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SAE TECHNICALPAPER SERIES 2000-01-0314

430LNb – A New Ferritic Wire forAutomotive Exhaust Applications

N. Renaudot, P. O. Santacreu, J. Ragot, J. L. Moiron and R. CozarUsinor Recherches

P. PédarréUgine Savoie Imphy

A. BruyèreSprint Métal

SAE 2000 World CongressDetroit, Michigan

March 6-9, 2000

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ISSN 0148-7191Copyright 2000 Society of Automotive Engineers, Inc.

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1

2000-01-0314

430LNb – A New Ferritic Wire forAutomotive Exhaust Applications

N. Renaudot, P. O. Santacreu, J. Ragot, J. L. Moiron and R. CozarUsinor Recherches

P. PédarréUgine Savoie Imphy

A. BruyèreSprint Métal

Copyright © 2000 Society of Automotive Engineers, Inc.

ABSTRACT

The increasing use of ferritic stainless steels (AISI 409,439, 436 and 441) in automotive exhaust systems, espe-cially for manifolds and catalytic converter canning, hasled the authors to develop a new ferritic welding wire,designated 430LNb. This new material is recommendedfor the GMAW and GTAW processes and provides bettermetallurgical compatibility with the ferritic base metals, interms of both thermal expansion and microstructure.

The composition of the new welding wire has beenadjusted in order to guarantee an entirely ferritic structurein the welds of ferritic sheet materials, together with goodresistance of the welds to both wet corrosion and hightemperature oxidation, corresponding to the conditionsencountered respectively in the colder and hotter parts ofthe exhaust line. This is achieved by limitation of the C(<0.02%) and N (<0.02%) contents, stabilisation with Nb,such that Nb > 0.05 + 7 (C + N) and Nb < 0.5%, and a Crcontent of 17.8-18.8% .

GMAW weldability tests on 1 and/or 1.5 mm thick sheetsof AISI 409, 436 and 441 grades, using 1 mm diameter430LNb welding wire, gave good quality beads (shape,structure, tensile properties, bending, intergranular corro-sion resistance), with a welding speed of the order of 2m/min. The quality of the weld seams obtained was atleast as good as that of welds made on the same sheetsusing austenitic filler materials (308LSi and 307Si). Testsspecific to the automotive exhaust application have beenundertaken in the laboratory in order to compare the ser-vice behaviour of the welds with that obtained using aus-tenitic filler wires. These included fatigue endurance testsperformed in the tension-compression mode between300 and 950°C, thermal fatigue tests involving cycling ofrestrained specimens between 250 and 900°C, and dip-dry corrosion-oxidation tests simulating the combination

of salt attack and high temperature corrosion. All theresults obtained confirm that the 430LNb welding wire isat least as good as the austenitic filler materials mostcommonly employed in Europe. Tests carried out on realcomponents by exhaust system manufacturers also con-firm the high productivity and good quality of the welds.

INTRODUCTION

The increasingly widespread use of ferritic stainlesssteels (AISI 409, 439, 436 and 441) for automotiveexhaust systems has led USI (Ugine Savoie Imphy, asubsidiary of Usinor) to develop a new ferritic stainlesssteel welding wire adapted to this application, designated430LNb. Until now, the welding wires most commonlyused in Europe are the 308LSi and 307Si austeniticstainless steels, capable of welding both carbon steelsand austenitic and ferritic stainless grades. However, inthe USA, 409Cb wire has been employed for many yearsfor welding low Cr ferritic sheets.

The 430LNb 18% Cr welding wire has been developed tomeet the requirements associated with the increasinguse of high Cr (18%) ferritic stainless steel sheets, suchas AISI 441, due to the trend towards higher tempera-tures in the upstream part of exhaust systems to improvethe efficiency of catalytic converters.

The present paper shows that the new wire has weldabil-ity and service properties equal to, and in some casesbetter than, those of the 308LSi and 307Si austeniticgrades. Weldability was evaluated by joining ferritic stain-less steel sheets corresponding to AISI grades 409, 436and 441. The functional properties were determined bymeasuring the corrosion and oxidation resistance,together with the mechanical and thermal fatiguestrengths. This was done with the aid of specially devel-oped tests, which are briefly described below.

2

The results of industrial welding operations on realexhaust system components using 430LNb wire are alsodescribed.

MAIN SECTION

EXPERIMENTAL PROCEDURE

Materials – The 430LNb wire was used to weld AISI 409,441 and 436 sheets and was compared with 308LSi and307Si filler materials. A comparison in terms of weldabil-ity was also made with a 409Cb wire, used to weld AISI409. The chemical analyses of the wires and sheets eval-uated are given in Table 1 below.

Welding procedure – In order to enable mechanical test-ing of the welds, gapless MIG butt welding was per-formed. The welding parameters were adapted to complywith standard exhaust system welding procedures:

• base metal thickness : 1.5 mm

• filler wire diameter : 1.0 mm

• voltage : U = 30 V

• current : I = 225 A

• welding speed : 208 cm/min

• wire speed : 965 cm/min

• heat input : 1.95 kJ/cm

• shielding gas : EN 439-M3 (Ar + 2%O2)

• back protection : EN 439 - I1 (pure argon)

The same welding parameters were used for all the weld-ing wires tested : 308LSi, 307Si, 409Cb and 430LNb.Figure 1 shows an example of the weld seams obtained(441/441 assembly with a 430LNb wire).

Corrosion tests

Intergranular corrosion test – 20 mm wide specimenswith a central weld, cut from the AISI 436 and 441assemblies, were subjected in the as-welded condition tothe ASTM A262-E test. In the case of AISI 409 assem-blies, the “modified” ASTM A262-E test [1] wasemployed.

Figure 1. 441/441 assembly produced with a 430LNb wire.

Dip-dry tests – From the corrosion standpoint, exhaustlines may be roughly divided into two parts :

• the front part, subjected mainly to high temperatureoxidation, together with short periods of wet corro-sion due to condensates (internal parts) or road saltprojections ( external parts ) ;

• the rear part, exposed to lower temperatures, andsubjected mainly to wet corrosion by condensates(internal parts), or by road salt projections (externalparts).

In order to simulate these conditions, USI has developeda specific “dip-dry” test, illustrated in Figure 2, whichinvolves the following cyclic procedure :

• periodic immersion into solutions carefully selectedto simulate exposure to condensates or salinesplashing, in order to reproduce wet corrosion mech-anisms ;

• transfer to a furnace simulating high speed runningand the associated high temperature corrosion andoxidation mechanisms.

Figure 2. Principle of the dip-dry test (alternate immersion in a solution and exposure in a furnace). For example : urban cycle, internal parts → 18 immersions for 5 min in a synthetic condensate with pH = 3 at 50°C separated by 5 min intervals, followed by 60 min furnace exposure, for a total of 30 days; motorway cycle, external parts → 1 immersion for 5 min in 0.5M NaCl, pH = 6.6, at 50°C + 120 min furnace exposure, for a total of 30 days.

High temperature fatigue testing

Mechanical fatigue – Figure 3a shows the drawing of thefatigue specimens cut from the weld assemblies. The lon-gitudinal specimen dimensions are chosen to allow forthe seam width “ w”. Special grips were made from a

Table 1. Chemical analyses of the welding wires and sheets (wt. %).

