Electroslag strip cladding

ABSTRACT:

Technological advancements have driven up temperature and pressure serviced in the petroleum, chemical, pulp, and environmental protection. Industries, and increased the possibility of severe corrosion and wear in process pressure vessels. The industries must upgrade the corrosion and wear performance of these main important parts .Economic features as a rule will not allow fabricating components from solid high alloyed materials. As a result it is essential to surface non-alloyed or low alloy base materials with high-alloy cladding. The submerged arc welding(SAW) and electroslag welding (ESW) process are appropriate for applying welded deposits over large surface areas by means of strip electrodes .Both processes are using a granular flux material. A strip electrode, fed continuously, is liquefied and fused to the substrate. In contrast with other processes it is very effective 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 exercised. After the welding to examine the defects NDT’s are carried upon it.

1. INTRODUCTION:

Electro slag strip cladding is an advancement of submerged arc strip cladding, which has speedily established itself as a reliable high deposition rate procedure. ESW is an arc less technique using Joules Effect to liquefy the strip material. The heating is an outcome of current flowing through the strip electrode and a relatively shallow layer of liquid electro conductive slag as shown in figure 1. The penetration is lesser for ESW than for SAW since the molten slag pool is used to liquefy the strip and some of the flux material rather than as an arc between the strip electrode and the flux material. As a rule of thumb, electro slag strip surfacing decreases dilution by up to 50% in contrast with submerged arc strip surfacing for the same heat input with a significantly higher deposition rate. However, as a consequence of the lower dilution levels for ESW, new strip compositions have been built for ESW, in particular for applications where the purpose is to obtain a certain weld metal chemistry (such as 304L, 316L) in one layer.

DILUTION:

A significant feature to consider in ESSC is dilution. In any overlay procedure, the weld metal accumulated gets blended with the base metal in the molten state, thus giving a slightly leaner composition. Therefore, the properties of this element of the bead will be somewhat compromised because of the alteration in the chemical composition. The quantity of dilution can be determined by using the formula: –

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

As shown in the figure 2, the width ‘a’ of the bead will preserve the necessary properties of the clad material, as it practices no change in chemical composition. The quantity of dilution is dependent on the overlay-procedure and procedure variables such as current, travel speed and width of material that will be coupled and also the welding process. Unnecessary base metal dilution, as with other metals, can instilled cracking along the fusion line and must be channeled by using appropriate welding consumable and welding method. Dilution is also an important consideration for ascertaining the number of weld-layers to be imparted for corrosion resistance. Some multi-pass welded beads may have a little change in composition in each weld layer due to the process control and method employed. The filler metal’s first layer should be competent to tolerate at least up to 30% dilution and still should be proficient to produce an acceptable deposit. Welding parameters must be chosen in such a way that fusion with minimum dilution is produced.

2. METHODOLOGY:

2.1. ESSC

2.1.1. WELDING PARAMETERS:

• Attitude of the electrode: The electrode is usually located 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 acceptable to a certain extent, but this will produce 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 generally about 30 mm.
• Flux depth: The depth of the flux determines the width of the slag layer obtained. If the flux is not deep enough, the slag pool will be too shallow, causing improved arcing of the strip. If the flux is too deep, the flux will liquefy only in the middle. The slag pool would be cooled by the flux lying on it, causing deterioration in electrical conductivity. At this point, again, the end result will be increased arcing. Normal depth should be 30 mm.
• Current density: Because of the absence of an arc, the penetration in the ESW procedure is very shallow; this represents that there will be slight mixing of the filler metal with the melted parent material. It is possible, in evaluation with the submerged arc welding (SAW), 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 calculating 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 predictable 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 attain thin layers (below 0.15 in – mm) and the strip electrode would break up either wholly or partially from the front edge of the liquid slag. This would outcome in the increased arcing. When using wider electrode strips -120*0.5 mm, for instance—current of > 2500 A may be necessary.
• Welding voltage: The welding voltage influences the specific resistance of the liquid slag and will decide how far the strip electrode is to be submerged in the slag pool. Inadequate immersion in the weld pool will affect the process and turn out the process into unstable one. The welding voltage must be lowered as current ascends. A range of 24 to 26 V when operating at 1250 A, or 22 to 24 V for 2500 A, is normal. The precise 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 turn out to be 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 suitable adjustment of the magnetic fields at the north and south poles of the magnets will make it possible to affect the shape of the bead. The South Pole is always place at the left side in the welding direction. Using additional magnetic fields for steering purposes are not required for 60*0.5 mm electrodes. The geometry of the bead may be unfavorably influenced by welding near the ground connection. The two yokes of the magnet are placed 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. This is how we can neutralize the natural magnetic blow effect by accurate correction of the two auxiliary magnetic fields.

