Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF TH~ INV~NTION
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The present invënti~n reIàtës ~o~a`ternary protective -
gas m~xture for arc-weld~ng or hard-facin~ of steel.
A protective atmosphere is used in the processes for
welding or hard-faclng steel in which an electric arc is
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struck between a consumable electrode~ comprising a steel
wire, and a molten metal zone; this is done in particular
for steel alloys with a structure of austenite or ;~
austenoferrite with low ferrite content, such as nickel-
chromium steels.
DESCRIPTION 0~ THE PRIOR ~RT
Welding or hard-facing of steel alloys can be achieved
by a ~ariety of processesO Under conditions of high current
intensity, the transfer o~ molten metal from the consumable
electrode to the molten metal zone takes place as a spray
of droplets. The use of this process under very hot
conditions, known as the "Spray-Arc" or spray-transfer
process, in which the rate o metal deposition is 5 to ~-
lO kg/hour depending on the diameter of the wire, is limited
20~ to the~welding of sheets over 4 mm thick, since sheets `;
below this measurement are perforated,
In another process in which the transfer takes place
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by contact involving successions o~ short-circuits with
the molten zone producin~ short-arcs, known as the "Short-
~, 25~ arc"~process, metal~deposl~ion takes place at a rate of
abo~t 1 kg/hour; with this process, it is possible to weld
sheets less than 4 mm thick down to about 1.5 mm thick.
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Similarly, a process involving conditions of , .:
intermediate heat between the "Spray-Arc" and the "Short-
Arc" processes is used for welaing or hard-facing elements
made of alloy steels. According to this technique, known
as the "Pulsed-Arc" process, the transfer of molten metal
from the electrode takes place through the application of
current impulses.
Welding techniques by TIG or by micro-plasma have
been used ~or very thin steel alloy sheets. The results are
~ 10 significant, but the welding process is slow, in the region
; of 10 to 30 cm per minute.
Certain known mixtures of protective gas are very
suitable for use in welding the metals cited above, part-
~ icularly a protective atmosphere with four constituents,
;~15 designated by the trade mark "Noxalic", comprising 20 to
gOyO helium, 0.5 to 2% hydrogen (free or combined), gases
and vapours containing carbon and oxygen, preferably 2 to
6% carbon dioxide, and the balance argon. Other atmospheres
are likewise suitable for electric arc-welding or electric
~2~0~ arc hard-facing of steel alloys under ~as protection using a
continuous~wire electrode.
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~ However, the development of materials and techniques `
,~ has shown that the prior art processes present certain
~ difficulties in the case o~ a very thin assembly to be
welded. Thus~ with materials below 1.5 mm in thickness,
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it~ls~unllkely that good results can be reliably attained
with the aid o~ the protective gases currently available............... -
Research has been conducted into providing a protective
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atmosphere allowing thin sheets to be welded, and into
achieving an attractive assembly with excellent current
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density with the advanta~eous use of the "Spray-Arc", -
"Short-Arc" and "Pulsed-Arc" processes cited above~ ;
SUMMARY O~^THE INV~NTION
A protective mixture has been found, based on a rare
gas, such as argon and containing an active gas such as carbon
dioxide and a reducing gas such as hydrogen. More
particularly, one aspect of the present invention provides
a ternary protective gas mixture for arc-welding or arc hard-
facing of steel alloys, which comprises by volume 2.5 to
3.5% carbon dioxide, and 0.8 to 1.2% hydrogen, the balance
being argon. The argon pre~erably has a purity greater than
99%. This gas mixture makes it possible to obtain excellent `
; lS - results, especlally with thin sheets.
;~ ~ The invention also provides a method of arc-welding -
or arc hard-facing of steel, wherein an electric arc is struck
between a consumable electrode comprising a steel alloy wire
and a molten metal zone under a protective atmosphere
~20~ compri~sing a ternary gas mixture containing, by volume, 2.5 `~
to~3.5% carbon~dioxide, 0.8 to 1.2% hydrogen and the balance
; argon.~
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present in~en~ion allows ~elding~ by the "Short- ;~;
~25 ~ Arc" process under good conditions, of elements whosê
thickness is between~ about O.5 and about 1.5 mm. Beyond ~ `~
~` this thickness range, it is-also possible to reliably attain , ;
~very satisactory results.
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In the case of welding involving transfer by metal
droplets - under conditions of high intensity e.g. by the
"Spray-Arc" process, and with the "Pulsed-Arc" process -
better results than those obtained previously-may be obtained
with the aid of the protective gas composition of the
invention.
In this gas mixture~ the presence of carbon dioxide
allows a finely divided transfer of the metal, which avo~ds
~ormation of metal projections; and the presence o~ hydrogen
has a positive, beneficial role, its reducing action allowing
the molten metal to be cleaned in such a way that the metal
is properly wetted without introducing porosity into the weld.
Analytical tests have not been able to reveal any significant ;
enrichment in carbon of the metal deposited.
Moreover, this gas mixture is advantageous from the
economic point of view because of the absence of helium, and
because of a simpler conditioning, since it contains only
three constituents instead of four. This conditioning can
take place in a gaseous or liquid state.
1201~ A mixture containing by volume 3% carbon dioxide, 1%
hydrogan and 96% argon,~forms a particularly valuable
protective atmosphere.
