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Messaggioda Aldebaran » 09/04/2010, 17:55

· What is welding?
Some general notions, with particular reference to different types of welding. · Welding and Cutting ossigas
or welding ossigas
The brazing or
or brazing
or cut ossigas
· Electric arc welding
or welding electrode
o The MIG / MAG
or TIG welding
or plasma cutting
Automates in welding and cutting
Welding, or Plasma
or cutting machines

Two pieces of metal to be welded need to be rendered by a
heat source.
The oxyacetylene torch, invented by Frenchman Charles Picard in the early century, is the
oldest tool that allows a mixing between their oxygen flame
(Oxidizing gas) and acetylene (fuel gas), it reaches temperatures of over
3000 ° C.
This tool is used today, both in large industrial complexes, both in smaller companies
craft up to the workgroups of fans of "going it alone."
While the oxyacetylene welding stated in various industrial sectors, was born the electric arc welding;
this new technique had no trouble finding the right position in the market thanks to its quality and
ease of use.
It uses a bow as a source of electric heat (5,000 ° C).
The electrode, melting gradually, serves as both a filler metal that polo
Electric arc furnace for the training.
The need to improve the quality and productivity has prompted engineers to improve
increasing the electric arc welding process. Hence the appearance of
new technologies include, for example, TIG (Tungsten Inert Gas)
Using an infusible tungsten electrode in a protective atmosphere of gases
This system allows high quality welds on
steels, copper alloys and light alloys and is used to
aerospace, chemical and food industry.
Subsequently, electric welding makes new and important progress with the adoption
of semiautomatic and automatic processes such as MIG (Metal Inert Gas) and
MAG (Metal Active Gas).
In semi-automatic procedure, protection of the weld pool is
provided by a gas, while the electrode formed by a wire
solid is pushed continuity in the bathroom of fusion: the operator has the task of monitoring the
joint with the torch. This process can be fully automated if
the progress of the torch is adjusted by a machine. provided by a gas, while
electrode, consisting of a solid wire, is pushed into the bathroom continuity of fusion:
the operator's task is to follow the joint with the torch.
By replacing the solid wire, use a cored wire that allows
improve the quality and aesthetics of welding, as well as offering a significant
improved productivity.
The cored wire is a tube that contains granules in a stream that protects the
weld pool and stabilizes the arc: its application has
uses very different.
It 'can replace the atmosphere of protective gas with a flow
granules that is responsible for protecting the weld pool of
stabilize the arc and adding alloying elements to the bath
merger itself.
Resulting in the welding process called "submerged arc, which is used for joining large
thicknesses as high-pressure tanks, components of thermal power plants and pipelines and aqueducts
New and more sophisticated automatic welding techniques were added such as, for example, the procedure
TIG four electrodes and submerged arc with three wires. These allow you to significantly accelerate the
welding speeds and are used especially in heavy carpentry and pipe production
The plasma process, which allows the welding of stainless steel sheets up to 10 mm
thickness in one pass, finds its most common location in the welded steel construction
steel, providing safe quality assurance.
The procedure for electron bombardment under vacuum and laser welds allow you to make more
complex of special metals such as titanium, magnesium and zirconium, with other difficult to weld
They are also particularly suitable where dimensional requirements do not allow deformations.
These processes find their greatest use in the automotive industry and in research laboratories.
The welding is to assemble metal parts, with or without filler metal,
by gradual melting of the base metal.
In the case of welding with filler metal, it must submit a melting temperature and
mechanical properties equivalent to those of the parts to assemble.
This process is used for applications to do-it-yourself, maintenance, repair ...
It 'necessary to use a strongly reducing flame Oxy-fuel. Acetylene can get
good welds quality.All oxyacetylene welding is a manual welding process autogenous
Fusion, which uses the combustion of acetylene by oxygen.
Oxygen [/ b
] The oxygen is in the air to a standard of 21%. It 's a gas combustion
colorless, odorless and tasteless. The system of storage and transported in
cylinders with a maximum pressure of 200 kg/cm2.
[B] Acetylene
Acetylene is a gas without color and tasteless, but with a very characteristic odor. It 'very
flammable when mixed with air or oxygen. In the presence of copper, silver, copper alloys containing
more than 70% of this metal, or mercury, can form an explosive product. To exert
industrially is dissolved nell'acetone filling the cylinders with a porous material that may be considered
acetone with acetylene dissolved in it.
