EMCON TECHNOLOGIES Brazil reduces cycle time for welding catalytic converters with ESAB Arcaloy 409Ti metal-cored wire.

In the automotive industry, stainless steels are increasingly being used for exhaust systems at the expense of carbon steel. The use of ferritic stainless steel for exhaust systems was first seen in the USA in the mid-70's and has since gained popularity due to its excellent resistance to corrosion, cyclic oxidation, fatigue, high temperatures and thermo mechanics. They are ideally suitable for environments with abrupt changes of temperature, mechanical wear and exposure to gases and corrosive condense.

Technical evolution and work temperature of catalytic converters
In Brazil, stainless steel exhaust systems became standard, from 1991, when a government law forced all cars – domestic or imported – to be equipped with catalytic converters. Since then, the maximum operating temperature of converters has increased, due to developments in the ceramic or aluminium-oxide core (honeycomb) and precious metal coatings, resulting in more dependable and efficient converters.

Figure 1 (above) shows the temperature zones in a complete exhaust system, with the traditional division of 'hot end' and 'cold end'. In the USA and Brazil, ferritic steels are predominantly used for the entire exhaust system – AISI type 439 being a popular choice for hot end applications and the lower alloyed AISI type 409 type for cold end components. In Europe, austenitic types are common for the complete system, although a trend towards the use of ferritic types is being seen.

Consumables
Consumables for welding ferritic stainless steel can be either austenitic or ferritic. Austenitic consumables have excellent weldability and give a weld with an austenitic microstructure and good mechanical properties. The addition of nickel, however, makes austenitic consumables more expensive.

Ferritic consumables contain 11-18% Cr, depending on classification, and are not alloyed with nickel. They are micro-alloyed with Ti and Nb as stabilisers to prevent sensitisation. They have good weldability and provide welds with excellent corrosion resistance and good mechanical properties – also at high temperatures. This group of consumables is ideal for welding as they do not have Ni and yet maintain the quality desired for the welding of vehicle exhaust systems components.

The test programme carried out by EMCON, in close co-operation with ESAB, investigated the advantages, in terms of weld quality and productivity, of replacing AWS ER308LSi solid wire with ESAB's Arcaloy 409Ti (AWS A5.9 EC409) ferritic stainless metal-cored wire, in the production of catalytic converters.

Test programme
The investigation was carried out in three phases, all focussing on a specific area of the consumables' performance:

• weldability and productivity
• weld profile
• chemical composition and micro-structure

All welding was done with a robot cell for the production of catalytic converters – under real production conditions – enabling an exact comparison between the solid and the cored wire.

Weldability and productivity
Arcaloy 409Ti was used successfully. Both wires were welded in the non-pulsing short arc mode in Ar/2%O2 shielding gas at 220A/18V (ER308LSi) and 240A/20V (Arcaloy 409Ti).

The metal-cored wire was impressive, with low level spatter, flat bead profile and uniform and linear welds (Figure 4 and 5). Arc ignition and arc stabilisation were deemed to be excellent - with minimum spatter at starts and stops - greatly improving the final appearance of the catalytic converter.

Economic results were equally convincing, with 45% higher welding speed and a cycle time reduction of 25% (table 1). Obviously, this accounts for an enormous improvement in productivity for a company in the competitive automotive market. The success of Arcaloy 409Ti in the welding of catalytic converters provides a basis for further testing to optimise welding productivity. The next stage of testing will explore the use of pulsed arc welding.

Weld profile
Macros were taken to verify weld profile and penetration. Figure 7 shows the weld dimensions measured on three individual catalytic converters welded with Arcaloy 409Ti and ER308LSi solid wire, respectively. Figure 6 indicates where the samples were taken.

Tables 2 and 3 show examples of individual values measured on test pieces welded with Arcaloy 409Ti and ER308LSi solid wire and Table 4 gives the average of three test welds for both wires.

From these results, it can be concluded that Arcaloy 409Ti shows a wider penetration "S" and "D1". This is because it has a less concentrated electric arc than solid wire. The wider penetration profile is beneficial in the avoidance of lack of fusion defects - particularly important for this industry. On average, the penetration depth "C" was smaller than from the ER308LSi solid wire. This is also beneficial as it involves thin-plate welding with a potential risk of burning-through.

Chemical composition and micro-structure
Figures 8 and 9 compare the micro-structure of welds made with both wires, the result being predictable. The weld metal structure is austenitic for the ER308LSi wire and ferritic for the metalcored wire. The microstructure of the base material and HAZ remain comparable in type and grain size. Tests with oxalic acid did not reveal any sign of sensitisation at the grain boundaries. Also visible is the wider weld bead with less reinforcement produced by Arcaloy 409Ti – in line with the findings of Table 4.

Conclusion
ESAB's partnership with EMCON TECHNOLOGIES in a test programme for the replacement of ER308LSi solid wire with Arcaloy 409Ti metal-cored wire in the robotic welding of catalytic converters, resulted in a dramatic reduction in cycle time, as well as a more favourable penetration profile for thin-plate welding. It formed the basis for wide-scale introduction of Arcaloy 409Ti in EMCON production and paved the way for the application of another type of ferritic metal-cored wire, Arcaloy 430LNb.

AcknowledgementWe express our appreciation to EMCON TECHNOLOGIES, in particular to Mr. and Mrs. Luiz Henrique Machado, José Eduardo Lepore and Edson Luiz Geniseli for their support and true team spirit during this project.

By Roberto Luiz de Souza, Welding Process Technologist and Technical Consultant Automotive Segment