WELDING DATA MANAGEMENT – PART IV – D. GTAW & FCAW

S.K.Gupta.
B.E.,C.E.,FIE.,FIIW.,
MISNT.,MAE.,MITD.

INTRODUCTION

                         Welding is used extensively as a major process in fabrication and manufacturing products ranging from Nano components to massive steel structures. Since the inception of shielded arc welding process  in 1907, a proliferation of welding processes and related technologies have been developed now covering a wide range of materials, products and applications in practice.

                         At present, SMAW, GMAW,FCAW,  GTAW, SAW, PAW, FSW, EBW , Resistance  Welding etc are considered to be the basic processes, based on which specialized branch processes have been  developed and effectively applied.

                           It is imperative to mention here that along with the process, development, manufacture and application of welding equipment and consumables have multiplied astronomically.

                           In order to keep up the Quality Standards of the Process and products a large number of Destructive, Nondestructive, Analytical and Statistical Process Control Techniques are in regular practice.

                          International Standards, Codes and  Specifications in conjunction of different National Specifications have been formulated on every aspects of Process, Equipment, Consumables, Operators, Operation and Products.

                       All the above mentioned areas covering Welding have generated vast number of Information in the form of Data and also continuing to generate equal volume of it every period of time. It is not possible for any individual or even any manufacturing organization to store, access, retrieve and apply useful data to the best advantage. But, individually if we can identify our needs, store methodically and systematically welding data used in industries, updating these to the current standards and use  Computer Software as the Welding Management System benefits will show all round Production – Operation  Management especially where Welding is used as the main Manufacturing Process.

 PARTS OF THE ARTICLE

                   In the process of writing and compiling the Data for the Article it is observed that it has become a bit lengthy. Dividing the Article into FIVE  Parts will be more appropriate and as such the following Parts will constitute the Article.

Part – I  deals with the Framework of Data Based Management system  outlined very briefly.

Part – II deals with the Materials used in fabrication and related data.

 Part – III  provides  details of the Data content to formation of WPS,PQR and WPQ.

Part – IV  deals with the data on Welding Processes, Equipment and Consumables.

Part – V  illustrates and explains how the Welding Mananagement Software works .

WELDING DATA MANAGEMENT – PART IV – D. GTAW & FCAW.

GAS TUNGSTEN ARC WELDING. (GTAW)

FIG . GTAW – I GAS TUNGSTEN ARC WELDING PROCESS

General Principles

          TIG (Tungsten Inert Gas) welding also known as GTA (Gas Tungsten Arc) in the USA and WIG (Wolfram Inert Gas) in Germany, is a welding process used for high quality welding of a variety of materials, especially, Stainless Steel, Titanium and Aluminium.  inductor to provide a smooth output. AC and AC / DC Power Sources are of a single phase design. Other important functions in TIG power sources are…

TIG Welding Benefits

  • Superior quality welds
  • Welds can be made with or without filler metal
  • Precise control of welding variables (heat)
  • Free of spatter
  • Low distortion

Shielding Gases

  • Argon
  • Argon + Hydrogen
  • Argon/Helium

 Helium is generally added to increase heat input (increase welding speed or weld penetration). Hydrogen will result in cleaner looking welds and also increase heat input,however, Hydrogen may promote porosity or hydrogen cracking.

 TABLE. GTAW – I . DIFFERENCE BETWEENGMAW AND GTAW

TIG Welding Limitations

  • Requires greater welder dexterity than MIG or stick welding
  • Lower deposition rates
  • More costly for welding thick sections

Equipment

  • DC or AC / DC Power Source
  • TIG Torch
  • Work Return Welding Lead
  • Shielding gas supply line, ( normally from a cylinder )
  • Foot Control Unit ( common option )

Power Source

 TIG welding can be carried out using DC for Stainless Steel, Mild Steel, Copper, Titanium, Nickel Alloys etc and AC for Aluminium and its Alloys and Magnesium.

