12. Thermal Cutting 2003

Transcription

1 12. Thermal Cutting 2003

2 12. Thermal Cutting 160 Thermal cutting processes are applied in different fields of mechanical engineering, br-er12-01e.cdr Figure 12.1 Classification of thermal cutting processes - physics of the cutting process - degree of mechanisation - type of energy source - arrangement of water bath Classification of Thermal Cutting Processes acc. to DIN shipbuilding and process technology for the production of components and for the preparation of welding edges. The thermal cutting processes are classified into different categories according to DIN 2310, Figure Figure 12.2 shows the classification according to the physics of the cutting process: - flame cutting the material is mainly oxidised (burnt) - fusion cutting the material is mainly fused - sublimation cutting the material is mainly evaporat The gas jet and/or evaporation expansion is in all processes responsible for the ejection of molten material or emerging reaction products such as slag. br-er12-02e.cdr Figure 12.2 Flame cutting The material is mainly oxidised;the products are blown out by an oxygen jet. Fusion cutting The material is mainly fused and blown out by a high-speed gas jet. Sublimation cutting The material is mainly evaporated. It is transported out of the cutting groove by the created expansion or by additional gas. Classification of Processes by the Physics of Cutting The different energy carriers for the thermal cutting are depicted in Figure 12.3: - gas, - electrical gas discharge and - beams. Electron beams for thermal cutting

3 12. Thermal Cutting 161 are listed in the DIN-Standard, they produce, however, only very small boreholes. Cutting is impossible. Figure 12.4 depicts the different methods of thermal cutting with gas according to DIN These are: - flame cutting - metal powder flame cutting - metal powder fusion cutting - flame planing -oxygen-lance cutting - flame gouging or scarfing -flame cleaning thermal cutting by: br-er12-03e.cdr Figure gas - electrical gas discharge - sparks - arc - plasma - beams - laser beam (light) - electron beam - ion beam Classification of Thermal Cutting Processes acc. to DIN In flame cutting (principle is depicted in Figure 12.5) the material is brought to the ignition temperature by a heating flame and is then burnt in the oxygen stream. During the process the ignition temperature is maintained on the plate top side by the heating flame and below the plate top side by thermal conduction and convection. thermal cutting processes using gas: oxygen cutting metal powder metal powder flame cutting fusion cutting However, this process is suited for automation and is, also easy to apply on site. Figure shows a commer- flame planing br-er12-04e.cdr Figure 1.4 oxygen-lance cutting flame gouging scarfing Thermal Cutting Processes Using Gas flame cleaning

4 12. Thermal Cutting 162 cial torch which combines a welding with a cutting torch. By means of different nozzle shapes the process may be adapted to varying materials and plate thicknesses. Hand-held torches or torches machine-type are equipped with different cutting nozzles: Standard or block-type nozzles (cutting-oxygen pressure 5 bar) are used for hand-held torches and for torches which are fixed to guide carriages. br-er12-05e.cdr Figure 12.5 heating flame cutting jet Principle of Oxygen Cutting cutting oxygen heating oxygen gas fuel workpiece The high-speed cutting nozzle (cutting-oxygen pressure 8 bar) allows higher cutting speeds with increased cutting-oxygen pressure. The heavy-duty cutting nozzle (cutting-oxygen pressure 11 bar) is mainly applied for economic cutting with flamecutting machines. A further development of the heavy-duty nozzle is the oxygenshrouded nozzle which allows even faster and more economic cutting of plates manual cutting equipment as a cutting and welding torch combination br-er12-06e.cdr Figure 12.6 cutting oxygen heating oxygen gas fuel mixing chamber gas mixing nozzle Cutting Torch and Nozzle Shapes block-type nozze within thickness certain ranges. Gas mixing is either carried out in the torch handle, the cutting attachment, the torch head or in the nozzle (gas mixing nozzle); in special cases also outside the torch in front

