Comprehensive Notes – Oxy-Acetylene Welding

Gas Cylinders

• Purpose: Storage of welding gases under high pressure.
  • Oxygen cylinder
  • Color: Black
  • Taller in height than acetylene cylinder
  • Stores pure $O_2$ at high pressure (commonly up to $15{,}000\,\text{kPa}$ in industry; course material may specify local max-fill values)
  • Acetylene cylinder (dissolved-acetylene type)
  • Color: Red or maroon
  • Shorter and wider than oxygen cylinder
  • Contains acetylene dissolved in acetone, held in a porous mass to keep pressure below the critical $206\,\text{kPa}$ safety limit
  • Handling rules
  • Always keep cylinders upright (prevents liquid acetone escape in acetylene and protects pressure-relief plugs)
  • Chain or strap cylinders securely to a wall, bench, or trolley during use and transport

Regulators (Pressure-Reducing Valves)

• Function: Reduce cylinder pressure to a safe, adjustable working pressure and indicate both cylinder & delivery pressures
• Two pressure gauges per regulator
  • High-pressure gauge: cylinder contents
  • Low-pressure gauge: outlet pressure to torch
• Color coding & threads
  • Oxygen regulator: Black body; right-hand thread (standard thread direction)
  • Acetylene regulator: Red / maroon body; left-hand thread (grooved nut & notch to avoid interchange)
• Adjustment screw: turned in to increase, out to decrease; always slacken when not in use to protect gauges

Welding Torches (Blowpipes)

• Purpose: Mix gases and deliver stable flame at nozzle
• Two basic designs
  • High-pressure or equal-pressure torch (“chamber mixer”)
  • Both gases admitted at similar (moderate) pressures, mix in chamber before tip
  • Low-pressure or injector torch
  • High-pressure oxygen jet entrains low-pressure acetylene in an injector, then mixes
• Key parts
  • Gas inlets with hose check valves (oxygen top, acetylene bottom)
  • Mixing chamber / injector
  • Control valves for fine adjustment of each gas at the handle
  • Copper nozzle/tip (size selected to suit plate thickness & desired flow rate)

Hoses & Hose Accessories

• Twin-hose set: supplies gases from regulators to torch
  • Oxygen hose: Black (sometimes blue or green in other standards)
  • Acetylene hose: Red or maroon
• Hose safety devices
  • Hose protectors / whip-checks – prevent kinks & chafing
  • Pre-flash flashback arrestors – stop flame propagation toward cylinder
  • Non-return (check) valves – prevent reverse flow during pressure fluctuations
• Avoid
  • Tight bends, mechanical damage, hot slag contact, oil/grease contamination (explosion risk with oxygen)

Nozzles (Tips)

• Removable copper or copper-alloy tips sized by gas-orifice diameter
• Selection factors
  • Plate thickness (heavier plate ⇒ larger orifice ⇒ higher gas flow)
  • Desired travel speed & heat input
• Alternative interchangeable tips available for special operations (heating, cutting, gouging)

Filler (Welding) Rods

• Composition: As similar as possible to parent metal for metallurgical compatibility
• Diameters specified in $\text{mm}$ (typical: $1.6, 2.4, 3.2, 4.8, 6.3$)
• Selection considerations
  • Base-metal type (mild steel, stainless, Cu-alloys, Al-alloys, cast iron, etc.)
  • Work thickness
  • Joint design (butt, fillet, lap)
  • Required mechanical properties (strength, ductility, corrosion resistance)
• Reference table (simplified from transcript)
  • Mild steel & wrought iron → Copper-coated mild-steel rod
  • High-carbon steel → Matching high-carbon rod
  • Stainless steel → Stainless filler
  • Cast iron → Super-silicon cast-iron rod
  • Aluminum & alloys → Pure Al rod

Weld Size vs. Rod Size Guide (typical)

• For plate thickness vs rod diameter:
  • $1.6\,\text{mm}$ plate → $1.6\,\text{mm}$ rod
  • $3.2\,\text{mm}$ plate → $2.4\text{–}3.2\,\text{mm}$ rod
  • $6.3\,\text{mm}$ plate → $3.2\text{–}4.8\,\text{mm}$ rod
  • $9.5\,\text{mm}$ plate → $4.8\text{–}6.3\,\text{mm}$ rod

