•
Introduction
Chromium
may occur in several valence states: chromium in metallic form (valence
state 0), trivalent form (Cr III), and hexavalent form (Cr VI) are
the most common and important. Trivalent chromium (Cr III) occurs
widely in nature and is an essential nutrient required by the human
body to promote the action of insulin in body tissues. Chromium as
a pure metal has no reported human or environmental toxicity effects.
Both acute and chronic toxicity of chromium are mainly caused by hexavalent
chromium compounds (Cr VI).
Fume particles from the welding of stainless steel may contain trivalent
chromium (Cr III) and hexavalent chromium (Cr VI) compounds. Welding
designates a joining operation that ensures the continuity of the
metallic state between two pieces to be joined. The main electric
arc methods which are employed for welding stainless steels are:
- Gas
Tungsten Arc Welding (GTAW)
- Plasma
Arc Welding (PAW)
- Gas
Metal Arc Welding (GMAW)
- Flux-Cored
Arc Welding (FCAW)
- Shielded
Metal Arc Welding (SMAW)
- Submerged
Arc Welding (SAW)
Various
gases and fumes can be generated during welding. Welding fumes are
metal-containing aerosols consisting of particles formed through complex
vaporisation-condensation-(oxidation) or vaporisation-(oxidation)-condensation
processes during welding. The fumes are therefore complex in their
composition and their structure.
The
composition of the welding fumes and their generation rate will depend
largely on the welding process employed and the filler material used
(if any), which is the major source of fumes and which has an influence
on metal speciation in the fumes.
The
rate of generation of fumes during arc welding of stainless steel
depends on various factors:
- welding
current (current density)
- arc
voltage (arc length)
- type
of metal transfer (type of filler material and/or welding process)
- shielding
gas or welding atmosphere
These
factors are interdependent and generally have a substantial effect
on the generation of fumes.
•
Electric Arc Processes for Welding of Stainless Steel
Gas Tungsten Arc Welding (GTAW)
The GTAW process is also known as TIG (Tungsten Inert Gas) or WIG
(Wolfram Inert Gas). In this process, the energy necessary for melting
the metal is supplied by an electric arc struck and maintained between
a refractory electrode and the work piece under an inert or slightly
reducing atmosphere. If a filler metal is employed, it is in the form
of either rods or coiled wire for automatic welding.
The inert gas flow which protects the arc zone from the ambient air
enables a very stable arc to be maintained. Depending on the base
material, shielding gases consist mainly of a mixture of argon (Ar),
helium (He) and hydrogen (H2).
The common work piece thickness range is 0.5 to 4.0 mm.
Plasma Arc Welding (PAW)
Plasma arc welding is similar to GTAW. The significant difference
is that the arc plasma is constricted by a nozzle to produce a high
energy plasma stream, in which temperatures between 10,000 and 20,000°C
are attained. Since the plasma jet is extremely narrow, it cannot
provide adequate protection for the weld pool, so that it is necessary
to add a larger diameter annular stream of shielding gas.
The gases used both for this purpose and for forming the plasma are
similar to those employed in GTAW, namely pure argon (Ar), Ar-hydrogen
(H2) up to 20%, Ar-helium (He)
- H2.
In manual plasma welding, where the torch is hand-held, the so-called
"micro-plasma" and "mini-plasma" processes are employed for currents
between 0.1 and 15 amperes and the "non-emergent jet" technique for
currents between about 15 and 100 amperes. By increasing the welding
current (above 100 amperes) and the plasma gas flow, a very powerful
plasma beam is created which can achieve full penetration of the work
piece. During welding, the hole progressively cuts through the metal
with the weld pool flowing in behind to form the weld.
The common work piece thickness range is 0.1 to 1.0 mm for micro-plasma
and mini-plasma processes, 1.0 to 3.5 mm for the non-emergent jet
technique and 3.5 to 10.0 mm for the key hole process (in a single
pass).
Gas
Metal Arc Welding (GMAW)
In the GMAW process, also known as the MIG (Metal Inert Gas) process,
the welding heat is produced by an arc struck between a continuously
fed metal wire electrode and the work piece. Contrary to the GTAW
and PAW processes, the electrode is consumable, an arc being struck
between the fusible filler wire and the work piece under a shielding
gas.
