Abstract

Exhaust Manifold is an integral part of the Exhaust System designed to collect exhaust gases from the cylinder and channel them to the exhaust outlet. This paper deals with the alternative material and manufacturing process to replace the convention material and manufacturing technique used in exhaust manifolds.

The newly developed Vanadium added cast iron has improved heat resistance than the conventional cast iron and lower cost when compared to Ni-resist. The addition of Vanadium provided high-temperature strength improvement and optimizing the amount of V   and Si greatly improved the high temperature properties. Tests conducted on this material has shown improvement in tensile strength and proof stress by more than 50% when compared to cast iron. Vanadium added cast iron can act as a suitable replacement for Ni-resist thereby reducing the cost by about 60%.

Loose Sand Vacuum Assisted Casting process combines the benefits of counter gravity and vacuum assisted casting process to develop thin walled exhaust manifold. This process employs light-weight bonded sand mold that is supported by unbonded backing sand. Test have shown that the ferritic stainless-steel cast sing this process has displayed superiority when compared to conventional stamped and tubular weld metal manifolds. This process can produce near-net shape thin walled sections for exhaust manifolds and cost reduction is achieved by reducing the usage of bonded sand.

Introduction

Exhaust manifolds are being used in extremely harsh conditions because the exhaust emissions from the engine are at very high temperature to achieve lower fuel consumption and to deliver high power. This demands the development of new materials and manufacturing techniques to withstand these harsh conditions and run efficiently under all circumstances.

Traditionally Gray cast iron, Nodular iron, High Si-Mo cast iron, Ni resist materials were used in the exhaust manifold. These materials were not able to achieve the desired service life and did not meet the emissions standards and hence the usage of Gray cast iron and Nodular iron was discontinued in the exhaust manifold. Ni resist displays excellent material properties but usage of 35% by weight of Ni makes the material expensive. Stainless steel is being used in the exhaust manifold in the form of tubes, stampings or the combination of tubes and stampings.

Casting is one of the techniques used to manufacture the exhaust manifold. Tubes are used as exhaust manifolds which are cut into sections of required dimensions and are subjected to bending to obtain the desired shape and are welded together to form the manifold. Stamped exhaust manifolds are fabricated using metal stampings which are bonded together or welded to form multi layered structure’s which are light in weight and generally made using stainless steel. Casting is the most common method used to manufacture exhaust manifold and new techniques in 3D printing like Selective Laser Sintering are being used in high performance race cars.

This research paper deals with Vanadium-added cast iron material for the exhaust manifold to replace the existing materials as it offers excellent mechanical properties and durability. This material is capable of handling high thermal loads and the test conducted on the material which are described in this paper has shown that it can satisfy the requirements of the exhaust manifold.

Counter gravity casting is the newly improved manufacturing process for exhaust manifold which combines the advantages of 2 casting process. Test conducted on the castings produced using this technique have shown thermal fatigue life 3 times better than stamped and conventional cast exhaust manifolds.

VANADIUM-ADDED HEAT RESISTANT CAST IRON FOR EXHAUST MANIFOLD

High-Si ferritic spheroidal graphite cast-iron (conventional cast iron) with small amounts of Mo was predominantly used in the exhaust manifold with high heat load. In recent years austenitic spheroidal graphite cast iron (Nickle resist) is being used to cope up with the increased thermal loads. Ni-resist cast iron contains 35wt% of Ni. Nickle is a rear metal and the international price of Ni can be subjected to variation based on the supply and demand therefore the rice of Ni-resist is very high. Development of the Vanadium Cast Iron (also referred to as Developed Cast iron) for the exhaust manifold offers improved heat resistance when compared to the conventional cast iron and is economical in comparison with Ni-resist.

The properties mainly considered for any exhaust manifold are excellent thermal fatigue life, antioxidation properties and durability to withstand vibrations

1)Thermal Fatigue Life

Factor that influences Thermal Fatigue in the exhaust manifold is the thermal stress induces in it due to the cyclic variation of temperature between extremes when is engine is running and when it is at a standstill. Improvement in thermal fatigue life can be achieved by reducing the plastic strain induced due to thermal stress. It is also necessary to reduce the thermal stress in the exhaust manifold by increasing the transformation temperature of ferritic cast irons beyond the operating temperature to prevent phase transformation into austenitic phase.

