Materials Selection For Automotive Exhaust System Engineering Essay

This assignment looks into the material choice and fabrication procedure for an automotive fumes system sing the operational demands. Towards the terminal, the application of alternate stuff is besides discussed.

Working Requirements

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The maximal temperature in an car fumes system is around 750-850 grade Celsius. So the selected stuff for fabricating the fumes system should be able to manage temperatures in this scope. Besides there are assorted other corrosion factors like high temperature oxidization, de-icing salt taint and wet corrosion by condensates [ chiefly towards the tail pipe subdivision ] .

The fumes system can chiefly be divided into 2 subdivisions based on the on the job temperature. The hot terminal [ temperatures above 600 degree Celsius ] which starts from the manifold till the catalytic convertor, and the cold terminal [ temperatures below 600 degree Celsius ] which extends from the pre-muffler till the tail pipe. The fumes system is besides subjected to high frequence quivers due to wash up gas discharge.

Normally, ferric metals are used in the fabrication of fumes system. These include C steel, unstained steel, metal steels and dramatis personae Fe. The intent of adding debasing elements is to assist in solid solution strengthening of ferrite, better the corrosion opposition and other features and the cause the precipitation of metal carbides. [ 5 ]

Mild C steel was extensively used for the fabrication of exhaust systems for a considerable period of clip. Although mild steel has the belongingss to defy exhaust temperature it has really hapless corrosion opposition. High exposure to route salt and exhaust condensate can awfully shorten the life span of a mild steel based exhaust system. Besides, over the old ages higher demands in power and environmental safety have seen the death of mild steel from exhaust systems. Nowadays mild steel is employed in applications where the environment is non-corrosive.

Stainless steel has replaced mild steel in exhaust systems today. The ferric debasing component used here is Cr. The minimal sum of Cr in unstained steel is 10.5 % . When unstained steel is heated, Cr signifiers a protective bed of Cr oxide over the unstained steel surface and delays farther oxidization procedure.

The fumes system in a vehicle is expected to be at its peak public presentation ever and the material failure is a unwanted feature for this. Performance and efficiency of a car engine is earnestly affected by minor alterations in the fumes system. Therefore, it is of import that fail cogent evidence and efficient stuffs be used in the building of automotive fumess.

Materials used for fabrication of exhaust systems should hold the undermentioned features:

High service temperature, high weariness strength, high break stamina, should be easy machinable and should be economic sing the overall cost of the car. Besides, the stuff should be extremely resistive to corrosion as the fumes system is majorly exposed to route salts and condensates from the engine fumes gases. Failure in the fumes system can do loss of back force per unit area which can significantly impact engine public presentation parametric quantities. [ 1 ]

Current tendency:

Development in the car industry has besides seen the development of exhaust systems. From the usual C steel fumess, we can see unstained steel being to a great extent employed in exhaust systems.

From the CES charts, we can see that chromium steel steel excels over low/medium C steel in all belongingss other than economic system. Ferritic chromium steel steel is used in the industry of exhaust systems. Ferritic chromium steel steel contains Cr as the chief alloying component. The per centum of Cr can change from 10.5 % to 18 % . These stuffs are immune to corrosion and have really low fiction features. Although, these can be improved by debasing. Classs such as 434 and 444 have improved fiction belongingss. Ferritic chromium steel steel is ever used in a annealed province as they can non be hardened utilizing heat intervention processes. They have vey high opposition to checking due to emphasize corrosion. They besides exhibit acceptable welding features in thin subdivisions. [ 1 ]

We can see in item the different parts of the fumes system and the stuff belongingss below:

Exhaust Manifold:

The exhaust manifold of a car engine is ever exposed to hot gases. Cast Fe has been in usage for the production of exhaust manifolds traditionally. The chief features required for the exhaust multiplex stuff include thermic fatigue strength required to defy the high temperature fumes gases, oxidization opposition, good fiction belongingss and low thermic capacity to heighten the catalytic map. Ferritic chromium steel steel exhibit all these belongingss and offers large weight decrease besides. The developments in vacuity casting procedure has helped in the fiction of chromium steel steel manifold with subdivision thickness of 2-5mm. Higher demands in pollution control will lift the exhaust temperatures excessively and hence, ferritic chromium steel steel will be in major usage for exhaust system fabrication. Ferritic chromium steel steel exhibits improves thermic weariness features when processed by solid solution beef uping with Mo or Nb. This procedure besides improves the oxidization opposition and microstructural stableness. Ferritic chromium steel steel besides has cost advantages because of the absence of Ni in its composing. Another discrepancy called the austenite chromium steel steel is used where ferritic chromium steel steel is unsuitable. Austenite unstained steel can heighten its belongingss when adequate C is added to it. But, the higher cost bounds its use compared to the ferritic discrepancy. [ 3 ]

