Welding is a key production process in the manufacture of modern automotive exhaust systems. The current automotive industry production trend is to directly weld stainless steel catalytic converter housings and ductile iron exhaust manifolds together. Since the austenitic and ferritic stainless steel welding consumables cannot withstand the dilution of a large amount of iron and carbon in the cast iron, the selection of the welding consumables becomes small.
The NI-ROD 44 filler metal developed by SMC International Superalloy Group Welding Products Co., Ltd. is a nickel-iron-manganese filler metal. It is the first consumable material successfully used in this type of dissimilar soldering, and its derivative NI- The ROD 44HT can be adapted to increasing engine temperatures. At the same time, the use of this type of nickel-based consumables not only reduces production costs, but also makes new progress in reducing exhaust gas leakage rate and emission levels as well as reducing noise, thus becoming a special welding material for optimizing the production of automotive cast iron exhaust systems.
Material for exhaust system silicon-molybdenum ductile iron exhaust manifold
In the past, exhaust manifolds were made and grey cast iron was an acceptable material. However, for the new engine, the operating temperature of the engine is constantly increasing, and the gray cast iron has poor oxidation resistance in a high temperature environment, so it is necessary to select a better material to make the exhaust manifold.
Ductile iron has been considered by the automotive industry to be another suitable exhaust manifold material, but when using space-saving design and higher requirements for catalyst "ignition" speed, the catalytic converter housing needs to be directly soldered to On the exhaust manifold, this requires correction of the welding specification for ductile iron. In order to reduce the formation of martensite and cementite in the heat affected zone (HAZ) of ductile iron, ductile iron is required to contain as much ferrite as possible and as little pearlite as possible (up to 10%). Silicon-molybdenum ductile iron has the highest ferrite content, especially when the silicon content is close to 40%, the oxidation resistance is very strong, and the temperature at which ferrite is transformed into austenite (A1 temperature) can be increased to exceed the operating temperature of the engine. At the same time, the empirical phase change of the material during the welding process and the consequent "condensation agglomeration" phenomenon are prevented, resulting in a decrease in the effective performance of the material.
Ford Engine Company recently studied the operating emissions characteristics of stainless steel exhaust manifolds and silicon-molybdenum ductile iron exhaust manifolds. The comparison shows that silicon-molybdenum ductile iron exhaust manifolds can improve fuel combustion efficiency, reduce noise, and are easy to process. Molding and greatly reducing costs, therefore, in the past few years, silicon-molybdenum ductile iron welded exhaust manifolds have been widely used.
Stainless steel catalytic converter and exhaust pipe
Due to the relatively low coefficient of thermal expansion (CTE), the 400 series stainless steel has good thermal fatigue resistance. And because they contain chromium, they have good oxidation resistance and can withstand temperatures up to 870 °C. The addition of niobium provides additional strength and microstructure stability. Therefore, 409 and 18 chrome-iridium stainless steels are the two steel grades available for catalytic converter housings and exhaust piping. Both materials have good weldability when soldered with NI-ROD 44 filler metal and NI-ROD 44HT filler metal.
Welded material of silicon-molybdenum ductile iron and stainless steel, low-nickel austenite and ferritic stainless steel welded cladding metal
It is rich in chromium and can form high temperature carbides of chromium; however, when exposed to high temperature air for a long time, too much chromium and insufficient nickel have an adverse effect on the weld metal. Ferritic stainless steel wire is usually not used to weld high carbon metal materials such as ductile iron. Even if the dilution effect of high carbon cast iron is small, it is enough to form a martensitic weld deposit metal with high crack sensitivity. Austenitic stainless steel wire is difficult to weld materials with a thermal expansion coefficient of 30% to 35% higher, so it is not suitable for welding between a silicon-molybdenum ductile iron exhaust manifold and a catalytic converter casing of ferritic stainless steel. If these stainless steel welding wires are used, the mismatch in thermal expansion coefficient will increase the stress in the heat-affected zone on the side of the ductile iron, thereby accelerating the generation of thermal fatigue fracture.
NI-ROD 44 filler metal
The NI-ROD 44 filler metal replaces the traditional NI-ROD 55 filler metal by 11% nickel with manganese, resulting in a stronger, more plastic weld filler metal with a lower solidification temperature. Until the 1890s, NI-ROD 44 filler metal was the welding consumable selected for the welding between cast iron and stainless steel when producing automotive exhaust units in Western Europe and North America.
