Electro Slag Strip Cladding

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27th Apr 2017 Education Reference this

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Electro Slag Strip Cladding

ABSTRACT:

Technological developments have driven up temperature and pressure used in the petroleum, chemical, pulp, and environmental protection. Industries, and increased the likelihood of severe corrosion and wear in process pressure vessels. The industries must improve the corrosion / erosion and wear performance of these major component .Economic factors as a rule will not permit fabricating components from solid high alloyed materials. As a consequence it is necessary to surface non-alloyed or low alloy base materials with high-alloy cladding. The submerged arc (SAW) and electroslag welding (ESW) process are suitable for applying welded deposits over large surface areas using strip electrodes .Both processes are using a granular flux material. A strip electrode, fed continuously, is melted and fused to the substrate. Compared to other processes it is very efficient in spite of the same equipments used but due to the wide strip used it procures a magnetic flow effect within to rectify it a magnetic steering device is used. After the welding to scrutinize the defects NDT’s are carried upon it.

1. INTRODUCTION:

Electro slag strip cladding is a development of submerged arc strip cladding, which has quickly established itself as a reliable high deposition rate process. ESW is an arc less method using Joules Effect to melt the strip/flux/parent material. The heating is a result of current flowing through the strip electrode and a comparatively shallow layer of liquid electro conductive slag as shown in 1. The penetration is lower for ESW than for SAW since the molten slag pool is used to melt the strip and some of the parent material rather than as an arc between the strip electrode and the parent material. As a rule of thumb, electro slag strip surfacing reduces dilution by up to 50% compared with submerged arc strip surfacing for the same heat input with a significantly higher deposition rate. However, as a result of the lower dilution levels for ESW, new strip compositions have been developed for ESW, in particular for applications where the aim is to obtain a certain weld metal chemistry (such as 304L, 316L) in one layer.

DILUTION:

An important factor to consider in ESSC is dilution. In any overlay process, the weld metal deposited gets mixed with the base metal in the molten state, thereby giving a slightly leaner composition. Thus, the properties of this part of the bead will be slightly compromised due to the change in the chemical composition. The amount of dilution can be calculated by using the formula: –

Dilution % = (b*100)/ (a+b)

As shown in the 2 the width ‘a’ of the bead will retain the required properties of the clad material, as it experiences no change in chemical composition. The amount of dilution is dependent on the overlay-process and process variables such as current, travel speed and thickness of material that will be joined and also the welding procedure. Excessive base metal dilution, as with other metals, can induce cracking along the fusion line and must be controlled by using proper welding consumable and welding technique. Dilution is also an important consideration for determining the number of weld-layers to be provided for corrosion resistance. Some multi-pass welded beads may have a little variation in composition in each weld layer due to the process control and technique employed. The filler metal’s first layer should be able to tolerate at least up to 30% dilution and still should be able to yield an acceptable deposit. Welding parameters must be selected in such a way that fusion with minimum dilution is obtained.

2. METHODOLOGY:

2.1. ESSC

2.1.1. WELDING PARAMETERS:

* Attitude of the electrode: The electrode is normally positioned at right angles to the work piece (vertical) and at right angles to the axis of its movement (relative to the work piece). Rotating the electrode around its longitudinal axis is permissible to a certain extent, but this will create a narrower, thicker bead.

* Spacing of current contact: The distance from the lower edge of the contact jaws to the surface to the work piece is normally about 30 mm.

* Flux depth: The depth of the flux determines the thickness of the slag layer produced. If the flux is not deep enough, the slag pool will be too shallow, causing increased arcing of the strip. If the flux is too deep, the flux will melt only in the middle. The slag pool would be cooled by the flux lying on it, causing deterioration in electrical conductivity. Here, again, the result will be increased arcing. Normal depth should be 30 mm.