5 12. Thermal Cutting 163 of the nozzle. As the design of cutting torches is not yet subject to standardisation, many types and systems exist on the market. br-er12-07e.cdr Figure 12.7 heating and cutting nozzle torch nozzle-to-work distance cutting length cut length cutting jet end of the cut Flame Cutting Terms kerf width kerf cut thickness start The selection of a torch or nozzles important and depends mainly on the cutting thickness, the desired cutting quality, and/or the geometry of the cutting edge. Figure 12.7 gives a survey of the definitions of flame-cutting. In flame cutting, the thermal conductivity of the material must be low enough to constantly maintain the ignition temperature, Figure Moreover, the material must neither melt during the oxidation nor form high-melting oxides, as these would produce difficult cutting surfaces. In accordance, only steel or titanium materials fulfill the conditions for oxygen cutting., Figure 12.9 br-er12-08e.cdr Figure 12.8 The heating flame has to perform the following tasks: - rapid heating of the material (about 1200 C) - substitution of losses due to heat conduction in order to maintain a positive heat balance - preheating of cutting oxygen - stabilisation of the cutting oxygen jet; formation of a cylindrical geometry over a extensive length and protection against nitrogen of the surrounding air Function of the Flame During Flame Cutting Steel materials with a C-content of up to approx. 0.45% may be flame-cut without preheating, with a C-content of approx. 1.6% flame-cutting is carried out with preheating, because an increased

6 12. Thermal Cutting 164 C-content demands more heat. Carbon accumulates at the cutting surface, so a very high degree of hardness is to be expected. Should the carbon content exceed 0.45% and should the material not have been subject to prior heat treatment, hardening cracks on the cutting surface are The material has to fulfill the following requirements: - the ignition temperature has to be lower than the melting temperature - the melting temperature of the oxides has to be lower than the melting temperature of the material itself - the ignition temperature has to be permanently maintained; i. e. the sum of the supplied energy and heat losses due to heat conduction has to result in a positive heat balance br-er12-09e.cdr Conditions of Flame Cutting regarded as likely. Some alloying elements form high-melting oxides which impair the slag expulsion and influence the thermal conductivity. Figure 12.9 The iron-carbon equilibrium diagram illustrates the carbon content-temperature interrelation, Figure As the carbon content increases, the melting temperature is lowered. That means: from a certain carbon content upwards, the ignition temperature is higher than the melting temperature, i.e., this would be the first violation to the basic requirement in flame cutting. Steel compositions may influence flame cuttability substantially - the individual alloying elements may show reciprocate effects (reinforcing/weakening), Figure The content limits of the alloying constitu- br-er12-10e.cdr Figure temperature [ C] 1000 solid steel pasty ignition curve solid cast iron liquid Liquidus Solidus 2,0 carbon content [%] Ignition Temperature in the Iron-Carbon-Equilibrium Diagram

7 12. Thermal Cutting 165 ents are therefore only reference values for the evaluation of the flame cuttability of steels, as the cutting quality is substantially deteriorating, as a rule already with lower alloy contents. br-er12-11e.cdr Maximum allowable contents of alloy-elements: carbon: up to 1,6 % silicon: up to 2,5 % with max. 0,2 %C manganese: up to 13 % and 1,3 % C chromium: up to 1,5 % tungsten: nickel: copper: up to 0,7 % up to 10 % and 5 % Cr, 0,2 % Ni, 0,8 % C up to 7,0 % and/or up to 35 % with min. 0,3 % C molybdenum: up to 0,8 %, with higher proportions of W, Cr and C not suitable for cutting Flame Cutting Suitability in Dependance of Alloy-Elements By an arrangement Figure of one or several nozzles already during the cutting phase a weld preparation may be carried out and certain welding grooves be produced. Figure shows torch arrangements for - the square butt weld, - the single V butt weld, - the single V butt weld with root face, - the double V butt weld and - the double V butt weld with root face. It has to be considered that, particularly in cases where flame cutting is applied for weld preparations, flame cutting-related defects may lead to increased weld dressing work. Slag adhesion or chains of molten square butt weld single-v butt weld single-v butt weld with rootface br-er12-12e.cdr Figure double-v butt weld double-v butt weld with root face Weld-Groove Preparation by Oxygen Cutting

