ManPro

Section: Welding
Back to Welding
Welding

Welding Introduction

Quick Cheat Sheet

Summary

Welding joins materials by coalescence — usually with heat, sometimes with pressure or filler. Classified into FUSION (most arc processes) and SOLID-STATE (friction, ultrasonic, diffusion).

Key Points

  • Fusion zone (FZ): melted & solidified — cast structure
  • Heat-affected zone (HAZ): not melted but altered — coarse grain, partial transformation, fine grain regions
  • Weldability depends on chemistry — measured by Carbon Equivalent (CE)
  • Arc is a sustained electric discharge through ionised gas (plasma) at 5,000–20,000 °C
  • AWS classifies welds by type (groove, fillet) and position (1G flat … 4G overhead)
  • Joint types: butt, lap, T, corner, edge

Remember This

  • 1CE = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15 (IIW formula)
  • 2CE > 0.45 → preheat needed to avoid hydrogen cracking
  • 3HAZ is the WEAKEST link in most welds — controlled cooling prevents cracking
  • 4Heat input H = (V · I · 60) / (v · 1000) in kJ/mm
  • 5Arc temperature: 5,000–20,000 °C; fusion zone ≈ pure cast metal

Quick Formulas

Heat input

H = (V · I · 60) / (v · 1000)

Carbon equivalent (IIW)

CE = C + Mn/6 + (Cr+Mo+V)/5 + (Cu+Ni)/15

Overview of Welding

Definition

Welding is a joining process that produces coalescence (fusion) of materials by heating them to suitable temperatures with or without the application of pressure, and with or without the use of filler material.

Classification of Welding Processes

1. Fusion Welding (Autogenous Welding)

  • Materials are heated to molten state
  • No pressure applied
  • Filler material may or may not be added
  • Examples: Arc welding, Gas welding, Resistance welding

2. Solid State Welding (Pressure Welding)

  • Materials joined without melting
  • Pressure is applied
  • Examples: Forge welding, Friction welding, Ultrasonic welding, Diffusion welding

3. Brazing and Soldering

  • Filler material melts at lower temperature than base metal
  • Base metal does not melt
  • Joint formed by capillary action

Arc Welding Fundamentals

Definition

Arc welding is a fusion welding process where coalescence is produced by heating with an electric arc, with or without the application of pressure, and with or without filler metal.

Electric Arc Characteristics

Arc Formation

  • Electric arc is a sustained electrical discharge through ionized gas (plasma)
  • Temperature: 3,000°C to 20,000°C
  • Arc column consists of:
    • Cathode region (negative electrode)
    • Anode region (positive electrode)
    • Arc plasma column

Arc Initiation

  1. Touch Start: Electrode touches workpiece, then withdrawn
  2. High Frequency Start: High voltage, high frequency pulse ionizes gap
  3. Scratch Start: Electrode scratched across surface

Heat Generation in Arc Welding

Heat Sources

  1. Resistance heating at electrode-arc interface
  2. Anode heating (positive terminal)
  3. Cathode heating (negative terminal)
  4. Arc column heating

Heat Distribution

  • Approximately 2/3 of heat goes to anode
  • Approximately 1/3 of heat goes to cathode
  • This affects electrode polarity selection

Electrode Polarity

Direct Current Electrode Positive (DCEP) / Reverse Polarity

  • Electrode connected to positive terminal
  • Workpiece connected to negative terminal
  • More heat at electrode (2/3)
  • Used when: deep penetration not required, thin materials, electrode melting rate should be high

Direct Current Electrode Negative (DCEN) / Straight Polarity

  • Electrode connected to negative terminal
  • Workpiece connected to positive terminal
  • More heat at workpiece (2/3)
  • Used when: deep penetration required, thick materials, high deposition rate needed

Alternating Current (AC)

  • Polarity alternates
  • Heat distribution balanced
  • Used for: aluminum, magnesium (breaks oxide layer)

Arc Welding Parameters

1. Welding Current (I)

  • Primary parameter controlling heat input
  • Higher current → deeper penetration, higher deposition rate
  • Typical range: 50-500 A (depends on process and application)