* Current density: Due to the absence of an arc, the penetration in the ESW process is very shallow; this means that there will be little mixing of the filler metal with the melted base material. It is possible, in comparison with the submerged arc welding, to use far higher power levels. For thin layers a normal current density of about 33 A/mm2 ,43 A/mm2 for thicker layers. For strip type electrodes measuring 60*0.5 mm this will cause 1000 or 1250 A, respectively. This increased current level will cause penetration, though the here is still at the lower limit of what would be expected in SAW surfacing using a strip type electrode. Power levels exceeding about 1000 A for 60*0.5 mm strip electrode would require very high welding speed to achieve thin layers (below 0.15 in – mm) and the strip electrode would separate either completely or partially from the front edge of the liquid slag. This would result in the increased arcing. When using wider electrode strips -120*0.5 mm, for example—current of > 2500 A may be required.

* Welding voltage: The welding voltage affects the specific resistance of the liquid slag and will determine how far the strip electrode is to be submerged in the slag pool. Insufficient immersion in the weld pool will cause the process to become unstable. The welding voltage must be lowered as current rises. A range of 24 to 26 V when operating at 1250 A, or 22 to 24 V for 2500 A, is normal. The exact value will depend on the properties of the flux and the dimensions of the strip. Arcing may be experienced if the voltage is too high and the electrode is not immersed far enough in the slag pool. The welding process will become unstable with increased arcing.

* Welding travel sound: The travel speed will depend on the desired thickness of the surfacing layer. The greater current density which can be applied along the high melting rate that can be achieved, make it possible to attain higher welding speeds than would be achieved, make it possible to attain higher welding speeds than would be possible with SAW surfacing. A layer of 4mm, is often specified encountered in processing equipment, the welding speed will be between 16 and 20 cm/min. The extent to which the thickness of the cladding can be reduced by increasing the welding speed is limited since, at sped exceeding 20 cm/min, the strip electrode will tend to “run away from “the slag pool. For this reason, lower current densities are used to apply thin layers about 3.5 mm. Not only can the surfacing depth be regulated by adjusting the welding speed; the degree of dilution by the substrate material can also be influenced for two different current levels, in comparison with submerged arc welding.

* Supplementary magnetic fields: With the auxiliary steering magnets switched on, the width of the bead will increase by1 to 2 mm; the depth is reduced accordingly since the filler material will be pulled toward the outer edges. With appropriate adjustment of the magnetic fields at the north and south poles of the magnets will make it possible to influence the shape of the bead. The south pole is always positioned at the left side in the welding direction. Using supplementary magnetic fields for steering purposes are not necessary for 60*0.5 mm electrodes. The geometry of the bead may be adversely affected by welding near the ground connection. The two yokes of the magnet are positioned 15 mm to the sides of the electrode strip and 1.5 mm above the surface of the base material. A strong magnetic field at the south pole (3A: 1A) will pull the liquid filler material against the natural magnetic blow direction, which would be to the left when looking at the rear of the electrode. A strong magnetic field at the north pole (2 A: 1A) would pull in the opposite direction. In this way it is possible to counteract the natural magnetic blow effect by exact adjustment of the two auxiliary magnetic fields.

2.1.2. ESSC Equipments and Machines:

· Strip feed unit: High powered, geared motors are required to unwind the strip from the reel and advance the strip electrode in welding process. This is true particularly when using wide strip electrodes. Grooved feed rollers matching the width of the electrodes will be required.

* Current pick-up unit: Broad surface power transfer contact units are used, designed to suit the size of the electrode. At least one edge of the contact shoe must be subdivided into fingers which are individually pressed against the strip to ensure uniform transfer of current across the entire width of the electrode. Off-center application of power will result in a non-symmetrical bead, particularly when using wider electrodes. When using higher currents in continuous duty operation, it is advisable to fit the rear contacts with water cooling as they are exposed to the thermal radiation from the open slag pool.

* Strip coil mount: The welding strip is normally delivered in coils. Strip measuring 60*0.5 mm, for instance, will weigh from 30 to 60 kg: the weight will increase accordingly for wider strip. The strip coil must be engineered to carry this load as shown in 3.

· Flux feed and removal of excess material: The granular flux material is normally fed out of the flux hopper, and only in front of the strip electrode. In this way the uncovered slag pool is formed behind the electrode. The resulting high temperature (2300 degree) ensures better electrical conductivity within the slag and a process which is free of arcing from the strip to the base metal. The flux which is not fused can be vacuum extracted immediately after the slag has solidified. It is done with the help of flux recovery or flux recycling machines.