2.1.2. ESSC Equipments and Machines:

• Strip feed unit: High powered, geared motors are necessary to unwind the strip from the reel and advance the strip electrode in welding process. This is accurate particularly when using wide strip electrodes. Grooved feed rollers matching the thickness of the electrodes will be mandatory.
• Current pick-up unit: Extensive surface power transfer contact units are used, designed to fit the size of the electrode. At least one edge of the contact shoe must be subdivided into fingers which are independently pressed against the strip to make sure the consistent transfer of current across the entire thickness of the electrode. Off-center application of power will result in a non-symmetrical bead, especially when using wider electrodes. When using higher currents in continuous duty operation, it is suitable 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 usually delivered in coils. Strip measuring 60*0.5 mm, for instance, will weigh from 30 to 60 kg: the weight will augment consequently for wider strip. The strip coil must be engineered to take this load as shown in figure 3.
• Flux feed and removal of excess material: The granular flux material is usually fed out of the flux hopper, and only in front of the strip electrode. Through these means the uncovered slag pool is shaped behind the electrode. The resulting high temperature (2300 degree) ensures improved electrical conductivity within the slag and a procedure which is free of arcing from the strip to the base metal. The flux which is not fused can be vacuum extracted directly after the slag has congealed. It is done with the help of flux recovery or flux recycling machines.
• Remote amperage and voltage control: A necessary component in the strip feed unit is a control unit to keep the welding voltage and current stable. This apparatus must provide an sufficient degree of accuracy while monitoring the welding parameter. Usually 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 figure 4.
• Magnet steering: The slag pool is electro conductive; as a result, the slag pool is subjected to EM forces that tend to make it flow from the edges towards the centre of the molten pool resulting in narrower beads and unfavorable wetting angles. Slag removal becomes tougher and there will be more probability of LF in bead overlap area. Effect of different forces is shown in figure 5.

To counterbalance for this, magnetic devices are used. This magnetic field is produce by means of two solenoids. The position of solenoids is very important. The tips should be located beside the strip electrode at a distance of approximately 15mm from the strip edge and about 15 mm above the base material surface.

The form of the solidification ripples should be used to direct the intensity of the magnetic field. The criterion for accurate intensity of the magnetic field is when the solidification lines become symmetrical.

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

It is observable that the intensity on each solenoid has to be modified according to the working conditions on a specific w/p, taking into account any magnetic blow effect that cannot be considered previously. After the application of the device the shape of the bead is shown in figure 6.

• Welding power supply: The DC current needed for welding is supplied by regular potential rectifiers which are designed to have a plane slope. Discontinuous 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: Instead of the boom, for regulating minute distances we use cyclomatic slide. It provides both vertical and horizontal movements to the welding head for fine alterations. It uses a 12V DC motor for the purpose; motor rotates the lead screw which transforms it into the linear motion of the nut frame for the alterations.

ESSC nozzle:

Main operations of nozzle:

• To direct the strip and to maintain it I the required position during welding operation.
• To transfer welding current from power source to the strip by means of suitable contact between shoes and fingers.
• The ESSC nozzle is also equipped with a water-cooling option for strip thickness more than 60mm. nozzle

2.1.3. Welding Consumables:

FILLER METAL USED

In the framework of Electro Slag Strip Cladding, the filler metal refers to the strip used. The composition of the filler metal is chosen according to the necessities of the job and the dilution expected. The most common prerequisite for cladding is a corrosion resistant internal surface. For this purpose, there are variety of strips available, of distinctive sizes and distinctive compositions. The table1 gives details of the strips:

Table1: Types of strips with different sizes and composition.

ESSC Flux:

It is required to use fluxes with a high proportion of CaF2 in order to accomplish the good electrical conductivity desired for the slag at high temperatures while at the same time designing a procedure which is resistant to arcing. Moreover, the fluxes may not include any components which may form gases because they would interfere with the contact needed between the strip electrode and the liquid slag: arcing may result. Flux composition of the flux generally used during ESSC is shown in table 2.

