Lesson 12 Oxyfuel Gas Welding and Cutting

Transcription

1 Department of Engineering Design Lesson 12 Oxyfuel Gas Welding and Cutting Professor Pedro Vilaça * * Contacts Address: P.O. Box 14200, FI Aalto, Finland Visiting address: Puumiehenkuja 3, Espoo pedro.vilaca@aalto.fi ; Skype: fsweldone February 2015 OXYFUEL GAS WELDING (OFW) is a manual process in which the metal surfaces to be joined are melted progressively by heat from a gas flame and are caused to flow together and solidify without the application of pressure to the parts OFW can be applied with or without filler metal The most important source of heat for OFW is the oxyacetylene welding (OAW) 1 1

2 Oxygen and fuel are stored in separate cylinders The gas regulator attached to each cylinder, whether fuel gas or oxygen, controls the pressure at which the gas flows to the welding torch The mixed gases then pass through the welding tip and produce the flame at the exit end of the torch tip 2 The equipment is versatile, low-cost, self-sufficient, and usually portable It can be used for preheating, postheating, welding, braze welding, and torch brazing, and it is readily converted into oxygen cutting The process can be adapted to short production runs, field work, repairs, and alterations The oxy-acetylene welding process (OAW) is by far the most important, and probably the most versatile, of the oxy-fuel gas welding (OFW) processes For welding applications the OAW is superceded by other welding processes, e.g., the TIG welding process 3 2

3 4 The gas acetylene (C 2 H 2 ) is the most important fuel gas employed, because it has the highest calorific (heat) value. Other hydrocarbon gases are also used, e.g. Liquid Petroleum Gas (LPG), Propane, etc. deg C Oxy-acetylene 3,100 to 3,300 Oxy-propane 2,500 Oxy-hydrogen 2,370 Oxy-coal-gas 2,200 Air-acetylene 2,460 Air-coal-gas 1,871 Air-propane 1,750 Table: Approximate Maximum Flame Temperatures 5 3

4 Ratio: Oxygen versus Fuel gases for higher flame temperature ACETYLENE PROPANE NATURAL GAS HYDROGEN Ratio: Oxygen / fuel gas Maximum temperature (ºC) Chemical and physical characteristics of Oxygen and relevant fuel gases 7 4

5 Chemical and physical characteristics of Oxygen and relevant fuel gases 8 Ignition ranges of several fuel gases with mixture with air and oxygen 9 5

6 In practical applications the oxy-acetylene welding process (OAW) is the only one applied in welding Higher heat power density Hotter for all the distances from the center of the flame More reducing action Easy to adjust with Oxygen The oxi-propane flame is more applied in brazing when temperature and heat power should be lower Propane and butane are more safe to use fuel gases The propane, butane and natural gas allow greater autonomy as they can be easily stored in large volumes. 10 Acetylene (C 2 H 2 ) storing: When under pressure of 203 kpa and above, acetylene is unstable, and a slight shock can cause it to explode, even in the absence of oxygen or air By dissolving purified and dried acetylene in liquid acetone, a cylinder such as that shown in Figure can be used to store about 7.79 m3 of acetylene under a pressure of 1.7 MPa The cylinders must be stored in an upright position to keep the acetone from escaping during use 11 6

7 The acetylene-acetone solution is in turn absorbed by a porous substance, such as kapok, charcoal or asbestos The porous substance must completely fill the cylinder but, by virtue of its porosity, it leaves small spaces holding in total a considerable amount of liquid acetone Internal details of dissolved-acetylene cylinder 12 Acetone is a chemical compound of hydrogen, oxygen and carbon (CH 2, CO, CH 2 ). It is a liquid at room temperature with a pungent smell (like nail varnish remover), boils at 56 C and is slightly poisonous It is inflammable and absorbs 25 times its own volume of acetylene gas at atmospheric pressure. When the pressure is increased to 14 kg / cm 2 it will absorb 370 times its own volume of acetylene When acetylene gas is introduced into the cylinder, it is promptly dissolved in the acetone, which in turn is contained in the tiny pores or cells of the filling material 13 7