2)Anti Oxidation Properties

The oxidation induced on the surface of the exhaust manifold decreases the wall thickness thereby reducing the strength. This promotes the occurrence of thermal fatigue cracks. Peeled oxidation film from the exhaust manifold can damage the downstream parts of the exhaust system. To improve the anti-oxidation, it is effective to increase the amount of Si in cast iron.

3)Durability to Vibration

Exhaust manifolds are subjected to stress due to vibrations originating from engine and the road. Exhaust manifold of the turbo specification receive higher stress repeatedly due to heavy parts like the turbo charger. To resist the vibration, it is necessary to improve the fatigue strength from room temperature to high-temperature.

ALLOY DESIGN AND OPTIMAL COMPOSITION RANGE OF ADDITIONAL ELEMENTS

Cast iron is used as the base metal for the exhaust manifold with alloying elements like Silicon, Vanadium and Molybdenum. Addition of Si in cast iron improves the anti-oxidation properties and transformation temperature. Therefore, the amount of Si added to cast iron was increased to optimize the properties of cast iron. Addition of Si improved the strength of the material but increasing the percentage of Si to 4.4% caused reduction in ductility by increasing the hardness of ferrite. Hence 4.3% is considered as the upper limit for the percentage of silicon added to base metal. Adding Molybdenum increased the high temperature strength, but the percentage used is same as to that of Conventional cast iron as indicated in Table 1.

Table 1. Chemical composition of conventional and base cast iron (mass %) (Ryo Yamauchi, 2010)

Amount of Si Addition

To determine the optimal amount of Si addition, it is important to determine to the relation between the amount of Si and transformation temperature. Thermomechanical analysis(TMA) is test which measures the change in mechanical properties of materials when subjected to temperature changes was carried out to determine the optimum amount of Si which can be added to the base cast iron. The Figure 1 depicts that minimum percentage of Si should be 4.1% or more to keep the target transformation temperature 890°C or more. Figure 2 compares the anti-oxidation properties of V added cast iron containing 4.1% of Si or more with conventional cast iron. Vanadium added cast displayed better anti oxidation properties when compared to convention cast iron by showing 45% less increase in wall thickness rate. To achieve target transformation temperature and desired anti oxidation properties it is necessary to maintain Si lower limit of 4.1%. The developed Vanadium added cast iron has the following composition:

Carbon-3.2 to 3.4%

Silicon-4.1 to 4.4%

Manganese-0.35%

Molybdenum-0.45-0.55%

Figure 1. Relation between Si amount and transformation temperature of conventional cast iron. (Ryo Yamauchi, 2010)

Figure 2. Change of wall-thickness increase rate by high-temperature oxidation. (Ryo Yamauchi, 2010)

Amount of V Addition

To determine the optimum amount of V addition, the tensile test was carried out at 700-900°C for high temperature tensile strength and at room temperature to determine elongation. It can be inferred from the graph that between 700-850°C, there was a remarkable increase in tensile strength by adding 0.25% of V and there was gradual increase beyond 0.25% of V. Hence, lower limit for V is maintained at 0.25%.

Figure 3. Influence of high-temperature tensile strength by Vanadium amount. (Ryo Yamauchi, 2010)

EVALUATION OF MATERIAL PROPERTIES

1)Tensile Properties

Tensile Test was carried out on the specimens made of Ni-resist, Conventional Cast Iron, and the Developed Cast Iron between room temperature and 900°C. Data from Table 2 clearly indicates tensile strength and the proof stress of the developed cast iron is more than conventional cast iron by 15%. Elongation achieved is less than Ni resist and conventional cast iron. Developed cast iron has higher tensile strength and twice the proof stress of Ni-resist at room temperature. The tensile strength and the proof stress of the developed cast iron at 700-800°C has shown improvement over conventional cast iron but it is inferior hen compared to Ni-resist. But tensile properties from room temperature to 800°C satisfy the required target from these results.

