ELECTRO SLAG WELDING

 1. What is Welding?

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FIG-1: Welding images

     Welding is a materials joining process in which two or more parts are coalesced at their contacting surfaces by a suitable application of heat and/or pressure. Many welding processes are accomplished by heat alone, with no pressure applied; others by a combination of heat and pressure; and still others by pressure alone, with no external heat supplied. In some welding processes a filler material is added to facilitate coalescence. The assemblage of parts that are joined by welding is called a weldment. Welding is most commonly associated with metal parts, but the process is also used for joining plastics.
In addition to melting the base metal, a filler material is typically added to the joint to form a pool of molten material (the weld pool) that cools to form a joint that is usually stronger than the base material. Pressure may also be used in conjunction with heat, or by itself, to produce a weld. Welding also requires a form of shield to protect the filler metals or melted metals from being contaminated or oxidized.
Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding may be performed in many different environments, including in open air, under water, and in outer space. Welding is a hazardous undertaking and precautions are required to avoid burns, electric shock, vision damage, inhalation of poisonous gases and fumes, and exposure to intense ultraviolet radiation.

2. Evolution of welding process:
Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for millennia to join iron and steel by heating and hammering. Arc welding and oxy-fuel welding were among the first processes to develop late in the century, and electric resistance welding followed soon after. Welding technology advanced quickly during the early 20th century as the world wars drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like SMAW (SMAW (Shielded Metal Arc Welding)), now one of the most popular welding methods, as well as semi-automatic and automatic processes such as MIG (Metal Inert Gas) Welding or GMAW (Gas Metal Arc Welding), SAW (Submerged Arc Welding), TIG (Tungsten Inert Gas) or GTAW (Gas Tungsten Arc Welding) FCAW (Flux-Cored Arc Welding) and ESW (Electro Slag Welding). Developments continued with the invention of laser beam welding, Electron Beam Welding, Magnetic Pulse Welding (MPW), and friction stir welding in the latter half of the century.
Today, the science continues to advance. Robot welding is commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality.

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FIG-2: Diagram of arc and weld area, in shielded metal arc welding. 1. Coating Flow 2. Rod 3. Shield Gas 4. Fusion 5. Base metal 6. Weld metal 7. Solidified Slag.

FIG-3: Diagram of different welding structure

3. Classification of Welding:
Welding processes divide into two major categories:
(1) Fusion welding, in which coalescence is accomplished by melting the two parts to be joined, in some cases adding filler metal to the joint; and
(2) solid-state welding, in which heat and/or pressure are used to achieve coalescence, but no melting of the base metals occurs and no filler metal is added.

Fusion welding is by far the more important category. It includes
(1) Arc welding, (2) Resistance welding, (3) Oxy-fuel gas welding, and (4) Other fusion welding processes

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FIG-4: Classification of Welding Process

4. What is Arc Welding?
Arc welding is one of several fusion processes for joining metals. By applying intense heat, metal at the joint between two parts is melted and caused to intermix – directly, or more commonly, with an intermediate molten filler metal. Upon cooling and solidification, a metallurgical bond is created. Since the joining is an intermixture of metals, the final weldment potentially has the same strength properties as the metal of the parts.

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FIG-5: Schematic Layout of Arc Welding

In arc welding, the intense heat needed to melt metal is produced by an electric arc. The arc is formed between the actual work and an electrode (stick or wire) that is manually or mechanically guided along the joint. The electrode can either be a rod with the purpose of simply carrying the current between the tip and the work. Or, it may be a specially prepared rod or wire that not only conducts the current but also melts and supplies filler metal to the joint. Most welding in the manufacture of steel products uses the second type of electrode.
a) Heat generated by a welding arc
(J) = Arc voltage (V) X Arc current (A) X Welding time (s)

b) If arc is moving at speed S (mm/min) then net heat input is calculated as:

Hnet= VI (60)/(S X 1000) kJ/mm

c) Welding heat input calculation:

  1.         Hinput = A x V x 0.06/S.
    Welding current A in Amps, Arc voltage V in volts & the welding speed in mm/min are the key elements of this calculation.
    5. Welding temperature:
    The arc welding is a fusion welding process in which the welding heat is obtained from an electric arc struck between the work(or base metal) and an electrode. The temperature of the heat produced by the electric arc is of the order of 6000°C to 7000°C. Both the direct current (D.C) and alternating current (A.C) may be used for arc welding, but the direct current is preferred for most purposes. Generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of about 3100 °C. In metal arc welding, the arc is produced between the metal electrode (also called filler rod) and the work piece. During the welding process, the metal electrode is melted by the heat of the arc and fused with the work piece. The temperature produced by the heat is about 2400° C to 2700° C
    6. Limitation of conventional and other welding process:
    1. Slow production rate
    a. Productivity rate =[work quantity]/activity duration*resource usage
    2. Thicker and longer plates can’t be welded economically in a single pass.
    3. Less deposition rate
    a. Rate that weld metal can be deposited by a given electrode or welding wire, typically expressed in lbs/hr or kg/hr. It is based on continuous production, not allowing time for stops/starts/cleaning or inserting new electrodes.
    b. Deposition Rate is directly proportional to the welding current being used.
    c. On a Constant Current Machine – increasing the amperage increases the deposition rate
    d. For a constant voltage machine – increasing the wire feed speed increases the deposition rate
    4. Deposition efficiency
    a. Relationship of the weight of weld metal deposited vs. the electrode consumed in making the weld. Mostly defined as a percentage.
    b. Example: 100lbs of coated electrodes with an efficiency of 65% will result in 65lbs of weld metal deposited.
    5. Extremely high deposition rates can’t be achieved (10-20 kg/hr electrode)
    6. Flux consumption high.
    7. Spattering and arc flashing occurs.
    a. A very common occurrence in gas metal arc welding (GMAW) is the creation of what welders call “spatter,” which is essentially droplets of molten material that are generated at or near the welding arc. Spatter is generally regarded as a nuisance and is a critical factor to consider when developing an application

7. Typical pain area in welding:

 

FIG-6: Typical component of ship where high volume welding is required.
RESEARCH BEGAN FOR THE INVENTION OF SUCH WELDING PROCESS WHERE THICKER PLATE CAN BE WELDED WITH EXTREMELY HIGH DEPOSITION RATE WITH HIGH PRODUCTION RATE.
8. History:
The process was patented by Robert K Hopkins in the United States in February 1940 (patent 2191481) and developed and refined at the Paton Institute, Kiev, USSR during the 1940s. The Paton method was released to the west at the Bruxelles Trade Fair of 1950. The first widespread use in the U.S. was in 1959, by General Motors Electromotive Division, Chicago, for the fabrication of traction motor frames. In 1968 Hobart Brothers of Troy, Ohio, released a range of machines for use in the shipbuilding, bridge construction and large structural fabrication industries. Between the late 1960s and late 1980s, it is estimated that in California alone over a million stiffeners were welded with the electro slag welding process. Two of the tallest buildings in California were welded, using the electro slag welding process – The Bank of America building in San Francisco, and the twin tower Security Pacific buildings in Los Angeles. The Structural Steel welding industry is well aware that, over one billion dollars in crack repairs were needed, after the Northridge earthquake, to repair weld cracks propagated in welds made with the gasless flux cored wire process. Not one failure or one crack propagation was initiated in any of the hundreds-of-thousands of welds made on continuity plates welded with the Electro slag welding process.
However the Federal Highway Administration (FHWA) monitored the new process and found that electro slag welding, because of the very large amounts of confined heat used, produced a coarse-grained and brittle weld and in 1977 banned the use of the process for many applications. The FHWA commissioned research from universities and industry and Narrow Gap Improved Electro Slag Welding (NGI-ESW) was developed as a replacement. The FHWA moratorium was rescinded in 2000.
9. Introduction to Electro Slag Welding:
Electro slag welding (ESW) is a highly productive, single pass welding process for thick (greater than 25 mm up to about 300 mm) materials in a vertical or close to vertical position. (ESW) is similar to electro gas welding, but the main difference is the arc starts in a different location. An electric arc is initially struck by wire that is fed into the desired weld location and then flux is added. Additional flux is added until the molten slag, reaching the tip of the electrode, extinguishes the arc. The wire is then continually fed through a consumable guide tube (can oscillate if desired) into the surfaces of the metal work pieces and the filler metal are then melted using the electrical resistance of the molten slag to cause coalescence. The wire and tube then move up along the work piece while a copper retaining shoe that was put into place before starting (can be water-cooled if desired) is used to keep the weld between the plates that are being welded.
Electro slag welding is used mainly to join low carbon steel plates and/or sections that are very thick. It can also be used on structural steel if certain precautions are observed, and for large cross-section aluminum bus bars.
This process uses a direct current (DC) voltage usually ranging from about 600 A and 40-50 V, higher currents are needed for thicker materials.