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

Drying of flux: The fluxes are packed dry. Flux used is hygroscopic and will soak up moisture. It is suitable to dry the flux at 300- 400oC for 2 hours prior to use. Transitional storage of flux which has been dehydrated in this way should be in a furnace or kiln at 100oC. It is required to use fluxes with a high proportion of fluoride (Ca F2 ) in order to attain the good electrical conductivity desired for the slag at high temperatures while at the same time obtaining a process which is resistant to arcing. Moreover the fluxes may not contain any components which would generate gasses – calcium carbonate (CaCo3 ) for instance—since the gasses would interface with the contact needed between the strip electrode and the melted slag; arcing might result . Four fluxes are obtainable, all alike in composition. Marathon 449 gives low silicon pickup and is mainly appropriate 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 pointed, whereby the melted pools are directed to one side, evidenced by off-center rippling of the bead and an off- center end crater. The troubles which result there from become more critical with higher current levels and smaller work pieces. Rising current levels will reinforce the intrinsic magnetic field formed 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 advantage 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 nevertheless happen that, under adverse circumstances, magnetic blow phenomena could be encountered even when working with large components. Like electric current paths within the slag can be influenced by external magnetic fields. This will initiate modifications in the current density and in the temperature distribution within the slag and will eventually alter 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 consequently displaced outward in each case and beads were created which were thinner in the overlapped area than on the outer sides.

2.1.5. HEAT TREATMENTS

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

Depending on the parent material and the specification, a PHWT at 12750F (6800C) and 32 h will be mandatory. 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 shield against disbonding.

2.1.6. Disbonding

In the early 1970’s the Japanese had made experience with hydrogen induced disbanding of stainless steel weld overlay in a desulfurizing reactor. Because the vessel was overlayed by ESSC, this process had become skimmish at many oil companies and therefore the oil companies require tow pass cladding by SAW for that type of work.

Hydrogen in metals tends to concentrate at inclusion, voids and precipitation, or close to them, rather than in the matrix. After PWHT of the thick walled pressure vessels there is a concentration between the overlay and the parent metal.

There is, therefore, under certain conditions, more hydrogen at these precipitates than in the matrix of the first layer of the overlay. During operation of a hydrocracker or hydroesulphurizer (operating conditions of about 450 c and 15MPa) hydrogen diffuses through the stainless overlay and into the 2 ¼ Cr 1 Mo parent material. Diffusibility of hydrogen in stainless steel is lower than in the base metal but the solubility is lower in the base metal. As temperature increases so does the difference in behavior of hydrogen in the two materials. The steady state condition is that there is a higher concentration in the austenitic overlay than the ferritic base metal with a concentration gradient from the weld overlay to the base metal at the transition zone. In conditions of abnormal rapid temperatures and pressure reduction the hydrogen tries to leave the steel but due to the diffusion and solubility characteristics of the material is distributed across the weld overlay with a concentration at the weld-base metal interface. The pressure of precipitate aggravates the situation and hydrogen embrittlement, so called disbanding can occur.

2.1.7. Comparison between single and double layer ESSC:

SINGLE LAYER ESSC: In single layer ESSC, the chief purpose is to attain the desired chemistry with just a single layer of the weld metal. To realize this, the strip chemistry is used in such a way that the diluted weld metal meets the final chemistry in a single layer. As a result, the chemistry of deposited weld metal remains constant throughout the bead height. Nonetheless, 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. – requirements to be maintained.

DOUBLE LAYER ESSC: Characteristically, a weld overlay procedure consists of minimum TWO layers. The first layer is accumulated 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 flux material. Subsequent layer (s) is accumulated with the weld consumable of same chemistry as that of final requirement. Table 3 shows the parameter for single and double layer.

**The above data is for 60*0.5mm strip size.

Table3: Effect of parameters on single layer and double layer essc.

From the above table, it is very clear that Single Layer ESSC provides considerable advantages in terms of:

• Overall Cycle time (Welding Time + NDT Time)
• Welding Consumable Cost
• Overall Heat Input.
• Lesser distortion due to lesser overall heat input

2.1.8. Comparison between ESSC and SAW:

Table4: Comparison between ELECTROSLAG and SUBMERGED ARC technique.

2.1.9. WELD DEFECTS: The major defects are slag inclusion, improper bead shape, crater crack, centerline cracks, porosity, under cut.

