TECHNICAL
SMAW PROCESS
Shielded Metal Arc Welding/Stick (M.M.A.) Welding
In this welding process, an electric arc is created between a coated consumable electrode and the work piece to be welded. This causes the parent materials to be fused together and the electrode to melt. The electrode is of similar material to the parent material, and by melting provides the weld (or joint) with a reinforcing filler material.
The electrode may be coated with basic, rutile or cellulose material, and as the coating burns it protects the arc and weld pool from the atmosphere with a gaseous shroud. The slag which solidifies over the newly deposited weld also protects it from the atmosphere.
TIG PROCESS
Gas Tungsten Arc Welding
In this welding process, the electric arc is generated between a non-consumable tungsten electrode and the work piece to be welded. An inert gas atmosphere, usually argon, protects the weld pool. Welding can be achieved by simply fusing the parent materials, or with the addition of a welding rod similar to the parent material.
The rod is allowed to melt in the electric arc and will fill and reinforce the weld joint. Arc ignition is achieved by touching the work piece with the electrode and lifting it by a few millmetres (contact ignition and lift ignition) or by using a high frequency voltage discharge, a gap of about 4 millmetres is maintained from the tungsten electrode and the work piece, high frequency (HF) spark is generated using HF, this makes the air gap conductive, this allows the arc to be established without touching of the work piece and tungsten electrode, therefore no tungsten contamination can occur ( High Frequency ignition).
The following can be used:
-DC (continuous) current is used with most metals(steel/stainless steel).
-AC (alternative) current is ideal for welding aluminium and other materials containing surface refractory oxide.
MIG PROCESS
Gas Shielded MIG Welding
The MIG/TIG welding process (or GMAW – Gas Metal Arc Welding) uses an electric Arc created between a continuous consumable wire and the work piece to be welded, all protected within a gas atmosphere. This atmosphere can be either inert (Argon) or active (CO2 or a mixture of Argon and CO2).
The wire is continuously fed through a gun to the weld pool by a wire feeder. Either Solid Wire (GMAW) or Cored Wire (FCAW-GS-flux-cored arc welding, gas shielding) can be used.
Self Shielded Flux-Cored Welding
Innershield is an Arc welding process in which welding heat is created from an arc between a continuous flux cored wire and the wire and the work piece. The flux provides gas shielding for the arc and a protective slag covering of the weld deposit.
Submerged Arc Welding
The mechanics of the Submerged Arc Welding process (SAW): Both the electrode and the base metal are melted beneath a layer of flux. This layer protects the weld metal from contamination and concentrates the heat into the joint. The molten flux rises through the pool, deoxidising and cleaning the molten metal. It then forms a protective slag covering and maintains the newly deposited weld.
The range of applications increases from 2mm, with no upper limit. Subarc is one of the most versatile of welding processes. All steel grades, from non to high alloyed, including Ni-based, can be welded with a combination of various application techniques.Ranging from a single electrode-single power source to a combination of four power sources feeding two wires each, Lincoln is proud to offer an extensive range of solutions to the market.
As a global supplier including equipment and consumables, Lincoln’s knowledge in the SAW process will support you in reaching the toughest productivity and quality targets.
TUNGSTEN ELECTRODES (Safety in Use and Effectiveness)
The tungsten arc welding concept, originally introduced as a practical tool in 1950, is now established as the most versatile technique for producing fusion welds to the highest quality standards.
A temperature of around 4,000ºC is generated in the arc during welding and the role played by the electrode is therefore crucial. It must have a high melting point and it must be non-consumable: tungsten quickly established itself as the most suitable material. As the knowledge of arc characteristics increased however it became clear that the use of pure tungsten presented some limitations on process development, particularly arc starting, stability and electrode wear. Early research showed that the addition of thoria resulted in overall improvements in performance and from this work a range of tungsten electrodes containing oxide additions or ‘dopants’ were introduced progressively.
Despite the clear significance of electrode composition the last 60 years has witnessed the publication of few scientific papers of practical use. Some of these advocate the widespread use of dopants on the basis of improved welding performance, some highlight the hazards associated with them. Evidence supporting results of these trials is flawed however and cannot be used as a basis on which to make generalised conclusions. Here we present an objective review of what has become an emotive issue – the use of dopants in tungsten electrodes. The issues under examination fall generally into two categories; those associated with technical and commercial advantages, and those relating to health hazards.
Read the full article published by Huntingdon Fusion Technologies here……..
WELDING FUMES
On 14 January 2019, the HSE (UK) shared with the Industry and Regulatory Forum on Local Exhaust Ventilation (LEV) that they have raised enforced control measures for welding operations in the UK. This follows an announcement from the International Agency for Research on Cancer (IARC), who classified welding fumes and UV radiation from welding as Group 1 carcinogens. IARC published their findings in Lancet Oncology in 2017 in a paper titled ‘Carcinogenicity of welding, molybdenum trioxide, and indium tin oxide’. [Citation.1]
The raised enforced control standards are highlighted below: ·All forms of welding fume can cause cancer. · Fume Control is required at all times ·Indoor welding tasks require the use of LEV. (Local Exhaust Ventilation) ·If LEV is unsuitable then RPE is also required. (Respiratory Protective Equipment) ·Outdoor welding requires use of RPE.
Michael Edwards, from the UK’s IOSH (Institute for Occupational Safety and Health), has explained what this means for industry……
“These changes to control standards for welding fumes currently only affect members working within the UK and will have implications to a whole range of different industries where welding operations occur. For those members outside of the UK, we can expect that your local regulators may also extend their enforcement position in the future, as this is based on evidence from an international agency”
The fume given off by welding and hot cutting processes is a varying mixture of airborne gases and very fine particles which if inhaled can cause ill health. Gases that may be present in welding and cutting fume are: nitrous oxide (NOx), carbon dioxide (CO2), carbon monoxide (CO), shielding gas (eg Argon, helium) and ozone (O3). The visible part of the fume cloud is mainly particles of metal, metal oxide and flux (if used). The predominant risk associated with aluminium welding is exposure to ozone. It is generally accepted that stainless steel fume is more hazardous than mild (carbon) steel fume due to the higher chromium and nickel content.