8 12. Thermal Cutting 166 edge defect: edge rounding chain of fused globules edge overhang cut face defects: kerf constriction or extension angular deviation step at lower edge of the cut excessive depth of cutting grooves br-er12-13e.cdr Figure cratering: sporadic craterings connected craterings cratering areas Possible Flame Cutting Defects adherent slag: slag adhearing to bottom cut edge cracks: face cracks cracks below the cut face globules have to be removed in order to guarantee process safety and part accuracy for the subsequent processes. Figure gives a survey of possible defects in flame cutting. In order to improve the flame-cutting capacity and/or cutting of materials which are normally not to be flame-cut the powder flame cutting process may be applied. Here, in addition to the cutting oxygen, iron powder is blown into the cutting gap. In the flame, the iron powder oxidises very fast and adds further energy to the process. Through the additional energy input the high-melting oxides of the highalloy materials are molten. Figure shows a diagrammatic representation of a metal powder arrangement. cutting water seperator br-er12-14e.cdr Figure powder dispenser compressed air oxygen acetylene Metal Powder Flame Cutting

9 12. Thermal Cutting 167 Figure shows the principle of flame gouging and scarfing. Both methods are suited for the weld preparation; material is removed but not cut. This way, root passes may be grooved out or fillets for welding may be produced later. br-er12-15e.cdr Figure flame gouging gas-heat oxygen mixture gouging oxygen scarfing Flame Gouging and Scarfing gas-heat oxygen mixture scarfing oxygen Figure shows the methods of thermal cutting processes by electrical gas discharge: - plasma cutting with non-transferred arc - plasma cutting with transferred arc - plasma cutting with transferred arc and secondary gas flow - plasma cutting with transferred arc and water injection - arc air gouging (represented diagrammatically) - arc oxygen cutting (represented diagrammatically) plasma cutting - with non-transferred arc - with transferred arc -with secondary gas flow -with water injection br-er2-16e.cdr Figure Thermal cutting processes by electrical gas discharge: carbon electrode arc air gouging compressed air = Thermal Cutting Processes by Electrical Gas Discharge arc oxygen cutting electrode coating tube cutting oxygen arc In plasma cutting the entire workpiece must be heated to the melting temperature by the plasma jet. The nozzle forms the plasma jet only in a restricted way and limits thus the cutting ability of plate to a thickness of approx. 150 mm,

10 12. Thermal Cutting 168 Figure Characteristic for the plasma cut are the cone-shaped formation of the kerf and the rounded edges in the plasma jet entry zone which were caused by the hot gas shield that envelops the plasma jet. These process-specific disadvantages may be significantly reduced or limited to just one side of the plate (high quality or scrap side), respectively, by the inclination of the torch and/or water addition. With the plasma cutting conductive materials may be separated. process, all electrically Nonconductive materials, or similar materials, may be separated by the emerging plasma flame, but only with limited ability. plasma gas cooling water nozzle br-er12-17e.cdr Figure electrode HF workpiece R Plasma Cutting - + power source In order to cool and to reduce the emissions, plasma torches may be surrounded by additional gas or water curtains which also serve as arc constriction, Figure In dry plasma cutting where Ar/H 2, N 2, or air are used, harmful substances always water curtain water bath br-er12-18e.cdr Figure nozzle plasma gas electrode workpiece Water Injection Plasma Cutting cutting water swirl chamber cone of water develop which not only have to be sucked off very carefully but which also must be disposed of. In water-induced plasma cutting (plasma arc cutting in water or under water) gases, dust, also the noise, and

11 12. Thermal Cutting 169 the UV radiation are, for the most part, held back by the water. A further, positive effect is the cooling of the cutting surface, Figure Careful disposal of the residues is here inevitable. cutting with water bath water injection plasma cutting with water curtain Figure gives a survey of the different cutting methods using a water bath. plasma cutting with workpiece on water surface br-er12-19e.cdr underwater plasma cutting Types of Water Bath Plasma Cutting Figure Figure shows a torch which is equipped with an additional gas supply, the socalled secondary gas. The secondary gas shields the plasma jet and increases the transition resistance at the nozzle front. The so-called double and/or parasite arcs are avoided and nozzle life is increased. Thanks to new electrode materials, compressed air and even pure oxygen may be applied as plasma gas therefore, in flame cutting, the burning of unalloyed steel may be used for increased capacity and quality. The selection of the plasma forming gases depends on the requirements of the cutting process. Plasma forming media are argon, helium, hydrogen, nitrogen, air, oxygen or water. br-er12-20e.cdr Figure plasma gas electrode secondary gas nozzle workpiece Plasma Cutting With Secondary Gas Flow