SLAG INCLUSION:

Slag inclusions are nonmetallic solid material trapped in weld metal or between weld metal and parent metal. Slag inclusions are regions inside the weld cross section or at the weld surface where the once-molten flux used to shield the liquefied metal is mechanically entrapped within the solidified metal. This solidified slag corresponds to a portion of the weld’s cross-section where the metal is not fused to itself. This can result in a weakened situation which could impair the serviceability of the component. Inclusions may also appear at the weld surface. Like partial fusion, slag inclusions can arise between the weld and parent metal or between individual weld passes. In fact, slag inclusions are frequently associated with partial fusion. Slag inclusion defect is shown in figure9.

Solution:

• Adjust the parameter to get bead proper slope at toe.
• Maintain accurate bead overlap before start of the overlay. It should be in the 2-5 mm not more than it.
• Appropriate cleaning of the bead with power wire brushing and blades.

IMPROPER BEAD SHAPE:

Improper bead shape defect is shown in figure 10.

Reasons:

• Incorrect / wrong settings of Magnetic Stripping Device
• Generating jerks in the welding head/ tank rotator.
• Current fluctuation.
• Wrong bead overlap.
• Flux height may be very high.

Solution:

• Connect shoes face should be polished & tight appropriately.
• Check ear thing lug and power source.
• Establish appropriate current & polarity.
• Set accurate bead overlap.

CRATER CRACK:

These are minute cracks which appear at the end of the weld where the arc has been cracked. Though small, these cracks are bothersome since they can propagate into the weld bead. A crater crack is shown in Figure 11. The main cause for this defect is the wrong technique for ending the weld. To appropriately end a weld, the crater should be filled. This is done by reversing the arc travel direction before breaking the arc. This method is depicted in Figure 12. In addition, if the welding control is designed to provide gas for a short time after the arc is cracked, the crater should be shielded until it is wholly solidified.

CENTERLINE CRACK:

Reasons:

• Use of inappropriate strip.
• Use of inappropriate flux.
• Irregular bead slope in the overlap area.
• Lesser flux height.
• Inappropriate setting of molten pool.
• Irregular 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.
• Fix the parameter before starting the welding.
• Surface must be clean appropriately.

POROSITY:

Porosity is gas pores created in the solidified weld bead. As seen in Figure 13, these pores may differ in size and are usually distributed in a random manner. On the other hand, it is possible that porosity can only be found at the weld center. Pores can arise either under or on the weld surface. The most usual reasons of porosity are atmosphere contamination, excessively oxidized work piece surfaces, inadequate deoxidizing alloys in the wire and the presence of foreign matter. Atmospheric contamination can be resulted by:

• Inadequate shielding gas flow.
• Excessive shielding gas flow. This can produce aspiration of air into the gas stream. 3)Severely clogged gas nozzle or broken gas supply system (leaking hoses, fittings, etc.) 4)An excessive wind in the welding area. This can blow away the gas shield.

Solutions:

• Ensure the flux baking period before use.
• Clean appropriately 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:

As shown in Figure 14, undercutting is a defect that seems as a groove in the flux metal directly along the sides of the weld. It is mainly common in lap fillet welds, but can also be encountered in fillet and butt joints. This type of defect is most frequently caused by irregular welding parameters; particularly the travel speed and arc voltage. When the travel speed is too high, the weld bead will be very peaked because of its extremely fast solidification. The forces of surface tension have drawn the molten metal along the sides of the weld bead and piled it up along the center. Melted portions of the parent plate are affected in the similar way. The undercut groove is where melted parent material has been drawn into the weld and not allowed to wet back properly because of the rapid solidification. Decreasing the arc travel speed will slowly reduce the size of the undercut and finally eliminate it. When only small or intermittent undercuts are present, raising the arc voltage or using a leading torch angle is also corrective actions. In both situations, the weld bead will become flatter and wetting will become better.

Solutions:

• Use appropriate parameter.
• Regulate the all device at the time of the trial.

VALLEY:

Reasons:

• Drift in shell rotation
• Minute 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 technique used to evaluate an item to by observation, such as: the accurate 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 requirement covers the norms for acceptable surface defects on weld overlay made using different welding procedures.