2. Arc Voltage (V)

  • Controls arc length and weld bead width
  • Higher voltage → wider, flatter bead
  • Typical range: 15-40 V

3. Travel Speed (v)

  • Speed at which electrode moves along joint
  • Higher speed → less heat input, narrower bead
  • Typical range: 2-10 mm/s

4. Heat Input (H)

Formula:

H = (V × I × 60) / (1000 × v)

Where:

  • H = Heat input (kJ/mm)
  • V = Arc voltage (V)
  • I = Welding current (A)
  • v = Travel speed (mm/min)
  • η = Process efficiency (typically 0.6-0.9)

Modified formula with efficiency:

H = (η × V × I × 60) / (1000 × v)

Weld Joint Design

Types of Joints

1. Butt Joint

  • Two pieces joined edge-to-edge
  • Most common joint type
  • Requires edge preparation for thick materials

2. Corner Joint

  • Two pieces joined at right angles
  • Forms L-shape

3. T-Joint

  • One piece perpendicular to another
  • Forms T-shape

4. Lap Joint

  • Two pieces overlap each other
  • Simple preparation

5. Edge Joint

  • Edges of two pieces joined
  • Used for thin materials

Edge Preparation

Square Butt

  • No edge preparation
  • Used for thin materials (< 6 mm)

Single-V Butt

  • One side beveled
  • Groove angle: 60-90°
  • Used for medium thickness (6-20 mm)

Double-V Butt

  • Both sides beveled
  • Reduces distortion
  • Used for thick materials (> 20 mm)

Single-U and Double-U

  • Curved groove
  • Less filler material required
  • Better for very thick materials

J-Groove and Double-J

  • One side curved, one straight
  • Compromise between V and U grooves

Weld Positions

1. Flat Position (1G, 1F)

  • Easiest position
  • Highest deposition rate
  • Best quality

2. Horizontal Position (2G, 2F)

  • Weld axis horizontal
  • Weld face approximately vertical

3. Vertical Position (3G, 3F)

  • Weld axis vertical
  • Can be welded upward or downward
  • Upward: better penetration
  • Downward: faster, less penetration

4. Overhead Position (4G, 4F)

  • Most difficult position
  • Requires skill to prevent sagging

Position Codes:

  • G = Groove weld
  • F = Fillet weld
  • Number indicates position

Weld Defects

1. Porosity

  • Gas pockets trapped in weld metal
  • Causes: contamination, improper shielding, moisture
  • Prevention: clean base metal, proper gas flow, dry electrodes

2. Slag Inclusions

  • Non-metallic solid material trapped in weld
  • Causes: improper cleaning between passes, incorrect technique
  • Prevention: thorough slag removal, proper welding technique

3. Incomplete Fusion (Lack of Fusion)

  • Base metal or previous weld bead not completely fused
  • Causes: insufficient heat, improper technique, wrong angle
  • Prevention: adequate current, proper manipulation, correct angle

4. Incomplete Penetration (Lack of Penetration)

  • Weld metal does not extend through joint thickness
  • Causes: insufficient heat input, improper joint design, excessive speed
  • Prevention: adequate current, proper joint preparation, slower travel speed

5. Undercut

  • Groove melted into base metal at weld toe
  • Causes: excessive current, wrong angle, excessive speed
  • Prevention: proper parameters, correct technique

6. Overlap

  • Weld metal flows onto base metal surface without fusion
  • Causes: insufficient heat, improper technique
  • Prevention: adequate heat, proper manipulation

7. Cracks

Hot Cracks

  • Occur at high temperatures during solidification
  • Types: centerline cracks, crater cracks
  • Causes: high restraint, improper composition, rapid cooling

Cold Cracks

  • Occur after cooling to ambient temperature
  • Causes: hydrogen embrittlement, high hardness, residual stress
  • Prevention: preheat, post-weld heat treatment, low hydrogen electrodes

8. Distortion and Residual Stress

  • Warping and dimensional changes
  • Causes: non-uniform heating and cooling
  • Control methods:
    • Proper joint design
    • Welding sequence
    • Fixtures and restraints
    • Preheat and post-weld heat treatment

