, located at the corners of an isosceles triangle. The main cutting jet moves ahead, cutting the metal to its full thickness. Two other jets, located on the sides and behind the main one, “protect” the hot edges formed by the main jet. The disadvantage of the sharp-angle method is the impossibility of shaped cuts and the large width of the cut.

Cutting with high pressure oxygen up to 5 MPa ensures an increase in cutting speed of metal up to 60 mm thick by 30-50%.

Steels up to 300 mm thick can be cut with conventional universal cutters. Welding thick steels is associated with additional difficulties: the need to use high oxygen pressures, the difficulty of heating the lower layers of metal and removing slag at a great distance from the cutter. Therefore, steel of large thickness (over 300 mm) is cut with special cutters, the nozzles of which have larger flow sections for cutting oxygen compared to universal cutters. A carburizing heated flame is used, since in this case it will be longer.

Surface oxygen cutting of metal. Surface oxygen cutting is the process of removing a layer of metal with an oxygen jet. In this case, the oxygen jet is directed to the processing surface at an acute angle of 15-40°, but unlike separation cutting, the direction of the jet coincides with the direction of cutting, and the metal located in front of the cutter is heated by the moving heated slag (Fig. 8).

Rice. 8. Surface cutting scheme:

1 - mouthpiece, 2 - slag, 3 - groove

Rice. 9. Scheme of burning a hole in concrete with an oxygen lance: 1 - lance holder, 2 - lance, 3 - protective screen, 4 - concrete product

Oxygen lance cutting(Fig. 9) is performed with a thin-walled steel tube (spear) with an outer diameter of 20-35 mm. The tube is connected to a handle with an oxygen valve and oxygen is supplied through it to the cutting site. Before cutting begins, the end of the tube is heated with a gas torch or electric arc to the ignition temperature. The burning end of the oxygen lance is pressed with sufficient force onto the product (metal, concrete, reinforced concrete) and thus a hole is burned. The slag formed during the burning of the hole is carried out by the pressure of oxygen and gases into the gap between the spear and the wall of the hole being burned. This process is facilitated by the reciprocating and rotational movements of the spear.

Oxygen flux cutting. For cutting chromium, chromium-nickel stainless steels, cast iron and non-ferrous metals that do not meet the conditions of oxygen cutting, the oxygen-flux cutting method is used. The essence is that a special powdered flux is introduced into the cutting zone along with cutting oxygen, the combustion of which releases additional heat and increases the temperature in the cutting zone. In addition, flux combustion products, interacting with refractory oxides, form fluid slags, which are easily removed from the cut zone without interfering with the normal course of the process.

The main component of powdered fluxes used in cutting metals is iron powder, which, when burned, releases a large amount of heat (about 1800 kcal/kg). The best results when welding stainless steels are achieved by adding 10-15% aluminum powder to the iron powder. For surface and separation cutting of stainless steels, a mixture of aluminum-magnesium powder with ferrosilicon or silicocalcium is used as a flux. The aluminum-magnesium powder included in the flux mixture, burning in a stream of oxygen, increases the flame temperature, and ferrosilicon or silicocalcium acts on chromium oxides as a fluxing additive.

The main task of flux when cutting cast iron is to dilute the flux with iron in the cut area, reduce the carbon content in the alloy, and also liquefy the slag, which contains a lot of oxygen. Fluxes for cutting cast iron include iron and aluminum powders, quartz sand and ferrophosphorus.

Non-ferrous metals and alloys are subjected to oxygen-flux cutting only with the use of fluxes. Installations for oxygen-flux cutting consist of two main parts: a cutter (manual or machine) and a flux feeder, which provides supply and regulation of flux flow.

End of work -

This topic belongs to the section:

Technological fundamentals of fusion and pressure welding

Lecture .. introduction .. Mr. Petrov discovery of the electric arc Mr. Benardos NN patented the welding of metals with an electric arc between carbon electrodes ..

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All topics in this section:

Hanging type devices
In devices of this type, as a rule, there is no mechanism for the welding movement, which makes them quite simple and portable. Typically, such devices include an electrode supply mechanism.

The essence of oxygen gas welding
Combustible gases. During gas-flame processing (welding, cutting, surface treatment, soldering), a gas flame is used as a heat source - a flame of combustible gas burned

Arc and beam types of metal cutting
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Equipment and equipment for gas welding and cutting
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Occupational safety requirements for gas welding and cutting
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oxygen explosions
Beam welding methods

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Limits of applicability

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Groups of materials: carbon, low and high alloy steels;

The essence and fundamentals of electric contact welding methods
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