* Remote amperage and voltage control: An essential component in the strip feed unit is a control unit to keep the welding voltage and current constant. This instrument must provide an adequate degree of accuracy while monitoring the welding parameter. Normally in ESSC process in L&T we use NA5 controller for this purpose. A prototype of the equipment with its different controls is shown in 4.

* Magnet steering: The slag pool is electro conductive; therefore, the slag pool is subjected to EM forces that tend to make it flow from the sides towards the centre of the molten pool resulting in narrower beads and unfavorable wetting angles. Slag removal becomes difficult and there will be more chances of LF in bead overlap area. Effect of different forces is shown in 5.

To compensate for this, magnetic devices are used. This magnetic field is creating by means of two solenoids. The location of solenoids is very important. The tips should be placed beside the strip electrode at a dist. off approx. 15mm from the strip edge and about 15 mm above the base material surface.

The shape of the solidification ripples should be used to control the intensity of the magnetic field. The criterion for correct intensity of the magnetic field is when the solidification lines become symmetrical.

In most cases, the south and north poles of the magnetic control requires different current through each of the solenoids. Start with 3.5Amps at the North and 3.0 at the South. Generally, set the North magnet 0.5 to 1 Amp higher than the South.

It is obvious that the intensity on each solenoid has to be adapted to the working conditions on a particular w/p, taking into account any magnetic blow effect that cannot be calculated previously. After the application of the device the shape of the bead is shown in 6.

· Welding power supply: The DC current required for welding is supplied by constant potential rectifiers which are designed to have a flat slope. Alternating current cannot be utilized. When using strip measuring 60 *5 mm for ESSC surfacing, the available output capacity should not be less than 1400 A at 100% duty cycle.

* Mechanization welding equipment: Apart from the boom, for adjusting minute distances we use cyclomatic slide. It gives both vertical and horizontal movements to the welding head for fine adjustments. It uses a 12V DC motor for the purpose; motor rotates the lead screw which translates it into the linear motion of the nut frame for the adjustment.

· ESSC nozzle:

Main functions of nozzle:

(a) To guide the strip and to maintain it I the required position during welding operation.

(b) To transfer welding current from power source to the strip by means of appropriate contact b/w shoes and fingers.

(c)The ESSC nozzle is also equipped with a water-cooling possibility for strip width more than 60mm.

2.1.3. Welding Consumables:

FILLER METAL USED: In the context of Electro Slag Strip Cladding, the filler metal refers to the strip used. The composition of the filler metal is selected according to the requirements of the job and the dilution expected. The most common requirement for cladding is a corrosion resistant internal surface. For this purpose, there are many types of strips available, of different sizes and different compositions. The table1 gives details of the strips:

Table1: Types of strips with different sizes and composition.

* ESSC Flux: It is necessary to use fluxes with a high proportion of CaF2 in order to achieve the good electrical conductivity desired for the slag at high temperatures while at the same time creating a process which is resistant to arcing. Furthermore, the fluxes may not contain any components which may create gases because they would interfere with the contact required b/w the strip electrode and the liquid slag: arcing may result. Flux composition of the flux usually used during ESSC is shown in table 2.

Composites

Percentage

Sio2

6%

Al2O3

25%

Li20+Na2O+K20

4%

MgO+CaO+CaF2

65%

Table 2: Flux composition (in %) used during ESSC process.

Drying of flux: The fluxes are packed dry. Flux used is hygroscopic and will absorb moisture. It is advisable to dry the flux at 300- 400oC for 2 hours prior to use. Intermediate storage of flux which has been dried in this way should be in a furnace or kiln at 100oC. It is necessary to use fluxes with a high proportion of fluoride (Ca F2 ) in order to achieve the good electrical conductivity desired for the slag at high temperatures while at the same time creating a process which is resistant to arcing. Furthermore the fluxes may not contain any components which would create gasses – calcium carbonate (CaCo3 ) for example—since the gasses would interface with the contact required between the strip electrode and the liquid slag; arcing might result . Four fluxes are available, all similar in composition. Marathon 449 gives low silicon pickup and is particularly suitable for surfacing using nickel based strip electrodes.

