Welding Processes and Weldability

• Welding joins similar or dissimilar metals by fusion with or without pressure or filler metal
• The fusion happens with heat, generated by gas combustion, electric arc/resistance, or chemical reactions
• Pressure is not always needed
• Welding makes permanent joints
• Welding affects the metallurgy of the components
• Post-weld heat treatment is needed for critical parts
• Welding is used for fabrication and repair in industries
• Typical applications include ships, pressure vessels, automobile bodies, offshore platforms, bridges, and pipes

Weldability

• Weldability is the ease a metal can be welded
• It depends on factors like metallurgical changes, hardness, gas evolution/absorption, oxidation, and cracking
• Plain low carbon steel (C-0.12%) has the best weldability
• Materials with high castability usually have low weldability

Terminological Elements

• Common welding joint terms include base metal, fusion zone, weld face, root face, root opening toe, and root

Edge Preparations

• Edges of joining surfaces are prepared before welding, with different preparations for butt joints

Welding Joints

• Common welding joints include lap joints and butt joints

Lap Weld Joints

• Single-Lap Joint:
  • Made by overlapping plate edges
  • Not recommended due to low bending resistance
  • Suitable for joining cylinders that fit inside each other
• Double-Lap Joint:
  • Stronger than single-lap
  • Requires twice as much welding
• Tee Fillet Weld:
  • Widely used
  • Avoid if an alternative design exists

Butt Weld Joints

• Single-Vee Butt Weld:
  • For plates up to 15.8 mm thick
  • Vee angle depends on the technique
  • Plates spaced approximately 3.2 mm
• Double-Vee Butt Weld:
  • For plates over 13 mm thick
  • Welding on both sides of the plate
  • Top vee angle at 60° or 80°
  • Bottom angle at 80°, depending on the technique

Welding Positions

• Welding positions include flat/down hand, horizontal, vertical, and overhead

Flat or Downhand Welding

• Welding performed from the upper side of the joint
• Weld face is approximately horizontal
• Simplest and most convenient position
• Produces excellent welded joints at a fast speed and minimum fatigue

Horizontal Welding

• Workpiece plane is vertical
• Deposited weld head is horizontal
• Metal deposition rate is high
• Commonly used in welding vessels and reservoirs

Vertical Welding

• Workpiece plane is vertical
• Weld is deposited on a vertical surface
• Difficult to produce satisfactory welds due to gravity
• Requires constant control
• Vertical-up welding is preferred for strength
• Vertical-down welding is used for sealing and sheet metal

Overhead Welding

• More difficult than vertical
• Greater pull of gravity on molten metal
• Flame counteracts gravity
• Workpiece plane is horizontal
• Welding is carried out from the underside
• Electrode is held upward
• Short arc and basic coated electrodes are recommended

Advantages of Welding

• More economical and faster than riveting, bolting, or casting
• Properly controlled welding makes permanent joints as strong or stronger than the base metal
• A wide variety of similar and dissimilar metals and alloys can be joined
• General welding equipment is not very costly
• Portable welding equipment is readily available
• Permits design freedom
• Can join jobs through spots, seams, end-to-end, and other configurations
• Can be mechanized

Disadvantages of Welding

• Results in residual stresses and distortion
• Welded joint needs stress relieving and heat treatment
• Emits harmful radiations (light), fumes, and spatter
• Jigs and fixtures may be needed
• Edge preparation is required
• Requires a skilled welder
• Heat alters the metallurgy of the welded joint

Welding and Allied Processes Classification

• Classified by heat source (blacksmith fire, flame, arc)
• Classified by type of interaction (liquid/liquid - fusion welding or solid/solid - solid state welding)
• Classified as plastic (forge) and fusion processes
• General Classifications:
  • Welding Processes
  • Allied Processes

Welding Processes

• Oxy-Fuel Gas Welding Processes:
  • Air-acetylene welding
  • Oxy-acetylene welding
  • Oxy-hydrogen welding
  • Pressure gas welding
• Arc Welding Processes:
  • Carbon Arc Welding
  • Shielded Metal Arc Welding
  • Submerged Arc Welding
  • Gas Tungsten Arc Welding
  • Gas Metal Arc Welding
  • Plasma Arc Welding
  • Atomic Hydrogen Welding
  • Electro-slag Welding
  • Stud Arc Welding
  • Electro-gas Welding
• Resistance Welding:
  • Spot Welding
  • Seam Welding
  • Projection Welding
  • Resistance Butt Welding
  • Flash Butt Welding
  • Percussion Welding
  • High Frequency Resistance Welding
  • High Frequency Induction Welding
• Solid-State Welding Processes:
  • Forge Welding
  • Cold Pressure Welding
  • Friction Welding
  • Explosive Welding
  • Diffusion Welding
  • Cold Pressure Welding
  • Thermo-compression Welding
• Thermit Welding Processes:
  • Thermit Welding
  • Pressure Thermit Welding
• Radiant Energy Welding Processes:
  • Laser Welding
  • Electron Beam Welding

