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:
2.1.2. ESSC Equipments and Machines:
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.
ESSC nozzle:
Main operations of 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:
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:
IMPROPER BEAD SHAPE:
Improper bead shape defect is shown in figure 10.
Reasons:
Solution:
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:
Solutions:
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:
Solutions:
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:
VALLEY:
Reasons:
Solution:
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:
Acceptance standard:
Advantages:
Disadvantages:
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:
Disadvantages:
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:
Disadvantages:
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:
Advantages:
Disadvantages:
2.3.5. Ferrite Test:
Why ferrite measurement is needed?
Acceptable limit of ferrite content for SS O/L weld:
HOW to function:
Correction factors:
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:
2.5. APPLICATIONS of ESSC:
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:
After welding:
REFERNCE:
