SMAW

Overview

Definition

Shielded Metal Arc Welding (SMAW), also known as Manual Metal Arc Welding (MMAW) or "stick welding," is an arc welding process that uses a consumable electrode covered with flux to lay the weld.

Process Characteristics

• Most widely used arc welding process
• Manual process requiring high skill
• Versatile and portable
• Can be used in all positions
• Suitable for both indoor and outdoor applications

Process Principle

Basic Setup

1. Power Source: Provides welding current (AC or DC)
2. Electrode Holder: Holds the consumable electrode
3. Work Clamp: Connects workpiece to power source
4. Electrode: Flux-coated metal rod

Working Mechanism

1. Arc is struck between electrode and workpiece
2. Arc heat melts both electrode and base metal
3. Flux coating melts and creates:
   • Protective gas shield
   • Slag layer over weld
4. Electrode metal transfers to weld pool
5. Slag protects cooling weld from atmosphere

Electrodes

Electrode Construction

Core Wire

• Provides filler metal
• Carries welding current
• Materials: mild steel, low alloy steel, stainless steel, cast iron, etc.
• Diameter: 1.6 mm to 8 mm (common: 2.5, 3.2, 4.0, 5.0 mm)

Flux Coating

• Covers core wire
• Thickness: 20-40% of electrode diameter
• Multiple functions (see below)

Functions of Flux Coating

1. Arc Stabilization

   • Contains ionizing elements (sodium, potassium)
   • Makes arc easier to strike and maintain
   • Provides smooth arc operation

2. Gas Shielding

   • Decomposes to produce CO₂, CO, H₂O vapor
   • Shields molten metal from atmospheric contamination
   • Prevents oxidation and nitrogen absorption

3. Slag Formation

   • Forms protective layer over weld
   • Slows cooling rate
   • Protects from atmospheric contamination
   • Easy to remove after cooling

4. Deoxidation and Purification

   • Contains deoxidizers (ferromanganese, ferrosilicon)
   • Removes oxygen from weld pool
   • Produces cleaner weld metal

5. Alloying

   • Adds alloying elements to weld metal
   • Improves mechanical properties
   • Compensates for element loss during welding

6. Arc Force Control

   • Controls penetration depth
   • Affects weld bead shape

7. Insulation

   • Allows welding in confined spaces
   • Prevents electrode from sticking to adjacent surfaces

Electrode Classification (AWS System)

Designation Format: E XX Y Z

Example: E 7018

• E: Electrode
• XX (70): Minimum tensile strength in ksi (70,000 psi = 482 MPa)
• Y (1): Welding position
  • 1 = All positions
  • 2 = Flat and horizontal positions only
  • 3 = Flat position only
• Z (8): Current type and coating type

Last Two Digits - Current and Coating

Common Electrode Types

E6010

• High cellulose sodium coating
• DCEP only
• Deep penetration
• All positions
• Fast freezing slag
• Used for: pipe welding, root passes

E6011

• High cellulose potassium coating
• AC or DCEP
• Similar to E6010 but works on AC
• Deep penetration
• All positions

E6012

• High titania sodium coating
• AC or DCEN
• Medium penetration
• Smooth arc, easy to use
• Good for beginners

E6013

• High titania potassium coating
• AC or DC (any polarity)
• Medium penetration
• Smooth, stable arc
• Excellent for thin materials
• Most versatile electrode

E7018

• Low hydrogen iron powder coating
• AC or DCEP
• Medium penetration
• All positions
• Produces high quality welds
• Used for: structural steel, pressure vessels
• Must be kept dry (store in oven at 120-150°C)

E7024

• Iron powder titania coating
• AC or DCEP
• High deposition rate
• Flat and horizontal positions only
• Heavy slag
• Used for: high-speed production welding

Electrode Storage and Handling

Storage Requirements

• Store in dry location (relative humidity < 50%)
• Temperature: 10-40°C
• Low hydrogen electrodes: store in heated cabinets (120-150°C)
• Keep in original sealed containers until use

Reconditioning

• If electrodes absorb moisture, they must be reconditioned
• Low hydrogen electrodes: bake at 370-430°C for 1-2 hours
• Other types: bake at 100-150°C for 1-2 hours
• Do not recondition more than twice

Moisture Effects

• Causes porosity
• Hydrogen cracking in susceptible steels
• Rough, spattered arc
• Excessive spatter

Welding Parameters

1. Welding Current

Selection Factors:

• Electrode diameter
• Electrode type
• Welding position
• Joint type

General Guidelines:

Current (A) ≈ 40 × Electrode diameter (mm)

Typical Ranges:

• 2.5 mm electrode: 60-100 A
• 3.2 mm electrode: 90-140 A
• 4.0 mm electrode: 140-200 A
• 5.0 mm electrode: 180-270 A

Effects:

• Too low: unstable arc, poor fusion, excessive spatter
• Too high: excessive penetration, undercut, electrode overheating

2. Arc Voltage

• Determined by arc length
• Typical range: 20-30 V
• Short arc: 18-22 V
• Medium arc: 22-26 V
• Long arc: 26-30 V

Arc Length:

• Should be approximately equal to electrode core diameter
• Short arc: better control, less spatter, deeper penetration
• Long arc: wider bead, more spatter, porosity risk

3. Travel Speed

• Typical range: 2-5 mm/s
• Too slow: excessive buildup, wide bead, slag inclusions
• Too fast: narrow bead, undercut, incomplete fusion

