In early automotive history little thought was given to the life expectancy of exhaust components. With heightened attention on improving fuel efficiency (while controlling emissions exhaust) temperatures in auto exhaust have risen dramatically. With the temperatures reaching 870 degrees Celsius, needs change.
The demanding service conditions led to an increase in incidents of cracking in the heat-affected-zone (HAZ) of the ductile iron manifold, limiting its service life. The weakness can be linked to stress-accelerated oxidation along aligned graphite particles.
The welding industry has reevaluated the old design of the cast iron manifold. Silicon Molybdenum modified ductile cast iron has refurbished the old technology. New designs also require placing the catalyst can closer to the engine, in turn, necessitating connection of Si-Mo cast iron manifolds to stainless steel of the catalyst can.
A new welding product for joining silicon-molybdenum ductile iron automotive engine exhaust manifolds to stainless steel converter housings became necessary.
Past Material Development
Since the 1940s nickel and nickel alloys have been used to arc weld cast iron. Nickel was needed to prevent cracking at the HAZ due to shrinkage and stress, but graphite was produced in the weld metal upon cooling. This caused volume expansion in the weld and reduced shrinkage stress. Other compositions were developed to meet strength and ductility requirements. Ni-Fe-Mn NI-ROD filler metal 44 was the most recent product that applied the graphite rejection principle.
Graphite formation in welds causes its own problems. Graphite can compromise performance in some applications. Since automotive manifold welds did not require machining, alloy designers began to turn their attention to stabilizing the graphite through carbide formation.
Ni-Fe-Mn in Exhaust Systems
Ni-Fe-Mn seemed the natural consideration for use in the close-coupled joint between Si-Mo cast iron manifolds ant the 409 stainless converter housing. Testing on the dynamometer (dyno) resulted in unexpected failure. Dyno testing cycled the engine through full-load wide-open throttle, followed by cold motoring with cold coolant. The maximum temperature was just below the 870 required on the weld. Cracking occurred in the HAZ.
Discovering the deficiency of secondary graphite formation and resulting stress-acceleration oxidation and cracking the Special Metals Corporation developed a new welding wire. NI-ROD filler metal 44HT contained sufficient amounts of carbide-forming elements that would combine with carbon dilution from the ductile iron to form carbides stable at the high temperature exacted upon the automotive exhaust system. Thermo-Calc software was used. Addition of niobium, chromium, and molybdenum was considered.
Niobium reduced the expected graphite formation, but needed to be balanced with the desired oxidation resistance. Chromium would capture carbon in carbides and enhance oxidation resistance. Chromium decreased the graphite solvus temperature.
After many calculations and compositions, researchers tested Ni-ROD filler metal 44HT. would it produce a more stable weld and reduce secondary graphite precipitation in the ductile iron HAZ? Cyclic exposures of vertical, electrically-heated furnace air were applied followed by cooling in ambient air. Filler metal 44HT and 44 were compared. The enhancement in oxidation resistance due to chromium addition is evident in 44HT – both by mass change and depth of oxidation.
The new alloy, NI-ROD filler metal 44HT, is currently being used in fabrication of close-coupled catalytic converter/Si-Mo ductile cast iron manifold assemblies. These parts fared well on the dyno and in actual service. Ni-Fe-Mn-Cr-Nb meets the demands posed by the automotive industry. The high integrity welds offer improved thermodynamic stability and oxidation resistance over past Ni-Fe-Mn product.