Cast Fe manifolds can be manufactured by the sand projecting procedure. The hardening of liquefied dramatis personae Fe can ensue in the formation of interstitial nothingnesss between the assorted atoms in the dramatis personae Fe construction. This can take to particle skiding over each other and as a consequence a diminution in the mechanical belongingss of the mold. Stress alleviating heat interventions are performed inorder to increase the dimensional stableness of the stuff. During heat intervention, the thermic enlargement of the stuff allows the nothingnesss to be filled. Although, attention should be taken as non to exaggerate the heat intervention procedure as excessively much thermic enlargement can convey approximately more emphasiss in the material microstructure and this can impair material belongingss. Changes in microstructure can decrease material belongingss like thermic weariness and break stamina that can ensue in the premature failure of the finished merchandise.

The alternate stuff that can be used in exhaust multiplex industry is ferritic chromium steel steel. These stuffs have enhanced belongingss for exhaust multiplex production. The chief characteristic is the low thermic enlargement that is one of the major jobs faced by dramatis personae Fe fumes manifolds. Molybdenum and Niobium alloying has shown enormous addition in the heat sweetening belongingss of ferritic chromium steel steel. Vacuum projecting procedure that is used in the production of ferritic chromium steel steel manifolds is the factor that increases its cost. Effective methods need to be developed to cut down the tooling and fabrication cost. Once this is achieved, cast Fe fumes manifolds can be replaced by unstained steel fumes manifolds that have improved belongingss and weigh less and can therefore adhere to stringent emanation norms that come in future. Nowadays chromium steel steel manifolds are merely used in vehicles that call for utmost public presentation. Commercialization of this engineering is non far off.

Catalytic convertor:

The catalytic convertor is used to clean the fumes gases and do it free of toxic effects. It is usually exposed to elevated temperatures like 1000 grade Celsius and therefore its lastingness is a really of import factor in proper operation of the fumes system. Conventionally the catalytic convertors are made of a stuff called the cordierite monolith that fundamentally acts as the absorbant in it. Cordierite is a silicate and therefore the constituent is manufactured by an bulge procedure. Then it is subjected to washcoating where the monolith is submerged in stuffs such as Al2O3, CeO2, ZrO2 and metals such as Pt, Rh and Pd. The chief belongingss of the catalytic convertor monolith are as follows

Fusion point: 1450 grade Celsius

Coefficient of Thermal Expansion: 0.002cal/s-cm-oC [ low ]

Thickness of monolith walls: 100 micrometers

Number of cells: 400-900 cpsi

The chief advantages of the cordierite monolith are lower cost and fabrication easiness.

Alternate stuff for the catalytic convertor is unstained steel. Alternatively of the silicate monolith metal metal stuffs are used in the building of the monolith. These metal monoliths are manufactured by the combination of level and corrugated beds of the metal metal. These are so made into the coiling shaped monolith. The exhaust gases escape through the channels formed between the level and corrugated planes. Washcoating is done in this instance besides. But the of import thing to be remembered is that this procedure needs to be done before the monolith is made into a coiling form. This is because the metal monolith is by and large smaller in size and that can blockade the proper washcoating of the full metallic monolith. The chief features of the metallicmonolith are as follows:

Fusion point: 1600 grade Celsius [ 150 grade Celsius more than cordierite ]

Coefficient of Thermal Expansion: really high [ straight influenced by aluminum content in metal metal ]

Thickness of monolith walls: 25 micrometers [ less compared to cordierite ]

Number of cells: 800-1250 cpsi

[ 2 ]

One of the major advantages of the metallic monolith is the high impact opposition. As the catalytic convertor is prone to impacts from route surfaces, it is of import that the catalytic convertor remains integral at all times. The cordierite monolith is really weak compared to metallic monolith in this instance. Metallic monoliths besides have the advantage of being placed near to the fumes manifold. This means that they can accomplish the operating temperature [ around 300 degree Celsius ] earlier and therefore cut down exhaust taint. These monoliths can besides be developed in smaller sizes compared to the silicate opposite numbers. Higher service temperature means that metallic monoliths are used in high public presentation applications where the conditions are really terrible.


