Discuss in the group.
1. Do you think welding is a dangerous/hazardous profession?
2. What type/types of welding do you consider the most/least hazardous?
Why?
2. Look through the text Health Risks of Welding Fume/Gases and list the risks generated during welding.
Health Risks of Welding Fume/Gases
Welding fume is a mixture of airborne fine particles.
Toxic gases may also be generated during welding and
cutting.
Particulate fume
More than 90 % of the particulate fume arises from
vaporisation of the consumable electrode, wire or rod as
material is transferred across the arc or flame. The range of
welding particles size is shown in relation to the more familiar types of dust
and fume. The respirable fraction of particles (especially less than 3μm) are
potentially the more harmful as they can penetrate to the innermost parts of
the lung.
Gases
Gases encountered in welding may be:
- Fuel gases which, on combustion, form carbon dioxide and, if the flame is
reducing, carbon monoxide;
- Shielding gases such as argon, helium and carbon dioxide, either alone or in
mixtures with oxygen or hydrogen;
- Carbon dioxide and monoxide produced by the action of heat on the
welding flux or slag;
- Nitric oxide, nitrogen dioxide and ozone produced by the action of hea
APPENDIX 1
Texts for written translation
Text 1
SUBMERGED ARC WELDING
Submerged arc welding (SAW) is a common arc welding process. It requires
a continuously fed consumable solid or tubular (metal cored) electrode.
The molten weld and the arc zone are protected from atmospheric contamination
by being “submerged” under a blanket of granular fusible flux. When molten,
the flux becomes conductive and provides a current path between the electrode
and the work. SAW is normally operated in the automatic or mechanized mode,
however, semi-automatic (hand-held) SAW guns with pressurized or gravity
flux feed delivery are available. Deposition rates approaching 100 lb/h (45 kg/h)
have been reported – this compares to 10 lb/h (5kg/h) (max) for shielded metal
arc welding. Currents ranging from 200 to 1500 A are commonly used; currents
of up to 5000 A have been used (multiple arcs). Single or multiple (2 to 5) electrode wire variations of the process exist. DC or AC power can be utilized, and
combinations of DC and AC are common in multiple electrode systems. Constant
voltage welding power supplies are most commonly used, however constant
current systems in combination with a voltage sensing wire feeder are
available.
Material applications are carbon steels, low alloy steels, stainless steels, nickel-based alloys, surfacing applications are wear-facing, build-up, and corrosion
resistant overlay of steels.
Advantages of SAW: 1) high deposition rates (over 100 lb/h (45 kg/h) have
been reported; 2) high operating factors in mechanized applications; 3) deep
weld penetration; 4) sound welds are readily made (with good process design
and control); 5) high speed welding of thin sheet steels at over 100 in/min
(2,5 m/min) is possible; 6) minimal welding fume or arc light is emitted.
Limitations of SAW: 1) limited to ferrous (steel or stainless steels) and
some nickel based alloys; 2) normally limited to long straight welds or rotated
pipes or vessels; 3) it requires relatively troublesome flux handling systems;
4) flux and slag residue can present a health and safety issue; 5) requires interpass
and post-weld slag removal.
Key SAW process variables: 1) wire-feed speed (main factor in welding
contact tip to work (CTTW); 5) polarity and current type (AC or DC).
Other factors: 1) flux depth / width; 2) flux and electrode classification and
type; 3) electrode wire diameter; 4) multiple electrode configuration.
Text 2
SAFETY
Users of our company’s welding equipment have the ultimate responsibility for ensuring that anyone who works on or near the equipment observes all the relevant safety precautions. Safety precautions must meet requirements that apply to this type of welding equipment. The following recommendations should be observed in addition to standard regulations that apply to the work-place.
All work must be carried out by trained personnel, well acquainted with the operation of the welding equipment, incorrect operation of the equipment may
lead to hazardous situations which can result in injury to the operator and damage
to the equipment.
1. Anyone who uses this welding equipment must be familiar with:
its operation;
location of energy stops;
its function;
relevant safety precautions.
2. The operator must ensure that:
no unauthorized person is stationed within the working area of the equipment when it is started up;
no-one is unprotected when the arc is struck.
3. The workplace must:
be suitable for the purpose;
be free from draughts.
4. Personal safety equipment:
always wear recommended personal safety equipment, such as safety
glasses, flame-proof clothing, safety gloves;
do not wear loose- fitting items, such as scarves, bracelets, rings, etc.,
which could become trapped or cause burns.
5. General precautions:
make sure the return cable is connected securely;
work on high voltage equipment may be carried out only by a qualified
electrician;
appropriate fire extinguishing equipment must be clearly marked and
close at hand;
lubrication and maintenance must not be carried out on the equipment
during operation.
Warning.
Arc welding and cutting can be injurious to yourself and others. Take precautions
when welding. Ask for your employer’s safety practices which shouldbe based on manufacturer’s hazard data.