In the presence of a protective atmosphere containing !:
2.5 to 3.5% carbon dioxide~ the stability of the arc
(stability of the intensity of current and stability of
voltag~) is excellent and the welding is even. Below 2% the
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stabili~y is not so good and around 1.5% it deteriorates
badly. Instability of the ar~ gives rise to difficult
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welding as a result of the metal projections and of a certain
unevenness of the weld.
The protective gas mixture of the invention allows
welding or hard-facing without the weld bead being enriched
in carbon. Carbon enrichment entails a risk of forming
chromium carbide in the case of alloy steels which brings
a~out a deterioration of the intercrystalline structure, -~
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resulting in an excessive tendency to corrosion and a
reduction in the mechanical properties of the joint.
~nalytical tests confirm the choice of the upper
limit of carbon dioxide. With a welding wire containing
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~ 0.02% carbon (0.019 to 0~02$7), in a ~'Spray-Arc'l technique,
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the carbon content of the transferred metal is as follows~
- under "Noxalic" atmosphere with 2.25% C02 C = 0.02~0% `
- under ternary mixture with 3% C02 C = 0.0263%
- under ternary mixture with ~.5% C02 C = 0.0375%
Once ihere is 4% C02 in the mixture, there is a significant
enrichment in carbon, and at 4.5% the effect of the C02 is
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dangerous. ~ -
~20~ The effect of the hydrogen is beneficial for cleaning
~ the edges of the weld only when used within very precise
j limits~ With an appropriate hydrogen content, cleaning of
material adjacent the weld takes place, facilitating wetting
and assisting bonding with the base sheet. The cleaning of
~25 ~ ~ the ~bead with reduction o~ the ox~des makes lt possible to
obtain shiny weld beads with clean`edges. ``~
A visual test of;tbe work pieces ~shows that below i~ -
0.8%, the hydrogen is not sufficiently active to ensure good
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cleaning. Between 0 and 0.5% hydrogen, no cleanin~ action
can be seen"and the-welds-aré dull because obviously
oxidised~
It has been shown by X-ray examination that beyond
1.2% hydrogen, its presence becomes a disadvantage with
regard to the porosity of the weld. Radiographic control i ~ -
shows occurrences of porosity when, under "Spray-Arc"
conditions with a hydrogen content of 1.5~, this untoward
incidence is all the more obvious because the level of carbon
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dioxide is lower. The ternary mixture of the invention
containing 3% car~on dioxide and 1% hydrogen gives welds which
are practical~ly free from porosity and are assigned the
porosity value M - 19.75 whereas a "Noxalic" atmosphere
gives welds having a porosity value of 18, making reference
to a standard weld joint with no porosity, assigned the
value 20, in the case of an assembly 6 mm thick and a ~oint
20 cm long. In the case of a ternaEy mixture containing
1.5% H2 and 3% C02, M - 17.25 (this figure corresponding,
under the same conditions as before, to 6 blisters with a
20~ diameter of 0.5 mm or 3 blisters with a diameter of 1 mm~. ;
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With a ternary mixture containing 2% hydrogen, the radio-
graphic examination value M is 14, i.e. on a 20 cm weld
bea~ there can be seen 9 to 10 blisters with a diameter of
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about a mi l l imetre .
;`25~ The invent~ion is illustrated by the following non-
limiti~g examples.
I ~ ~ Example 1
~ Netal ~s deposited-in a mould starting with a stainless
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steel wire of the 18/8 type wi~h a low carbon content and a
diameter of 0.8 mm, ~he welding current has an average :
~ntensity of 120A and an average voItage of 17 V. The
operating conditions chosen are those employing short- ~
circuits and short-arcs occurring at a rapid rate. Welding :
takes place in the upward vertical position~
The protective atmosphere, supplied by a cylinder .
in which the gas mixture is condensed7 has the following
compo~ition by volume~
C2 3%
H2 1% :
Argon 96%
The oscillograph recordings demonstrate the regularity ::
of the arc.
Mechanical test-pieces are taken from the bead of
deposited metal; the characteristics are very good, as .
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shown below.
Mechanical properties
Assembly 12 mm, sheet 18/87 upward vertical manual
welding, 120 ~/17 V.
Tensile strenyth (cylindrical test-piece with a diameter
~ of 9.78 mm): 61.11 k~/mm ::.
; - Elongation (length équal to 7 times the diameter~ : 40%
- Contraction coeficient : 57.7%
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: 25 - Resilience (Charpy V) ; KcVu ~8.37 Kg.cm at ~ 200C.
18.25 at 0C~ 14.85 at^-20~C-; 15.2 at -400C, 14.67 at-
-60~C.~ 11.07 at _800C. and-5.67 at - 196C~
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Corrosion : -
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- ` Resistance to corrosion, in a copper and sulphuric :
solution at boiling point, is perfect.
Chemical Analysis
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C Si Mn Ni Cr Mo - :
Wire 0.026 0089 1.68 9.6 20.70 0.07 :
Deposition without dilution in "Short-Arc" (HD) gives: j .
C Si Mn Ni Cr
0.026 0.75 ~.56 10.02 20.62
Porosity value: 19.5; sheet 12 mm vertical; value 20
sheet 1.5 flat.
Example 2 `.
Under the same conditions as in Example 1~ welding :.
takes place by the "Spray-Arc" method. The current has an
average intensity of 280 A and an average voltage of 30 V.
C Si Mn Ni Cr
Compoiition o.o26 0.75 1.5610.02 - 20.62
: HD 0.026 0.88 1.58 9.44 17.7
: Porosity value, sheet 6 mm flat, 19.75. ~-
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