12 12. Thermal Cutting 170 The advantage of the use of oxygen as plasma gas is in the achievable cutting speeds within the plate thickness range of approx mm (400 A, WIPC). In the steel plate thickness range of approx mm the application of 40 A-compressed air units is recommended. In comparison with 400 A WIPC systems, these allow vertical and narrower significantly cutting kerfs, but with lower cutting speeds. Figure different shows cutting speeds for different units and plasma gases. cutting speed [m/min] br-er12-21e.cdr Figure machine type and plasma medium 1 WIPC, 400 A, O2 2 WIPC, 400 A, N A, s < 8 mm: N2 s > 8 mm: Ar/H A, compressed air plate thickness [mm] Cutting Speeds of Different Plasma Cutting Equipment for Steel Plates In the thermal cutting processes with beams only the laser is used as the jet generator for cutting, Figure Variations of the laser beam cutting process: br-er12-22e.cdr Figure Thermal cutting processes by laser beam - laser beam combustion cutting - laser beam fusion cutting - laser beam sublimation cutting Thermal Cutting With Beams - laser beam combustion cutting, Figure laser beam fusion cutting, Figure laser beam sublimation cutting, Figure

13 12. Thermal Cutting 171 The process sequence in laser beam combustion cutting is comparable to oxygen cutting. The material is heated to the ignition temperature and subsequently burnt in the oxygen stream, Figure Due to the concentrated energy input almost all metals in the plate thickness range of up to approx. 2 mm may be cut. In addition, it is possible to achieve very good bur-free cutting qualities for stainless steels (thickness of up to approx. 8 mm) and for structural steels (thickness of up to 12 mm). Very narrow and parallel cutting kerfs are characteristic for laser beam cutting of structural steels. In laser beam cutting, either oxygen (additional energy contribution for oxidising materials) or an inactive cutting gas may be applied depending on the cutting job. Besides, the very high beam powers (pulsed/superpulse d mode of operation) allow a direct evaporation of the material (sublimation). In laser beam combustion cutting and laser beam sublimation cutting the reflexion of the laser beam of more than 90 % on the workpiece surface de- laser focus thin layer of cristallised molten metal br-er12-23e.cdr Figure br-er12-24e.cdr absorption factor Figure λ = µ slag jet Laser Beam Cutting 1,06 m (Nd:YAG-laser) 10,06 m (CO -laser) λ = µ heating-up 2 melting point Tm melting temperature lens Qualitative Temperature Dependency on Absorption Ability cutting oxygen workpiece evaporating boiling point Tb

14 12. Thermal Cutting 172 creases unevenly when the process starts. In laser beam fusion cutting remains the reflexion on the molten material, however, at more than 90%! Figure shows the absorption factor of the laser light in dependence on the temperature. This factor br-er12-25e.cdr Figure laser cutting (with oxygen jet) - the laser beam is focused on the workpiece surface and the material burns in the oxygen jet starting from the heated surface materials: - steel aluminium alloys, titanium alloys cutting gas: - ONAr 2, 2, criteria: - high cutting speed, cut faces with oxide skin Characteristics of the Laser Beam Cutting Processes I mainly depends on the wave length of the used laser light. When the melting point of the material has been increases unevenly factor reached, the absorption and reaches values of more than 80%. During laser beam combustion cutting of structural steel high cutting speeds are achieved due to the exothermal energy input and the low laser beam powers, Figure In the above-mentioned case (dependent on beam quality, focussing, etc.), above a beam power of approx. 3,3 kw, spontaneous evaporation of the material br-er12-26e.cdr Figure laser fusion cutting: - the laser beam melts the entire plate thickness (optimum focus point 1/3 below plate surface) - high reflection losses (>90%) materials: - metals, glasses, polymers cutting gas: - N, Ar, He 2 criterions: - cutting speed is only 10-15% in comparison to cutting with oxygen jet, characteristics melting drag lines Characteristics of the Laser Beam Cutting Processes II takes place and allows sublimation cutting. Significantly higher laser powers are necessary to fuse the material and blow it out with an inert gas, as the reflexion loss remains constant. Important influence quantities for the cutting speed and quality in laser beam cut-

