This document discusses material selection for automobile silencers and catalytic converters. It provides details on the manufacturing process, technical aspects, and criteria for selecting materials for silencers. Common materials used include steel, stainless steel, cast iron, and alloys for their mechanical properties, corrosion resistance, and ability to withstand high exhaust temperatures. The document also discusses the material requirements and alternatives for catalytic converters, focusing on a ferrite alloy and stainless steel sheet that is pre-oxidized to provide a surface for catalyst adhesion.

1. G.H.PATEL COLLEGE OF ENGINEERING
AND TECHNOLOGY
Made By: Nishit Karkar, Kalp Shah
Enrollment No. : 160110120024, 160110120055
Branch: Mechatronics
Semester: 3rd
Course Title: Principles of Material Science and
Physical Metallurgy
Course Code: 2132004
Acadamic Year: 2017-18{odd}

2. CASE STUDY FOR
MATERIAL SELECTION

3. What is Automobile Silencer?
 Automobile Silencer is a device used to reduce the noise
produced by the engine.
 Silencer is used in automobile vehicles to reduce the noise
produced by the exhaust gases of the engine. Silencer is
also used in many other engines and generators.
 The size, shape and construction varies according to the
type and size of the engine.

4. TECHNICAL ASPECTS
 Process of Manufacture:
 The process of manufacturing auto silencer does not involve any special
technology, it is a simple press working process. The sheets are cut into the
required size and shape. These sheets are then given the desired shape with the
help of power presses, edge folding machines and bending rollers. The mufflers
are also made out of sheet with the help of presses. These mufflers are then gas
welded in one half of the silencer, the other half of the silencer with MS tubing’s
according to the requirement and are gas welded. These are now assembled and
spray painted. The silencers are then cleaned and packed for dispatch.
 Quality Control and Standards:
 Good workmanship and best quality of raw material must be used to satisfy the
consumers.
 Pollution Control:
 The assembled product will be spray painted or nickel plated for which
arrangements for exhaust fans will be made in the plating section.

5. Purpose of Silencer
 Automotive silencer should sustain vibration generated due
to high pressure exhaust gas.
 An automotive requires a silencer to reduce the amount of
noise emitted by a vehicle.
 Silencers use neat technology to cancel out the noise.
 Silencers are installed along the exhaust pipe as a part of the
exhaust system of an I.C. engine to reduce its exhaust noise.
 The silencer reduces exhaust noise by dampening the
pulsations in the exhaust gases and allowing them to expand
slowly.

6. Working requirements of an silencer
 The maximum temperature in an automobile exhaust system is around
750-850 degree Celsius. So the selected material for manufacturing the
exhaust system should be able to handle temperatures in this range.
 Also there are various other corrosion factors like high temperature
oxidation, de-icing, salt contamination and wet corrosion by
condensates.
 Normally, ferrous alloys are used in the manufacturing of exhaust
system. These include carbon steel, stainless steel, alloy steels and
cast iron.
 The purpose of adding alloying elements is to help in solid solution
strengthening of ferrite, improve the corrosion resistance and other
characteristics and the cause the precipitation of alloy carbides.

7.  Mild carbon steel was extensively used for the manufacturing of exhaust systems for
a considerable period of time. Although mild steel has the properties to withstand
exhaust temperature it has very poor corrosion resistance. High exposure to road salt
and exhaust condensate can terribly shorten the life span of a mild steel based
exhaust system.
 Also, over the years higher demands in power and environmental safety have seen
the demise of mild steel from exhaust systems. Nowadays mild steel is employed in
applications where the environment is non-corrosive.
 Materials used for manufacturing of exhaust systems should have the following
characteristics:
High service temperature, high fatigue strength, high fracture toughness,
should be easily Machinable and should be economic considering the overall cost of
the automobile. Also, the material should be highly resistive to corrosion.
 Failure in the exhaust system can cause loss of back pressure which can significantly
affect engine performance parameters.

8. Criteria for Material selection of silencer
 No two materials have the same properties and the choice is usually decided
by the best possible combination of material properties and economical
factors which necessitates an optimum solution. Material selection task
requires a through and scientific approach and the following major aspects
need to be satisfied:
 a) Functional properties: The correct combination of mechanical, physical
and chemical properties to meet the function and operating conditions of the
component.
 b) Manufacturing properties: Processing or fabrication properties of the
material for the conversion processes needed to convert the material into
required shape, such as ease of casting, forming, joining, and machining.
 c) Economic: Cost of the material as well as cost of processing the material
into required shape. As part of overall economics, both availability and
recycling aspect should also be taken into account.