FIG-7: Schematic Layout of Electro Slag Welding
10. Working principle of Electro slag Welding
Electro slag welding (ESW) is a fusion-welding Process in which coalescence is achieved by hot, electrically conductive molten slag acting on the base parts and filler metal as shown in Figure 8, the general configuration of (ESW) is similar to electro gas welding.

FIG-8: Schematic Layout of Electro Slag Welding
It is performed in a vertical orientation (shown here for butt welding), using water-cooled molding shoes to contain the molten slag and weld Metal. At the start of the process, granulated conductive flux is put into the cavity. The Consumable electrode tip is positioned near the bottom of the cavity, and an arc is generated for a short while to start melting the flux. Once a pool of slag has been created, the arc is extinguished and the current passes from the electrode to the base metal through the Conductive slag, so that its electrical resistance generates heat to maintain the welding Process. Since the density of the slag is less than that of the molten metal, it remains on top to protect the weld pool. Solidification occurs from the bottom, while additional molten metal is supplied from above by the electrode and the edges of the base parts. The process gradually continues until it reaches the top of the joint.

FIG-9: Schematic Layout of Electro Slag Welding preparation

The temperature of this molten slag pool is approximately 16500°C at the surface and 1950°C inside, under the surface which is sufficient to weld thick sections in a single pass. Several electrodes are used for longer welds so that the heat is more uniformly spread. Water-cooled shoe or copper dam plate fastened to the sides of the work piece prevents the molten metal from running off. These plates also assist the solidification process by transferring heat and move up as the weld progresses.
Generally, combination of oxides of silicon, manganese, titanium, calcium, magnesium and fluorspar are used as flux in this process. It also shields the molten metal and clears the impurities from the molten metal.
11. Steps in Electro Slag Welding:
11.1. Prior to welding the gap between the two work pieces is filled with a welding flux.
11.2. First current is flow between welding electrode and base plate. This establishes an arc between electrode and base plate which heat the flux or filler wire.
11.3. This heat leads to melt the filler metal and deposits into the weld cavity.
11.4. The filler metal continuously provide through roller arrangement as shown in figure.
11.5. Now the cooled copper shoe comes into action and start solidified this filler metal into weld cavity. This will made to avoid flowing out the weld metal.
11.6. As the filler metal solidified into weld cavity, the current flow through it. It will generate heat due to electric resistance. This heat is further use to continuous melting down the filler metal into weld Cavity.
11.7. During welding both the copper shoe and feed mechanism moving upward until the whole cavity is formed.
11.8. This will create a strong joint in single pass. The single or multi-pass weld is used according to plate thickness.

Wire feeder Feed roller Drive motor Provide oscillation by rack and pinion motion

Retaining Blocks maintain the slag and molten metal with in the cavity

Solid copper and water cooled copper shoe travel upward as welding progress

For low melting point graphite or steel can be used Maintain the slag and molten metal with in the cavity

Welding head and Controls: It Consists: – wire feeder Electrode supply Wire guide tube Oscillation drive

Consumables Electrode: Solid and metal-cored Wire range 1.6 to 4.0 mm diameter.

12. Welding Terminology:
Electrodes: In arc welding, an electrode is used to conduct current through a work piece to fuse two pieces together. Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding.
Generally two types of electrodes that are solid and metal-cored are used. Though solid electrodes are more popular than the metal-cored electrodes.

FIG-6: Electrode of Arc Welding
Flux: In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of flux is to prevent oxidation of the base and filler materials. For example, tin-lead solder attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures.
Flux is the most important consumable material of Electro slag welding. In its molten state it transforms the electrical energy into heat energy which helps in melting the electrode wire and the base metal to form a weld joint. It is also required to protect the molten weld metal from the atmosphere and to ensure stability. The flux in its molten state is required to conduct electricity but at the same time it should offer sufficient resistance to its flow for generating enough heat to do welding.