The place oxyacetylene
A post modern oxyacetylene generally consists of:
§ gas cylinders (oxygen: white sleeve - Dissolved Acetylene: orange cone)
§ The pressure (oxygen and acetylene)
§ accessories (safety valves, rubber hoses, quick couplings for connection
clamp rings)
§ torch welding and cutting
Gas cylinders
Gas cylinders are high pressure vessels in steel or aluminum alloy constructed
specifically in order to contain compressed gas at 200 atmospheres.
Differ for the various types of gas, the color of the object (white for oxygen, acetylene orange, gray for argon, etc.)..

The pressure
The pressure regulators are designed to reduce and stabilize the gas pressure
Safety valves
The safety valves or safety devices, prevent the return of
flame and gas into the cylinders, reducing the risk of outbreak of
cylinders themselves, and discharging excess pressure to avoid explosion
valves for cylinders or rubber hoses.
The welding torch
The welding torch is a device that allows a convenient gas mixture
fuel with the combustion gases which, fire at the exit, giving rise to the formation of a flame
stable shape, power and certain property.
Oxyacetylene welding mild steel
Mild steel, commonly called "iron", is a ductile metal, malleable and tenacious, its melting point
is 1510 ° C. Heated to red and in contact with air it oxidizes rapidly on the weld pool
tends to form a surface layer of iron oxide. Mild steels and extra sweet commerce
can be welded to perfection, medium hard steels are less easily weldable, hard steels and extra hard
are practically weld with a torch.
Oxyacetylene welding of cast iron
Cast iron is an alloy of iron and carbon and its melting point varies from 1,050 to 1,200 ° C according to the
proportion of carbon and other elements that compose it. The cast iron having no toughness hot nor
elongation, is likely to break, what makes difficult the preparation and execution of the weld. In
practical welding of cast iron is limited to repair work.
Oxyacetylene welding of copper
Copper is a reddish metal, tough, malleable and ductile, and its melting point is 1083 ° C. E '
a good conductor of heat, very fragile when hot. At 500 ° C, the toughness is reduced by 60%. Copper is strong
well when pure, but when this oxide contains the action of heat and makes the changes
Copper unfit for any mechanical work.
Oxyacetylene welding brass
Brass is an alloy of copper and zinc, which may also contain small amounts of tin, lead and aluminum.
Brass welding results in the following phenomena that make it difficult to implement: oxidation alive
metal species at the expense of zinc volatizza; uptake molten metal gas, with
production of blowholes.
Oxyacetylene welding aluminum
When pure aluminum is a silvery white metal, very malleable, pliable and tough. The
its thermal conductivity is very high, its tenacity is weak, hot, its melting temperature is
657 ° C. Aluminum oxidizes very easily produce a aluminum oxide (alumina), which melts at
2200 ° C. and that, during welding greatly hinders the union of the fusion molecules.
Braze welding
The brazing is a technique of assembly with a filler metal, whose melting temperature is
less than that of parts to assemble.
The operative technique is similar to that of the weld: the implementation is gradual, without melting of borders by
The brazing is commonly used for vehicle bodies, but also to work
repair and maintenance.
We recommend the use of a powerful flame Oxy-fuel and reducing agent (acetylene).
The soldering is assembling metal parts using a filler metal whose temperature
fusion is less than that of parts to assemble.
The weld metal penetrates by capillary action between the pieces to be assembled, having been subjected to pickling.
Brazing is often used for the installation of sanitary facilities and water for manufacturing or
For brazing requires a powerful and reducing flame (acetylene tetrene).
Brazing is the oldest man to join the process used for fusion, two pieces of metal. In fact
was already known and used by the Phoenicians and Etruscans.
He is currently a popular procedure among both industry and is used by both craftsmen.
Brazing using the same equipment oxyacetylene welding, but it is executing
very fast.
The temperature performance of the joint is lower than that required by oxyacetylene welding.
Brazing does not require mechanical finishing operations and the deformation of the workpiece, due to the expansions,
are negligible or minimized.
Brazing is used in place of autogenous welding when:
· E 'need to reduce the heating of the workpiece
· The joints are made of materials difficult to weld
· The pieces are of different nature and their welding is impossible
· The appearance of the joint is a priority or essential
Depending on the melting temperature of filler metal, as mentioned, can be used
different means of heating. The melting temperature then determines the soldering or brazing.
Oxy-fuel heating
Oxy-fuel flame can heat up locally pieces of any shape and volume, surface or
heart, between 150 ° C and 1,500 ° C, in whole or in part.