2.1.4. MAGNETIC BLOW EFFECT IN WIDE STRIPS:

The repellent effect of the grounding point has already been mentioned, whereby the liquid pools are displaced to one side, evidenced by off-center rippling of the bead and an off- center end crater. The problems which result there from become more serious with higher current levels and smaller work pieces. Rising current levels will reinforce the intrinsic magnetic field created around the electrode in the base material; with small work pieces one is always welding near the grounding connection. It has been found that there is no point in using wide electrodes >4.7 in (> 120 mm) to apply trial cladding to test panels smaller than 40*20 in (1000*500 mm). It can nonetheless happen that, under unfavorable circumstances, magnetic blow phenomena could be encountered even when working with large components. Like electric current paths inside the slag can be affected by external magnetic fields. This will cause changes in the current density and in the temperature distribution within the slag and will ultimately change the natural direction of flow. When welding the two beads, the grounding pole was attached on the right-hand side, about even with the overlapping point. The pools were therefore displaced outward in each case and beads were formed which were thinner in the overlapped area than on the outer edges.

2.1.5. HEAT TREATMENTS: The work piece preheating and interpass temperature will depend on the base material (as a rule 3000C – 1500 C). Thermal post treatments (annealing temperature and time, heating and cooling rates) are to be specified so that the properties of neither base material nor the cladding will be adversely affected. To be given particular consideration are the temperature curve for the base material, resistant to intercrystalline corrosion, and brittleness of the austenitic cladding. Where multiple layers are applied, stress relief annealing may under certain circumstances be undertaken before depositing the final layer. This appears to be favorable for base materials which require annealing at temperatures exceeding (9850 F – 5300 C), one example of which is ASTM A387 Gr 22.

Depending on the base material and the specification, a PHWT at 12750F (6800C) and 32 h will be required. Here it is advisable when dealing with two layers, corrosion-resistant, austenitic ES claddings to weld the first layer with a type 309L strip or a stabilized 309L Nb strip, in order to protect against disbonding.

2.1.6. Comparison between single and double layer ESSC:

SINGLE LAYER ESSC: In single layer ESSC, the primary aim is to achieve the desired chemistry with just a single layer of the weld metal. To realize this, the strip chemistry is adjusted in such a way that the diluted weld metal meets the final chemistry in a single layer. Consequently, the chemistry of deposited weld metal remains uniform throughout the bead height. However, to successfully carry out single layer ESSC, a very stringent control over the welding parameters – i.e. current, voltage, welding speed, stick out, bead overlap, flux burden etc. – needs to be maintained.

DOUBLE LAYER ESSC:

Typically, a weld overlay procedure consists of minimum TWO layers. The first layer is deposited with weld metal of richer chemistry (e.g. for a typical Austenitic Stainless Steel, the weld consumable for barrier layer consists of higher Cr & Ni) to take care of dilution from C-Mn steel or Low Alloy Steel base material. Subsequent layer (s) is deposited with the weld consumable of same chemistry as that of final requirement. Table 3 shows the parameter for single and double layer.

2.1.7. Comparison between ESSC and SAW:

ELECTROSLAG technique

Electro slag strip cladding differs from the submerged arc technique in that the energy required to melt the strip, the base metal and the flux is produced by the Joule effect.

There is no electric arc in the electro slag cladding technique. The agglomerated flux is fed from one side only.

The resulting liquid pool (strip, base metal and melted flux) is electrically conductive. The dimensions of the deposit can be adjusted using two magnetic rods, one on each side of the liquid pool.

The electro slag strip cladding technique is characterized by:

• A higher rate of deposition than the submerged arc technique (22 kg/h),

• Low dilution with the base metal (7 to 10%),

• The required chemical analysis can be obtained with one or at most two layers, with exceptionally stable and regular operation (this is because of the previous two features),

• The ability to use strips with widths in the range

SUBMERGED ARC technique

There is no fundamental difference between submerged arc welding and cladding. The welding wire is merely replaced with the cladding strip. The equipment is the same, except the head must be adapted to guide the strip.