Sheets C Si S P Mn Ni Cr Mo Ti Nb N

409 .007 .5 .004 .021 .2 .1 11.2 .01 .18 - .010

436 .040 .4 .004 .022 .5 .1 17.2 1.20 - .54 .024

441 .014 .5 .003 .020 .5 .2 17.5 .02 .15 .55 .018

Wires C Si S P Mn Ni Cr Mo Ti Nb N

307Si .077 .7 .009 .016 7.1 8.3 18.9 .12 - - .056

308LSi .014 .9 .008 .018 1.8 10.3 19.9 .11 - - .045

409Nb .056 1.3 .017 .020 1.6 .2 12.2 .06 .36 .93 .037

430LNb .017 .4 .002 .018 .3 .3 18.0 .04 .01 .32 .017

1.5 mm

Furnace Samples

Beakers, acid

and/or salt solution

3

nickel-base superalloy, designed to both clamp the speci-men ends and to guide them to within 1.5 mm of the edgeof the seam (Figure 3b), in order to avoid buckling duringtension-compression loading (R = σmax/σmin = - 1). Inthe case of tests on unwelded sheet, the same type ofspecimen was used, conserving a free zone 3 mm long(twice the sheet thickness) between the guided parts.

(a)

(b)

Figure 3. (a) Geometry of the mechanical fatigue specimens - the dimensions are measured from the edges of the central weld seam; (b) Specimen mounting system, with part of the top grip removed and without the refractory string used to attach and protect the thermocouple.

Heating was performed in a radiation furnace. The tem-perature was controlled by a thermocouple attached byrefractory string to the free part of the specimen, thestring also protecting this zone from direct radiation. Thetests were performed on a 100 kN servohydraulicmachine, at a frequency of 20 Hz, at temperatures from300 to 950°C, to a maximum of about 2.105 cycles.

Thermal fatigue – A special thermal fatigue test hasbeen developed to evaluate the thermal fatigue resis-tance of stainless steel sheets. The testing rig and theexperimental procedure are described in detail in refer-ences [2] and [3]. In summary, thermal cycles areimposed on a clamped V-shaped specimen, by alternateresistance heating and air cooling (Figure 4). Damageaccumulation at the top of the V due to the thermally-induced plastic strain eventually leads to failure of thespecimen. The thermal fatigue life of the specimen isexpressed as the number of cycles to failure anddepends on the maximum and minimum temperatureduring the thermal cycle, the specimen thickness, andthe material concerned. The test has been adapted tothe case of welded specimens as shown in the insert inFigure 4, in order to characterise the HAZ. The thermalcycle is defined by a maximum temperature of 900°C anda minimum temperature of 250°C, with no holding time.The duration of each cycle is about 200 seconds. Thethickness is kept constant and equal to 1.5 mm, 3 or 4specimens being tested for each type of welding wire.Homogeneous 441, 409 and 436 assemblies producedwith the different welding wires were compared.

Figure 4. Schematic principle of the thermal fatigue test.

RESULTS AND DISCUSSION

Weldability and basic properties of the weld seams –Weldability, in terms of fluidity, wettability and ease ofexecution, was essentially the same for all the weldingwires tested. In contrast, large differences were observedin the microhardness of the weld seams :

• with austenitic filler metals, there is a sharp disconti-nuity in hardness between the fusion zone and theferritic base metal (cf. example hardness profiles for409/409 and 441/441 assemblies in Figure 5b) ; thisis due to extensive martensite formation in themelted zone, especially in the 409/409 assembly.

• with ferritic filler metals, especially 430LNb, the hard-ness of the fusion zone is the same as that of thebase metal.

.

.

w (weld seam width)

45.5 15 1.5

Clamped parts Clamped parts

Guided parts(0.08 mm clearance on either side of the sheet)

thickness = 1.5

weld seam

124 + w

Thermocouple�������� �� �

�����������

�������� � �������

��������

HAZ FZ

4

Figure 5a. Location of hardness measurements.

Figure 5b. Hardness profiles on 409/409 and 441/441 assemblies (1.5 mm thick sheet) made with different welding wires (4370M = 307Si).

In order to characterise the drawability of the weldseams, Ericksen cupping tests were performed on bothwelded and unwelded specimens, the punch beingapplied against either the top or rear surface of the weld.The results are given in Table 2 in terms of the limitingcup heights as a percentage of those obtained in theabsence of a weld. In this test also, ferritic filler metals,and especially 430LNb, prove to be far better than auste-nitic ones. Cross-weld tensile tests were also performed,but essentially showed that fracture always occurs in thebase metal.