GMAW requires a shielding gas to prevent oxidation in the welding
arc. Argon with 2% oxygen (O2)
gives a stable arc and is suitable for most applications. Argon with
3% carbon dioxide (CO2) gives about
the same result. The welding speed and penetration can be increased
when helium (He) and hydrogen (H2)
are added to the argon + O2
or argon + CO2 shielding
gas mixtures.
Confusion often arises between MIG and MAG (Metal Active Gas) welding.
In the MIG process applied to stainless steels, the oxidising nature
of the shielding gas is negligible, whereas it is deliberately enhanced
in the MAG process. However, in the GMAW/MIG process, a low percentage
of oxygen (O2) or
carbon dioxide (CO2)
is often needed in the shielding gas (argon) to improve both arc stability
and wetting by the molten metal. Higher levels of O2
or CO2 give excessive
oxidation of chromium (Cr), manganese (Mn) and silicon (Si) and excessive
pick-up of carbon (C) in the weld pool. For single V-joints and square
butt joints welded in one run, the common work piece thickness range
is 1.0 - 5.0 mm.
Flux
Cored Arc Welding (FCAW)
A variant of the GMAW process is the FCAW (Flux Cored Arc Welding)
process, in which the electrode wire consists of a stainless steel
sheath filled with a solid flux, whose role is similar to that of
the electrode covering in the SMAW (Shielded Metal Arc Welding) process.
The core provides deoxidisers and slag forming materials and may provide
shielding gases in the case of self-shielded FCAW electrodes.
The gas which can be added extraneously serves as a gas shielding
to avoid the harmful effects of the oxygen in the air. Argon with
3% CO2 can be used
for this welding process.
The FCAW technique combines the advantages of the SMAW method with
the high productivity of an automatic or semi-automatic process due
to the possibility of continuously feeding the cored wire. Compared
to a conventional solid electrode, the flux provides a slag cover.
Both FCAW and GMAW have similar bead sizes. For single V joints and
square butt joints, welded in one run, the common thickness range
is 1.0 to 5.0 mm.
Shielded
Metal Arc Welding - covered electrode (SMAW)
Although the SMAW process, also known as the MMA (Manual Metal Arc)
process is very old, since the first applications were reported by
Kjelberg in 1907, it remains widely employed due to its great flexibility
and simplicity of use.
The electrode consists of a metal core covered with a layer of flux.
The core is usually a solid stainless steel rod. The covering, which
plays an essential role in the process, is extruded on the core, and
gives each electrode its specific "personality". It serves three main
functions : electrical, physical and metallurgical. The electrical
function is related to initiation and stabilisation of the arc, while
the physical action concerns the viscosity and the surface tension
of the slag, which control the transfer of metal droplets, the effective
shielding of the weld pool and its wetability. The metallurgical role
involves chemical exchanges between the weld pool and the slag, i.e.
refining of the weld metal.
The following is a short description of the most frequently used covered-electrodes:
- Rutile (titanium covering) electrodes: slag formation is the main
shielding mechanism in rutile-based electrodes, which are easy to
handle, ensure low spatters and produce welds with smooth surfaces.
A typical composition of this type of covering is : TiO2
: 42% ; CaF2 : 4%
; CaCO3 or / and MgCO3
: 20% ; Mica : 10% ; Fe-Cr : 10% ; Fe-Mn : 4% ; Feldspar : 4% ; slipping
agents (kaolin, talc, bentonite) ; binding agents (sodium and /or
potassium or lithium silicates)
- Basic (limestone covering) electrodes : limestone is a main constituent
of basic covered electrodes due to its favourable arc-stabilising
and metallurgical characteristics. It also evolves carbon dioxide
which provides a gas shield. However, a major disadvantage of limestone
is its high melting point. This is counteracted by additions of fluorspar
(CaF2) which helps
to lower the slag melting point. A typical composition of this type
of covering is : TiO2:
4% ; CaF2: 35% ; CaCO3
and/or MgCO3: 30%
; Mica: 6%; Fe-Cr: 8%; Fe-Mn: 2%; slipping agents (kaolin, talc, bentonite);
binding agents (sodium and/or potassium or lithium silicates).