However, Hyundai Motor has increased the engine operating temperature to improve fuel combustion efficiency, resulting in an exhaust manifold temperature exceeding 750 ° C, and a new heat affected zone cracking mode. Due to the combined effect of dense mesh secondary graphite precipitation, oxidizing environment, and cyclic stress caused by thermal cycling, such cracks appear on the fusion line along the heat affected zone. This phenomenon has been defined as stress accelerated oxidation (SAO). When the exhaust manifold is assembled to the engine of the car, with repeated engine operation, the martensite will be tempered and both martensite and cementite will be dissolved and re-precipitated as secondary graphite. The more secondary graphite is produced, the more likely it is to generate SAO cracks.
The dynamometer test of the car also showed that the NI-ROD 44 filler metal used well at temperatures below 750 °C. However, at temperatures above about 750 ° C, the NI-ROD 44 filler metal is subject to SAO formed by ordered secondary graphite deposits near the heat affected zone of the silicon-molybdenum ductile iron, increasing the likelihood of cracking.
NI-ROD 44HT
With the increase in engine exhaust temperatures and the need for higher oxidation resistance, SMC has designed a new filler metal numbered NI-ROD 44HT. It not only maintains the advantages of 11% manganese, but also contains a carbide stabilizing element to prevent graphitization during the welding process. At the same time, in order to improve the solubility of carbon in the deposited metal, the minimum nickel content is set in the filler metal. The carbide and the lower overall nickel content reduce the total amount of carbonization in the heat-affected zone of the silicon-molybdenum ductile iron, which reduces the formation trend of secondary graphite and these precipitates, and greatly reduces the SAO. And the moderate nickel content of the filler metal provides sufficient strength and stability and a good coefficient of thermal expansion that matches the ductile cast iron.
How to produce high quality weldments
There are two very important things that must be considered when designing the joining process for these dissimilar materials. One is to choose ductile iron and welding materials suitable for welding; the other is to carefully design and execute the welding procedure.
Choose ductile iron and welding consumables suitable for welding
The best silicon-molybdenum nodular cast iron matrix contains the most ferrite, which reduces the amount of martensite and cementite formed in the heat affected zone of the cast iron weld. It is necessary to have a minimum pearlite content (10%) in silicon-molybdenum ductile iron with ferrite as the main component, so that the heat affected zone of silicon-molybdenum ductile iron will be kept reasonable during welding and use. distributed.
As for the use of NI-ROD 44 or NI-ROD 44HT as the welding consumables of this type, the foregoing has been described in detail.
Reasonable interface design and welding procedure
* Design of welded joints - Reasonable design of welded joints should make it easier to access the root of the weld and to more easily control the dilution rate. Fillet welds are preferred. There must be enough shelf space to prevent the weld toe from extending beyond the edge of the casting. Other forms of welded joint design are acceptable as long as the design method minimizes self-constraint and optimum dilution.
* Convex weld bead - to prevent hot cracks along the centerline of the fillet weld and the undercut weld, not only when using NI-ROD(r)44HT, but also when using other nickel-based weld materials. Depositing metal is very necessary.
* Properly shaped weld toe - a well-shaped or raised surface weld is very important to reduce the "cleaning and rounding" of the weld and to reduce the local dilution of the deposited metal.
* Adequate overlap—The overlap area at the start/end of the deposited metal should be close to 12 to 19 mm, and the deposited metal here should be smoothly melted to minimize the occurrence of crater cracks.
* Correct welding parameters - Controlling the shape of the deposited metal is critical to ensure weld quality. The welding parameters selected for NI-ROD(r)44HT welding shall be controlled as follows:
Protective gas: 95% Ar + 5% CO2
Welding method: pulsed gas metal arc welding (P-GMAW)
Wire diameter: 1.2mm
Welding current: 250 ~ 350A
Welding voltage (average value): 24 to 27V DCRP (DCEP)
Welding speed: 10 ~ 16mm / sec
Distance between the end of the wire and the workpiece: 16~19mm
Stress treatment of other manufacturing parts
It is important to reasonably delay/support the weld assembly and provide sufficient space for unconstrained thermal expansion and contraction, which will minimize the dynamic stresses in the weld zone during use.
to sum up
The automotive industry has solved some of the problems associated with cast iron welding and has yielded considerable returns. When using NI-ROD(r)44HT filled metal-welded pearlite with less than 10% silicon-molybdenum ductile iron without preheating, the production cost is only 20% to 25% of other designs. And the emissions data indicates that the ductile iron manifold is comparable to a steel manifold in at least controlling combustion emissions. Due to these outstanding advantages, the application of NI-ROD(r)44HT filler metal in the welding of silicon-molybdenum ductile iron exhaust manifold will undoubtedly continue to grow rapidly.
Electric Winches,Permanent Motor Electric Winches,Electric Winch On Winch Bar,Wound Recovery Electric Winch
JINHUA RUNYE TECH. CO.,LTD , https://www.cnirunwinch.com