The silencer is the concluding portion in the fumes system. The debut of catalytic convertor has led to the formation of condensates inside the silencer thereby ensuing in corrosion. Aluminized C steel is employed in the industry of the fumes silencers traditionally. The fumes gas condensates have amplified the corrosion rates in the silencer and it is seen that they do non execute to their maximal service life when the catalytic convertor is introduced. This usually happens in the short tallies of the vehicle where the fumes gas temperature does non lift high plenty to zap these condensates. During long tallies, the fumes gas temperature goes over 100 grade Celsius which prevents the formation of condensates and reduces corrosion. Stainless steel silencers have really high corrosion opposition and the add-on of Cr and Mo can better these belongingss. High production rates can be achieved by the usage of chromium steel steel by tandem factory cold peal, tempering and descaling procedure. Titanium can besides be added in little hints to better silencer belongingss in chromium steel steel. [ 3 ]

Titanium exhaust systems [ a future position ] :

Environmental bureaus now prefer guaranteed corrosion opposition on exhaust systems for more than 100,000 stat mis. Titanium achieves this and is much better than unstained steel systems that are used now. A typical catalytic convertor and silencer pipe in chromium steel steel weighs around 10kg. A redesigned Ti system weighs merely about 3.2 kilogram. This weight salvaging is really good when sing high public presentation and high preciseness cars.

Pure Ti tubing and sheet are the stuffs of pick for silencers, and pipes. Decrease of weight and cost are achieved by choosing the thinnest gages of stuffs consistent with the technology and acoustic parametric quantities of the fumes system. Titanium may non be suited for the full system, and will most likely be limited to constituents in which the temperature of metal does non travel over 400 degree Celsius for longer periods of clip. The parts instantly behind the catalytic convertor continue to execute good under trial. Use of Ti Lugs welded to the pipe will surely turn out to be the most efficient method to repair the fumes to the vehicle frame ( via gum elastic isolators ) .

Material conforming to ASTM Grade 2, ( e.g. Timetal 50A ) offers the optimum in footings of cost, handiness, fabricability, weldability and mechanical belongingss. Rate 45A is somewhat less strong and more malleable than 50A and may be required where extended forming is portion of the fabrication procedure e.g. lock seaming ( Table 4 ) . Both metals are to the full weldable, necessitate no intermediate or post-forming heat intervention, and are available in broad sheet spiral. These metals are besides used to fabricate low-priced continuously welded tubing.

Properties of pure Ti 45A and 50A



Thermal enlargement ( 10-6degree Celsius )



Hardness ( HV )



Tensile Modulus ( 103N.mm2 )



Density ( )



Specific Heat ( )



RA min ( % )



Titanium Production:

Pure Ti is cold formable, and sheets or tubing can be shaped readily at room temperature utilizing techniques and equipment that are usually for steel. Following factors must be taken into consideration while machining Ti.

· The ductileness of Ti is by and large less than that of steel. More generous crook radii may be required during flexing applications.

· The modulus of snap of Ti is about half that of steel. This means that Ti will return back to original province after organizing. Compensation for this can be done by little overforming.

· Titanium tends to chafe against ungreased forming tools. Proper lubrication of the tool can work out this job.

Both lock seaming and opposition welding are suited to fall in the catalytic convertor, and traditional TIG is suited to seal the terminals and pipe articulations with a torch draging shield to the external surface of the box terminal articulations.

Production Economy:

A taking fumes system maker in the United States, successfully rolled and seam welded a batch of Ti boxes at one of its production units. This was done with no excess cost when compared with the production of steel constituents. It was besides seen that the flexing units that are presently employed for the bending of steel are suited in the instance of Ti besides. The lone factor is the procurement cost of Ti and hence important attempts are necessary to convey down tooling and fabrication costs. [ 3 ]


It can be concluded that unstained steel can replace conventional stuffs in automotive fumes systems. We have to number on developments in tooling and fabricating methods that can convey down overall costs to minimum. Stainless steel is easy the best option in service facets of the fumes system. High public presentation stuffs like Ti may besides come into commercial production shortly. Improved thermic, mechanical and chemical belongingss are major factors that govern stuff choice for an fumes system.



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