Slipping agents are used to aid the extrusion and alkaline silicates,
which are used to assist the binding of covering fluxes, are also
the prime source of alkali ions in the plasma arc and can lead to
the formation of Cr VI compounds. On the other hand, the alkali ions
in the arc plasma are an essential aid to the arc formation and its
stability.
The common work piece thickness range is 1.0 to 2.5 mm for a single
run process and 3.0 to 12.0 mm for a multi-pass technique.
Submerged
Arc Welding (SAW)
The welding heat is generated by the passage of a heavy electric current
between one or several continuous wires and the work piece under a
powdered flux which forms a protective molten slag covering.
This molten blanket covering is very effective in shielding the arc
and the molten weld metal from the atmosphere and consequently is
an essential aid to prevent the release of welding fumes.
The process is suitable for butt and fillet welding in the flat position
and horizontal - vertical fillet welding. For welding stainless steels
a "lime / fluoride" type flux is most widely used, its typical composition
being : 25% = CaO + Mg O = 40% ; SiO2
= 15% ; 20% = CaF2
= 35%.
Two forms exist, produced either by melting or bonding. Fused fluxes
are produced by heating to temperatures of the order to 1600 - 1700°
C and are converted to powder form either by atomisation on leaving
the furnace, or by crushing and screening the solidified bulk material.
Bonded fluxes are produced from raw materials of appropriate grain
size, bonded together with an alkali silicate binder. The mixture
is dried, then mechanically treated to obtain the desired final particle
size. Only part of the flux is fused during welding and the unfused
material is picked up, usually by a suction hose and returned to a
hopper for further use. The fused flux solidifies behind the welding
zone and on cooling contracts and can be readily detached.
The SAW process is generally used for joining heavy work pieces in
the thickness range 10 - 80 mm, after the root run has been completed,
using another welding process.
•
Welding Fume Formation
In
the electric arc, the temperature is very high (of the order of 6,000
- 8,000° C for the GTAW, GMAW, FCAW and SAW processes, and up to 10,000
- 20,000° C for the PAW process) and well above the boiling point
of the base and filler materials (2,680°C for Cr, 2,860°C for Fe and
2,915°C for Ni).
With the exception (in most cases) of GTAW and PAW processes, the
melted metal is transferred in the form of droplets from the filler
material (GMAW, FCAW, SMAW and SAW processes) to the molten pool or
weld pool. A small fraction of the filler metal is vaporised-condensed
(oxidised) or vaporised (oxidised)-condensed, creating small particles
with an individual diameter ranging from 0.05 to 0.2 µm.
Fume particles formed during welding, whatever the filler metal, can
be split into three categories:
- Spherical
particles which are agglomerated in the form of clusters with a
linear size of the order of 4 µm and a maximum of 10 µm.
- Particles
which are agglomerated in the form of chains with a linear size
of the order of 4 µm and a maximum of 10 µm.
- Spherical
particles which are agglomerated in the form of globules (spherical
agglomerates) with a diameter up to 100 µm.
This
typical situation is summarised in Table 1.
Table
1: Size and morphology of welding fumes that can be formed during
the welding of stainless steel
|
Category
|
Morphology
|
Size
(µm)
|
|
1
|
Cluster
|
Average:
4
|
|
2
|
Chain
|
Average:
4
|
|
3
|
Globule
(spherical agglomerate)
|
Up
to 100 µm
|
Different
investigations have shown that the main source of welding fumes is
the consumables and whatever the process involved, less than 5% of
the fume comes from the weld pool.
According to the welding process the fume formation rate (FFR) can
range from zero to a maximum of the order of 0.50 g / min. as it is
shown in Table 2.
Table
2: Fume formation rate related to welding processes
|
Process
|
Shielding
Gas
|
Fume
Formation Rate (FFR)
|
|
g
/ min
|
g
/ kg. deposit |
GTAW
PAW
|
Ar
or
95% Ar + 5% H2
|
0.005
max.
|
(*) |
|
GMAW
|
98%
Ar + 2% O2
|
0.10
/ 0.50
|
3.0
/ 6.0 |
| FCAW |
No
(Self-Shielded) |
0.40
/ 0.60 |
2.0
/ 3.0 |
| SMAW |
No |
0.15
/ 0.30 |
3.5
/ 8.0 |
| SAW |
No
(granulated flux blanket) |
0.005
max |
0.08
max |
(*) In most cases no filler metal is used
|
Particularly
with the GMAW process, there is a close dependence between the fume
formation rate (FFR) and the two main welding parameters, i.e. the
couple formed by the voltage and the welding current which have a
strong influence on the metal transfer.
