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Correction of Distortion Using Line Heating

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Correction of Distortion Using Line Heating
25
CORRECTION OF DISTORTION USING THE OXY-ACETYLENE FLAME PROCESS The Process Flame straightening is used for correcting deformations which occur in welded constructions. Shrinkage in the weld it self and the surrounding material due to cooling occurs in all welding. Shrinkage causes deformation and buckling in the surrounding plate, even at some distance from the welds. Deformation can also stem from thermal stresses created by rolling or thermal cutting, but the extent of deformation is largely dependent on how welding is executed. Distortions are especially visible on painted or lacquered surfaces. It is usually impossible to stretch areas that have shrunk due to welding. One solution is to attempt to shrink the over extended areas, a job for which flame straightening has proved to be perfectly suitable. Flame straightening is particularly convenient since no equipment other than a blowpipe is normally required. But the operator must be well aware of how the work piece will react to heating and how shrinkage forces straightening can best be utilized for straightening. In contrast to mechanical straightening the results of flame straightening are not evident until the structure has cooled. Principle The Principle of flame straightening is based on using the hottest flame possible – acetylene – oxygen – for rapid heating of a limited portion of plate to a temperature of approx 600 0 C, at which the plasticity of the steel has been substantially increased. Since the surrounding material remains cold, the heated parts will be restrained during heating and upset so that the excessively long parts will 1 of 25
Transcript
Page 1: Correction of Distortion Using Line Heating

CORRECTION OF DISTORTION USING THE OXY-ACETYLENE FLAME PROCESS

The Process

Flame straightening is used for correcting deformations which occur in welded constructions. Shrinkage in the weld it self and the surrounding material due to cooling occurs in all welding. Shrinkage causes deformation and buckling in the surrounding plate, even at some distance from the welds. Deformation can also stem from thermal stresses created by rolling or thermal cutting, but the extent of deformation is largely dependent on how welding is executed. Distortions are especially visible on painted or lacquered surfaces.

It is usually impossible to stretch areas that have shrunk due to welding. One solution is to attempt to shrink the over extended areas, a job for which flame straightening has proved to be perfectly suitable.

Flame straightening is particularly convenient since no equipment other than a blowpipe is normally required. But the operator must be well aware of how the work piece will react to heating and how shrinkage forces straightening can best be utilized for straightening. In contrast to mechanical straightening the results of flame straightening are not evident until the structure has cooled.

Principle

The Principle of flame straightening is based on using the hottest flame possible – acetylene – oxygen – for rapid heating of a limited portion of plate to a temperature of approx 6000C, at which the plasticity of the steel has been substantially increased. Since the surrounding material remains cold, the heated parts will be restrained during heating and upset so that the excessively long parts will shorten during cooling. The plate may also be restrained by external means such as clamps or loads.

Fig.1 illustrates the principle of flame straightening.If a bar is restrained during heating longitudinal expansion is prevented. This leads to a build up of compression stresses. Finally the compressive yield strength limit is reached and the material is plastically upset. Assisting in the upset is the fact that as the temperature increases the upset limit decreases. During cooling the steel bar will be shortened due to the upsetting or plastic deformation which has been made to occur.

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Fig.1

Page 2: Correction of Distortion Using Line Heating

Equipment

The most effective straightening result is obtained by using the high heat penetration characteristic of the oxy-acetylene flame applied in a number of localized heat patterns over the surfaces to be rectified.

Blowpipes with multi-flame heat configurations (3 nozzle and 5 nozzle) have been developed to achieve to such desired patterns, thus giving greater control of the straightening process in the most economical manner.

These multi-headed blowpipes are especially suited for the straightening of large areas of plate such as decks and deck superstructures on ships.

The 3 nozzle blowpipe of used for plate thickness between 4mm and 12mm, and the 5 nozzle head assemblies.

On plate thickness of 4mm to approx 8mm it may be necessary to limit the heat input. In such cases series 1390 nozzle size 6 are recommended. Both sizes of nozzle are supplied with each blowpipe.

For the treatment of beams, angles of deformation existing prior to the corrective treatment commencing, the operator should have available a straight edge bar of length approx 1.5m.

In certain cases it may be necessary to supplement the flame process by the use of props or jacks.

Fig.2

Fig.3

Illustrates that 3 nozzles blowpipe version comprising sharks, mixer, straight and bent neck extensions, and adaptors and head assembly. This blowpipe version id used for most horizontal applications (decks) with plate thickness up to 12mm.Also illustrated alongside is he alternative 5 nozzle head assembly for use on plate above 12mm thickness.

Illustrates the shorter blowpipe version using the bent extension only, and more suited for use in vertical applications (deck superstructures) and/or use in confined areas.

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I don’t know which one of these explanations illustrates this bellow fairing line (Fig2 or Fig 3 ?), and how the picture of the other.