FIG GTAW II.  POWER SOURCE

   The Power Source is of a transformer design with or without a rectifier, with a drooping characteristic (constant current power source). The output is generally controlled by either a moving core within the main transformer of the power source or by using electronic control of power thyristors. mDC power sources could be of 1 phase or 3 phase design, with an

Arc Starting Circuit

                HF : – Sparks of high tension jump across the gap between electrode and workpiece rapidly to carry the welding current across to start welding in DC TIG welding.This will stop once the arc is struck, in AC TIG welding, this will normally continue to keep the arc alive as the AC output changes from a Positive half cycle to a Negative halfcycle and back again.

                Lift Arc : – The electrode is touched onto the workpiece, the TIG Torch switch or foot control switch is operated, the equipment circuits detect a short circuit on the output and allow only a very low current typically 5 – 8 amps to flow. The electrode is lifted off the workpiece, the equipment circuits now detect a voltage between electrode and workpiece and welding current strikes across that very tiny gap as the electrode lifts off and welding continues.

GTAW TORCHES

           GTAW welding torches are designed for either automatic or manual operation and are equipped with cooling systems using air or water. The automatic and manual torches are similar in construction, but the manual torch has a handle while the automatic torch normally comes with a mounting rack. The angle between the centerline of the handle and the centerline of the tungsten electrode, known as the head angle, can be varied on some manual torches according to the preference of the operator. Air cooling systems are most often used for low-current operations (up to about 200 A), while water cooling is required for high-current welding (up to about 600 A). The torches are connected with cables to the power supply and with hoses to the shielding gas source and where used, the water supply.

  FIG.GTAW – III. GTAW TORCH.   DISASSEMBLED TORCH

The internal metal parts of a torch are made of hard alloys of copper or brass so it can transmit current and heat effectively. The tungsten electrode must be held firmly in the center of the torch with an appropriately sized collet, and ports around the electrode provide a constant flow of shielding gas. Collets are sized according to the diameter of the tungsten electrode they hold. The body of the torch is made of heat-resistant, insulating plastics covering the metal components, providing insulation from heat and electricity to protect the Welder.

GTAW ELECTRODES

              The electrode used in GTAW is made of tungsten or a tungsten alloy. Tungsten has the highest melting temperature among pure metals, at 3,422 °C (6,192 °F). As a result, the electrode is not consumed during welding, though some erosion (called burn-off) can occur. Electrodes can have either a clean finish or a ground finish—clean finish electrodes have been chemically cleaned, while ground finish electrodes have been ground to a uniform size and have a polished surface, making them optimal for heat conduction. The diameter of the electrode can vary between 0.5 and 6.4 millimetres (0.02 and 0.25 in), and their length can range from 75 to 610 millimetres (3.0 to 24.0 in).

A number of tungsten alloys have been standardized by the International Organization for Standardization and the American Welding Society in ISO 6848 and AWS A5.12, respectively, for use in GTAW electrodes, and are summarized in the adjacent table.

  • Pure tungsten electrodes (classified as WP or EWP) are general purpose and low cost electrodes. They have poor heat resistance and electron emission. They find limited use in AC welding of e.g. magnesium and aluminum.
  • Thoriumoxide (or thoria) alloy electrodes offer excellent arc performance and starting, making them popular general purpose electrodes. However, thorium is somewhat radioactive, making inhalation of vapors and dust a health risk, and disposal an environmental risk.
  • Ceriumoxide (or ceria) as an alloying element improves arc stability and ease of starting while decreasing burn-off. Cerium addition is not as effective as thorium but works well,[28] and cerium is not radioactive.
  • An alloy of lanthanumoxide (or lanthana) has a similar effect as cerium, and is also not radioactive.
  • Electrodes containing zirconiumoxide (or zirconia) increase the current capacity while improving arc stability and starting while also increasing electrode life
  • Filler metals are also used in nearly all applications of GTAW, the major exception being the welding of thin materials. Filler metals are available with different diameters and are made of a variety of materials. In most cases, the filler metal in the form of a rod is added to the weld pool manually, but some applications call for an automatically fed filler metal, which often is stored on spools or coils.