Corrosion resistance

Intergranular corrosion resistance – None of the assem-blies tested (AISI 409, 441 and 436, with 430LNb, 308LSiand 307Si filler metals) showed any sign of intergranularcorrosion.

Dip-dry tests – In the temperature range studied (furnacetemperatures of 300°C- 800°C) , no significant differencein corrosion resistance was revealed between assem-blies produced with 430LNb wire and those made withaustenitic filler metals, whatever the corrosive medium,either synthetic condensate (internal parts) or salt solu-tion (external parts). Figure 6 shows an example of aspecimen after testing.

308LSi 307Si 430LNb

Figure 6. dip-dry test (urban cycle-synthetic condensate-600°C-30 days) on 441/441 assemblies.

High temperature fatigue

Mechanical fatigue – The results of tests on specimenswith and without welds for AISI 441 and AISI 436 areshown in Figure 7, together with the static tensile proper-ties, for the 4 test temperatures studied (300, 750, 850

FZ

HAZ

BM

HAZ

AISI 409

0

50

100

150

200

250

300

350

400

1 2 3 4 5 6 7 8 9 10 11

Har

dnes

s H

V10

0g

308LSI

4370M

430LNb

409Cb

AISI 441

0

50

100

150

200

250

300

350

400

1 2 3 4 5 6 7 8 9 10 11

Har

dnes

s H

V10

0g

308LSI

4370M

430LNb

Table 2. Ericksen cupping test results on weld assemblies compared to those for the base metal (BM)

Base metal AISI 409 AISI 436 AISI 441

Filler metal Ericksen % BM

Ericksen % BM

Ericksen % BM

Punch side top rear top rear top rear

307Si 68.5 70.8 66 63 86.6 86.6

308LSi 77.6 78.8 76 77 90.4 87.3

409Cb 86.5 84 - - - -

430LNb 98 96 93 93 96 94.5

5

and 950°C). For both sheet alloys and for all test temper-atures, the stress for a life of 200000 cycles (σNf = 200000)can be seen to vary very little with the type of filler metalemployed.

AISI 441 AISI 436

AISI441

AISI 436

Figure 7. 200000 cycle endurance limits (σNf = 200000) for both weld assemblies (denoted 430LNb, 308LSi or 307Si) and unwelded sheets; (a) AISI 441 and (b) AISI 436, compared to the static tensile properties (0.2%YS and UTS) of the unwelded base materials, for test temperatures of 300, 750, 850 and 950°C.

Furthermore, the results on the welded assemblies areclose to those obtained on unwelded sheet, the values ofσNf = 200000 being 10 to 25% lower at 300°C and slightlyhigher (2 to 15%) at 850 and 950°C. The ratio betweenthe endurance limit and the static tensile strength(σNf = 200000/UTS) is logically less than 1 at 300°C (≈0.45) and 750°C (≈ 0.70), but becomes greater than 1 at850°C (≈ 1.25) and 950°C (≈ 1.45). This high tempera-ture behaviour can probably be explained by the stabilityof strain hardening at the high strain rates employed inthe fatigue tests, whereas dynamic recrystallisation dur-ing the slower static tensile tests leads to relative soften-ing.

Thermal fatigue – Figure 8 shows the results of thermalfatigue lives obtained for 308LSi , 307Si and 430LNbwires used for welding of 409/409 and 441/441 assem-blies. We choose to express fatigue lives by the ratio ofthe life of welded specimens over the life of the basemetal. 409 specimens welded using the 430LNb wireexhibit a very good thermal fatigue resistance compare tospecimen welded using 308LSi and 307Si wires.

Figure 8. Thermal fatigue lives for 409, 436 and 441 assemblies produced with 307Si, 308LSi and 430LNb welding wires, expressed as a percentage of the lives for the unwelded base metals.

In the case of 441 and 436 assemblies, the fatigue ther-mal life of the specimen welded with 430LNb wire isalways included in the range of lives of specimen weldedusing 308LSi and 307Si (see figure 8) . Metallographicobservations evidence two type of thermal fatigue crackpropagation in specimen : in Heat Affected Zone (HAZ)or Base Metal (BM) or at interface between HAZ andFusion Zone (FZ), as shown in Table 3 and Figure 9.