Page 3: Correction of Distortion Using Line Heating

Fig.2

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Page 4: Correction of Distortion Using Line Heating

Fig.3

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Page 5: Correction of Distortion Using Line Heating

Safety

BOC equipment is manufactured to highest standard of quality and safety and will give first class service for many years if operated correctly.

Safety precautions to be observed when using compressed gases with welding and cutting equipment are described in the BOC handbook ‘SAFE UNDER PRESSURE’ available free of charge from your nearest BOC Sales Office.

Any user requiring more derailed information on safety precautions should refer to Home Office Memorandum ‘Safety Measures for the use of Oxy-acetylene Equipment in Factories’. (Form 1704).

The following notes may, however, prove useful to ensure your equipment is efficient and safe.

Gas Cylinders

Leakage round a valve spindle will be revealed by hissing or in some cases by a smell of gas. Tighten the gland nut on the cylinder valve with a spanner and test with soapy water. NEVER USE A FLAKE.

Pressure Regulators

Always treat a regulator as a precision instrument, do not expose it to knocks, and jar sudden pressure surges caused by the rapid opening of the cylinder valve.

Never use a regulator with other than the gas for which it was designed. Release pressure on the control spring when shutting down, after pressure in the hoses has been released.

If gauge pointers do not return to zero when pressure is released, the mechanism is faulty and the gauge should be replaced.

If the regulator ‘creeps’ (passes gas when the pressure regulating screw is released), or builds-up pressure on the low pressure side when the blowpipe valve is shut, it should be replaced (see details of BOC Service Exchange Scheme for safety and economy).

Hoses

Use only hose which is in good condition and fitted with hose connections attached by permanent ferrules.

Do not expose hoses to heat, tragic, slag, sparks, oil or grease. Test for leakage at working pressure by immersing in water; leak may be repaired by cutting out a faulty suction of hose and inserting an approval coupling which ends should be cut back and re-fitted with hose connectors and clips.

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Page 6: Correction of Distortion Using Line Heating

Saffire Hose Check Valves are recommended for fitting to all hoses. The valve is an automatic safeguard incorporating a spring loaded non-return valve, its purpose being to prevent oxygen and fuel gases mixing in the hoses. The saffire Hose Check Valve has eliminated the conditions of flashback which are present when oxygen contaminates a fuel gas hose or vice versa and it is well worth ensuring that your welding and cutting equipment is protected as far as possible against flashback which as often serious and may cause extensive damage to hoses is particularly severe.

Flashback can be avoided by adherence to recommended operating procedure, and the use of hose check valves does not enable the operator to ignore good operating practices.

Blowpipes

Leakage can be detected by soapy water bubbles or hissing and in the case of fuel gases also by smell or ignition. Leakage at valves should be cured by tightening gland nuts; leakage at the head nut or welding nozzle by cleaning with a soft cloth. If leakage continues the blowpipe shield be Service Exchanged. In the case of a backfire, with gas burning inside the blowpipe, first close the oxygen then the fuel gas valve; check delivery pressure, purge the hoses and if these are correct after a pause relight the blowpipe, if backfiring persists it indicates that the blowpipe is in need of replacement.

Protective Clothing

Goggles should be worn at all times whilst welding and cutting. Leather or asbestos protective clothing should be worn for heavy cutting or welding. The feet should be protected from sparks, slag or falling off-cuts.

Ventilation

In a confined space ensure that there is a suction fan to give adequate ventilation (a fume hood at the source of fumes is the best method); DO NO USE OR AN AIR BLOWER and always post a helper outside for emergencies. Test all equipment for leaks before entering and move it outside during interruptions and on completion of daily work.

Operation

Operating data for all versions of the blowpipe are as follows :

Blowpipe Nozzle Type

Oxy. Pressure bar (lbf/in2)

Acet. Pressure bar

(lbf/in2)

Oxy. Flow l/h (ft3/h)

Acet. Flow l/h (ft3/h)

FS3FS3FS5FS5

No.6No.7No.6No.7

2.4(35)2.4(35)2.4(35)2.4(35)

0.34(5)0.34(5)0.48(7)0.48(7)

1330(47)2067(73)2435(86)2888(102)

1218(43)1870(66)2208(78)2633(93)

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Page 7: Correction of Distortion Using Line Heating

Blowpipe Position

The blowpipe should be adjusted by means of the wheel bracket fixings so that the tip of the inner flame cone is approx 4mm from the plate surface. (Fig.4)

(Fig.11 illustrated the alternative fixing positions for nozzle sizes 6 and 7 )

Heating commences above the stiffener, (Fig.5) with one of the multi-flames always positioned directly above the stiffener. The remaining flames are displaced according on the plate adjacent to the stiffener. (see Method of Applications).