Fields of application
Boilers, mechanical, agricultural materials, railway construction, metal and naval foundry ....
Heating before forming (forging, pressing, stretch-forming lathe ...), preheating and post-heating,
recoverable deposits of metallic, shrink fit. The torch is a procedure that combines the action of a
Oxy-fuel flame heating with that of a jet of oxygen.
Flame heating
The action of flame heat is used to bring the point of ignition at the required temperature (1,300 ° C for
steel) and keep it.
Performance Factors
The use of a fuel efficient (specific power and high flame temperature) reduces the
starting time, increase the cutting speed, a better quality of cut, reducing the width of
cut. The jet of cutting oxygen affords a means of burning metal.
Terms of oxyfuel
Three conditions are necessary: ·
. The oxidation reaction must be exothermic
• The ignition temperature must be below the melting temperature of the metal.
In practice, non-alloy steel or low alloy can be easily subjected to oxyfuel.
Certain materials that do not meet these conditions may be tested by torch
the use of powdered iron (stainless steel, cast iron and alloyed steels).
Oxygen cutting
The cutting speed depends on the nature and quantity of impurities in oxygen.
Nature of metal
The torch parameters depend largely on the chemical composition (carbon content and
elements additives), the homogeneity of metal from the surface layer of parts (oxides, paints) and
initial temperature of the workpiece.
Fields of application
The torch is used with automatic process to obtain a quality cutting, machinery or equipment
robot, with manual process, maintenance or demolition, in steel, for cutting
slabs or blooms output of continuous casting.
Rectifiers mechanical adjustment
Machines are simpler and cheaper to settle.
Receive an input alternating current voltage and provide output voltage (V) and current (A) Continuous
as the battery of a car.
The current supplied by the welder must be adjusted to the electrode diameter, bearing in mind
that if the electrode is low value does not melt, while if the electrode is high is red hot on the whole
length instead of one bit, making it impossible to weld.
Electronically controlled rectifiers
Machines are most suitable shipyards and fabrication medium-large.
Receive an input alternating current voltage and provide output voltage (V) and current (A) Continuous
as a car battery.
They differ by rectifying mechanical adjustment as in the regulation of these machines
current is acting on a potentiometer (as the volume of a radio).
The most obvious benefit of this solution is more professional the opportunity to have a remote control
with which to adjust the current required to melt the electrode.
Welding Inverters
In recent years there has been increasingly widespread use of electronic device called an inverter
on welding.
Its success, despite increased cost made to facilities, has been enormous, particularly
welding electrodes up to 200 amperes.
One of the features introduced by the inverter is, in fact, reducing the weight of the welder
especially valuable where this decrease has transformed plants weighing about 20-30 kg in
portable equipment weighing 5-10 kg
But how was this possible?
You must know that the weight of a welder is determined largely by the processor dimensionei
with which it is built. The weight of the transformer is in turn determined by the amount of iron and copper
with the quality and size.
More parameters are "tension" and "current" in play, the greater must be the weight of the transformer.
When sizing a transformer, however, in addition to voltage and current to be reached, it
must also take into account the frequency with which it is fed (Hz). The higher the frequency and
child may be the weight of the transformer.
But there is a problem ....
The frequency is determined by the power line and is not editable.
It was thought then to introduce a mechanism upstream of the transformer, called inverter,
as the input voltage swings up to 40-50000 Hz, could thus reduce, without prejudice to other
parameters, the size of the transformer up to one third of its size and hence its weight.
Choosing the welding electrode, adequate power
To determine the welding point according to the electrode diameter to be welded, just multiply the
same electrode diameter to 40 and then choose a welder who give at least the current value
resulting with cyclic 35%.
For eg a welding electrode diameter 3.25 [3.25 x 40 = 130 (A)], the welder must give at least
130 A to cyclic 35%.
The thickness of the electrode is chosen from those available on the market (from 1.0 to 6.0 mm in diameter), according to
thickness of the material being welded.
MIG / MAG welding
The wire welding in protective atmosphere is now
well known for some time: it is marked GMAW
(Gas Metal Arc Welding) and in the international symbols
replaced the previous M.I.G. (Metal Inert Gas) and M.A.G. (Metal
Active Gas), also still in common use.
E 'characterized by the fusion of a weld metal (filament) by a protective atmosphere created by a
gas line and gas are led by a torch that supplies electricity directly to the wire melting through
an arc body from the tip of the wire and the workpiece.
The power supply is provided by a source of special features, you will usually current
continues with positive polarity to the wire.