300°C

0

50

100

150

200

250

300

350

400

450

500

AIS

I 44

1A

ISI

441

AIS

I 44

1

430

LNb

308

LSi

307

Si

stre

ss (

MP

a)

0.2%

YS

UT

S

σσσσNf = 200000

300°C

0

50

100

150

200

250

300

350

400

450

500

AIS

I 436

AIS

I 436

AIS

I 436

430L

Nb

308L

Si

307S

i

stre

ss (

MP

a) 0.2%

YS

UT

S

σσσσNf = 200000

750°C 850°C 950°C

0

20

40

60

80

100

120

140

160

AIS

I 44

1

AIS

I 44

1

AIS

I 44

1

430

LNb

308

LSi

307

Si

AIS

I 44

1

AIS

I 44

1

AIS

I 44

1

430

LNb

308

LSi

307

Si

AIS

I 44

1

AIS

I 44

1

AIS

I 44

1

430

LNb

308

LSi

307

Si

stre

ss (

MP

a)

0.2%

YS

0.2%

YS

0.2%

YS

UT

S

UT

S

UT

S

σσσσNf = 200000

σσσσNf = 200000

σσσσNf = 200000

750°C 850°C 950°C

0

20

40

60

80

100

120

140

160

AIS

I 436

AIS

I 436

AIS

I 436

430L

Nb

308L

Si

307S

i

AIS

I 436

AIS

I 436

AIS

I 436

430L

Nb

308L

Si

307S

i

AIS

I 436

AIS

I 436

AIS

I 436

430L

Nb

308L

Si

307S

i

stre

ss (M

Pa)

0.2%

YS

0.2%

YS

0.2%

YS

UT

S

UT

S

UT

S

σσσσNf = 200000

σσσσNf = 200000

σσσσNf = 200000

0

10

20

30

40

50

60

70

80

90

100

308LSi 307Si 430LNb

Welding wires

The

rmal

fatig

ue r

atio

10

0=ba

se m

etal

409

441

436

Base metals

6

(a)

(b)

Figure 3. Thermal fatigue cracks : a. 441 assembly welded with 430LNb (crack in base metal/HAZ, �) ; b. 441 assembly welded with 307Si ( crack at interface HAZ/FZ, ×).

The low resistance obtained for the assemblies weldedwith 307Si wire can be explained by a systematic fatiguecrack propagation occuring at the interface the betweenmelting zone and the heat affected zone. In case of 409assemblies the advantage of durability for the ferriticwelding wire is more evident. We can notice a larger plas-tic strain of the melting zone in case of austenic wiresdue to a higher thermal expansion coefficient comparedto ferritic base metals, and which can lead to a thermally-induced deformation of the exhaust component near thewelded seam (catalytic converter, manifold).

TESTS CARRIED OUT ON REAL EXHAUSTCOMPONENTS – Within the framework of a partnershipwith Faurecia, a major exhaust system manufacturer,MIG welding tests using both 430LNb wire and 308LSiwire for comparison have been carried out on differentassembly configurations, two of which are shown in Fig-ure 10.

The behaviours of the two filler wires are very similar. Inparticular, the welding parameters are fairly close, with aslightly higher welding current for 430LNb, probably dueto a difference in electrical conductivity. In the tests on theracetrack converter can, somewhat higher weldingspeeds were found to be possible with 430LNb wire.

(a)

(b)

Figure 10. Fillet welds on Faurecia catalysts : (a) straight weld on an AISI 409 racetrack type catalytic converter can; (b) circular weld on an AISI 441 cylindrical catalytic converter can.

Table 3. Area of thermal fatigue cracks

Welding wiresBase metals 308LSi 307Si 430LNb

409 � × �

441 × × �

436 × × �

× indicates a crack at interface between Heated Affected Zone and Fu-sion Zone ; � indicates a crack in Heated Affected Zone or Base Metal.