8 Should disintegration of acetylene occur, it is localised by these tiny pockets and so prevented from spreading to the remainder of the acetylene in the cylinder Not more than 20% of each cylinder s gas capacity must be drawn off per hour; otherwise acetone used to dissolve the acetylene in the cylinder may be drawn out with the gas In the high-pressure acetylene system the gas is stored in cylinders having an average gas capacity of 3,398 to 6,230 litres These cylinders may be used singly by individual welders, in which case pressures of up to 1,05 kg/ cm 2 may be used at the blowpipe 14 Alternatively as a centralised manifold system feeding multiple welding points, in which case, for safety reasons, the line pressure is not allowed to exceed 0,63kg/cm 2 The number of acetylene cylinders required on a manifold is related to the total amount of acetylene required at any one time. 15 8

9 Manifold system on high-pressure acetylene supply line Note: The oxy manifold may be basically the same or may be supplied for liquid-oxygen evaporation plant. 16 Manifolded supplies may be used for both acetylene and oxygen cylinders, although where the oxygen consumption is high it is more practical to use an oxygen generator/evaporator plant, the oxygen being delivered and stored in the more compact liquid form. In each instance the gas is stored away from the workshop in a special building designed to relevant safety requirements, the gas is piped from the store to the supply points in the workshop. The acetylene is conveyed in steel tubes (copper is not used because of the danger of producing the explosive compound: copper acetylide). The line is protected by non-return valves and flash-back arresters at each outlet. The cylinder is also protected by largecapacity flashback arresters. 17 9

10 Oxygen cylinder (Capacity, 220ft 3 at lb / in 2, Weight 145 lb) Acetylene cylinder (Capacity, 250ft 3 at 250 lb / in 2, Weight 215 lb) 18 OXYFUEL GAS WELDING (OFW) STATION - Equipment 19 10

11 1. Welding torch: efficient and light, producing a flame of the right shape and temperature with controls that are easy to adjust allowing for the quick and easy changing of nozzle tips, thus affording a wide range of blowpipe power 2. Oxygen and acetylene gas regulators: to reduce the high pressure in a gas storage cylinder to a lower working pressure, and at the same time maintaining a steady supply, free from pressure fluctuations 3. Supplies of oxygen and acetylene gases, which must be safely stored in cylinders and suitably piped to the welding areas Protection equipment, such as suitable colour-tinted goggles, manufactured to the appropriate standard, e.g. BS 679, and other protective clothing, such as chrome- Ieather gloves, aprons, etc Filter glasses for goggles are invariably green, their main function being to reduce the glare from the flame cone and molten metal to a level comfortably acceptable to the eyes of the welder, while also protecting the eyes from hot sparks They are produced in different densities or shades, appropriate to light or heavy glare. Modern filters are photo chromatic, i.e. they change shade automatically 21 11

12 Protection goggles Fire lighters Supplies of suitable filler rods in convenient diameters and lengths. These are usually from 1,2mm to 4,8mm in diameter and 1mm in length 6. Certain other equipment, such as flexible, high-pressure rubber hoses, coloured red for fuel gas and black for oxygen, with properly designed connections, threaded left-hand for fuel gas and right-hand for oxygen, to prevent accidental exchange Also, safety devices, such as flash-back arresters, or hydraulic backpressure valves. Equipment such as refractory-surfaced work tables, gas economizers, etc are desirable but cannot be regarded as absolutely essential 23 12

13 Safety/security devices 24 Although individual designs from different manufacturers vary to some extent in appearance and performance, welding torches are of two main types, namely low-pressure and highpressure Essential features of high-and low-pressure blowpipes 25 13

14 Features of low-pressure with injector blowpipe 26 Types of torches/burners architecture Features of high-pressure with no injector blowpipe 27 14

15 The high pressure blowpipe is lighter and simpler. In operation it is less troublesome since it does not suffer from backfires to the same extent It does not need an injector, so that the gases are fed to the torch at equal pressures, and when the flame setting is neutral, in equal proportions To change the power of the blowpipe, it is only necessary to change the nozzle tip size and increase or decrease the gas pressures appropriately 28 Selecting a filler rod diameter The following working formulae may be used: For butt welds up to t=4,8mm then D = t / 2 For vee welds up to t=6,4mm then D = t / 2 + 0,8mm Where: D is the rod diameter and t the plate thickness

16 Selecting a filler rod diameter (cont.) The diameter of the welding rod can considerably affect the ease of welding and the weld quality Too large a rod is slow to melt and can chill or freeze the weld pool, leading to lack of fusion, cold laps and other defects Too small a rod melts too fast and other metal tends to drop through the joint. The rod must then be fed into the joint more rapidly, requiring extra dexterity on the part of the operator Quite a small increase in rod diameter greatly increases the total available volume of weld metal, because V is proportional to D 2, where V is the volume of weld metal, and D the rod diameter 30 Procedure for adding a filler rod into the weld pool 31 16