Allow sufficient time at the beginning of the cycle for the plate to become dull red in color (temperature approx 6000C/7000C).

Heat penetration for maximum effect should be 1/3 or plate thickness as indicated in Fg.6

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Nozzle

-

Fig.4

Fig.6

Plate

Approx90o

4mm

1 T 1/3 T

Page 8: Correction of Distortion Using Line Heating

Constant progress of the blowpipe at the correct speed for the size of nozzle selected can be maintained by observation of the heat zones formed by the individual nozzles.Fig.7 indicated the desired heat pattern, the gap between the heat zones being approx 1/3 width of the heat zone itself.

Where the gap is as wide as or wider than the heat zone (Fig.8) heat input is the insufficient indicating the speed is too fast.

Where the gap is non existent with heat zones overlapping the heat is excessive indicating the speed is too slow. (Fig.9)

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Fig.7

Fig.8

Fig.9

Page 9: Correction of Distortion Using Line Heating

Heat bands should be approximately 100mm long on a 4-6mm thick plate and approximately 250mm long on a 8-12mm thick plate. The distance between the heat bands should be approx 100mm to 250mm dependent upon the plate thickness. (Fig.10)

Method of Application

Different constructions with varying deformations require differing straightening methods. It is not possible therefore to be specific of the method of application in all cases. External factors such as residual stress due to rolling, thermal cutting and welding greatly affect the results straightening. Tolerance requirements can also vary from case to case.

A few examples of how flame straightening can be executed will be given here. The straightening involves a deck section and the wall of a deckhouse, both with a plate thickness of approx 8mm.

Both these constructions are flame straightened in the same manner except that the deck can usually only be reached from one side.

Begin by straightening the plate directly above the stiffeners straightened the side of the plate which is too long, i.e. the convex.Always locate one of the flames directly above the stiffener while we remaining flames are displaced according to the type of deformations in the surrounding plate. The following examples illustrate the location of the blowpipes.

In this case the plate has been symmetrically deformed on both sides of the stiffener either upwards (+) or downwards (-). The blowpipe is then located symmetrically with one frame on either side of the stiffener, despite the fact that this is the concave side of the stiffener.

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Fig.10

Fig.12

- -

+ +

Blowpipes

Page 10: Correction of Distortion Using Line Heating

This plate is asymmetrically deformed it is deformed upwards in one side of the stiffener and downwards on the plate. The blowpipe should be applied eccentrically. One of the outer flames is located directly above the stiffener while the remaining flames are over the side of the stiffener where the plate is deformed upwards.

However deformations as distinct as those shown in the two examples in Fig.12Above are rare. It is therefore essential that the deformations in the vicinity of the stiffener, i.e. the area directly affected by the flames, be studied carefully.

In the case of Fig.14 the blowpipe is located eccentrically, since the plate is flat in the middle. The plate there will rise and then revert back to the original shape. In this case as well the flames are placed over the high parts.

The stiffeners are distinctly visible in Fig.15 since the plate has been deformed downwards in both of the stiffeners. Only one flame located directly over the stiffeners should be used in this case.

In Fig.16 the plate I the middle is not deformed. In any case, the operator should apply the blowpipe eccentrically while carefully observing how the plate reacts to the heating. If the plate drops down, the blowpipe must be returned to the normal position with the center flame directly over the stiffener.

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+ 0-

Fig.13

Fig.14

Fig.15

Fig.16

+ -

+ +-0 + 0--

+ - +- --

Page 11: Correction of Distortion Using Line Heating

If extreme deformation still remains after the first straightening pass (1-2), further straightening may be necessary. These passes are marked with the numbers 3-6 in Fig.17

In cramped areas, Fig.18 the blowpipe can be displaced by half the distance between the flames line 3 and 4 – since heating the same area twice is not recommended.Also ensure that there is space between the heat bands during the second straightening. Otherwise the stiffener may bend out.

The appearance of the plate after the initial straightening with the blowpipe eccentrically located must be studied carefully. Indicated heating is not always advisable in every case. Buckles with large bumps, for example, are difficult to straighten.

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3 & 4

31

42

46 2531Fig.17

Fig.18

1& 2

Page 12: Correction of Distortion Using Line Heating

A buckle suitable for renewed heating appears in Fig.19 this buckle has the smooth contours which are necessary for a second straightening pass to be possible.

Straightening Buckles

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Fig.19

After the 1st straightening pass

After the 2nd straightening pass

21

Page 13: Correction of Distortion Using Line Heating

After the area around the beams is straightening, the remaining buckles in the intermediate plate surfaces may be treated.Convex buckles on the deck must be straightened first.

The rule “the big ones follow the small ones”, which means that the small buckles are treated before the larger ones, applies here. This is illustrated in this example.

First 2 or 3 spots are heated (1 in Fig.20a). After they have cooled, their extended areas are then heated (2). However, it is important that heating not be resumed until the first spots have completely cooled down.