                             The electrode used in GTAW is made of tungsten or a tungsten alloy, because tungsten has the highest melting temperature among pure metals, at 3,422 °C (6,192 °F). As a result, the electrode is not consumed during welding, though some erosion (called burn-off) can occur. Electrodes can have either a clean finish or a ground finish—clean finish electrodes have been chemically cleaned, while ground finish electrodes have been ground to a uniform size and have a polished surface, making them optimal for heat conduction. The diameter of the electrode can vary between 0.5 and 6.4 millimetres (0.02 and 0.25 in), and their length can range from 75 to 610 millimetres (3.0 to 24.0 in).

 TABLE GTAW – II GTAW ELECTRODE TYPES AND USES

TABLE- GTAW – III: Typical Manual Gas Tungsten Arc Welding Parameters (Flat Position)

Joint
Thickness
Tungsten
Electrode Diameter
Filler
Wire Diameter
Welding 
Current
Arc
Voltage
in mm in mm in mm Amps Volts
0.030-0.062 0.8-1.6 0.062 1.6 0.062 1.6 15-75 9-15
0.062-0.125 1.6-3.2 0.062/0.093 1.6/2.4 0.062/0.093 1.6/2.4 50-125 9-15
0.125-0.250 3.2-6.4 0.093/0.125 2.4/3.2 0.093/0.125 2.4/3.2 100-175 12-18
> 0.250 >6.4 0.093/0.125 2.4/3.2 0.093/0.125 2.4/3.2 125-200 12-18

 

TABLE – GTAW – IV GTAW GENERAL WELDING PARAMETERS

TABLE. GTAW – V. WELDING PARAMETERS FOR ALUMINIUM  ALLOYS

TABLE. GTAW – VI . TUNGSTEN ELECTRODE COLOUR CODING

TABLE. GTAW – VII . TUNGSTEN ELECTRODE COLOUR CODING

ADVANCED GAS TUNGSTEN ARC WELD SURFACING

TABLE. GTAW – VIII. EXPERIMENTAL TESULT FOR STRENGTH AND  S/N RATIO

EX. NO Welding Current (amp) WeLDING Speed (mm/sec) Distance of Electrode from the workpiece (mm) Strength (Mpa) S/N Ratio
1 130 3.2 2.3 34.68 12.11
2 130 3.5 2.4 40.35 2.57
3 130- 3.8 2.5 39.06 5.99
4 135 3.2 2.4 25.9 14.89
5 135 3.5 2.5 55.55 8.26
6 135 3.8 2.3 19.95 11.74
7 140 3.2 2.5 43.23 11.59
8 140 3.5 2.4 14.65 3.94
9 `140 3.8 2.3 34.47 9.24

 

TIG WELDING PARAMETERS

Tungsten Electrode Nominal Dia 2.4 mm
Filler rod Dia. 2.4 mm
Welding Current 80,90,100 and 120 amp
Welding Speed 0.3 cm/sec
Argon Flow 15 litres / min
Backing Flow 10 litres / min

PREPARATION OF TUNGSTEN ELECTRODE

Tungsten Electrode Geometry

FLUX CORED ARC WELDING

                   Flux cored arc welding,  (FCAW), is  evolved from the gas metal arc welding, or GMAW process to improve arc action, increased metal transfer, weld metal properties, and weld appearance. The heat is provided by an arc between a continuously fed tubular electrode wire and the workpiece. The major difference is that FCAW utilizes an electrode very different from the solid electrode used in GMAW. In fact, it is closer to the electrodes used in shielded metal arc welding, or SMAW or stick welding, except the flux is on the inside of a flexible electrode instead of on the outside of a very stiff electrode.  

                  Metal cored gas-shielded wire combines the high deposition rates of a flux cored wire with the high efficiencies of a solid wire. Productivity improves with the increased deposition rates and higher travel speeds, combine this with the minimal spatter and lack of clean up required due to the slag free welds and your labour costs will reduce. The up-front cost of metal cored wires is higher but filler metals only make up at most 10% of a weld’s cost and gas around 3%. In almost all applications, labour accounts for 85% of the total cost so any significant productivity increase from switching to metal-cored wire will out-weigh the increase in the filler metal cost.