17 Fluxes: A flux prevents the oxidation of molten metal. The flux (material) is fusible and non metallic During welding, flux chemically reacts with the oxides and a slag is formed that floats to and covers the top of the molten puddle of metal and thus helps keep out atmospheric oxygen and other gases Except for lead, zinc, and some precious metals, OFW of nonferrous metals, cast irons, and stainless steels generally requires a flux In welding carbon steel, the gas flame shields the weld adequately, and no flux is required Adjustment for correct flame atmosphere is important, but the absence of flux results in one less variable to control

18 The maximum temperature of the oxy-acetylene flame is 3,100 to 3,300 C and the centre of this heat concentration is just off the extreme tip of the white cone. Combustion is recognised as taking place in two main stages of combustion 34 Types of Flames in OAW In oxy-acetylene welding the character of the flame is most important. Certain technical terms must be learned in this connection When the acetylene and oxygen are in equal proportions the resultant flame is said to be neutral; when there is an excess of oxygen the flame is said to be oxidising; and if more acetylene is present than oxygen the flame is said to be carburising, or reducing A reducing flame is on that, because of its need for oxygen will reduce surface oxides, such as iron oxide. A strictly neutral setting is correct, but the slightest excess of acetylene may keep oxidation to a minimum, particularly when welding stainless steels For example, non-ferrous alloys and carbon steels may require a reducing flame, while zinc-bearing materials may need an oxidising flame 35 18

19 Three types of flame setting: mixture ratio Oxygen and acetylene (O 2 and C 2 H 2 ), in equal proportions by volume, burn in the inner white cone. In the cone two separate reactions take place, the oxygen combining with the carbon of the acetylene to form carbon monoxide (CO), while the hydrogen (H 2 ) is liberated. 2. Upon passing into the outer envelope of the flame two more separate reactions take place as combustion is completed. The carbon monoxide takes up oxygen from the atmosphere, and as a result of burning forms carbon dioxide (CO 2 ). The hydrogen also burns with oxygen from the atmosphere to form water vapour (H 2 O)

20 While the quantities of acetylene and oxygen taken from the supply are equal, something like two and a half times as much oxygen is actually consumed, the balance being taken from the surrounding air. The combustion products are the reason for maintaining good ventilation in gas welding bays, together with the fact that the flame itself uses large quantities of oxygen from the air. 38 FERROUS AND NONFERROUS METALS THAT CAN BE WELDED BY OAW: (a) - Match base metal ; (b) - No Flux required 39 20

21 Combustion with other fuel gases Acetylene: 2 C 2 H 2 +5 O 2 =4 CO 2 +2 H 2 O Propane: C 3 H 8 +5 O 2 =3 CO 2 +4 H 2 O Natural Gas: CH 4 +2 O 2 =CO 2 +2 H 2 O 40 Flame Setting: Influence of the Speed of Flow 41 21

22 Lighting a blowpipe The correct pressures, as recommended for the appropriate nozzle, should first be set, initially the fuel gas by opening slightly the blowpipe acetylene valve and regulating it to the correct pressure by the pressure-regulator screw This procedure is repeated for the oxygen supply, the oxygen valve then being closed. The fuel gas is turned on, ignited and adjusted so that the flame just ceases to form soot but is not blown away from the nozzle tip The oxygen is now turned on at the blowpipe valve and adjusted until the acetylene feather just disappears, to obtain a neutral flame setting 42 Lighting a blowpipe (cont.) Each nozzle size will impose the flame conditions at the neutral setting ranging from a soft quiet flame to a hard or harsh flame. The average gas velocity is (approximately) 182,88 m / sec To extinguish the flame, the fuel gas should be turned off first, followed by the oxygen. In the event of backfires with either design of torch, the fuel gas should be turned off first to prevent the internal temperatures from being destructively high and damaging the blow-pipe body 43 22

23 Welding techniques The usual techniques in oxy-acetylene welding are: the leftward (push technique) the rightward (pull technique) (of rather less prominence are variations, such as) the all-position rightward technique and Linde welding For all descriptive purposes it is assumed that the operator is right handed: should he be left handed, it is only necessary to interchange the words 'right' and 'left' and 'rightward' and 'leftward' 44 When plate exceeds 6-4 mm, the combined power of the two blowpipes is much higher than using a single blowpipe to weld an equal thickness, even if employing the rightward technique This means not only that less metal is needed, but also that the operators are subjected to less physical discomfort and fatigue Whether the weld calls for one operator or two, the weld can be completed with a single pass without any need for multi-passes Welding speed overall is much higher, and the consumption of gas and filler rod is lower, up to about 50% saving, where the plate is over 6,4 mm thick, than with the down-hand leftward or rightward technique 45 23