4. Electrode Angle

Longitudinal Angle (Travel Angle):

• Push angle (5-15°): shallow penetration, wider bead
• Drag angle (5-15°): deeper penetration, narrower bead
• Perpendicular (90°): balanced

Transverse Angle (Work Angle):

• Depends on joint type
• Butt joint: 90° to surface
• Fillet joint: 45° to both surfaces
• T-joint: 45-60° to vertical member

Welding Techniques

Arc Starting

1. Scratch method: Scratch electrode on workpiece like striking a match
2. Tap method: Tap electrode perpendicular to workpiece, then lift

Electrode Manipulation

Stringer Bead

• Straight line motion
• No weaving
• Fast, good penetration
• Used for: root passes, thin materials

Weaving

• Side-to-side motion
• Various patterns: zigzag, crescent, figure-8
• Wider bead coverage
• Used for: fill and cover passes
• Maximum weave width: 3 × electrode diameter

Crater Filling

• Fill crater at end of weld
• Prevents crater cracks
• Reverse direction briefly before breaking arc

Multi-Pass Welding

Sequence for Thick Materials:

1. Root pass: First pass, establishes penetration
2. Hot pass: Second pass, cleans root, adds reinforcement
3. Fill passes: Build up weld to required thickness
4. Cover pass: Final pass, provides smooth surface

Inter-pass Temperature:

• Maximum temperature before next pass
• Typical: 200-300°C for carbon steel
• Control to prevent excessive heat input
• Prevents grain growth in HAZ

Advantages of SMAW

1. Equipment Simplicity

   • Simple, inexpensive equipment
   • Portable
   • No external gas supply needed

2. Versatility

   • All positions
   • Wide range of materials
   • Indoor and outdoor use
   • Wind resistant (slag protection)

3. Accessibility

   • Can weld in confined spaces
   • Short electrode allows access to tight areas

4. Material Range

   • Carbon steel, low alloy steel
   • Stainless steel
   • Cast iron
   • Some non-ferrous metals

5. Cost

   • Low initial investment
   • Low operating cost for small jobs

Limitations of SMAW

1. Low Productivity

   • Low deposition rate (1-5 kg/h)
   • Frequent electrode changes
   • Time for slag removal
   • Operator dependent

2. Skill Requirement

   • Requires high operator skill
   • Long training period
   • Quality depends on welder

3. Electrode Stub Loss

   • 10-20% of electrode wasted as stub
   • Cannot use last 50 mm of electrode

4. Limited Thickness Range

   • Not economical for very thin materials (< 1.5 mm)
   • Slow for very thick materials

5. Fume Generation

   • Produces significant fumes
   • Requires ventilation
   • Health hazard

Applications

Typical Uses

• Structural steel fabrication
• Pipeline welding
• Maintenance and repair work
• Ship building
• Pressure vessel fabrication
• Construction
• Farm equipment repair
• General fabrication

Material Applications

• Carbon steel (most common)
• Low alloy steel
• Stainless steel
• Cast iron
• Some nickel alloys

Thickness Range

• Minimum: 1.5-2 mm
• Maximum: unlimited (with multi-pass)
• Optimal: 3-20 mm

Weld Quality Considerations

Common Defects in SMAW

1. Porosity

   • Causes: moisture in electrode, contaminated base metal, long arc
   • Prevention: dry electrodes, clean base metal, proper arc length

2. Slag Inclusions

   • Causes: incomplete slag removal, improper technique
   • Prevention: thorough cleaning between passes, proper electrode angle

3. Undercut

   • Causes: excessive current, wrong angle, fast travel speed
   • Prevention: proper parameters, correct technique

4. Incomplete Fusion

   • Causes: insufficient current, improper technique, contamination
   • Prevention: adequate heat, proper manipulation, clean surfaces

5. Spatter

   • Causes: long arc, high current, moisture, wrong polarity
   • Prevention: proper arc length, correct parameters, dry electrodes

Quality Improvement Measures

• Proper electrode selection and storage
• Correct welding parameters
• Adequate joint preparation and cleaning
• Proper welding technique
• Appropriate preheat and interpass temperature
• Post-weld heat treatment when required

Safety Considerations

Specific SMAW Hazards

• Electric shock (open circuit voltage: 50-100 V)
• Arc radiation (UV and IR)
• Fumes and gases
• Hot slag and spatter
• Fire hazard from slag and sparks

Safety Measures

• Dry gloves and clothing
• Proper welding helmet (shade #10-#14)
• Adequate ventilation
• Fire-resistant clothing
• Remove flammables from area
• Proper grounding

Economics and Productivity

Deposition Rate

• Typical: 1-5 kg/h
• Depends on: electrode size, current, duty cycle

Duty Cycle

• Percentage of time actually welding
• Typical: 20-30% for manual SMAW
• Lost time: electrode changes, slag removal, repositioning

Operating Factor

Operating Factor = Arc time / Total time

• Typical: 0.2-0.3 for SMAW

Cost Factors

• Labor cost (largest component, 70-80%)
• Electrode cost
• Power cost (minimal)
• Equipment depreciation
• Overhead

Improving Productivity

• Use larger diameter electrodes when possible
• Iron powder electrodes for higher deposition
• Proper electrode selection
• Minimize electrode changes
• Efficient slag removal
• Operator training