Stainless steel metal coredwires for welding automotiveexhaust systems
by Stanley E. Ferree and Michael S. Sierdzinski, ESAB Welding and
Cutting Products, Hanover (PA), USA
Over the past two decades, advances in stainless steel materials and the designs of automotive exhaust systemshave led to longer life cycles and extended warranties. Thispaper will describe the development of stainless steel metal-cored (SSMC) wires and their expanded use for weldingthese improved automotive exhaust systems. Information on manufacturing methods, core ingredients, available alloytypes and application data will be presented.
to ten years. Table 1 shows the major stainless steel al-
In the past, automotive exhaust systems were expected
loy types that are now used to make automotive ex-
to last for only three to four years. A major change oc-
curred in the United States in 1970 with the Clean Air
Although ferritic stainless steels have been used for
Act, which forced car makers to add catalytic convert-
certain automotive exhaust components since 1961 (3),
ers to exhaust systems. In the mid-1970s, environmental
their expanded use to include complete exhaust
mandates for catalytic converters with five-year/50,000-
systems has only occurred over the past 10 years.
mile warranties forced car makers to abandon carbon
Today’s automotive exhaust systems can be divided
steel systems in favour of 409 ferritic stainless steel (1).
into two parts: a hot and a cold end (4). The hot end in-
In addition, austenitic stainless steels and hot-dipped
cludes the exhaust manifold, downpipe, flexible
aluminium-coated ferritic stainless steels were used for
coupling and catalytic converter. The cold end includes
some demanding applications. Because of competitive
the resonator, intermediate pipe, silencer and tail pipe.
pressure and consumer demands, today’s exhaust
Important material properties and considerations for
systems usually have life cycles and warranties of seven
the hot end of the exhaust system include high-
Carbon and stainless steels/aluminium- coated
Table 1. Typical alloy composition (wt%) of materials for exhaust systems (2).
temperature oxidation resistance, fatigue strength andthe coefficient of thermal expansion. The importantpoints when it comes to the cold end of the exhaustsystem are condensate corrosion and aqueous salt cor-rosion.
The ferritic 11% Cr alloys are popular choices for
many exhaust components and systems. However, tocomply with the long-term durability requirements,such as a 10-year/100,000-mile warranty, the higherchromium (17–20% Cr) ferritic stainless steel gradesare often used.
A number of joining processes are used to fabricate
these advanced automotive exhaust systems. They in-clude high-frequency resistance welding, laser welding,
resistant spot welding, gas tungsten arc welding, brazing
and gas metal arc welding (GMAW) with solid or
metal-cored wires. Welding stations may also be de-
signed for semi-automatic, mechanised, or fully robotic
welding applications. The process chosen by exhaust
system fabricators depends on various factors. The com-
plexity of the parts, the number of varieties and quan-
complexity of joint and varie-ty of thicknesses. Weld
tities produced, the required capital expenditure and
production cost-profit analyses all play an important
role in determining the process. However, the GMAW
process using solid or metal-cored stainless wires has
evolved as one of the favourites for welding the new au-
tomotive exhaust systems. In addition, it soon became
apparent to exhaust system fabricators using the
GMAW process that it was much easier and less costly
to obtain customised stainless steel metal-cored wires
than solid wires for welding the modified or new stain-
less steels which are used in these systems. Along with
the availability of a wider range of alloy modifications,
the SSMC wires also offered welding fabricators advan-
tages in terms of improved quality, increased productiv-ity and reduced costs.
alloys, while a 304L grade of sheath is usually used for
Design of stainless steel metal-cored wires
Some of the advantages of SSMC wires are a result of
The tubular structure of SSMC wires offers several
their tubular design. SSMC wires are made by forming
advantages compared with solid wires. As previously
a steel sheath into a U-shape, filling it with ferroalloys
mentioned, many alloy types are easily made by chang-
and other core ingredients, closing the sheath into a tu-
ing the ferroalloys in the core ingredients. Some auto-
bular shape and then drawing or rolling the formed
motive exhaust system manufacturers want 409 types
tube down to the required size. A low carbon mild steel
stabilised with Ti or Nb and/or with Ni additions, while
sheath is commonly used to make ferritic stainless steel
others prefer modified 17–18% Cr alloys with similar al-
Arcaloy 18CrCb SSMC wireand a solid 439Ti wire.
