Basic Welding: Submerged Arc Welding

Submerged-arc Welding

Overview: SAW involves formation of an arc between a continuously-fed bare wire electrode and the work piece. The process uses a flux to generate protective gases and slag, and to add alloying elements to the weld pool. A shielding gas is not required. Prior to welding, a thin layer of flux powder is placed on the work piece surface. The arc moves along the joint line and as it does so, excess flux is recycled via a hopper. Remaining fused slag layers can be easily removed after welding. There is no visible arc light, welding is spatter-free and there is no need for fume extraction.

SAW is usually operated as a fully-mechanised or automatic process, but it can be semi-automatic. Welding parameters: current, arc voltage and travel speed all affect bead shape, depth of penetration and chemical composition of the deposited weld metal. Because the operator cannot see the weld pool, greater reliance must be placed on parameter settings.

Applications of SAW process: SAW is used for longitudinal and circumferential butt and fillet welds, in flat position. For circumferential joints, the workpiece is rotated under a fixed welding head with welding taking place in the flat position. Depending on material thickness, either single-pass, two-pass or multipass weld procedures can be carried out. There is virtually no restriction on the material thickness, provided a suitable joint preparation is adopted. Most commonly welded materials are carbon-manganese steels, low alloy steels and stainless steels, although the process is capable of welding some non-ferrous materials with judicious choice of electrode filler wire and flux combinations.

Hazards in SAW: This process has all the hazards of MMA, except for Welders’ eye, since the molten weld pool is always covered under flux.

Basic Welding, Metal Inert Gas Welding

Solid wire MIG Welding

Overview: MIG is similar to MMA in that heat for welding is produced by forming an arc between a metal electrode and the workpiece; the electrode melts to form the weld bead. The main difference is that the metal electrode is a small diameter wire fed from a spool. As the wire is continuously fed, the process is often referred to as semi-automatic welding.

Applications: MIG is widely used in most industry sectors and accounts for almost 50% of all weld metal deposited. Compared to MMA, MIG has the advantage in terms of flexibility, deposition rates and suitability for mechanisation. However, it should be noted that while MIG is ideal for 'squirting' metal, a high degree of manipulative skill is demanded of the welder.

Hazards in MIG Welding: This process has similar hazards that of MMA Welding. You could read the same here.

Basic Welding: Gas Tungsten Arc Welding

Tungsten Inert Gas (TIG) Welding




Overview: In the TIG process the arc is formed between a pointed tungsten electrode and the workpiece in an inert atmosphere of argon or helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding. Because the electrode is not consumed during welding, the welder does not have to balance the heat input from the arc as the metal is deposited from the melting electrode. When filler metal is required, it must be added separately to the weldpool.
Applications of TIG welding : TIG is applied in all industrial sectors but is especially suitable for high quality welding. In manual welding, the relatively small arc is ideal for thin sheet material or controlled penetration (in the root run of pipe welds). Because deposition rate can be quite low (using a separate filler rod) MMA or MIG may be preferable for thicker material and for fill passes in thick-wall pipe welds.
Hazards in GTAW welding: All hazards related to MMA welding are always applicable to TIG welding also. In addition the following hazards are present in TIG welding:
a) Asphyxia: TIG welding involves use of inert gases to protect weld pool form atmosphere. When this process is used in confined spaces, it may lead to asphyxiation. Welders have to be cautious about ventilation available to them.
b) Leakage of water, short circuit, hot water bath: This process uses water to cool the tungsten electrodes. This water is circulated by a pump & is recycled through a tank. For some reasons if the water circuit is jammed or level of water in the tank is insufficient, the PVC pipes carrying water may burst, giving the welder a hot water bath. Also leaks through pump & tank may interfere with the electric circuits causing short circuits.

Basic Welding : Manual Metal Arc Welding

Manual Metal Arc Welding
Overview: Perhaps the most primitive of welding processes, this process involves a metal rod with flux coating to give a protective gas shield. When an arc is struck between the metal rod (electrode) and the work piece, both the rod and work piece surface melt to form a weld pool. Simultaneous melting of the flux coating on the rod forms gas and slag which protects the weld pool from the surrounding atmosphere. The slag solidifies and cools and must be chipped off the weld bead once the weld run is complete (or before the next weld pass is deposited).

The process allows only short lengths of weld to be produced before a new electrode needs to be inserted in the holder. Weld penetration is low and the quality of the weld deposit is highly dependent on the skill of the welder.

Applications of MMA Welding: Due to the simplicity of apparatus, this process is widely used for welding applications in difficult locations, like at heights or where the joint is in abnormal welding position. This process is normally used where the weld metal deposition is less than 20Kg. If the deposition is more than 20 Kg, MMA will be used only if automated processes like MIG / SAW cannot be utilized.

Hazards in MMA Welding: The following hazards are common for MMA process:

a) Welders’ Eye or Arc Eye: This normally happens dues to accidental or inadvertent eye contact with the welding arc. Operators & people in close vicinity of the weld process must wear suitable glasses to safeguard from this hazard.

b) Fumes: The flux coating on the electrodes create gas fumes that protect the weld pool from environmental hydrogen. However these fumes are harmful to the health of the welder. Sufficient air draft has to be provided such that while the fumes protect the weld pool, they do not come in the respiratory zone of the welder.

c) Electric Shock: This may be caused due to improper welding cable connections, improper grounding of workpiece, damaged insulations of power cables etc. thw welder has to check for these points, every time he starts a fresh job, to safeguard against this hazard.

d) Burns: This happens due to contact with hot metal surfaces, contact with flying weld spatters, contact with molten slag etc. Welders must use appropriate clothing, gloves & guards so as to prevent burn injuries.

e) Fire: Since the process involves a naked arc & flying spatters, a danger of fire is always present. There shall be no flammable articles near the welding process. Also the welder has to be cautious enough to see where the spatters & electrode stubs are falling so as to prevent occurrence of a fire.

f) Fall: When working at heights, the falling hazard is eminent. Relevant fall arrestors have to be worn, if proper platform cannot be provided.

What is fabrication ?

What is Fabrication ?

The definition of the word “Fabrication”, as per the Oxford Dictionary is “ to construct or manufacture an industrial equipment”. That is the simplest way to define fabrication for industry.

A fabrication engineer will put to use a variety of processes in his armoury to construct an equipment as desired by his customer. Even as the fabrication technology has developed considerably over past 2 decades, the basic processes involved in fabrication of equipment remain the same.

The core processes involved in fabrication are Gas Cutting, Plasma Cutting, Welding, Plate bending & rolling, Hot / Cold Forming, grinding, heat treatment, surface treatment & coating. This list is however only indicative. Each of these processes has many variants. Also this list is not exhaustive. There are a lot of support processes in fabrication which may not be termed as core processes though. It can be noted that there is huge element of human involvement in these processes. A workshop that carries out such processes will normally be termed a fabrication shop or fab shop for short. A fab shop will typically employ fitters (some times called fabricators), welders and grinders (small tool operators).

However, machining processes too are integral part of construction of equipments. In industry these processes are collectively termed as machining. It may include operations like turning, milling, drilling, tapping, thread whirling. Machines used may vary from simple lathes & ,milling machines to sophisticated CNC machines.

While customers will normally refer to their equipment manufacturer as fabricator, the manufacturer may have distinct shops for fabrication and machining.

Therefore hereon I will be refering manufacturer as fabricator, shop that builds an equipment by manual / semi-automatic processes mentioned above as fab shop and shop that carries out major machining operations as machine shop.

I will be discussing each of the core fabrication processes to the details that I have in my limited knowledge, in future posts.