Arc Welding Processes Overview

Major Arc Welding Processes

  1. Shielded Metal Arc Welding (SMAW) - Manual Metal Arc Welding (MMAW)
  2. Gas Tungsten Arc Welding (GTAW) - TIG Welding
  3. Gas Metal Arc Welding (GMAW) - MIG Welding
  4. Submerged Arc Welding (SAW)
  5. Flux Cored Arc Welding (FCAW)
  6. Plasma Arc Welding (PAW)

Selection Criteria

  • Material type and thickness
  • Joint design and position
  • Production requirements
  • Quality requirements
  • Cost considerations
  • Skill level available

Safety in Arc Welding

Hazards

  1. Electric Shock

    • Can be fatal
    • Prevention: proper insulation, dry gloves, avoid wet conditions

  2. Arc Radiation

    • UV and IR radiation
    • Can cause arc eye (photokeratitis)
    • Prevention: proper helmet with correct shade lens

  3. Fumes and Gases

    • Metal fumes, ozone, nitrogen oxides
    • Prevention: adequate ventilation, respiratory protection

  4. Fire and Explosion

    • Sparks and hot metal
    • Prevention: remove flammables, fire watch, proper storage

  5. Burns

    • From arc, hot metal, slag
    • Prevention: proper protective clothing, leather gloves

Personal Protective Equipment (PPE)

  • Welding helmet with proper shade (typically #10-#14)
  • Safety glasses underneath helmet
  • Leather gloves and jacket
  • Long pants (no cuffs)
  • Safety boots
  • Respiratory protection when required

Weld Quality and Testing

Non-Destructive Testing (NDT)

  1. Visual Inspection

    • First and most important test
    • Check for surface defects

  2. Liquid Penetrant Testing (PT)

    • Detects surface-breaking defects
    • Dye penetrates cracks, then drawn out

  3. Magnetic Particle Testing (MT)

    • For ferromagnetic materials
    • Detects surface and near-surface defects

  4. Radiographic Testing (RT)

    • X-ray or gamma ray
    • Detects internal defects
    • Permanent record

  5. Ultrasonic Testing (UT)

    • High frequency sound waves
    • Detects internal defects
    • Can measure defect size and location

Destructive Testing

  1. Tensile Test

    • Measures strength of weld

  2. Bend Test

    • Checks ductility and fusion

  3. Impact Test

    • Measures toughness (Charpy V-notch)

  4. Macro and Micro Examination

    • Examines weld structure

Metallurgical Aspects

Weld Zones

  1. Fusion Zone (Weld Metal)

    • Completely melted and resolidified
    • Mixture of base metal and filler metal

  2. Heat Affected Zone (HAZ)

    • Base metal affected by heat but not melted
    • Microstructure changed
    • Often weakest zone

  3. Unaffected Base Metal

    • Beyond HAZ
    • Original properties retained

Solidification

  • Weld pool solidifies from fusion boundary toward center
  • Columnar grain structure typical
  • Grain growth direction: opposite to heat flow

Cooling Rate

  • Affects microstructure and properties
  • Fast cooling: fine grain, higher hardness, risk of cracking
  • Slow cooling: coarse grain, lower hardness, better ductility

Welding Symbols

Basic Symbol Components

  • Reference line (horizontal)
  • Arrow pointing to joint
  • Weld symbol on reference line
  • Tail (for specifications)
  • Dimensions and other data

Arrow Side vs Other Side

  • Symbol below line: arrow side
  • Symbol above line: other side
  • Symbol both sides: both sides welded

Common Weld Symbols

  • Fillet weld: triangle
  • Groove welds: various shapes (V, U, J, etc.)
  • Plug/slot: rectangle
  • Spot/projection: circle

Economics of Welding

Cost Factors

  1. Labor cost (largest component)
  2. Electrode/filler material cost
  3. Power cost
  4. Equipment cost
  5. Overhead

Improving Productivity

  • Increase deposition rate
  • Reduce weld metal volume (proper joint design)
  • Mechanization and automation
  • Proper process selection
  • Operator training

Next

SMAW