loy additions. Customised austenitic alloys are also easy
The optimum wire diameter for welding most joints
to make using a tubular designed metal-cored wire.
on exhaust systems is 0.9 mm (0.035 in.) for solid wire
Potassium, sodium or lithium arc stabilisers are of-
and 1.2 mm (0.045 in.) for SSMC wires. However, on
ten added to the core ingredients to reduce welding
some very thin-gauge parts (< 1 mm), 1.0 mm (0.040
spatter and/or to produce good arc stability at the low
in.), SSMC wires may be used to prevent burn-through
welding currents required for thin-gauge applications.
problems. For the same reason, the pulsed-arc transfer
Small amounts of fluoride compounds and/or oxide
mode is more commonly used than a spray transfer, ex-
compounds also may be added to improve weld metal
cept on thicker tube-to-flange joints.
wetting. In general, a spray transfer with lower spatter
The shielding gas for solid and SSMC wires is usual-
levels and a wider bead flow can be achieved at lower
ly a mixture of argon and oxygen or carbon dioxide (95-
current settings than is possible with solid wires. SSMC
98% Ar/Rem. O2 or CO2). Sometimes, tri-mixes that in-
wires are therefore less prone to burn-through prob-
clude hydrogen or helium are used in these applications.
lems on thin-gauge sheet metal and are more tolerant
The shielding gas is selected to optimise the welding op-
of joint gap variations. In overall terms, a higher level
erability or performance characteristics. However, the
of quality, with fewer defects and lower part rejection
proper Ti:C or Nb:C ratios must be maintained in the
rates are often achieved with SSMC wires.
weld deposit to tie up the free carbon and nitrogen andreduce the formation of Cr carbides. Sensitisation or the
formation of Cr carbides reduces wet corrosion resis-
In many applications, faster welding travel speeds are
tance and leads to intergranular attack (1).
attainable with SSMC wires than with solid wires be-
Any change in gas composition requires a weld met-
cause of the tubular design of SSMC wires. Their inher-
al analysis to ensure acceptability. Table 3 shows the typ-
ent higher current density characteristics produce fast-
ical effects of the most common shielding gases on the
er wire melt-off rates than solid wires. Some exhaust
weld metal composition of an SSMC 18CrCb wire. As
system fabricators have doubled their welding travel
expected, the carbon content increases slightly when us-
speeds when changing to SSMC wires because of their
ing a shielding gas containing CO2. The shielding gas
faster melt-off rate. Table 2 shows the improved pro-
composition has little effect on the other elements.
ductivity results for an SSMC 18CrCb wire compared
A similar trend is found with solid wires (6, 7).
with a solid 439Ti wire, when welding flange joints ondecouplers. The arc time per weld was reduced from 30
ESAB’s stainless steel metal-cored wires
seconds with the solid wire to 12 seconds with the
metal-cored wire. Less penetration and better bead
Five ferritic SSMC wires are being produced for the au-
profiles were also found with the SSMC wire.
tomotive exhaust system industry: Arcaloy 409Ti,
Some of the components in the newer exhaust
409Cb, 436, 439 and 18CrCb. The choice of filler metal
systems also have complex joints containing a variety of
depends on cost, availability and performance, such as
stainless steel alloys and sheet thicknesses (Figure 1).
superior corrosion, oxidation and creep resistance.
The SSMC wires appear to be more tolerant than solid
A description of each product now follows and Table 4
wires of the complexities involved in welding these
shows the typical weld metal analysis.
joints. The special additions made to their core ingre-
(AWS A5.9-93, EC409) Arcaloy
dients and their tubular design produce this advantage.