Tab  (Ryo Yamauchi, 2010) le 2. Mechanical property at room temperature

Figure 4. High-temperature tensile properties of developed cast iron and rival materials. (Ryo Yamauchi, 2010)

2) Thermal Fatigue Life

As shown in Figure 7 below, maximum temperatures of 800°C to 850°C were attained during the heating and the cooling cycles. Thermal Fatigue life of the developed cast iron lies between conventional cast iron and the Ni-resist. Thus, the developed cast iron has achieved the target vale of thermal fatigue life necessary for an exhaust manifold.

Figure 5. Thermal fatigue life of developed cast iron and rival materials. (Ryo Yamauchi, 2010)

PART EVALUATION FOR EXHAUST MANIFOLD

Exhaust manifolds of identical shape and thickness were made using conventional cast iron, Ni-resist and developed cast iron. Durability bench test was conducted to evaluate the life of the exhaust manifold under same conditions. Figure 6 shows the results of the Life span test and it can be seen clearly that the developed cast iron has higher life span than Ni-resist and its life span is 1.8 times as that of conventional cat iron.

Figure 6. Comparison of durability to thermal cycle with exhaust manifold of each cast iron. (Ryo Yamauchi, 2010)

Ni-resist with excellent thermal fatigue life has displayed inferior Life span when compared to developed cast iron because Ni-resist is austenitic cast iron which has lower thermal conductivity and large coefficient of thermal of linear expansion while the developed cast iron is ferritic.

Figure 7. Change of thermal conductivity and coefficient of linear thermal expansion by temperature. (Ryo Yamauchi, 2010)

The developed Vanadium cast iron has displayed excellent mechanical properties and has proven to be the appropriate material for use in the exhaust manifold. Figure 8 below shows the Exhaust manifold developed using Vanadium added cast iron.

Figure 8. Exhaust manifolds of developed vanadium cast iron. (Ryo Yamauchi, 2010)

COUNTER-GRAVITY CASTING PROCESS for MAKING THINWALL STEEL EXHUAST MANIFOLD 

Advancements in Casting Technology has led to the manufacturing process that enables the casting of thin-wall (2-3 mm) het resistant ferritic stainless-steel exhaust manifold which can replace tubular weldments and stampings where there is high temperature requirement.

This casting technique combines the benefits of counter gravity and vacuum-assist casting process. This combination is called Loose Sand Vacuum Assisted Casting(LSVAC). This process will contribute to the production of thin walled and near-neat shape stainless steel exhaust manifold.

CASTING PROCESS

Casting of oxidation and corrosion resistant alloy into thin sections is difficult using traditional casting process due to the low fluidity and high melting point of these alloys. Vacuum Assisted Casting(VAC) process also called Counter Gravity Low Pressure Air melt sand casting can produce thin walled section like exhaust manifolds. This process has been optimized to a new process called Loose Sand Vacuum Assisted Casting(LSVAC). LSVAC can produce can produce large and thin walled casting, but also reduce the core sand use and the cost of production.

CASTING PROCEDURE

The LSVAC Process uses special thin molds instead of the thick, block type bonded sand molds as in VAC or CLAS process. These thin molds are placed in casting flasks and dry unbonded sand is vibrated around them which provides the necessary support and strength to the mold. Vacuum is applied to this assembly. Metal is cast into the mold cavity by lowering the bottom of the mold into the molten metal bath. Vacuum is applied to the mold which causes the molten metal to flow into the mold cavity through the gates positioned in the lower part of the mold. Applying vacuum ensures smooth flow of molten metal into mold cavity as opposed to the turbulent flow of molten metal in conventional casting where molten metal is poured through the runner. Mold is removed from the molten metal bath after the casting have solidified. Vacuum is then released, and the cast manifold is removed from the sand.   Loose sand vacuum assisted casting process can produce thin walled castings and high metallurgical quality like Vacuum Assisted Castings, but at a lower cost.