If the deformation is not corrected by heating the middle of the plate new heating bands (3) must be applies to the sides. Fig 20b

Continued heating is carried out according to either a or b.

If the buckles are uniformly distributed over the entire plate, head bands shall be applied in accordance with Fig.20c

If the buckles occur mostly in the middle, the new heat bands shall be applies within the previous heat bands in accordance with Fig.20d.

If there are any sections where the upper part in convexly distorted and the lower part is concavely distorted, straighten in accordance with Fig.21a, b, c, d.

First, the part which is convexly distorted is straightened from the outside (1 and 2).

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Fig.20a

Fig.20b

Fig.20c Fig.20d

Fig.21a

1

2

3 3

4 4

3 3

44

2

1

2

1+

-

+0- +0-+

a b

-

Page 14: Correction of Distortion Using Line Heating

The lower part is then heated from the inside (3 and 4). Observe that the heating bands on both the outside and the inside form a common pattern and that heating is executed from the convex side of the plate.

Whether the final heating (5) in the transitional area between (+) to (-) is to be done from the outside or the inside must be determined in each individual case. Heating shall be carried out from the convex side which means from outside in the above figure. In this case, the blowpipe shall be moved upwards. If head is applied from the outside the blowpipe is moved in the opposite direction.

In most cases the flame straightened section will new be within the tolerance limits and straightening is concluded.There may, however, be some special cases such as short buckles, which require further treatment.

Short Buckles

Experience has shown that short, prominent buckles are difficult to straighten. If the blowpipe is applies in the manner described in Fig.22a & 22b the buckles will be extended, thereby lowering their rigidity.

The flame is applied in the middle of the buckles and directed forward. Thos preheats the material aver a large area and the plate will then rise towed the flame.

After the buckle has been extended in one direction, the blowpipe is directed at the middle and the buckle is extended in the other direction. The buckle then loses its rigidity and becomes easier to straighten.

Thicker Plate

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Fig.21b

Fig.21c

Fig.21d

3 3

44

5 5

Starting point

Starting point

Forward direction

Forward direction

Fig.22a

Fig.22b

Page 15: Correction of Distortion Using Line Heating

For plates approx, 12mm thick or more, it is usually sufficient to treat only the angular deformation. If the stiffeners are thick and the angular deformation is not too great a five flame torch is used. In this case as well, the length of the heat bands shall be approx. 250 – 350mm. If the initial heating pass is not sufficient to straighten the deformation, further heating may be requited between first heat bands.

Fig.23 Shows how a ship hull with a plate thickness of 12mm is straightened. Note how the plate around the butt weld has sunk down.

First the plate surrounding stiffeners 1, 2, 3, 4 and 6 is straightened using a five flame blowpipe. Due to the extreme deformation at stiffener 5, only a three flame blowpipe was used there, since a five flame blowpipe could make the deformation worse. After the first heating pass, it was apparent that insufficient straightening had been obtained around the butt weld. Therefore, continuous heat bands were applied over stiffeners 1 and 2. Straightening was further increased by using hacks to lift the butt weld form the inside and using wedges on the outside.Plate Straightening Of Pre-Fabricated Parts

Pre-fabricate parts to be flame straightened must be firmly fixed to prevent movement. It is therefore impossible to straighten open plate surfaces effectively.

It is quite common for shipyards to construct deckhouses as separate units from the ship and then place them on the deck with all straightening completed.

This construction method is only possible if the bottom plates that are to be welded to the ship deck are sufficiently stiffened daring straightening.

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7001 2 3 4 5 6

12

+ 4- 7 + 3 - 6 - 3

2000

(+ 1)(- 1)(- 1)(± 0)(- 2)

V

5

6 63

4 41

2 2

Page 16: Correction of Distortion Using Line Heating

Flame straightening cannot begin until all welding is completed. The deck plates in the deckhouse should be straightened first and then the underlying walls. As indicated in fig.24

FLAME STRAIGHTENING OF BEAMS

L – BEAMS

Begin by heating the horizontal flange at the point indicated by the arrow

Heat both flanges. Begin at 1 and continue with the other at 2.

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Fig.24

2

Page 17: Correction of Distortion Using Line Heating

I - BEAMS

Heat both flanges simultaneously, starting at the points indicated by 1.

Begin by heating the web at 1 and continue with the flange at 2.

FLAME STRAIGHTENING OF BEAMS

T – BEAMS

Begin by heating the horizontal flange at the point indicated by the arrow

.

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1

2

1

2

1

Page 18: Correction of Distortion Using Line Heating

.

Heat both flanges.Begin with the horizontal flange at 1 and continue at 2

Heat both flanges.Begin with vertical flange at 1 and continue with the other at 2.

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1

1

2

2


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