409Ti is a 10.5–13.5% Cr alloy stabilised with Ti for arc
Welding conditions 235 A, 24 V, DCEP, 7.9 m/min wire feed speed, 12.5 mm contact tip to
weld metal analysis (%) with98 Ar/2 O
100% Ar shielding gas foraustenitic SSMC wires.
stability and to form carbides to improve corrosion re-
welding fabricators in this industry. They offer the fol-
sistance, increase strength at high temperatures and
lowing advantages compared with solid wires:
promote the ferritic microstructure.
1. Customised chemistry requirements are readily
(AWS A5.9-93, EC409Cb) Arcaloy
409Cb is the same as Arcaloy 409Ti, except that Cb(Nb)
2. Increased travel speeds and deposition rates help re-
is used instead of Ti to achieve similar results.
(no AWS classification) Arcaloy 436 is
3. More tolerant to poor fit-up with better wetting
a 16.5–18% Cr alloy stabilised with Ti and with a
1–1.5% Mo addition for improved resistance to con-
4. Higher level of quality — fewer weld defects
densate corrosion and aqueous salt corrosion.
5. Overall welding costs are usually lower
(no AWS classification) Arcaloy 439 is
a 17–19% Cr alloy stabilised with Ti. The higher chro-
mium content provides an increased level of oxidation
1. The Catalyst, Beyond 409, ARMCO Inc., Issue No. 3,
and corrosion resistance compared with the 409 grades.
(no AWS classification) Arcaloy
2. Automotive Exhaust Systems Materials Compara-
18CrCb is a 17.5–19.5% Cr alloy similar to Arcaloy 439
but stabilised with both Ti and Cb(Nb). The dual stabil-
3. Heat Resisting Ferritic Stainless Steels for Automo-
isation helps to prevent carbide sensitisation during
tive Exhaust System Components, Allegheny Lud-
welding and high-temperature exposure.
4. Dowthett, Joseph A. 1997. Designing Stainless Ex-
haust Systems, ARMCO, Inc. Technology Center.
Three austenitic grades are also available for these ap-
5. Ferree, S. E. 1992. Status Report on Small Diameter
plications: Arcaloy MC 308L, MC309L and MC316L. A
Cored Stainless Steel Wires. Welding Journal. 71 (1):
description of each product now follows and Table 5
shows the typical weld metal analysis.
6. Stenbacka, N. and Persson, K.A; Shielding Gases for
(AWS A5.9-93, EC 308L) Arca-
Gas Metal Arc Welding. Welding Journal. November
loy 308L can be used to weld AISI types 301, 302, 304,
7. Geipl, H. and Pomaska, H., MAGM Welding Stain-
(AWS A5.9-93, EC 309L) Arca-
less Steel—Effect of Shielding Gas. Sonderdruck
loy 309L is designed for welding dissimilar joints
#101. Linde AG, Hoellriegelskreuth, Germany.
between carbon steels and various stainless steels.
(AWS A5.9-93, EC 316L) Arca-
loy 316L is used to weld AISI 316 and 316L grades ofstainless steel when pitting corrosion is a problem.
PackagingThe Arcaloy family of ferritic and austenitic SSMC
wires is sold to major car manufacturers and their sup-
is vice president, technical for ESAB Welding
pliers of exhaust components or assemblies. They are
and Cutting Products, Hanover, PA, USA. He is
packaged in 15-kg (33 lb.) and 20-kg (44 lb.) spools and
responsible for the research and development of welding
227-kg (500 lb.) Marathon Pacs. The Marathon Pac has
become the preferred package for many exhaust system
is a research scientist for ESAB
fabricators. Down time is reduced as fewer changeovers
Welding and Cutting Products, Hanover, PA, USA. He
are needed with this larger package size.
has worked in a number of welding consumabledevelopment areas, from manual electrodes to coredwires. He is currently involved in the development of
very low hydrogen flux-cored wire and stainless steelmetal-cored wire.
Advances in the design of automotive exhaust systemshave led to a new family of stainless steel metal-coredwires that are rapidly becoming the favourite choice of
different studies, and then applied it to studies that were different in their subject selection criteria, treatments employed, and statistical methods used. They published their study because both they and their peers considered their findings to have considerable merit. The meta-analysis was conducted on 19 studies which fulfilled the following criteria: patients with a primary diagnosis of dep
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