               Concentrating the welding current through the outside tube of MCW results in a broader, bowl-shaped arc with finer droplets. This means MCWs fill imperfections and bridge gaps better, giving a quality weld with high structural integrity.

               In  addition, a quality metal-cored wire like KOB MX-A70C6LF, or NSSW’s SM-3A, runs much cleaner and smoother than solid wire, so not only extremely low spatter but less wear and tear on tips, liners, drive rollers, etc, leading to savings in other consumable costs as well.

              Welders usually prefer working with MCW as not only can they weld faster and smoother and easily produce great welds, their working environment is safer and healthier because of the much lower levels of fume

          The essential  process, equipment and consumable data requirement are on :

  • welding power source,
  • remote controller,
  • wire feeder,
  • welding torch,
  • welding wire , filler rod

EQUIPMENT for WELDING

                   The equipment used for FCAW is very similar to that used for GMAW. The basic arc welding equipment consists of a power source, controls, wire feeder, welding gun, and welding cables. A major difference between the gas-shielded electrodes and self – shielded electrodes is that the gas shielded wires also require a gas shielding system. This may also have an effect on the type of welding gun used. Fume extractors are often used with this process. For machine and automatic welding, several items, such as seam followers and motion devices, are added to the basic equipment.

POWER SOURCE

          The FCAW welding process needs a suitable and constant voltage power source (DC). FCAW equipment is generally more robust than GMAW plant and requires some skill to set up properly. Typical uses for the FCAW process include: heavy fabrication general engineering. FCAW has a better deposition rate and fusion than GMAW.

   FIG.  Power Source  Water-cooled gun.   Air Cooled Gun

Types of Current

                FCAW Process uses direct current, which can be connected in one of two ways: electrode positive (reverse polarity) or electrode negative (straight polarity). The electrically charged particles flow between the tip of the electrode and the work. Flux-cored electrode wires are designed to operate on either DCEP or DCEN.

SHIELDING GAS

The choice of the proper shielding gas for a specific application is based on:

  1. Type of metal to be welded
  2. Arc characteristics and metal transfer
  3. Availability
  4. Cost of the gas
  5. Mechanical property requirements
  6. Penetration and weld bead shape.

Carbon Dioxide

                The carbon dioxide shielding gas during welding breaks down into components  as carbon and oxygen. Because carbon dioxide is an oxidizing gas, deoxidizing elements are added to the core of the electrode wire to remove oxygen. The oxides formed by the deoxidizing elements float to the surface of the weld and become part of the slag covering. Some of the carbon dioxide gas will break down to carbon and oxygen

                If the carbon content of the weld pool is below about .05%, carbon dioxide shielding will tend to increase the carbon content of the weld metal. Carbon reducing  the corrosion resistance of some stainless steels, is a problem for critical corrosion applications.

                Extra carbon can also reduce the toughness and ductility of some low-alloy steels. If the carbon content in the weld metal is greater than about .10%, carbon dioxide shielding will tend to reduce the carbon content. This loss of carbon can be attributed to the formation of carbon monoxide, which can be trapped in the weld as porosity deoxidizing elements in the flux core, reducing the effects of carbon monoxide formation.

Argon-Carbon Dioxide Mixtures

             Argon and carbon dioxide are sometimes mixed for use with FCAW. A high percentage of argon gas in the mixture tends to promote a higher deposition efficiency due to creating less spatter. This mixture also creates less oxidation and lower fumes. The most commonly used argon-carbon dioxide mixture contains 75% argon and 25% carbon dioxide. This gas mixture produces a fine globular metal transfer that approaches a spray. It also reduces the amount of oxidation that occurs, compared to pure carbon dioxide. The weld deposited in an argon-carbon dioxide shield generally has higher tensile and yield strengths.