3. Electrode Guide Tube: It is used to guide the electrode wire at desired position where the welding is to be done. Ceramic guides are very precise and, in addition to presenting the cooling hole, they have a dual ceramic cores, at the both end of guide which ensures the perfect isolation from the electric discharge between tube and guide. Brass, copper and tungsten carbide tubes, both in single hole and in several multichannel solutions:
• 2 channel, normal type or with bigger holes;
• 3 channel Y type, normal type or with bigger holes;
• 4 channel.

13. Images of Electro slag Welding machines :

14. Electro Slag Welding variables,
14.1. Welding Current: it depend upon welding voltage and Electrode feed rate. Increases with increase in wire feed rate. Increasing in current means increasing in welding speed. Due to increase in welding speed depth of penetration will reduce and lack of fusion is likely occur. It may cause cracking Max level of current used bellow 500 A for wire dia 3.2 mm and bellow 400 A for 2.4 dia. It depend upon welding voltage and Electrode feed rate

14.2. Welding Voltage: Welding voltage Effect the depth of penetration and stability of process. Excessive voltage may cause overheating of metal, gassing of slag pool and even sparking. With Low voltage electrode may short-circuit to the pool of molten metal. Selection of voltage is governed by the type of flux used and is usually 32 to 55 volt per electrode. Higher voltage is used with thicker section. Effect the depth of penetration and stability of process.

14.3. Electrode diameter: Electrode diameter Greater the diameter of electrode more the depth of penetration and the suitable operation of process. In such cases use is made of electrode plates instead of large diameter wires. Greater the diameter of electrode more the depth of penetration and the suitable operation of process. In such cases use is made of electrode plates instead of large diameter wires.
14.4. Electrode extension: Electrode Extension is the distance between contact tube and slag pool surface is referred to as “dry electrode extension”. And length of electrode dipped in slag is called “wet extension”. Electrode extension of 50-75mm are normally used. Bellow 50 mm resulting in overheating of contact tube and more than 75 mm resulting in overheating of electrode because of increasing in electrode resistance leads to melt the electrode at the surface of slag pool instead of inside it.
14.5. Electrode oscillation: Plates up to 75 mm thick can be welded without oscillation but with high voltage. To achieve better fusion it is necessary to oscillate the electrode horizontally across the thickness. Oscillation speed normally varies between 10-40 mm/sec. Increasing in speed result in reduce weld width. Plates up to 75 mm thick can be welded without oscillation but with high voltage. To achieve better fusion it is necessary to oscillate the electrode horizontally across the thickness. Oscillation speed normally varies between 10-40 mm/sec. Increasing in speed result in reduce weld width
14.6. Slag Pool depth: Slag Pool depth Excessive pool depth resulting slag inclusion. Lead to reduce weld penetration. Too shallow result arcing on the slag surface. Optimum depth of weld pool is about 40 mm and the range can be used between 25 mm to 60 mm. Excessive pool depth resulting slag inclusion. Lead to reduce weld penetration. Too shallow result arcing on the slag surface. Optimum depth of weld pool is about 40 mm and the range can be used between 25 mm to 60 mm.
14.7. Number of electrode and their spacing: One electrode is commonly used to make welds on materials with a thickness of 25 to 75 mm (1 to 3 in), and thicker pieces generally require more electrodes. The maximum work piece thickness that has ever been successfully welded was a 0.91 m (36 in) piece that required the simultaneous use of six electrodes to complete.

14.8. Root gap: Root gap it effect the depth of penetration Decrease in root gap result in decrease in penetration and vice-versa. Narrow gap increase in short-circuit. Large gap required an extra amount of filler metal. Feasible root gap should between 20-35 mm. It effect the depth of penetration Decrease in root gap result in decrease in penetration and vice-versa. Narrow gap increase in short-circuit. Large gap required an extra amount of filler metal. Feasible root gap should between 20-35 mm
14.9. Vertical Angle:

15. Applications
 Fabrication of high pressure vessels, frames of heavy mechanical and hydraulic presses.
 Rolling mill frames, ship hulls, locomotive frames, etc.
 It is used in heavy industries where plate thickness is up to 80 mm to be joined.
 Welding of thick walled large diameter pipes is done by this welding process.
 Welding of storage tanks is done by it.
 It is used to construct big and thick parts of ships. Applications of ESW Welding of structure, machinery, ships, pressure vessel and casting. Applicable to long butt weld. Welding of structure, machinery, ships, pressure vessel and casting. Applicable to long butt weld.