24 Combined power of the two blowpipes (cont.) Advantages are obvious from the point of view of both the economies and the quality and soundness of the weld deposit Absence of heavy oxide scale and smaller tendency to distort because of uneven heating or weld-metal distribution is another advantage 46 Some useful data will be found in Tables 1 to 6 Table 1 - Tack welds Thickness of plate 6,4 mm 9,5 mm 12,7 mm Dimensions of tack 12,7 mm long 19,1 mm long 25,4 mm long Distance apart of tacks 152 mm 229 mm 305 mm Distance between edges after tacking 3,2 mm bare 3,2 mm 3,2 to 4,0 mm Size and form of tack weld 47 24

25 Table 2 - Welding rate in relation to plate thickness Plate thickness Welding rate Leftward method Rightward method m / h mm mm 6,1 to 7,6 0,8, 2,4-7,6 to 9,1 1,6-5,5 to 6,1 3,2-4,6 to 5,5 4,0-3,7 to 4,6 4,8 4,8 3,0 to 3,7-6,4 2,1 to 2,4-7,9 48 Table 2 - Welding rate in relation to plate thickness (continued) Plate thickness Welding rate Leftward method Rightward method m / h mm mm 1,8 to 2,1-9,5 1,4 to 1,5-12,7 1,1 to 1,3-15,9 0,9 to 1,0-19,1 0,6 to 0,7-25,

26 Table 7 - Nozzle power in relation to plate or tube wall thickness Plate or tube wall thickness Power of blowpipe (volume of acetylene per hour) mm litres 6,4 1,133 to ,9 1,274 to 1,416 9,5 1,416 to 1,557 11,1 1,472 to 1,614 12,7 1,557 to 1, Linde Technique The Linde welding process is a special one basically used for the butt welding of steel tubes at a temperature below the melting point of the parent metal When carbon is added to steel the melting points is lowered; e.g. a 0,8% carbon steel has a lower melting point than a 0,2% carbon steel When a carburising (or excess acetylene) flame is played on the surface of mild steel, the steel at high temperature absorbs carbon on its surface and the surface sweats because its melting point is lowered The rightward technique is used, the flame being set with an excess acetylene feather about one and a half times as long as the standard neutral cone 51 26

27 Nozzles larger than normal are used (Table 7) and special doublenozzle blowpipes allow pre-heating of the vee and increased welding speed Preheating the Vee 52 The filler rod may also be of a larger diameter than usual (Table 8), as the blowpipe flame is played upon this more than upon the joint walls Normal-carbon mild-steel rod is unsuitable, and must be replaced by one containing suitable proportions of silicon and manganese Linde welding is at its best for materials, tubes, etc over 6,4 mm thick; the joint edges should have a 70 included-angle vee with clean oxide and scale-free surfaces The welding should be carried out in the flat position, tubes being rotated to achieve this. Tack welds should be about three times the parent-metal thickness in length and taper formed 53 27

28 As welding approaches completion, particularly on a tube, special precautions become necessary. About 12,7 mm from the end of the run, the blowpipe is adjusted to give a smaller neutral flame This is now played on the original start of the weld, which is reheated for about 50mm until red hot. The original start and finish are then fused together at the root of the joint without adding filler wire After fusion has been achieved, filler rod is again introduced to complete the weld reinforcement. Tables 9 and 10 indicate the high speeds which can be obtained by the Linde welding technique 54 Table 9 - Welding rate in relation to thickness Plate or tube wall thickness Rate of welding mm m / h 6,4 4,6 to 4,9 7,9 4,0 to 4,6 9,5 3,7 to 4,0 11,1 3,0 to ,7 2,7 to 3,

29 Plate or tube wall thickness Outside diameter mm 152 mm 203 mm 254 mm 305 mm 457 mm min min min min min 6,4 6, , ,9 6,75 10, ,5 7,50 11,50 13,50 15, ,1 9 12, ,50 27,50 12,7 10, Table 10 - Average time for butt pipe joint for different outside diameters of pipe 56 Department of Engineering Design Lesson 12a Oxyfuel Gas Cutting Professor Pedro Vilaça * * Contacts Address: P.O. Box 14200, FI Aalto, Finland Visiting address: Puumiehenkuja 3, Espoo pedro.vilaca@aalto.fi ; Skype: fsweldone February