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

Lighting: The light in the visual inspection area shall be enough to provide adequate contrast so that the detection of relevant objects and discontinuities is accomplished with a high degree of success. The essential 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 required lightning

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 verified under illumination.
• Variation in height of O/l b/w adjacent beads shall be less than 1.6 mm.
• No sharp corners allowable.
• Porosity, slag inclusion, spatter, mechanical damage not tolerable.
• If undercut >.5 mm the sharp edges shall be merged.
• Fisher/ crater cracks not allowable.

Advantages:

• No heavy or complicated equipments needed.
• Saves cost and time.
• Can be done everywhere .No restrictions of place.
• Some points can only be covered by this technique e.g. spatter, thickness etc.

Disadvantages:

• Only for macro surface defects can be detected.
• Manually done, so vulnerable to error.

2.3.2. Magnetic Testing:

This NDT technique is attained 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 errors produce magnetic poles or distort the magnetic field in such a way that the iron particles are attracted and concentrated. This creates a visible indication of defect on the surface of the material. Magnetic field lines around the crack and magnetic particles stretch over the crack is shown in figure 15.

Longitudinal Magnetization: When the length of a component is several times larger than its diameter, a longitudinal magnetic field can be formed in the component. The component is often positioned longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization method is often referred to as a “coil shot.” It is shown in figure 16.

Yoke for Longitudinal Magnetization

Circular Magnetization: When current is passed through a solid conductor, a magnetic field develops in and around the conductor. The subsequent statements can be made about the distribution and intensity of the magnetic field. The field strength fluctuates 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 consistent. (However, a larger conductor is capable of carrying more current.). Examples of Magnetic testing indication are shown in figure 17.

Advantages:

• Large surface areas of complicated parts can be checked quickly.
• Can detect surface and subsurface flaws.
• Magnetic particle signs are created immediately on the plane of the part and create an image of the discontinuity.
• Equipment costs are relatively low.

Disadvantages:

• Only ferromagnetic materials can be checked.
• Exact positioning of magnetic field and defect is crucial.
• Large currents are required for very large parts.
• Requires comparatively smooth surface.
• Paint or other nonmagnetic coverings unfavorably affect sensitivity.
• Demagnetization and post cleaning is generally required.

2.3.3. Ultrasonic testing:

A usual UT inspection method consists of several functional units, such as the pulsar/receiver, transducer, and display devices. A pulsar/receiver is an electronic apparatus that can produce high voltage electrical pulse. Driven by the pulsar, the transducer produces 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, fraction of the energy will be reflected back from the flaw surface. As shown in figure 18. The reflected wave signal is changed into electrical signal by the transducer and is displayed on a screen. The reflected signal strength is displayed against the time from signal generation to when an echo was received. Signal travel time can be directly correlated to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.

Couplant: A couplant is a material (usually liquid) that facilitates the transmission of ultrasonic energy from the transducer into the test specimen. Couplant is usually necessary because the acoustic impedance mismatch between air and solids, such as the test specimen, is large and, consequently, nearly all of the energy is reflected and very little is transmitted into the test material. Application of the couplant between transducer and the workpiece is as shown the figure 19.

The couplant displaces the air and makes it probable to get more sound energy into the test specimen so that a usable ultrasonic signal can be acquired. In contact ultrasonic testing a thin film of oil, glycerin or water is normally used and in immersion testing water is between the transducer and the test surface.

Advantages:

• It is sensitive to both surface and subsurface discontinuities.
• The intensity of penetration for fault recognition is better than other NDT techniques.
• Only single-sided access is required when the pulse-echo method is used.
• It is high accuracy in determining reflector position and calculating size and shape.
• Minimal part preparation needed.
• Electronic equipment provides immediate results.
• Detailed images can be created with automated systems.
• It has other uses such as width measurements, in addition to error detection.

Disadvantages:

• Surface must be accessible to transmit ultrasound.
• Skill and training is more extensive than with some other techniques.
• It generally needs a coupling medium to promote transfer of sound energy into test specimen.
• Materials that are rough, improper in shape, very small, exceptionally thin or not homogeneous are difficult to check.
• Cast iron and other coarse grained materials are difficult to check due to low sound transmission and high signal noise.
• Linear faults leaning corresponding to the sound beam may go unnoticed.
• Reference standards are essential for both equipment calibration, and characterization of errors.

2.3.4. Penetration Testing:

Penetrant solution is practiced to the surface of a pre-cleaned component. The fluid is drag into surface-breaking faults by capillary action. Excess penetrant material is cautiously dusted from the outside. A developer is practiced to drag the trapped penetrant back to the plane where it is spread out and create an indication. The indication is much simpler to notice than the actual error.

