classification of joining methods
In an oil and gas plant, we can see a thousand pieces of equipment. A piece of mechanical equipment (pipeline, tank, pressure vessel) is a collection of several metal components which are joined together into one unit. The joining methods can broadly be classified into three categories by the joining mechanism:
-mechanical joining by means of such mechanical methods as riveting and bolting,
-adhesive bonding by using glues or plastics
-welding by means of metallurgical bonding.
welding can be defined as “the operation that produces coalescence of two or more members by applying heats or pressure or both, with or without the use of filler metal.” Welding (Metallurgical joining) can further be classified by the joining mechanism into 4 groups :
- fusion welding consists of 5 categories: gas welding, arc welding, electroslag welding, thermite welding and high energy beam welding (electron beam welding & Laser Welding).
- solid phase joining (pressure welding),
- diffusion welding that
- joining by liquid-solid phase reaction (Brazing & soldering).
In fusion welding, members to be joined (base metals) are heated to melt with or without the use of filler metal and then the molten parts produce coalescence of the base metals, followed by solidification. In pressure welding, the joining parts are given the thermal energy by friction or gas flame, and then mechanical pressure is applied to complete the welding process. In brazing or soldering, the brazing filler metal or solder that has a lower melting point than the base metal is fused, without melting the base metal, and then the fused filler metal is distributed between the closely fitted faying surfaces of the joint by capillary action. The brazing process uses a brazing filler metal, and the soldering process uses a solder with a relatively lower melting point. Of these metallurgical joining processes, the fusion welding process, particularly the arc welding process that uses the arc discharge for the heat source, is most widely used.
Types of arc welding, can be classified into 8 welding processes:
- Shielded metal arc welding / SMAW
- TIG welding / GTAW
- Plasma arc welding
- MIG/ MAG welding
- Self-shielded arc welding
- Submerged arc welding
- Electrogas arc welding
- Stud welding
pressure welding can be classified into 7 welding processes:
- Forge welding
- Electric resistant welding
- Friction welding
- Explosion welding
- Pressure gas welding
- Ultrasonic welding
- Cold welding
Advantages of the welding process
- Simple joint configuration
- Economical due to possible material savings
- High joint efficiency and excellent leak tightness
- The almost unlimited thickness for joining
- Less noise during welding work
disadvantages of the welding process
- Higher tendency to cause distortion due to local h ting and cooling
- Higher tendency to generate residual stresses which adversely affect fatigue strength and stress corrosion cracking
- higher tendency to degrade the characteristics of the base metal being affected by the welding heat, possibly causing weld cracks and reduced toughness and corrosion resistance
- higher possibility of brittle fracture of the whole structure caused by a crack, without arresting the crack at the weld joint
- Special consideration is required about the weld metal which has different properties from the base metal.
- Not easy to conform to weld qualities because of strong dependence on the skill of welding personnel.
It is important to control the weld quality, that’s why many organization develop welding Code and Standard. The famous welding codes are ASME Boiler and pressure vessel code Section IX, API 1104 for pipeline and AWS D1.1 for structural steel. These codes control and standardize the welding requirement such as WPS – Welding procedure specification, welder qualification, welding position, and other requirements. with this regulation, the manufacturers can reduce welding defects, reduce the welding repair rate and improve equipment integrity. As the last guard, a welding inspector is available to inspect the preparation of welding, process of welding and welding result.