30 Introduction to Material Cutting Technologies General Classification Cutting Technologies, include: Sectional cutting (e.g. beveling) Drilling Marking Removing partial layer from surface 58 Introduction to Material Cutting Technologies Applicability Analysis of Cutting for Beveling Welding Joint preparation (1/2): Welding Joint Process 59 30

31 Introduction to Material Cutting Technologies Applicability Analysis of Cutting for Beveling Welding Joint preparation (2/2): Welding Joint Process 60 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Fundaments of OXYFUEL GAS CUTTING (OFC): A group of cutting processes that use controlled chemical reactions to remove preheated metal by rapid oxidation in a stream of pure oxygen. A fuel gas/oxygen flame heats the workpiece to ignition temperature, and a stream of pure oxygen feeds the cutting (oxidizing) action. The OFC process, which is also referred to as burning or flame cutting, can cut carbon and low-alloy steel plates of virtually any thickness

32 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Fundaments of OXYFUEL GAS CUTTING (OFC): 62 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Fundaments of OXYFUEL GAS CUTTING (OFC): 63 32

33 Introduction to Material Cutting Technologies Analysis of Cutting for Beveling Geometrical characteristics of edges after cutting: 64 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Chemical Reaction in Iron Combustion, by Oxidation at Ignition Temperature: 1st- Fe + O = FeO kj ( 64 kcal ) 2nd- 3Fe + 2O2 =Fe3 O kj ( 266 kcal ) 3rd- 2Fe O2 = Fe2O kj ( 190 kcal ) The products (oxides / slag) from the chemical reactions are: 50% FeO ; 40% Fe2 O3 ; 10% Fe 65 33

34 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Note 1: In 1776, the French scientist Lavoisier conducted an experiment showing that introducing a small slot of red-hot iron into a bottle containing oxygen it burned continually: Lavoisier proved that the iron is fuel. Note 2: The ignition temperature does not depend on the oxygen pressure. A carbon steel foil ignites in oxygen according to the Semenov-Frank-Kamenetskii mechanism at an initial surface temperature not lower than 1233 K. The Iron (Fe) presents significant Advantages for OFC: Burns in Oxygen at Ignition Temperature. The combustion is highly exothermic. Ignition Temp. < Slag Fusion Temp < Base Material Fusion Temperature 66 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) CRITERIA: Suitability of Materials for Flame Cutting (OFC) 67 34

35 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Equipment of OXYFUEL GAS CUTTING (OFC): The simplest equipment consists of two cylinders (one for oxygen and one for the fuel gas, typically acetylene), gas flow regulators and gages, gas supply hoses, and a cutting torch with a set of exchangeable cutting tips. 68 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Oxyfuel Gas Cutting Torch for Manual and Automatic Operation: 69 35

36 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Oxyfuel Gas Cutting TIPS for Several Operations: 70 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Fundaments of OXYFUEL GAS CUTTING (OFC): The OFC equipment manually operated is portable and inexpensive

37 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Fundaments of OXYFUEL GAS CUTTING (OFC): Manual Cutting 72 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Fundaments of OXYFUEL GAS CUTTING (OFC): Cutting machines, employing one or several cutting torches guided by solid template pantographs, optical line tracers, numerical controls, or computers, improve production rates and provide superior cut quality. Machine cutting is important for profile cutting--the cutting of regular and irregular shapes from flat stock

38 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Fundaments of OXYFUEL GAS CUTTING (OFC): Machine Cutting 74 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Application of OFC in Beveling: 75 38

39 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Application of OFC in Beveling: 76 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Cutting Of Oxidation Resistant Metals: With oxidation-resistant materials, either a chemical flux or metal powder is added to the oxygen stream to promote the exothermic reaction. Basics Of Powder Cutting: In the powder cutting process, a finely divided "iron rich" powder is introduced into the reaction zone. This iron powder, because of its finely divided state, combines rapidly with the Oxygen stream and increases the temperature of the reaction, resulting in an increase in the fluidity of the refractory oxides. A clean surface is exposed to cutting oxygen stream, and cut progresses through the metal. The quality of the cut is slightly inferior to that of oxygen cut low carbon steel