Description of a plant
A system for wire welding in protective atmosphere consists essentially of:

1) a source of DC
2) a device wire
3) a torch cable.
For power supplies are installed:
4) a gas cylinder with regulator and flow meter (if certain pre-heater for gas)
5) a skein of yarn.
The current source
The current source is almost universally a rectifier in constant feature. It
therefore, provides working voltages vary within a wide range of values in order to meet the needs
This welding system, delivering certain current density.
The variation in the voltage of power source can be done in different ways:
Switch · various outlets of the transformer
· Diary on sliding brushes by magnetic amplifier
¯ by using thyristors or transistors.
The first two systems are mechanical, the latter is electronic, the second and third allow
continuously adjustable. The amplifier, controlled diodes and transistors can also be adjusted during the
welding. In the DC circuit of the sources of good construction is an embedded device
Electrical (ballast) that facilitates the arc stability, it is connected to various statements by mass (negative) for
an exact determination of its action.
The wire
The wire is usually the best possible development, driven by a DC motor with adjustable
mail. It is suitable for pushing wires with diameter from 0.6 to 2.4 mm, changing the rollers between which hold the wire.
Since the wire speed is recalling more or less current, the exact adjustment of this parameter is
essential for good welding. For the use of solid wire feeder with two rollers proved to be the most suitable, for
cored wires made from thin ribbon 4 rollers are preferable to distribute the pressure of the device.
Are connected to the wire, in addition to the source, the cable with the torch and gas bottle.
The torch cable
The torch cable have
lengths between 2 and 4 m
depending on use;
greater lengths are
possible using special
torches system push-pull "with
built an engine that pulls the
wire to help push the normal engine towing. Another solution is given by guns microrocchetto
incorporated. Along the cable (usually within a protective sheath) are prepared: the holder of
welding current, control cables, tubing and any gas pipes for water, in the case of
torches are not natural cooling, in addition to wire hose. The handle of the torch makes a button
command to the insertion of the welding current, the leakage of gas and the progress of the wire.
The torch body, externally insulated, lead wire, gas and electricity, ending with a nozzle, from which
leaking gas through a special loudspeaker around a tube leading to the current thread that runs
Filler materials and gases
For the welding of all carbon steel, normally used only one type of wire suitable for use CO2 or
Argon/CO2 mixtures, such as shielding gases. The mechanical properties of the deposit obtained with this wire will vary, depending
the operative technique used.
The wire welding
The wire is wound on spools or support rings, the surface is copper, for protection and good contact
electric the coils free must not effect the propeller, the rigidity to enable the thrust coupling, but
should not be excessive. The diameters used are 0.6 to 0.8 - 1 - 1.2 to 1.6 (rarely 2 to 2.4) mm.
Welding gases
Carbon dioxide (CO2) used as protective gas must be perfectly dry (type SS) to avoid problems
during melting (spray) and the risk of hydrogen cracking. It results in a weld pool very
penetrated and relatively narrow. It 'totally harmless to the operator who must also be insured
a suitable replacement of ambient air (smoke evacuation).
The addition of argon results in mixtures that can contain 70 to 90% Ar. Is used for both
improve the appearance of the cord, is to raise the mechanical values, other things being equal conditions. The
penetration is greater at the center of the cord, the lower sides and this may present some risk of
The amount of protective gas ranges from 8 to 15 l / min. The flow is regulated, generally by reducing with
Operational techniques
Depending on the goals to be achieved adjusting the welding parameters so that the wire rests so
appropriate, namely:
• With the formation of droplets that pass through the arch
• With droplets are immersed in the bath before breaking the wire.
In the first technique is called "ARC-SPRAY" that arc spray in the second case of "SHORT-ARC" ie
short arc. Typically up to 200 A and about 24 V, the metal transfer occurs SHORT-ARC; above
of these values forms the arc spray.
On thin sheets, the bottom of welds and joints in the floor is not necessary technical SHORT-ARC, for
thickness and material for large penetrations using the technique SPRAY-ARC.
Reasons for choice:
· Productivity
· Ease of use in all positions
· Big use of the process
Ways of use:
· In most cases the use is manual. Since the activity of the wire is forcibly
automatic use of the procedure manual MIG / MAG gave rise to the term "semiautomatic"
In cases when auto welding robot
· Unalloyed or low alloy
· Stainless steels
· Light alloys or copper
· Boiler
· Production automotive, railway, shipbuilding.
TIG means "Tungsten Inert Gas."