Electric shock can kill:
install and earth the welding unit in accordance with applicable standards;
do not touch live electrical parts with bare skin, wet gloves or wet clothing;
insulate yourself from earth and the workpiece;
ensure your working stance is safe.
Fumes and gases can be dangerous to health:
keep your head out of the fumes;
use ventilation, extraction at the arc, or both, to take fumes and gases
away from your breathing zone and the general area.
Arc rays – can injure eyes and burn skin:
protect your eyes and body. Use the correct welding screen and filter
lens and wear protective clothing;
protect by-standers with suitable screens or curtains.
Fire hazard:
sparks (spatter) can cause fire. Make sure therefore that there are no inflammable
materials nearby.
Noise:
protect your ears. Use earmuffs or other hearing protection;
warn by-standers of the risk.
Malfunction: call for expert assistance in the event of malfunction.
Text 3
PLASMA ARC WELDING
Plasma arc welding (PAW) is an arc welding process similar to gas tungsten
arc welding (GTAW). The electric arc is formed between an electrode,
which is usually but not always made of sintered tungsten and the work-piece.
The key difference from GTAW is that in PAW, by positioning the electrode
within the body of the torch, the plasma arc can be separated from the shielding
gas envelope. The plasma is then forced through a fine-bore copper nozzle
which connects the arc and the plasma exits the orifice at high velocities, approaching
the speed of sound, and temperature approaching 20 000 ºC.
Plasma arc welding is an advancement over the GTAW process. It can be
used to join all metals that are weldable with GTAW, i.e. most commercial metals
and alloys. Several basic PAW variations are possible by varying the current,
plasma gas flow rate, and the orifice diameter, including: micro-plasma (< 15 Amperes);
melt-in mode (15–400 Amperes); key-hole mode (100 Amperes). Plasma
arc welding has a greater energy concentration as compared to GTAW. A deep narrow
penetration is achievable, reducing distortion and allowing square-butt joints in
material up to 12 mm thick. Greater arc stability allows a much longer arc length
and much greater tolerance to arc length changes. Its limitation is that PAW requires
relatively expensive and complex equipment as compared to GTAW.
At least two separate (and possibly three) gas flows are used in PAW: plasma gas which flows through the orifice and becomes ionized; shielding gas,
which flows through the outer nozzle and shields the molten weld from the atmosphere;
back-purge gas, required for certain materials and applications. These
gases can be the same, or of differing composition.
Key process variables are: current type and polarity; usually DCEN from a
CC source; AC square wave, common on aluminum and magnesium. Current
can vary from 0,5 to 1200 A; current can be constant or pulsed at frequencies up
to 20 kHz. Gas flow rate is a critical variable and must be carefully controlled
based upon the current, orifice diameter and shape, gas mixture, and the base
material and thickness.
Depending upon the design of the torch, e.g. orifice diameter, electrode design,
gas type and velocities, and the current levels, several variations of the plasma process are achievable, including plasma arc welding, plasma arc cutting,
plasma arc gouging, plasma arc surfacing and plasma arc spraying.
Text 4
UNDERWATER WELDING
Underwater welding refers to a number of distinct processes that are performed
underwater. The two main categories of this techniques are wet underwater
welding and dry underwater welding, both classified as hyperbaric welding.
In wet underwater welding, a variation of shielded metal arc welding is
commonly used, employing a waterproof electrode. Other processes that are
used include flux-cored welding and friction welding. In each of these cases, the
welding power supply is connected to the welding equipment through cables and
hoses. The process is generally limited to low carbon equivalent steels, especially
at greater depths, because of hydrogen-caused cracking. In dry underwater welding the weld is performed at the prevailing pressure in a chamber filled with
a gas mixture sealed around the structure being welded. For this process, gas
tungsten arc welding is often used, and the resulting welds are of high integrity.
The applications of underwater welding are diverse – it is often used to repair
ships, offshore oil platforms and pipelines. Steel is the most common material
welded. For deep water welds and other applications where high strength is
necessary, dry underwater welding in most commonly used. Research into using
dry underwater welding at depths of up to 1000 meters is ongoing. In general,
assuring the integrity of underwater welds can be difficult, but it is possible using
non-destructive testing applications, especially for wet underwater welds,
because defects are difficult to detect if they are beneath the surface of the weld.
For the structures being welded by wet underwater welding, inspection following
welding and assuring the integrity of such welds may be more difficult
than for welds deposited in air. There is a risk that defects may remain undetected.
The risks of underwater welding include the risk of electric shock to the
welder. To prevent this, the welding equipment must be adaptable to a marine
environment, properly insulated and the welding current must be controlled.
Underwater welders must also consider safety issues that normally divers face;
most notably, the risk of decomposition sickness due to the increased pressure of
inhaled breathing gases. Another risk, generally limited to wet underwater welding,
is the build-up of hydrogen and oxygen pockets, because these are potentially
explosive.
Text 5
