Welding By Laxman Singh Sankhla, Jodhpur India.doc

Click here to load reader

  • date post

    08-Nov-2014
  • Category

    Documents

  • view

    52
  • download

    0

Embed Size (px)

description

Welding technic is used for metal joining throughout world.

Transcript of Welding By Laxman Singh Sankhla, Jodhpur India.doc

TRAINING MANUAL MECHANICAL

ARC & GAS WELDINGCONTENTS PAGE 2.0 Introduction 3 3.0 Types of Welding 3 4.0 Electric Arc welding 3 4.1 Basic Principle of arc welding 4 4.2 Welding Station and Welding Equipment 7 4.3 Arc-Welding Hazards 9 4.4 Welding Operators Protection 10 4.5 Protective clothing for manual arc-welding 11 5.1 Safe Practices Before Operation 12 5.2 Safe Practices During Operation 12 5.3 Key Safety Points 12 6.1 Power supplies & General Arrangement of Arc Welding Equipment 14 6.2 Arc-Welding circuit 15 6.3 Polarity of the electrode: 15 7.1 Shielded Metal Arc Welding (SMAW) or Stick Welding Processes 15 7.2 Function of coating on electrode 19 8.1 Types of weld joint and Welds 20 8.2 Various Parts of welds 22 8.3 Butt weld and its terminology 23 8.4 Fillet Welds and its terminology 23 9.1 Weld Positions 25 10.1 Weld joint preparation 26

Welding By Laxman Singh Sankhla, Jodhpur, India Mobile:+91-9928829491 Mail ID: [email protected]

1

TRAINING MANUAL MECHANICAL 10.2 Factors affecting the method of edge preparation 27 11.1 Weld joint fit up before welding 29 11.2 Striking the arc technique 30 11.3 Tack welding 32 12.1 Welding sequence 33 12.2 Effect of the gap in weld penetration 36 12.3 T- Joint 13.1 Examples of Good & Bad Stick Welds 38 13.2 Heat affected Zone 40 14.1 Safety issues 42 15.1 Welding Defects 42 15.2 Porosity and its prevention 45 15.3 Wormholes and its Prevention 47 15.4 Incomplete Penetration or Fusion 48 15.5 Trapped Slag 50 15.6 Cracks and Distortion 52 16.1 Geometrical imperfections 57 17.1 Introduction to Oxy-Acetylene Welding 61 17.2 Gas Welding Station 63 17.3 Protective Clothing for Gas Welding and Cutting 64 17.4 Safe Work Practices Electric & Gas Welding 65Welding By Laxman Singh Sankhla, Jodhpur, India Mobile:+91-9928829491 Mail ID: [email protected] 2

37

TRAINING MANUAL MECHANICAL 17.5 Welding, Cutting and Brazing Accidents Causes 66 18.1 Oxy-acetylene Welding Process 67 18.2 Basic Gas-Welding Equipment 69 18.3 Types of flame 73 19.1 Safe Work Practices 77 20.1 Fluxes 78 21.1 Welding Techniques 78 22.0 Comparison of oxy-acetylene and metallic arc-welding 81 23.1 Oxy-Acetylene Cutting 82

Welding By Laxman Singh Sankhla, Jodhpur, India Mobile:+91-9928829491 Mail ID: [email protected]

3

TRAINING MANUAL MECHANICAL

2.0 IntroductionWelding is used when a permanent joint which has high strength is required. It is often better to fabricate steel by welding than using a casting because the mechanical properties of steel are better than those of cast iron. In fusion welding the edges of the metal to be joined are melted. The molten metal runs together and so a joint is made whose strength is equal to that of metal being joined. Melting the workpieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces. Fabrication, when used as an industrial term, applies to the building of machines, structures and other equipment, by cutting, shaping and assembling components made from plate metal, formed and expanded metal ,tube stock,square stock ,sectional metals (I beams, W beams, Cchannel...) 3.0 Types of Welding 1. Fusion Welding: Melting base metals (a) Arc Welding (AW): Heating with electric arc (b) Oxy-fuel Welding (OFW): Heating with a mixture of oxygen and acetylene (c) Resistance welding (RW): Heating the resistance by an electrical current (d) Other fusion welding: Electron beam welding and laser beam welding 2. Solid State Welding: No melting, No fillers (a) Diffusion welding (DFW): Solid-state fusion at an elevated temperature (b) Friction welding (FRW): Heating by friction (c) Ultrasonic welding (USW): Moderate pressure with ultrasonic oscillating motion 4.0 Electric Arc welding When electricity passes through an air gap from one conductor to another intense heat is produced. The temperature of an arc jumping

Welding By Laxman Singh Sankhla, Jodhpur, India Mobile:+91-9928829491 Mail ID: [email protected]