7

CONCLUSIONS

The aim of the present study was to determine whetherthe 430LNb ferritic stainless steel welding wire developedby USI (Ugine Savoie Imphy) could provide a satisfactorynew solution for welding the ferritic stainless steel sheetscontaining 18% Cr (or less) used in automotive exhaustsystems.

GMAW welds were first of all made on 1.5 mm thicksheets of AISI 409, 436 and 441 grades, using 1 mmdiameter 430LNb, 308LSi and 307Si welding wires.Whatever the sheet material, the 430LNb wire gave goodquality beads (shape, structure, tensile and bendingproperties, intergranular corrosion resistance), with awelding speed of the order of 2 m/min. The quality of theweld seams obtained was at least as good as that ofwelds made on the same sheets using austenitic fillermaterials (308LSi and 307Si).

Tests specific to the automotive exhaust application havebeen undertaken in the laboratory in order to comparethe service behavior of welds produced using 430LNbwire with that of similar welds obtained with austeniticfiller metals. These included dip-dry corrosion-oxidationtests simulating the combination of salt attack and hightemperature corrosion, mechanical fatigue tests per-formed in the tension-compression mode between 300and 950°C, and thermal fatigue tests involving cycling ofrestrained specimens between 250 and 900°C. All theresults obtained confirm that the 430LNb welding wire isat least as good as the austenitic filler materials mostcommonly employed in Europe, and sometimes better(e.g. the thermal fatigue results on 409/409 assemblies).

Finally, tests carried out on real components by exhaustsystem manufacturers also confirm the high productivityand good quality of the welds.

REFERENCES

1. T.M. DEVINE and J. DRUMMOND : ‘An AcceleratedIntergranular Corrosion Test for Detecting Sensitiza-tion in Low Chromium Ferritic Stainless Steel’, inCorrosion NACE, 38 (6), 1982

2. H. SASSOULAS, P-O. SANTACREU : ‘Elémentspour prédire la durée de vie en fatigue thermiqued’élements de lignes d’échappement réalisés enacier inoxydable’, 18ème Journée de Printemps de laSF2M- Dimensionnement en Fatigue des Structures :Démarche et outils, Paris, 2-3 June 1999, p. 161.

3. P-O. SANTACREU et al. : ‘Thermal Fatigue of Stain-less Steels and its Application to Life Prediction ofAutomotive Exhaust Lines’, Thermal Stresses’99,Cracow, Poland, June 13-17 1999, p. 245

CONTACT

1. N. Renaudot, Research Metallurgist, Usinor Recher-ches, Centre de Recherches, 73403 Ugine cedex(France), fax : 33 4 79 89 35 00

2. P.O. Santacreu, Research Metallurgist, UsinorRecherches, Centre de Recherches, 73403 Uginecedex (France), tel: 33 4 79 89 35 43, fax: 33 4 79 8935 00.

3. J. Ragot, Corrosion Laboratory Manager, UsinorRecherches, Centre de Recherches, 73403 Uginecedex (France), tel: 33 4 79 89 38 35, fax: 33 4 79 8935 00.

4. J.L. Moiron, Welding Laboratory Manager, UsinorRecherches, Centre de Recherches, 71130Gueugnon (France), tel: 33 3 85 85 78 53, fax: 33 385 85 79 56.

5. R. Cozar, Senior Research Metallurgist, UsinorRecherches, Centre de Recherches, 58160 Imphy(France), tel: 33 3 86 21 32 19, fax: 33 3 86 21 31 11.

6. P. Pédarré, Wire Rod Products Manager, UgineSavoie Imphy, Direction Commerciale, 73403 Uginecedex (France), tel: 33 4 79 89 30 08, fax: 33 4 79 8931 92.

7. A. Bruyère, Welding Products Manager, Sprint Métal,BP1103, Maginot, 01000 Bourg-en-Bresse (France),tel: 33 4 74 45 94 20, fax: 33 4 74 45 94 19.