Chapter – 17 : JSSC JE Mechanical Note

Updated on 05/05/26 by Mananjay MahatoShare on WhatsApp
  • Brazing
  • Soldering
  • Welding
  • Arc welding
  • Gas Welding
  • resistance Welding
  • Special welding Techniques
    • TIG
    • MIG
  • Welding defects and Testing; NDT,
  • Foundry & Casting
    • methods
    • defects
  • different casting processes
      • Forging
      • Extrusion
  • Brazing, Soldering, and Welding: Joining processes are classified based on the state of the parent metals and the temperature used. 

1. Welding (Joining by Fusion/Pressure)

Welding is a permanent joining process that involves the melting of parent metals or the application of pressure.

  • Classification:

    • Autogenous: No filler material used.

    • Homogenous: Filler material same as parent metal.

    • Heterogenous: Filler material different from parent metal.

  • Solid State Welding: Joining without melting (e.g., Friction, Ultrasonic, Explosive, Forge welding).

  • Fusion Welding: Parent metal is melted (e.g., Arc, Gas, TIG, MIG, Thermit).

  • Heat Affected Zone (HAZ): The area of base metal which has not melted but whose mechanical properties/microstructure have been altered by heat.

  • Welding Polarities (DC Arc Welding):

    • Straight Polarity (DCEN): Electrode (-), Workpiece (+). 66% heat at workpiece. Best for thick plates.

    • Reverse Polarity (DCEP): Electrode (+), Workpiece (-). 66% heat at electrode. Best for thin plates and overhead welding.

2. Brazing (Hard Soldering)

Brazing is a joining process where a filler metal is melted and distributed between two or more close-fitting parts by capillary action.

  • Temperature: Performed at temperatures above 450°C but below the melting point of the base metal.

  • Filler Metal (Spelter): Usually an alloy of Copper and Zinc (Brass) or Copper and Silver.

  • Flux: Commonly uses Borax. It prevents oxidation and promotes wetting.

  • Clearance: Requires a very small gap between parts (typically 0.025 to 0.1 mm) for effective capillary action.

  • Strength: Stronger than soldering but weaker than welding.

  • Applications: Pipe fittings, carbide tips on tools, and heat exchangers.

3. Soldering (Soft Soldering)

Soldering is similar to brazing but occurs at lower temperatures. It is primarily used for electrical connectivity and leak-proofing.

  • Temperature: Performed at temperatures below 450°C (typically around 180°C to 250°C).

  • Filler Metal (Solder): Traditionally an alloy of Tin (Sn) and Lead (Pb).

    • Eutectic Solder: 63% Tin and 37% Lead. It melts at a single temperature (183°C).

  • Flux: Zinc Chloride (ZnCl2) or Resin (Rosin).

  • Strength: Lowest among the three processes. Not suitable for high-load mechanical joints.

  • Applications: Printed Circuit Boards (PCBs), radio/TV electronics, and plumbing.

Feature Soldering Brazing Welding
Joining Temp $<450° C > 450° C
 > Melting Point of Base Metal
Base Metal Does not melt Does not melt Melts
Mechanism Capillary Action Capillary Action Fusion / Diffusion
Joint Strength Low Medium High
Heat Treatment Not required Not required Often required (to relieve stress)
Skill Level Low Medium High
  • Thermit Welding: Uses a mixture of Aluminum powder and Iron Oxide in a ratio of 1:3 by weight. Highly exothermic; used for repairing railway tracks.
  • TIG Welding: Uses a non-consumable Tungsten electrode and Helium/Argon as shielding gas.
  • MIG Welding: Uses a consumable wire electrode.
  • Neutral Flame (Gas Welding): Oxygen and Acetylene ratio is 1:1. Used for mild steel and stainless steel.
  • Oxidizing Flame: More Oxygen (O2 > C2H2). Used for Copper and Brass.
  • Carburizing Flame: More Acetylene (C2H2 > O2). Used for High Carbon steels and hard facing.
Chapter – 17 : JSSC JE Mechanical Note