4

TRAINING MANUAL MECHANICAL between two conductors is in the region of 3500 degree Celsius. In electric arc welding it is this arc which provides the source of heat. During welding an electric current of high amperage and low voltage is fed into a metal electrode and the arc is drawn between the electrode and the metal to be joined. The high temperature of the arc melts the parent metal and the end of the electrode (or welding rod). Small drops of molten metal from the rod are deposited and this unites with the metal being welded. Coated Electrodes: When a base wire is used as an electrode the arc is difficult to control and also the weld tends to be brittle and porous. When coating forms a slag over the weld which protects the weld from the atmosphere and reduces the rate at which the weld cools, making it less brittle. 4.1 Basic principles of Metallic-Arc Welding The arc is produced by a low voltage, high-amperage electric current jumping an air gap between the electrode and the joint to be welded. The heat of the electric arc is concentrated on the edges of two pieces of metal to be joined. This causes the metal edges to melt. While these edges are still molten additional molten metal, transferred across the arc from a suitable electrode, is added. This molten mass of metal cools and solidifies into one solid piece. As soon as the arc is struck, the tip of the electrode begins to melt, thus increasing the gap between electrode and work. Therefore, it is necessary to cultivate a continuous downward movement with the electrode holder in order to maintain a constant arc length of 3 mm during welding operation. The electrode is moved at a uniform rate along the joint to be welded, melting the metal as it moves. The greatest bulk of electrodes used with manual arc welding are coated electrodes. A coated electrode consists mainly of a core wire having a concentric covering of flux and/or other material, which will melt uniformly with the core wire forming vaporized and partly molten screen around the arc stream. This shield protects the arc from contamination by atmospheric gases. The liquid slag produced performs three important functions. 1. Protects the solidifying weld metal from any further contamination from the atmosphere. 2. Prevents rapid cooling of the weld metal.

Welding By Laxman Singh Sankhla, Jodhpur, India Mobile:+91-9928829491 Mail ID: [email protected]

5

TRAINING MANUAL MECHANICAL 3. Controls the contour of the completed weld. The core wire melts in the arc and tiny globules of molten metal shoot across the arc into the molten pool (arc crater in parent metal) during welding. These tiny globules are explosively forced through the arc stream. They are not transferred across the arc by the force of gravity; otherwise it would not be possible to use the manual arc-welding process for overhead welding. The chemical coating surrounding the core wire melts or burns in the arc. It melts at a slightly higher temperature than the metal core and, therefore, extends a little beyond the core and directs the arc. This extension also prevents sideways arcing when welding in deep grooves. Shielded metal-arc welding with the transformer welding machine depends upon this fundamental fact: that when one side of the welding circuit is attached to a piece of steel, a welding electrode connected to the other side and two brought into contact, an arc will be established. If the arc is properly controlled, the metal from the electrode will pass through the arc and be deposited on the steel. When the electrode is moved along the steel at the correct speed, metal will deposit in a uniform layer called a bead. Electrodes consist of a core of steel wire, usually called mild since it contains a low percentage of carbon (0.100.14). Around this core is applied a special coating which assists in creating the arc and at the same time protects the molten metal as it transfers across the arc.

Welding By Laxman Singh Sankhla, Jodhpur, India Mobile:+91-9928829491 Mail ID: [email protected]

6

TRAINING MANUAL MECHANICAL

The arc stream and basic features of manual gas shield metal-arc welding

Welding By Laxman Singh Sankhla, Jodhpur, India Mobile:+91-9928829491 Mail ID: [email protected]

7

TRAINING MANUAL MECHANICAL

4.2 Welding Station and Welding Equipment

Welding By Laxman Singh Sankhla, Jodhpur, India Mobile:+91-9928829491 Mail ID: [email protected]

8

TRAINING MANUAL MECHANICAL

Mains operated arc-welding equipmentWelding By Laxman Singh Sankhla, Jodhpur, India Mobile:+91-9928829491 Mail ID: [email protected]

9

TRAINING MANUAL MECHANICAL This equipment is designed to change the high-voltage Alternating Current mains supply into a safe, low-voltage, heavy-current supply suitable for arc welding. Output can have an AC or DC. For safety the output voltage is limited between 50 and 100 V; however, the output current may be as high as 500 A. A welding set is basically a transformer to breakdown the high mains voltage and a tapped choke to control the current flow to suit the gauge of electrode used

4.3 Arc-Welding Hazards

Welding By Laxman Singh Sankhla, Jodhpur, India Mobile:+91-9928829491 Mail ID: [email protected]

10

TRAINING MANUAL MECHANICAL Hazards that may arise from mains operated welding equipment are set out in table:

To eliminate these hazards as far as possible, the following precautions to be