With GMAW, the mechanism of metal transfer is therefore of great importance.
Three principal modes are distinguished:
- The
short-circuiting or dip transfer mode, in which the metal melts
to form large droplets. This mode is associated with a low FFR which
is of the order of 0.10 g / min.
- The
globular or gravity transfer mode. As in the previous case, melting
occurs in the form of large droplets, which break away when their
mass is sufficient to overcome surface tension forces. This mode
is associated with a high FFR which can reach 0.60 g/min.
- The
spray transfer mode involves current densities above a certain transition
level, of the order of 200 A /mm2.
The electrode melts to give a stream of fine droplets. This mode
is associated with a moderate FFR ranging from 0.10 to 0.40 g /
min.
The effect
of transfer mode on fume formation rate is summarised in Table 3.
Table
3: Effect of transfer mode on fume formation rate (FFR) in GMAW
|
Transfer
Mode
|
FFR
- (g/min)
|
|
Short
circuiting or Dip transfer
|
0.10
-> 0.40
|
|
Globular
or gravity transfer
|
0.50
(maximum)
|
|
Spray
transfer
|
0.10
-> 0.40
|
Some
investigators(1,2,3)
have tried to identify the relative significance of welding parameters,
i.e. welding current and voltage on the FFR. In fact, this distinction
is difficult to make because for each welding current, there is an
associated voltage which is selected to give the best welding condition
in order to minimise the formation of spatter. For this reason, it
is better to associate the FFR to the transfer mode which takes into
account both voltage and welding current(4).
A
few studies devoted to the SMAW (covered electrode) process have considered
fume formation. The two most comprehensive surveys were carried out
by Kobayashi et al.(5)
and Voitkevich(6).
From these, it has been demonstrated that the droplet surface during
the transfer in the arc plasma zone (high temperature zone) is the
main source of metal vapour which can be oxidised and condensed. Welding
current density (A/mm2)
is a very important factor affecting fume formation rate. Increasing
current density, increases droplet temperature and could increase
vaporisation.
On the other hand, covering of the electrode provides slag which contributes
to fume formation through chemical reactions which can decrease the
FFR. The electrode covering also provides shielding gases through
decomposition of carbonates, increasing protection against the air
immediately surrounding the welding zone.
Submerged arc welding (SAW) uses a bed of granulated flux which will
be fused to shield the arc and molten weld metal from the atmosphere.
As the arc is not exposed to the air, there is a negligible emission
of fume through the blanket covering.
•
Composition and Particle Structure of Welding Fumes
In
order to take into account the valency state and the solubility of
chromium in stainless steel welding fumes, it is necessary to classify
chromium as shown in Table 4.
Table
4: Classification of chromium states in stainless steel welding fumes(7)
|
Total
Chromium
|
|
Soluble
Cr
|
Insoluble
Cr
|
|
Soluble
Cr VI
|
Soluble
Cr III
|
Insoluble
Cr VI
|
Insoluble
Cr III
|
To determine (by wet chemical analysis) Cr VI, because of the possible
oxidation of Cr III into Cr VI or the possible reduction of Cr VI
into Cr III, it was recommended to use an adequate dissolution medium
such as a water solution containing 3% of sodium carbonate (Na2
CO3) and 2% of sodium hydroxide
(NaOH)(4).