Steps of PT:

• Surface Preparation: One of the most essential steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering faults. The sample may also need etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear the surface of the sample, thus mitigating the defects.
• Penetrant Application: Once the surface has been systematically cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the parts in a penetrant bath as shown in figure 20.
• Penetrant Dwell: The penetrant is left on the surface for an adequate time to allow as much penetrant as possible to be drawn from or to seep into an error. Penetrant dwell time is the entire time that the penetrant is in contact with the part surface. Dwell times are normally recommended by the penetrant producers or needed by the specification being followed. The times change depending on the application, penetrant materials used, the material, the form of the material being checked, and the type of error being checked. Minimum dwell times normally range from 5 to 60 minutes. Normally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not permitted to dry. The ideal dwell time is often ascertained by experimentation and is often very specific to a certain application.
• Excess Penetrant Removal: This is a most delicate part of the inspection process because the surplus penetrant must be removed from the surface of the sample while removing as small penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treated with an emulsifier and then rinsing with water .
• Developer Application: A thin layer of developer is then applied to the sample to draw penetrant entrapped in faultss back to the surface where it will be noticeable as shown in figure 21. Developers come in a number of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).
• Indication Development: The developer is permitted to stand on the part surface for a period of time enough to permit the extraction of the trapped penetrant out of any surface faults as shown in figure 22. This development time is normally a minimum of 10 minutes and significantly longer times may be required for tight cracks.
• Inspection: Inspection is then performed under proper lighting to detect indications from any faults which may be present.
• Clean Surface: The final step in the procedure is to thoroughly clean the part surface to remove the developer from the parts that were found to be tolerable.

Advantages:

• Large quantity of parts can be checked quickly and at less cost.
• Parts with complex geometry are routinely checked.
• Indications are generated directly outside of the part giving a visual image of the discontinuity.
• Apparatus investment is minimal.

Disadvantages:

• Detects only surface breaking flaws.
• Surface grounding is unfavorable as contaminants can mask flaws.
• Needs a comparatively smooth and nonporous surface.
• Post cleaning is required to remove chemicals.
• Needs multiple operations under controlled situations.
• Chemical handling precautions are required (toxicity, fire, waste).

2.3.5. Ferrite Test:

Why ferrite measurement is needed?

• Ferrite is a non equilibrium structure, an outcome of very fast freezing and very rapid cooling.
• Ferrite in SS has proved as challenging and appealing to researchers as hydrogen in CS and LAS.
• The differences lies in the requirement to control hydrogen at the lowest possible level, so required to control ferrite within the specified range.
• After PWHT is the effect of ferrite decomposition products on the weld metal, in which case ferrite measurement in direct concern.
• AWS D4.2 is the standard for ferrite determination by magnetic technique for industrial use.
• Specifications of ferrite content should be made only in terms of FN and it should be taken as the mean value measurements.

Acceptable limit of ferrite content for SS O/L weld:

• Suitable limit of ferrite content in SS weld O/L is given by customer.
• Generally it is within 4-10 FN ferrite content (except UREA Grade/ NIL ferrite).

HOW to function:

• Ferrite is ferromagnetic, but austenite is not, and this has become the basis for magnetic -determination of ferrite at room temp.
• In working principle it uses the magnetic induction test procedure whereby the ferrite content is obtained from the magnetic permeability.
• Probe tip should be kept at a distance of 50mm from any metal object to prevent erroneous measurements
• The probe has to be positioned vertically on the surface of the measuring object
• The instrument is ready to measure again.

Correction factors:

• Curvature of the measuring instrument.
• Width of the measuring object.
• Layer width.
• Distance of the measuring position to the edge.

2.4. ADVANTAGES of ESSC:

Conventional welding procedure being used for overlay such as SMAW, FCAW, SAW, and SASC are all arc welding procedures. This procedure results in high dilution because of concentrated arc forces, which tend to produce a digging action on the parent metal, which is in molten form. This eventually affects the chemistry of the overlay, making it making to deposit more number of layers to attain the desired chemistry’s. This problem is not found in ESSC welding procedure. By controlling various interaction parameters of ESSC, dilution can be limited to 7-10%. This gives ESSC a huge advantage over the other overlay procedure in productivity. The further main advantages of ESSC are:

• Lower penetration level (about 0.5mm)
• Lower defect and rework possibilities.
• Better bead characteristics.
• Problem free operation.
• Higher Overall Productivity.
• Simple equipment (similar to SAW equipment)
• Lesser number of layers to attain desired chemistry.