Argon-oxygen mixture

Argon-oxygen mixtures containing 1 or 2% oxygen are used for some applications. Argon-oxygen mixtures tend to promote a spray transfer that reduces the amount of spatter. A major application of these mixtures is in welding stainless steels where carbon dioxide can cause corrosion problems.

Electrode Specification

 An example of a carbon-steel electrode classification is E70T-4 where:

  1. The “E” indicates an electrode.
  2. The second digit indicates the minimum tensile strength in units of 10,000 psi 69Mpa).
  1. The third digit indicates the welding position. A “0” indicates flat and horizontal positions only, and a “1” indicates all positions.
  1. The “T” stands for a tubular (flux-cored) wire classification.
  2. The suffix “4” gives the performance and usability capabilities

When a “G” classification is used, no specific performance requirements are indicated. This classification is intended for electrodes not covered by another classification. The chemical composition requirements of the deposited weld metal for carbon steel.

FIG.  FCWA ELECTRODES

STEEL ELECTRODES

 An example of a low-alloy steel classification is ES1T1-Ni2 where:

  1. The “E” indicates an electrode.
  2. The second digit indicates the minimum tensile strength in units of 10,000 psi (69 Mpa). The mechanical property requirements for low-alloy steel electrodes are shown in Table.
  1. The third digit indicates the welding position capabilities of the electrode. A “0” indicates flat and horizontal positions only, and a “1” indicates all positions.
  1. The “T” stands for a tubular (flux-cored) wire classification.
  2. The fifth digit describes the usability and performance characteristics of the electrode. These digits are the same as used in carbon steel electrode classification but only EXXT1-X, EXXT4-X, EXXT5-X and EXXTS-X are used with low-alloy steel flux-cored electrode classifications.
  1. The suffix tells the chemical composition of the deposited weld metal The classification system for stainless steel electrodes is based on the chemical composition of the weld metal and the type of shielding to be used during welding. An example of a stainless steel electrode classification is E30ST-1 where:
  1. The “E” indicates an electrode.
  2. The digits between the “E” and the “T” indicate the chemical composition of the weld as shown in Table 11-10.
  1. The ‘T’ stands for a tubular (flux-cored) wire classification.
  2. The suffix indicates the type of shielding to be used

MECHANICAL PROPERTIES AND DEPOSIT COMPOSITION

FCAW –S  ELECTRODE USABILITY DESIGNATION

                                                                                                       

T-3 ELECTRODES

Ø DC +  Polarity

Ø Spray Type Transfer

Ø High Welding Speeds

Ø Single Pass Only

Ø Low Penetration

Ø Sheet Metal Application.

T-10 ELECTRODES

Ø DC – Polarity

Ø High Travel Speeds

Ø Single Pass Only

Ø Down Hand Only

Ø Unlimited Material Thickness

T-11 ELECTRODES

Ø DC- Polarity

Ø Spray Type Transfer

Ø General Purpose

Ø All Position

Ø High Speed

Multiple Pass

T-4 ELECTRODES

Ø Dc + Polarity

Ø Globular Type Transfer

Ø High Deposition

Ø Low Penetration

Ø Down Hand Only

T – 8 ELECTRODES                   

Ø DC – Polarity

Ø All Position

Ø Multiple Pass

Ø Good Low-Temperature Impacts

T-13 ELECTRODES

Ø DC – Polarity

Ø All Position (Except V U)

Ø Single Pass Only

Ø For Root Pass On Pipes

T-6 ELECTRODES

Ø DC + Polarity

Ø Spray Type Transfer

Ø Deep Penetration

Ø Good Low-Temperature Impacts

Ø Multiple Pass

Ø Down Hand Only.

T-7 ELECTRODES

Ø DC – Polarity

Ø High Deposition

Ø Down Hand Only (Large Dia)

Ø Multiple All Position (Small Dia)

Ø Multiple Pass.

T-14 ELECTRODES

Ø DC – Polarity

Ø All Position (Except V U )

Ø Single Pass Only

For Galvanized,, Aluminized and Other Coated Steels

 FLUX CORED WIRE PROCESS OUTCOME

ARC FORCE EQUILIBRIUM