The principle is the same: the energy to melt the strip and the base metal is supplied by the electric arc struck between them. On melting, the agglomerated flux protect the liquid metal and where applicable enriches it with alloying elements.

Submerged arc cladding is characterized by:

• Low penetration generating a low level of dilution (16 to 20%) limiting the number of layers of deposit required (two to three depending on the required chemical analysis),

• Good chemical homogeneity of the deposit,

• Low sensitivity to hot cracking (when cladding with nickel alloy) due to the absence of chemical segregation around the deposit (unlike using wire),

• Possible transfer of alloying elements from the flux (compensation and enrichment),

• A very flat coating surface,

• A low investment cost on changing over from wire to strip,

• The ability to use strips with widths in the range 30 to 90 mm.

Table4: Comparison between ELECTROSLAG and SUBMERGED ARC technique.

2.1.8. WELD DEFECTS:

The major defects are:

-Slag inclusion.

-Lacks of inter bead fusion.

-Improper bead shape.

-Crater crack.

-Centerline cracks.

-Porosity.

-Under cut.

· SLAG INCLUSION:

Slag inclusion is a very common defect in the any welding process. Slag inclusion defect is shown in 8.

-Improper bead shape.

-More bead overlapping

-Improper cleaning of the slag.

-Poor slag detachability of the flux

Solution:

-Adjust the parameter to get bead proper slope at toe.

-Maintain correct bead overlap before start of the overlay. It should be in the 2-5 mm. Not more than it.

-Proper cleaning of the bead with power wire brushing and blades.

· LACK OF INTER BEAD FUSION:

When we are doing overlapping of the bead in the any welding process, at that time if we are not take care of proper overlap so the over bead may not be fuse with the under bead 100%, and there the chances of the lack of fusion.

Reasons:

-Improper bead shape

-Thickness of the layer is very high.

-Use of less current.

Solution:

-Take care at the time of the welding that the bead must be sat on the other bead.

-Use correct welding parameter and especially travel speed.

-Check the ampere meter and other electronics device before starting the welding and change it if required

· IMPROPER BEAD SHAPE:

Reasons:

-Incorrect / wrong settings of Magnetic Stripping Device

-Generating jerks in the welding head/ tank rotator.

-Current fluctuation.

-Incorrect bead overlap.

-Flux height may be very high.

Solution:

-Connect shoes face should be polished & tight properly.

-Check ear thing lug and power source.

-Set proper current & polarity.

-Set correct bead overlap.

· CRATER CRACK:

Crater crack defect is shown in 10.

-Improper cleaning of surface.

-Less flux height.

Solutions:

-Surface must be cleaned properly before starting the welding.

-Flux height may not be higher than stick out +7mm.

· CENTERLINE CRACK:

Reasons:

-Use of improper strip.

-Use of improper flux.

-Improper bead slope in the overlap area.

-Lesser flux height.

-Improper setting of molten pool.

-Improper cleaning of the surface.

-Use of very high current.

-Inter pass temperature may not be maintained.

Solutions:

-Check the strip and flux before use in the welding.

-Overlapping must not be more then 2-5mm.

-Flux height should be stuck out +7mm.

-Set the parameter before starting the welding.

-Surface must be clean properly.

· POROSITY:

Porosity defect is shown in 11.

Reasons:

-Improper cleaning of the surface.

-Improper shielding of the molten pool.

-Use of cold/unbaked flux.

-Contamination of foreign particles with the strips and flux.

Solutions:

-Check the flux baking period before use.

-Clean properly before use of the recycled flux.

-Clean the surface before start the welding.

-Take care of the proper shielding of the molten pool.

· UNDER CUT:

Reasons:

-Less current.

-High travel speed.

-Wrong setting of the polarity in the magnetic stripping device.

Solutions:

-Use proper parameter.

-Check the all device at the time of the trial.

· VALLEY:

Reasons:

-Drift in shell rotation

-Slight error in magnet steering device setting.

Solution:

-Nullify the drift in shell rotation.

-Correct the magnet steering device setting.

2.3. NON DESTRUCTIVE TESTING:

2.3.1. Visual Inspection:

Introduction: It is a non-destructive method used to evaluate an item to by observation, such as: the correct assembly, surface conditions or cleanliness of materials, parts and components used in fabrication.