2.5. APPLICATIONS of ESSC:

• Heavy plates, forgings and castings can be butt welded.
• Where plates or castings of consistent width are involved or if they taper at a consistent rate, electroslag welding has virtually replaced thermit welding, being much simpler.
• Subsequent alloys can be welded:
• Low carbon and medium carbon steels.
• High strength structural steels
• High strength alloy steels such as stainless steel and nickel alloys.
• Longitudinal stiffeners of the upper deck of ships.
• Longitudinal welds in cylindrical pressure vessels.
• Shells for blast furnaces and basic oxygen furnaces.

3. CONCLUSION:

Electro slag strip cladding is the most widely used welding procedure in the industry. Electro slag strip cladding is an advancement of submerged arc strip cladding, which has rapidly established itself as a reliable high deposition rate procedure. In ESSC for each application, the efficiency and quality of weld can be controlled by controlling the process variables: attitude of electrode, spacing of current contact, flux depth, current density, welding voltage, welding travel sound, supplementary magnetic fields. There are certain safety measureswhich are to be taken care of before and during welding.

Before welding:

• Inspect all the cable connection to the job and the welding head-allow no loose connection all cables be crimp with the lugs.
• Connect welding cables to the welding head using cu-strip. The cables shall be connected to the cu-strip in such a way that the cables are not exactly above the weld pool.
• The welding cables nearer to the weld pool must be appropriately insulated from heat using fiberglass tape.
• Ensures cleanliness of the surface to be overlaid. It should be free from the rust, dirt, grease, dents, pitting marks etc.
• Job to be suitably position on the tank rotator to eliminate drift.
• Inspect the smooth flow of strip through the contact shoe and contact surface.
• No flux or slag should be entrapped between the strip and the contacts – this will lead to scratches on the contact surfaces – thus damaging them.
• Inspect mounting arrangement of the spool to ensure consistent feeding without interruption aligns the spool holder in line with the weld.
• Each station must have 2 spool holders to reduce the spool change over time.
• Provide water-cooling arrangement for the contact shoe holder for 90 mm and 120 mm wide strips. The water-cooling hoses must be insulated from heat near the weld pool using fiberglass tape.
• Start preheating where needed.
• Magnet is to be switched on only if needed.

After welding:

• Strip should be cut with a sharp tapered end for simple arching.
• Position the strip to touch the job with its pointed cut end.
• Cover starting spot with flux on the front and rear of the strip.
• Switch on the magnet if needed.
• During welding flux is feeding from the side of the strip only. Adjust flux-feeding height to give strip layers of approximately 30 mm.
• Use only backed flux. Ensure flux is free from the particles after 2 – 3 seconds.
• Inspect the height each bead and ensure the required width. Typical height is 4 to 4.5 mm. Inspect and adjusts the travel speed if the bead height is more than 4.5 mm – excessive width of deposit shall lead to strip and flux shortage.
• Use SS wire brush or grinding wheels appropriate for interposes cleaning.
• During restart cut minimum length of strip to prevent unnecessary wastage.
• Mark the starting point by chalk on the shell while starting of weld.
• While completing the pass overlap the starting point by 10 m before stopping the welding.

REFERNCE:

• Kuskov, Yu. M., Skorokhodov, V. N., Ryabtsev, I. A., and Sychev I. S. 2001. Electroslag welding and surfacing. Science and Technology, p. 179.
• YU. M. KUSOV 2001. A New Approach to Electroslag Welding-Welding journal.
• R.D. Jr. Thomas, 1960. Electroslag Welding- A New Process for Heavy Fabrication, p.111 International journal.
• George E. Linnert, Welding Metallurgy, 2nd Edition
• T.W. Eagar W.S. Ricci 1982. A Parametric Study of the Electroslag Welding Process, p.397 Welding journal.
• B. E. Paton, Electroslag Welding, 2nd Edition
• F. Eichhorn J. Remmel B. Wubbels 1984. High Speed Electroslag Welding,p.37
• Y. M. Kuskov 2003. Welding, A New Approach to Electroslag-Welding journal.
• B. E. Paton 1997. Electroslag Welding: A Status Report -International journal.
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