Fluxes for Oxy-Acetylene Welding

• General purposes
  • Dissolve & float away surface oxides/impurities
  • Shield molten pool from atmospheric oxygen (prevents porosity & weak welds)
  • Promote wetting & flow of filler metal
• Typical selections
  • Ferrous metals: Borax, $\text{Na}<em>2\text{CO}</em>3$ (sodium carbonate), $\text{NaHCO}_3$ (bicarbonate), sodium silicate
  • Mild steel generally welded flux-free due to protective CO/CO₂ environment of neutral flame
  • Copper & Cu-alloys: Mixtures of sodium/potassium borates, chlorides, carbonates, sulfates, boric acid
  • Aluminum & Al-alloys: Alkaline fluorides, chlorides, bisulfates (must remove tenacious Al₂O₃ film)

Flame Types & Applications

1. Neutral Flame (Balanced)
   • Oxygen : Acetylene ratio $\approx 1:1$
   • Visible zones: sharp inner cone, short acetylene feather outer envelope
   • Temperature peak $\approx 3{,}200\,^{\circ}\text{C}$
   • General purpose → mild steel, cast iron, copper, aluminum, etc.
2. Oxidizing Flame (Excess $O_2$)
   • Inner cone shorter, hissing sound; outer envelope smaller
   • Slightly higher temp (up to $3{,}500\,^{\circ}\text{C}$) but introduces oxide layer
   • Used on copper-based alloys (brass, bronze) where a thin oxide skin prevents zinc loss
3. Carburizing / Reducing Flame (Excess $C<em>2H</em>2$)
   • Three zones; long white acetylene feather
   • Lower temp; adds carbon to molten pool
   • Used for surface-hardening steels, welding high-nickel alloys (Monel), or where carbon pick-up desired

Welding Techniques

• Left-hand (forehand) technique
  • Torch in right hand angled $\approx 30\text{–}45^{\circ}$, travels left to right
  • Suitable for sheet up to $5\,\text{mm}$
  • Low deposition rate; gives good visibility and control
• Right-hand (backhand) technique
  • Torch in right hand angled $\approx 30^{\circ}$ reversed, travels right to left
  • For plates thicker than $5\,\text{mm}$
  • Higher deposition; deeper penetration due to pre-heating from reflected flame

Safety & Handling of Oxy-Fuel Equipment

• Pre-use leak test
  • Crack each cylinder valve momentarily to blow out dirt
  • Assemble regulators with correct threads (RH for $O<em>2$, LH for $C</em>2H_2$)
  • Apply soap-water to acetylene connections, fresh water to oxygen joints – watch for bubbles
• Opening valves
  • Use correct spindle key; stand aside (never in line with valve outlet)
  • Open oxygen valve slowly, fully; acetylene only $1\text{–}1.5$ turns (quick shut-off possible)
• Hose management: keep clear of sparks, traffic, sharp edges; avoid coiling around hot work
• Personal protective equipment (PPE)
  • Shade $5\text{–}7$ goggles, flame-resistant gloves, leather apron, closed-toe shoes
• Cylinder transport
  • Use trolley with chain, never roll acetylene on its side, avoid oil/grease on $O_2$ fittings (fire hazard)

Backfire & Flashback Prevention

• Backfire: Flame extinguishes with loud pop inside tip; causes
  • Tip touches work, flame set too small, tip dirty/damaged, overheating
• Flashback: Flame burns back into torch/hose → hissing/whistling, possible hose fire/cylinder explosion
• Immediate actions
  • Close oxygen then fuel valve (or both simultaneously) at torch
• Prevention devices
  • Flashback arrestor: stops flame propagation via a sintered filter & non-return valve
  • Check valves: allow one-direction flow; located at regulator outlet & torch inlet
• Maintenance: keep tips clean (tip cleaners), correct pressures, proper tip size

Equipment Setup Procedure (Start-Up)