40 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Basics Equipment for Powder Cutting: In the normal cutting nozzle is surrounded by a powder nozzle which introduces the powder which in turn is fed from a powder dispenser. The medium of carrying the powder to the nozzle is compressed air or nitrogen. The "fluid" oxides are now removed by a combined melting and fluxing operation and to a certain extent by the eroding action of the iron particles themselves. The intense heat generated by the powder eliminates the preheating of Oxidation Resistant Metals, and therefore "flying starts" can be made 78 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Basics Equipment for Powder Cutting: Figure: Multi-jet powder nozzle Figure: Single tube powder feed 79 40

41 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Cutting Of Oxidation Resistant Metals: Stainless Steel When certain alloys are added to Stainless Steel, they become oxidation resistant rendering them unsuitable for cutting by means of the normal oxy-fuel process. With stainless steel and non-ferrous metals, the oxide formed when the jet of oxygen is impinged on to the heated plate, has a higher melting point than the material itself, and this forms a film on the surface of the metal which prevents any further oxidation. Before the introduction of powder cutting, stainless steel had to be cut by mechanical means which is expensive and very slow and when these high costs are added to the cost of an already expensive material, the final costs of products become prohibitive. 80 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Cutting Of Oxidation Resistant Metals: Cast Iron and High Alloy Steels Some cast irons and high alloy steels may be cut in a manner similar to stainless steel. However, when cutting high alloy and tool steels, it is advisable to preheat to avoid cracking which may result from local heating. Gray cast irons are almost impossible to cut due to lamellar graphic form of the carbon content

42 Introduction to Material Cutting Technologies Oxyfuel Gas Cutting (OFC) Cutting Of Oxidation Resistant Metals: Copper and Copper Alloys When "cutting" copper and copper alloys, powder cutting appears to be a "melting action", coupled with the eroding action of the high velocity particles of iron powder rather than a true oxygen cutting action. The main problem with copper is its high thermal conductivity. The rapid dissipation of heat through the metal being cut poses problems due to the large amount of heat required to maintain the high temperature to allow the cut to progress Correct Conditions Appearance of cut Sharp top edge Smooth surface, drag lines barely visible A very light scale of oxide easily removed Square face Sharp bottom edge Remarks The very light drag lines should be almost vertical for profile cutting. For straight cutting a drag of up to 10% would be permissible 83 42

43 2. Speed Too Slow Appearance of cut Melted and rounded top edge Lower part of the cut face fluted or gouged very irregularly Bottom edge rough Heavy scale on cut face which is difficult to remove Remarks The bad gouging in the lower half of the cut is caused by molten steel scouring the cut surface and the hot metal and slag which congeals on the underside is always difficult to remove. Secondary cause of this condition is oxygen pressure being too low Speed Too Fast Appearance of cut Top edge not sharp and may be beaded Undercutting at top of cut face Drag lines have excessive backward drag Slightly rounded bottom edge Remarks The excessive backward drag of the cut line would result in the cut not being completely severed at the end. The occasional gouging or fluting along the cut indicates that the oxygen pressure is too low for the speed, but possibly not too low for a normal speed. In other words, if the speed was dropped and the oxygen pressure maintained, a perfectly good cut would result 85 43

44 4. Nozzle Too High Above Work Appearance of cut Excessive melting and rounding of top edge Undercut at top of cut face with lower part square and sharp bottom corner Remarks The melting at the top edge is due to heat spread each side of the cut and the undercutting is caused by the oxygen stream being above the work so that it spreads or trends to bell out as it traverses down the kerf Nozzle Too Low Appearance of cut Top edge slightly rounded and heavily beaded Cut face usually square with fairly sharp bottom corner Remarks Having a nozzle too low does not usually spoil the cut face unduly, but will badly burn the top corner. Very often it retards the oxidation reaction and makes it appear that the cut has been done too slowly 87 44

45 6. Pressure Of Cutting Oxygen Too High Appearance of cut Regular bead along top edge. Kerf wider at top edge with undercutting of face just below Remarks Probably the commonest fault in cutting, causing rounding of the top part of the cut face through turbulence within the oxygen stream which is set at too high pressure. On thinner material it may cause a taper cut which sometimes leads to the incorrect supposition that the cutter is incorrectly mounted in relation to the plate Preheat Flame Too Large Appearance of cut Rounded top edge with melted metal falling into kerf Cut face generally smooth, but tapered from top to bottom Excessive tightly adhering slag Remarks This is the easiest and most obvious condition to correct. Providing other conditions are normal, the appearance is of a clean but heavily oxidised face combined with very heavy rounding at the top edge 89 45