Allied Processes

• Metal Joining or Metal Depositing Processes:
  • Soldering
  • Brazing
  • Braze Welding
  • Adhesive Bonding
  • Metal Spraying
  • Surfacing
• Thermal Cutting Processes:
  • Gas Cutting
  • Arc Cutting

Gas Welding

• Uses heat from the combustion of oxygen/air and fuel gas which includes acetylene, hydrogen propane or butane to join metal
• Intense heat melts and fuses metal edges, often with filler metal
• Acetylene is commonly used but other gases can be employed
• Oxy-acetylene flame is versatile with high heat

Oxy-Acetylene Welding

• Acetylene is mixed with oxygen and ignited
• The flame melts and joins the parent metal
• Oxy-acetylene flame reaches around 3300°C
• Can melt most ferrous and non-ferrous metals
• Filler metal rod is generally added

Types of Welding Flames

• Flame controls the welding joint and the welding process
• Flame requires proper size, shape, and condition for efficiency
• Three basic types of oxy-acetylene flames:
  • Neutral welding flame
  • Oxidizing welding flame
  • Carburizing welding flame

Neutral Welding

• Equal proportions of oxygen and acetylene
• Has a clear, well-defined inner cone and a slightly feathered outer flame
• Used for welding steel, stainless steel, copper, aluminum, and other common metals

Oxidizing Welding

• Excess oxygen compared to acetylene
• Inner cone is shorter and more pointed
• Flame is hotter than a neutral flame
• Used for welding brass and bronze

Carburizing Welding

• Excess acetylene compared to oxygen
• Three distinct flame zones: the inner cone, an acetylene feather, and an outer flame
• Used for welding high-carbon steel and for applying hard surfacing

Welding Equipment Safety

• Use welding goggles or face shields to protect eyes
• Wear gloves and protective clothing
• Provide adequate ventilation
• Remove flammable materials
• Inspect equipment for damage before use
• Use a spark lighter or stationary pilot flame

Cylinder handling safety

• Handle with care and store properly
• Valve protection caps should always be in place when the cylinders are not in use
• When transporting cylinders by a crane or derrick, use a cradle or suitable platform
• Never drop cylinder or permit them to strike against each other violently
• Do not release the cylinders valve forcibly
• Avoid tampering with safety devices in cylinder valves and regulators
• Keep cylinders away from heat, sparks, and flame; do not drape welding cables over them
• Never use cylinder lying on its side for it may spill acetone
• Never use a hammer or wrench to open or close cylinder valve if they are fitted with hand wheels
• Use a friction lighter for lighting and never use matches for lighting
• Store oxygen cylinders separately from fuel gas cylinders, and dry materials
• Do not store reserve stock of calcium carbide also with other materials.

Regulator handling safety

• Before connecting regulator to cylinder valve, stand to one side of outlet and open valve slightly for an instant to clear opening of dust to prevent from entering regulators
• Always release adjusting screw fully before the cylinder valve is opened
• Open cylinder valve slowly
• Always use a pressure regulator designed for the gas with which it is to be used
• Always see the threads on regulators should match the threads on the cylinder valves
• Regulators should always be repaired by trained personnel and should not be oiled
• Prevent oil and grease from coming into contact with regulators

Hose safety

• Repairs, calibrations and adjustments of regulators should be done by experts
• Do cracking before connecting pressure regulator to the gas cylinder
• Always inspect union nuts before using.
• Hose connections properly fitted and clamped
• Protect hoses from sparks, hot slag, hot workpieces, and open flames
• Remove all the dirt from hose
• Store hoses on a reel when not in use.
• Hoses shouldn't come in contact with grease
• The correct color hose should be used for oxygen (green/black) and acetylene (red)
• Never use oxygen hose for acetylene or vice versa
• Protect hoses from being trampled or run over
• Minimize fumes, gases, and dusts by improving ventilation or wearing respiratory equipment

Arc Welding

• Uses an electric arc between an electrode and a workpiece to weld metals
• Employs shielding gas, coatings, or fluxes to protect the weld pool
• Arc welding processes include Carbon Arc Welding, Shielded Metal Arc Welding, Flux Cored Arc Welding Gas Tungsten Arc Welding, Gas Metal Arc Welding Plasma Arc Welding, Atomic Hydrogen Welding, Electroslag Welding Stud Arc Welding, Electrogas Welding.