The arc between an electrode body infusible tungsten that acts as the cathode (positive
negative) while the piece - protected by a non-oxidizing gas flow - plays the role
anode (positive).
These gas mixtures to be used need not be chemically oxidizing.
Key features of this procedure
The advantages of TIG give great flexibility of use, ensuring a quality
impeccable with minimum thickness (tenths of mm).
For several years, arc welding, protective atmosphere of argon with refractory electrode
(SistemaTIG = Tungsten Inert Gas) is applied worldwide, almost all fields of work Metals.
Even in Italy has considerably bringing widespread benefits of this system of their union, in many cases
the only one who can solve certain problems.
As is known, the principle is: it triggers the electric arc between a tungsten electrode (refractory
so do not fuse) and the workpiece, the latter is locally melted by the heat of the arc and the edges to be
solidify and unite together, with the possible addition of other materials of suitable composition, made
form of wire, around the arc.
It 's a process similar to the Oxy-acetylene welding, where the flame is replaced by the arc power and where
the necessary protection of the weld pool harmful influence of air is obtained by sending a current of
Argon, concentrically to the electrode, so as to create a protective cone.
Special torches, cooled by various means, ensure the two functions:
· Bring the current electrode
· Channel the gas to a nozzle surrounding the electrode itself.
An auxiliary system ensures the desired hourly outflow of Argon and his arrest
during work breaks.
Of course, good planning and robust and secure execution of all equipment is in the final
analysis, given the success of the entire plant.
Influence of electrode polarity
Quite different is the behavior of a tungsten electrode, depending on whether the same applies - in
Argon welding - the direct polarity (-) or the reverse (+).
In the first case the electrode is located Termoelettronica emission and electric arc concentrates the heat
on the piece. It has a strong penetration of the weld pool. On the other hand the bow has no strong power
Electric pickling bath, so the possible presence of surface oxides prevent the molten material
to bind well with that made.
When the polarity is reversed, the piece, connected to the negative, it emits electrons that break any film
oxide, clean the bathroom and provide a good union.
From the foregoing it is clear that can be welded with direct polarity (negative electrode) all
weld, except that the surface oxide which is always present, such as light alloys.
The latter, moreover, also can not be welded with reverse polarity (electrode positive) if
not present when small thicknesses. The problem is solved by alternating current, useful for welding
thin brass, especially if they contain much zinc.
Stainless steels are welded with polarity direct current (electrode negative).
To facilitate the trigger you can apply a lighter high frequency.
In welding wire there is adequate intake of any combustion of carbon and no evidence of
The thickness than 2.5 mm must be blunted. The filler material must be suited to the quality
stainless steel weld, never use wire electrode normal after removing the coating.
It can weld without filler material to a thickness of 2.5 mm. Never point with normal electrodes; do
pointings in Argon long about 20 mm at a distance of 100 to 150 mm.
We have seen the light alloys are welded AC and require, for good performance
cord, applying a high frequency generator with adequate characteristics.
If there is a strong oxidation, it is good to remove it by brushing or pickling.
For thickness over 6 mm should be a chamfer 60 to 90 ° C.
The thicknesses can be welded without filler, the latter in any case be
suitable quality compared to the workpiece. And 'unwise use of cuttings, always oxidized or dirty.
In addition to the light alloy and stainless steels can be welded under Argon, with electrode
tungsten, the following materials: mild steel and alloy, nickel alloys, copper and its alloys, titanium and
noble metals.
They can also be deposited weld and wear-resistant surfaces. For all these metals and alloys is used
direct current electrode negative polarity.
More than 2 mm thick is best to use a product gearbox of the type normally used for welding
oxyacetylene torch.
More than 4 mm is necessary to preheat to 260 - 300 ° C. In our past coverage, a little commuter; recovery
with little or no filler material.

Reasons for choice
· High quality metallurgical
· Welds clean and handsome
· Manual and automatic
Processed metals
· Unalloyed or low alloy steels
· Stainless
· Read or copper alloys
· Special alloys (nickel. titanium, zirconium, tantalum etc.).
Fields of application
All fields where quality takes precedence over productivity
· Aerospace industry
˛ Chemicals and Food
· Production of steel tubes
· Machining delicate and precision (boiler).
Plasma cutting compressed air is a patent of Air Liquide Group and plants FROMOS CUT
enclose all the experience gained over the years in this specific field of application.
Rugged and reliable, allowing, for the same output current, cutting thicker gauges or equal
thick, to use lower current and thus save energy.
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