16. Advantages of Electro Slag Welding:
16.1. Benefits of the process include its high metal deposition rates—it can lay metal at a rate between 15 and 20 kg per hour (35 and 45 lb/h) per electrode.
16.2. Ability to weld thick materials. Many welding processes require more than one pass for welding thick work pieces, but often a single pass is sufficient for electro slag welding.
16.3. The process is also very efficient, since joint preparation and materials handling are minimized while filler metal utilization is high.
16.4. The process is also safe and clean, with no arc flash and low weld splatter or distortion.
16.5. Electro slag welding easily lends itself to mechanization, thus reducing the requirement for skilled manual welders.
16.6. Low slag consumption (about 5% of the deposited metal weight);
16.7. Low distortion.
16.8. Unlimited thickness of work piece.
17. Disadvantages of Electro Slag welding:
17.1. Coarse grain structure of the weld: Less Strength and toughness

17.2. High HAZ (Heat Affected Zone):

Figure 53. HAZ CRACKS IN ESRW RESULTING FROM EXCESSIVE WELD SPEEDS.
17.3. Low toughness of the weld;
17.4. Only vertical position is possible.
17.5. For joints below 60 mm electro slag welding is less economical as compared to submerged arc welding.
17.6. Cylindrical welds are difficult to be closed.
18. Hardness Pattern:

19. WELD FINISHING
Chemical products are available for improving the surface finish and corrosion of resistance or Arc welds. These include root flux for root protection during welding, pickling paste for post weld treatment and neutralization paste for neutralizing pickling residues to pH value 7-10, facilitating rapid precipitation of metallic.
a. Root flux
Root flux is used for weld root protection during welding of tube joints and other cases when it is impractical or accessibility restricts the use of shielding gases. When shielding gas cannot be used root flux minimizes the risk of porosity, weld oxides and burn-through. The root flux is brushed on approximately 20 mm on each side of the joint. During welding the flux is activated and creates a thin layer of protective slag. When the tubes are in operation, the slag will be washed off by the process fluid.
b. Pickling paste
The optimal corrosion resistance in stainless steel weld metals and heat affected zones (HAZ) is achieved when the weld oxide and chromium depleted region are removed. This is achieved by brushing, grinding or pickling. Pickling gives the best corrosion resistance.
c. Pitting resistance potential
The pickling paste is thixotropic, which means it is easy to apply in different welding positions and does not drip or splash.
d. Neutralization paste
After pickling, aggressive acid residues always remain on the weld. These residues are easily neutralized with Sandvik Neutralization paste. Corrosion at a later date can thereby be avoided and the best possible environmental care achieved. The neutralization paste is thixotropic, which means it is easy to apply in various welding positions and it can afterwards be flushed off safely with water.

20. Determining Total Cost of Weld
Total Weld Cost= Total Arc Time + Non Arc Time + Filler Metal
How much will it cost to make the weld? (Total Arc Time)
Determine Weld Metal Volume Needed
Determine Deposition rate for Given Process
Calculate Total Time needed to make weld
Manpower Cost
Investment cost
Electricity Cost
Depreciation Cost
Area Rent Cost
Utility Cost

Toe Length’s will be Equal to Sides of a Triangle
Calculation = Leg X Leg ÷ 2
EX. Equal Leg ¼” Fillet Area = .03125 Sq.In
Volume = Length X Area
¼” Fillet Weld 100’ Long = (.03125 X 1200) = 37.5 Cu. In
Volume = Total Area X Density = 37.5 X .283 = 10.6125Lbs

Reference:
1. Books by Michel Groovers, Manufacturing Process
2. Brown Brother’s Journal on Ship Building and Different Manuals
3. Google
4. Wikipedia

INDRAJIT SANTRA

2 Comments

  1. KAUSIK RAY says:

    Very informative and in one place..

    Liked by 1 person

    1. Thank you Kaushik Da

      Like

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