Arc welding safety

• An electric arc created between the electrode and the workpiece provides heat for the weld
• Most processes use shielding gas while others employ coatings or fluxes to prevent the weld pool from the surrounding atmosphere

Arc Welding Equipment

• Setup including switch box, terminals, welding machine, scales, hand wheel and apron
• Equipment has asbestos hand gloves, glasses strap, holder and hand shield
• Other equipment is the cable, hammer, wire brush, earth clamp and welding table

Arc Welding power source

• Direct current (DC) and alternating current (AC) are used
• DC welding uses generators driven by electric motor or internal combustion engines
• AC welding uses transformers

Power source selection

• Type of electrodes and metals
• Available power source (AC or DC)
• Required output
• Duty cycle
• Efficiency
• Initial and running costs
• Available floor space
• Versatility of equipment

Welding cables

• These conduct current from the power source, through the electrode and back
• Cables are insulated copper or aluminium

Electrode holder

• It is used for holding the electrode and conducting current
• Sizes range from 150 to 500 Amps

Welding Electrodes

• Electrodes are pieces of wire or rod with or without coatings
• An arc is between the electrode and the workpiece
• Electrodes are classified into consumable and non-consumable

Consumable

• Made of different metals and alloys
• Start melting when an arc is struck
• Act as a filler metal
• Bare electrodes consist of metal or alloy wire without flux coating
• Coated electrodes have flux coating that melts and prevents joint contamination, and are arc stabilizers

Non-consumable

• Made of high melting point materials like carbon, tungsten, etc
• Electrodes do not melt during welding
• Electrode length decrease over time because of oxidation
• Materials are copper coated carbon or graphite, pure tungsten, thoriated or zirconiated tungsten

Hand screen

• Hand screen used for eye protection and weld bead supervision

Other arc welding equipment

• Chipping hammers used for removing slag
• Wire brush used for cleaning the weld surface
• Protective clothing used to protect against heat

Carbon Arc Welding

• A graphite or carbon rod is used as a non-consumable electrode to create an electric arc
• Welding occurs with and without a filler
• Two types: single electrode and twin carbon electrode
• Single electrode arc welding uses DCSP to reduce disintegration, and for brazing, soldering, and repairing iron/steel castings
• Twin carbon arc welding uses AC to maintain temperature, and joins copper alloys as well as aluminum, nickel, zinc and lead alloys

Shielded Metal Arc Welding

• Commonly used arc welding process done manually by a welder
• Heat for welding is produced through an electric arc
• Electrodes are flux coated
• MMAW setup is depicted in Fig 17.9
• Multi pass beads are shown in Fig 17.19.

Arc Welding Advantages

• Carried out in any position with high weld quality
• Simplest of all arc welding processes
• Process finds many applications
• Big range of metals and alloys can be welded easily
• Very well employed for hard facing and metal resistance
• Joints are welded by flux shielded metal arc welding
• Portable and fairly low costs

Arc Welding Limitations

• Due to flux coated electrodes, chances of slag entrapment are higher
• Arc and metal transfer is not very clear
• Limited length of electrodes
• Progressed with the new electrode
• Slower compared to MIG welding

Arc Welding Applications

• Almost all commonly employed metals and alloys can be welded
• Commonly used in construction fabrication and maintenance
• Applications include Bridge construction, Automotive and Aircraft Industry, tanks and boilers, ship building and penstock joining

Electrode Coating Ingredients

• Covering coating consists of many materials
• Listed below are functions of the electrode coating ingredients
• Welding electrodes join similar/dissimilar metals like carbon steels, cast iron, aluminum, magnesium and stainless steel
• Slag forming are light weight and protect from contamination
• Arc stabilization helps with arc stability and helps strike the same
• Gas shielding (Cellulose, wood flour, starch, calcium carbonate) forms a shield around electrodes, arc and weld pool
• Deoxidizing refines the molten metal
• Coating limits spatter, allowing removal of slag.
• Alloying elements add properties and strength
• Iron powder improves arc appearance and speed
• Wire melts faster than the covering, sleeve arc constriction
• Coating radiation protection
• Proper coating resistance

Welding defects

• Lack of Penetration: Failure of metal, which is due to inadequate or incorrect edge penetration and technique
• Lack of Fusion: Filer melts to the parent metal is duo to fast travel
• Porosity: Group of holes in Metal
• Slag inclusion: From previous runs causes insufficient cleaning
• Undercuts- Fast Travel
• Cracking- Metals used are unstable
• Weld Bead Appearance
• Distortion - Caused by high cooling rate
• Blowholes - Holes are caused when Gas is trapped Burn through- Collapse due to head Excessive penetration