15 12. Thermal Cutting 173 ting are the focus intensity, the position of the focus point in relation to the plate surface and the formation of the cutting gas flow. A prerequisite for a high intensity in the focus is the high beam quality (Gaussian intensity distribution in the beam) with a high beam power and suitable focussing optics. Laser beam cutting of contours, especially of pointed corners and narrow root faces, requires adaptation of the beam power in order to avoid heat accumulation and burning of the material. In such a case the beam power might be reduced in the continuous wave (CW) operating mode. With a decreasing beam efficiency decreases the cuttable plate thickness as well. Better suited is the switching of the laser to laser evaporation cutting: - spontaneous evaporation of the material starting from 10 W/cm with high absorption rate and deep-penetration effect - metallic vapour is pressed from the cavity by own vapour pressure and by a supporting gas flow materials: - metals, wood, paper, ceramic, polymer cutting gas: - N, Ar, He (lens protection) 2 criteria: - low cutting speed, smooth cut edges, minimum heat input br-er12-27e.cdr Figure Characteristics of the Laser Beam Cutting Processes III 5 2 pulse mode (standard equipment of HF-excited lasers) where pulse height can be selected right up to the height of the continuous wave. A super equipment pulse (increased excitation) allows significantly higher pulse efficiencies to be se- laser 600 W 1500 W 600 W 1500 W 1500 W plasma 50 A 5 kw 250 A 25 kw 500 A 150 kw oxy-flame br-er12-28e.cdr 1 Stahl Cr-Ni- Stahl steel Cr-Ni-steel aluminium steel Cr-Ni-steel aluminium plate thickness [mm] lected than those achieved with CW. Further fields of application for the pulse and super pulse operation mode are punching and laser beam sublimation cutting. Fields of Application of Cutting Processes Figure 12.28

16 12. Thermal Cutting 174 Laser beam cutting of aluminium plates thicker than appx. 2 mm does not produce bur-free results due to a high reflexion property, high heat conductivity and large temperature differences between Al and Al 2 O 3. The addition of iron powder allows the flame cutting of stainless steels (energy input and improvement of the molten-metal viscosity). The cutting quality, however, does not meet high standards. cuttig speeds [m/min] br-er12-29e.cdr ,1 Figure CO2-laser (1500 W) oxygen cutting (Vadura 1210-A) 10 plate thickness [mm] plasma cutting (WIPC, A) 100 Cutting Speeds of Thermal Cutting Processes Figure shows a comparison of the different plate thicknesses which were cut using different processes. For the plate thickness range of up to 12 mm (steel plate), laser beam cutting is the approved precision cutting process. Plasma cutting of plates > 3 mm allows higher cutting speeds, in comparison to laser beam cutting, the cutting costs [DM/m cut length] br-er12-30e.cdr Figure laser plasma plate thickness [mm] Thermal Cutting Costs - Steal total costs machine costs flame cutting with 3 torches quality, however, is significantly lower. Flame cutting is used for cutting plates > 3 mm, the cutting speeds are, in comparison to plasma cutting, significantly lower. With an increasing plate thickness the dif-

17 12. Thermal Cutting 175 ference in the cutting speed is reduced. Plates with a thickness of more than 40 mm may be cut even faster using the flame cutting process. Figure shows the cutting speeds of some thermal cutting processes. Apart from technological aspects, financial considerations as well determine the application of a certain cutting method. Figures and show a comparison of the costs of flame cutting, plasma arc and laser beam cutting the costs per investment total (replacement value) calculation for a 6-yearaccounting depreciation /h flame cutting (6-8 torches) 170, plasma cutting (plasma 300A) Cost Comparison of Cutting Processes laser beam cutting (laser 1500W) maintenance costs /h energy costs /h production cost unit rate costs/1 operating hour /h br-er12-31e.cdr Figure extract from a costing acc. to VDI , , shift, 1600h/year, 80% availability, utilisation time 1280h/year m/cutting length and the costs per operating hour. The high investment costs for a laser beam cutting equipment might be a deterrent to exploit the high cutting qualities obtainable with this process.