1. Position cylinders upright & secure
2. Inspect equipment; clean dirt & oil
3. Crack cylinder valves briefly to purge debris
4. Attach regulators (RH $O<em>2$, LH $C</em>2H_2$); tighten with spanner
5. Connect hoses (black $O<em>2$ top, red $C</em>2H_2$ bottom)
6. Attach torch to hoses with check valves/arrestors in place
7. Select correct nozzle; install tightly
8. Re-check all joints for tightness
9. Open cylinder valves slowly; set working pressure (typical: $O<em>2 = 200\text{–}400\,\text{kPa}$, $C</em>2H_2 \le 100\,\text{kPa}$)
10. Open acetylene torch valve slightly; ignite with flint lighter
11. Adjust acetylene until smoke ceases (feather just visible)
12. Open oxygen valve; adjust to desired flame (neutral/oxidizing/carburizing)

Shutdown Procedure

1. Close acetylene valve at torch → flame extinguishes
2. Close oxygen valve at torch
3. Close cylinder valves (fuel then oxygen preferred)
4. Bleed hoses: reopen torch valves until gauges drop to $0\,\text{kPa}$
5. Close torch valves & back out regulator adjusting screws

Complete Welding Kit Components (Field Checklist)

• Oxygen & acetylene cylinders with safety plugs/valves
• Cylinder key/spindle wrench
• Regulators with contents & delivery gauges
• Flashback arrestors & check valves
• Twin hoses with protectors
• Welding torch/blowpipe with assorted nozzles
• Flint spark lighter
• Filler rods & fluxes as required
• PPE: goggles, gloves, apron
• Cylinder trolley w/ chaining attachment

System Flow & Functional Blocks (per Transcript Diagram)

1. Cylinders → high-pressure gas storage
2. Regulators (reducers) → step down & stabilize pressure
3. Hoses → convey low-pressure gases; contain check valves
4. Mixer (torch body) → uniform gas mixing for stable flame
5. Nozzle → controls exit velocity; sized to match flow
6. Flame → delivers heat for welding/cutting/brazing

Edge & Joint Preparation (Butt Joints)

• Objective: achieve full penetration & sound weld
• Common configurations
  • Square butt (no bevel) for $0.8\text{–}3.2\,\text{mm}$ sheet; small root gap $\approx 0.8\,\text{mm}$
  • Single-V bevel $60^{\circ}$ included angle; gap $1.6\text{–}3.2\,\text{mm}$ for >3.2\,\text{mm} plates
• Key terms
  • Root gap: distance between faying edges at bottom of bevel
  • Fusion face: prepared surface to be melted
  • Throat thickness: minimum weld section between root & reinforcement
  • Reinforcement: buildup above plate surface (controlled to spec limits)
• Thin sheet ((<3.2\,\text{mm})) often welded autogenously (no filler rod) using tight fit-up and forehand technique

Practical & Ethical Considerations

• Quality assurance: correct flame, filler, and technique critical for mechanical integrity; bad practice can lead to catastrophic failures (e.g., pressure vessel seams)
• Environmental impact: Acetylene production (calcium carbide + water) consumes energy; proper gas management minimizes waste
• Safety ethics: Neglecting flashback arrestors or using oil-contaminated oxygen fittings creates explosion/fire hazards for personnel; adherence to safety codes (AWS, ISO, local regulations) is obligatory

Numerical / Reference Data Snapshot

• Maximum safe withdrawal rate of acetylene: \le 1\/7 of cylinder capacity per hour
• Neutral flame peak temperature: $\approx 3.2\times10^3\,^{\circ}\text{C}$
• Typical working pressures
  • Welding up to $3\,\text{mm}$ plate: $O<em>2 = 70\text{–}140\,\text{kPa}$, $C</em>2H_2 = 35\text{–}70\,\text{kPa}$
  • Cutting: $O_2$ jet can exceed $550\,\text{kPa}$ depending on tip size
• Backfire sound = sharp “pop”; flashback sound = shrill whistle/hiss followed by torch heating

Quick Troubleshooting Guide

• Excessive soot → increase oxygen (carburizing flame)
• Loud hissing & short inner cone → reduce oxygen (oxidizing flame)
• Porous weld bead → use flux / clean metal / lower tip distance
• Popping sounds while welding → clean or tighten tip; verify pressures