All finished weld O/L on products are subjected to eld visual examination. This specification covers the norms for acceptable surface defects on weld overlay made using different welding processes.

Apparatus:

The usually used aids are mirrors, artificial lights, magnifiers, rulers and special weld gauges, boroscope, endoscope.

Lighting:

The light in the visual inspection area shall be sufficient to provide adequate contrast so that the detection of relevant objects and discontinuities is accomplished with a high degree of success. The required luminance to perform visual inspection shall be 1000 Lux (> 100 Foot candles).

Pre requisite:

Cleanliness of weld O/l area( i.e. free from dirt, grease oil, paint, embedded foreign material, spatters, loose scales/ slag, chalk paste etc.)

Presence of necessary lightning

Points to be checked by inspector:

-Dimensional accuracy of O/L thickness.

– Conformity to drawing requirements, this includes determination of all required O/L is performed and whether finished O/l confirm with regard to size and contour ass per drawing.

– Acceptance of weld O/l with regard to appearance (including such as weld spatter, inter bead valley, start & end points, overlapped beads etc.).

– The presence of unfilled crater cracks, pockmarks, undercuts, overlaps and cracks.

– Evidence of damage from mis-handling.

– Markings of excessive grinding.

Procedure:

-Visual inspection of overlay shall be carried out grid wise where overlaid surface is a large area after marking grid lines.

-All the defects shall be marked with permanent maker.

-Rectification shall be carried out grid wise.

Acceptance standard:

-Cleanliness of weld O/l shall be checked under illumination.

– Variation in height of O/l b/w adjacent beads shall be less than 1.6 mm.

– No sharp corners acceptable.

– Porosity, slag inclusion, spatter, mechanical damage not acceptable.

– If undercut >.5 mm the sharp edges shall be merged.

– Fisher/ crater cracks not acceptable.

Advantages:

-No heavy or complicated equipments required.

-Saves cost and time.

-Can be done anywhere .No restrictions of place.

-Some points can only be covered by this method e.g. spatter, thickness etc.

Disadvantages:

-Only for macro surface defects can be detected.

-Manually done, so susceptible to error.

2.3.2. Magnetic Testing:

This NDT method is accomplished by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles (either dry or suspended in liquid). Surface and near-surface flaws produce magnetic poles or distort the magnetic field in such a way that the iron particles are attracted and concentrated. This produces a visible indication of defect on the surface of the material. Magnetic field lines around the crack and magnetic particles spread over the crack is shown in 12.

Magnetic field lines around the crack and magnetic particles spread over the crack.

Longitudinal Magnetization: When the length of a component is several times larger than its diameter, a longitudinal magnetic field can be established in the component. The component is often placed

2. Circular Magnetization: When current is passed through a solid conductor, a magnetic field forms in and around the conductor. The following statements can be made about the distribution and intensity of the magnetic field. The field strength varies from zero at the center of the component to a maximum at the surface. The field strength at the surface of the conductor decreases as the radius of the conductor increases when the current strength is held constant. (However, a larger conductor is capable of carrying more current.). Examples of Magnetic testing indication are shown in 14.

Advantages:

-Large surface areas of complex parts can be inspected rapidly.

-Can detect surface and subsurface flaws.

-Magnetic particle indications are produced directly on the surface of the part and form an image of the discontinuity.

-Equipment costs are relatively low.

Disadvantages:

-Only ferromagnetic materials can be inspected.

-Proper alignment of magnetic field and defect is critical.

-Large currents are needed for very large parts.

-Requires relatively smooth surface.

-Paint or other nonmagnetic coverings adversely affect sensitivity.

-Demagnetization and post cleaning is usually necessary

2.3.3. Ultrasonic testing:

A typical UT inspection system consists of several functional units, such as the pulsar/receiver, transducer, and display devices. A pulsar/receiver is an electronic device that can produce high voltage electrical pulse. Driven by the pulsar, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. As shown in 15. The reflected wave signal is transformed into electrical signal by the transducer and is displayed on a screen. The reflected signal strength is displayed versus the time from signal generation to when an echo was received. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size

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