At present the wet chemical analysis making possible chrome speciation
is mainly based on the following methods:
- Atomic-Absorption-Spectroscopy
(AAS)
- Spectroscopy
with Induction-Coupled-Plasma (ICP) and for solid phase methods:
- X-ray
Absorption Near Edge Spectroscopy (XANES)
- X-ray
Absorption Fine Structure (XAFS)
- Laser
Ablation Mass Spectroscopy (LA - MS)
- Secondary
Ion Mass Spectroscopy (SIMS)
Combining
results of these various methods (6,8,9)
and according to the arc welding process involved, the chemical composition
of welding fumes is given in Table 5.
| From
the examination of welding fume composition (Table 5), it is obvious
that there is no relationship between the chemical composition
of the fumes and the chemical composition of the base metal and/or
the filler metal. |
For
stainless steel welding fumes complex metal oxides and sodium and
potassium species were identified (Table 6) in SMAW. According to
Eagar et al (10)
in GMAW performed with an ER 308 filler metal (ER 308 is a 20% Cr
and 10% Ni containing filler metal) under a shielded gas mixture of
98% Ar and 2% O2, the theoretical
metal vapour can contain approximately 20% Cr, 6% Cr O, 0.5% Cr O2
and a negligible amount of Cr VI. It was concluded that chromium and/or
its oxides must subsequently oxidise.
| Spinel
type structures were found in all particle fumes which are
basically different from both base material and/or filler metal
which have metal structures i.e. face-centred cubic structure
(fcc) for the austenitic grades and a body-centred cubic
(bcc) structure for the ferritic ones. |
Table
5: Fume Composition related to welding processes
|
PROCESS
|
SHIELDING
GAS
|
FUME
COMPOSITION, Wt (%) |
| Fe |
Mn |
Si |
Ca |
K |
Na |
F |
Mg |
Ni |
Cr
VI |
Cr
Total |
|
SMAW
|
No
|
5
- 20 |
1
- 12 |
0.2
- 13 |
0.2
- 10 |
1
- 25 |
1
- 15 |
0.1
- 20 |
-- |
0.1
- 6 |
0.5
- 6 |
1
- 10 |
|
GMAW
|
98%
Ar / 2% O2
|
30
- 40 |
1
- 12 |
1
- 4 |
-- |
-- |
-- |
-- |
-- |
2
- 8 |
0.2
- 1 |
8
- 13 |
| FCAW |
75%
Ar / 25% CO2 |
10
- 20 |
4
- 8 |
2
- 8 |
0.1
- 5 |
1
- 3 |
5
- 12 |
3
- 6 |
-- |
1
- 3 |
0.3
- 2 |
8
- 13 |
| No
(Self-shielded ) |
13 |
6 |
4 |
1 |
18 |
5 |
21 |
0.01
- 3.0 |
3 |
4 |
9 |
| SAW |
No
(Granulated flux blanket) |
-- |
-- |
-- |
-- |
-- |
-- |
-- |
-- |
<
1.0 |
<
0.1 |
2 |
Table
6: Phase Composition of Chromium in Stainless Steel Welding Fumes
|
Element
|
Phases
|
|
SMAW
|
GMAW
|
|
Cr
III
|
Cr2
O3
Fe Cr2 O4
K Cr F3
|
Cr2
O3
(Fe, Mn, Ni) (Fe, Cr, Mn)2
O4 |
|
Cr
VI
|
K2
Cr O4
Na2 Cr O4
NaK3 (Cr O4)2
K2 Na Cr F6
|
|
•
Conclusion
Based
on the speciation concept, this paper provides information on the
different forms of chromium compounds in fumes generated during arc
welding of stainless steels.
With most arc welding processes (GMAW, FCAW, SMAW and SAW) a small
fraction of the filler metal is vaporised-condensed (oxidised) or
vaporised-(oxidised)-condensed creating a fume consisting of small
particles containing chromium which is mainly in the trivalent form
(Cr III). Hexavalent chromium (Cr VI) is only present in small proportions.
The fumes from SMAW (covered electrode) and FCAW processes contain
the highest proportion of Cr VI compounds but with these two processes
it is possible to employ very efficient fume extraction systems. GTAW,
GMAW and SAW processes do not generate signification amounts of hexavalent
chromium.
With the GMAW process, the fume formation rate (FFR) is closely dependent
on the transfer mode. Selecting the right welding parameters and/or
employing pulsed GMAW is a way to reduce the FFR.
From the examination of both chemical composition and structure of
the welding fume particles, it is obvious that there is no relationship
between the fume characteristics and those of the base material and/or
the filler metal. The chemical composition and the structure of fume
particles reflect reactions between the filler metal and its arc environment.
•
References
1.
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A. LEROY, Soudage et Techniques Connexes, 39, 1985, 156 - 170
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