Figure 9. LSVAC casting of the manifolds. (Chandley, Redemske, & Johnson)

Figure 9. LSVAC Process Schematic. (Chandley, Redemske, & Johnson)

Advantages of LSVAC Process:

Molds in LSVAC are thin and contoured when compared to the large, thick bonded sand molds in VAC and CLAS. This method uses simple material handling equipment and reduces the usage of bonded sand. Reduction in the overall cost of production can be achieved.

Reduction in bonded sand usage means the reduction in usage of binder. This improves the recyclability of the sand and results in less disposal.   

LSVAC process provides the freedom of orientation of the mold in the casting flasks results in more castings per machine cycle for a given molten metal bath. Improved foundry productivity can be achieved on products such as exhaust manifolds.

MANIFOLD THERMAL FATIGUE TEST

Thermal fatigue test was conducted on a cast high silicon-molybdenum cast iron manifold, a cast stainless steel manifold and a fabricated type 409 stainless steel manifold using a 3.8L production engine on a dynamometer. The test was conducted by running the engine at full power and then turning it off which as depicted in Figure 10.

Figure 10. Thermal fatigue conditions.

Castings have the following advantages over fabricated manifolds:

Improved Quality

Increased Design Freedom

Shorter Lead Time

Lower Unit cost

Lower Tooling cost

Faster Validation

Fabricated manifold developed cracks after 600 cycles. Test on the cast stainless steel manifold was carried on till 1800 cycles and no visual sign of thermal cracking was observed. Test rests are depicted in Figure 11. This test has demonstrated that fabricated and stamped exhaust manifold cannot withstand cyclic thermal loading for long durations of time. 

TAIL PIPE EMISSIONS

Tail Pipe emission of the Cast Stainless steel is like that of fabricated tubular 409 Stainless Steel. This indicates that Cast Stainless Steels is the suitable material for use in the exhaust manifold. The Table 3 depicts the quantity of pollutants released per mile.

Table 3. Tail pipe emission data for a fabricated and a cast manifold.

Material

TPHC

gm/mile

TPCO

gm/mile

TPNOx

gm/mile

409 Stainless steel fabricated, base line

0.136

1.8

0.3

Cast stainless steel

0.151

1.84

0.28

Cast Iron, Low and High-alloy Steels can be used to manufacture exhaust manifolds using Loose Sand Vacuum Assisted Casting process. This process can be automated to achieve mass production and reduce the overall cost of production. L

Conclusion

Addition of Vanadium to cast iron improves the high temperature properties of high-Si ferritic spheroidal cast iron. Optimizing the amount of V and Si has resulted in improved high-temperature tensile properties, better anti-oxidation properties and good transformation temperature when compared to conventional cast iron which is confirmed by conducting tensile and oxidation tests. Thermal fatigue test conducted to determine the durability has shown that the exhaust manifold manufactured using Vanadium added cast iron has higher durability when compared to Conventional cast iron and Ni-resist. V added cast iron has better Thermal conductivity that Ni-resist at room temperature and at elevated temperatures and low coefficient of liner expansion. Vanadium added cast iron is the perfect blend of excellent mechanical properties and low cost, thus it the effective choice of material for current exhaust manifolds.

Loose Sand Vacuum Assisted Casting process is an improvement of Vacuum Assisted Casting process. LSVAC process can cast exhaust manifolds that can withstand extreme operating conditions that fabricated exhaust manifolds cannot handle. This process is highly economical as it uses very less quantity of bonded sand for casting. Very accurate surface finish can be obtained using LSVAC as the flow of molten metal inside the cavity is controlled using vacuum. High volumes of production can be achieved using this process. LSVAC enables the casting of thin walled complex shape exhaust manifolds which were impossible to achieve using conventional casting techniques.

Chandley, G. D., Redemske, J. N., & Johnson, J. N. (n.d.). Counter-Gravity Casting Process for Making Thinwall Steel Exhast Manifold. Detroit,Michigan: SAE International.

Kalpakjian, S., & Schmid, S. R. (2014). MANUFACTURING ENGINEERING AND TECHNOLOGY.

Ryo Yamauchi, S. I. (2010, 09 28). Development of Vanadium-added Heat Resistant Cast Iron for Exhaust Manifold. SAE International.