9. An additional factor in selecting a material for a particular component is
consideration of the manufacturing process. A process must be found that is
capable of making the component shape, with the correct accuracy, and with an
acceptable cost.
In reality, both material and process selection must be considered
simultaneously since not all materials are compatible with every process.
For example steel, nickel and titanium cannot be die cast, ceramic materials
cannot be machined using conventional techniques and the complexity of
component shape limits the process choice further.

10. Metal combinations presently used
Worldwide
•Aluminized steel:
Aluminized steel is an attempt to make the
metal more corrosion resistant. The
aluminized coating oxidizes to protect the
underlying metal (like galvanized metal).
However, any abrasion that removes this
coating compromises the underlying steel
and can allow rust to set in.
•Stainless steel:
Several grades of stainless steel are used on
aftermarket exhaust systems, particularly
the muffler and tips. Stainless steel offers
some protection against weathering and
damage, but it will also eventually rust.

11. •Cast iron: Cast iron is used mostly in stock exhaust
systems, and is used to manufacture the exhaust manifold
that connects the engine to the piping. Cast iron is very
strong, but very heavy. It will also eventually rust and can
become brittle over time.
•Other metals: There are many other metals used in
automotive exhaust systems, but they’re generally used
as alloy materials with steel or iron in order to promote
better corrosion resistance. These include chromium,
nickel, manganese, copper and titanium.

12. Common alloying elements for Stainless Steel
used in Silencer
Element Effect
Chromium
 Essential in forming the passive film. Oxidation resistance
increases at Cr levels above 10.5%
 High Cr content can adversely affect mechanical properties,
fabric ability and weld ability
Nickel
 Stabilize the austenitic structure to enhance mechanical
properties and fabrication characteristics
 Resistance to stress-corrosion cracking is poorest at
approximately 8 to 10% Ni but is restored at levels of about
30% Ni
 Can minimize damage of surface oxide during temperature
cycling

13. Element Effect
Manganese
 In moderate quantities and when nickel is present,
performs many of the functions attributed to nickel
 Interacts with sulphur to form manganese sulphides
which can effect corrosion resistance
 Can improve the adhesion of oxide film and improve
oxidation resistance
Molybdenum
 In combination with chromium is very effective for
passive film stabilization in the presence of chlorides
 Especially effective in increasing resistance to the
initiation of pitting and crevice corrosion
Carbon
 Provides strength in the high-temperature applications
 Detrimental to corrosion resistance through reaction with
chromium to form chromium carbides
 Detrimental to toughness in ferrite grades

14. Criteria For Material Selection Of
catalytic converters
Catalyst support materials have several required properties for
optimum operation.
 These include mechanical strength for structural integrity
 Low thermal capacity to reduce pre-heat time
 High surface area for slurry adhesion.
 Easily machineable for complex shape
 and long-term oxidation resistance

15. Which material is necessary for catalytic
converter?
There are multiple ceramic and metallic material candidates for this
application, each with its respective benefits. Currently, ACS employs
a ferrite alloy termed Fe-Cr Alloy which satisfies many of the
requirements .
 It has good ductility and formability during manufacturing.
 High strength for mechanical stability during operation.
 Low thermal capacity for shorter thermal start-up time .
 This alloy is oxidized during manufacturing in order to provide a
surface morphology suitable for Al2O3 slurry adhesion.

16. How oxidation is done
 This alloy is oxidized during manufacturing in order to
provide a surface morphology suitable for Al2O3 slurry
adhesion.
 This occurs during heat treatment by aluminium diffusion
to the surface, oxidation, and subsequent growth of an α-
Al2O3surface layer, which provides a stable, high-surface
area structure.
 Due to the high cost of the oxidation process and the
alloy, ACS is considering an alternative material

17. Which alternative material is used as
catalytic converter
 SCR is comprised of a stainless steel sheet dip coated in a hot aluminium
bath with subsequent bright annealing.
 The manufacturer claims this pre-oxidized surface provides sufficient slurry
coat adhesion as a catalyst support . A table with the compositions of alloy is
provided below

18. Comparison of 2 different materials used in
Catalytic converter
Fe-Cr Alloy Cr Al Alloy (SCR)
The Fe-Cr Alloy is a patented alloy that
is cast and then cold rolled to the
desired gauge.
The SCR material process involves a Fe-
Cr alloy sheet that is then dipped
through a hot aluminium bath, dried,
rolled to gauge and recoiled.
Composition :78 wt.% Fe, 17 wt.% Cr, 5
wt.% Al
Composition :16 - 20 wt.% Cr, 87 wt.%
Al, 10 wt.% Si, 3 wt.% Fe
Thickness of layer is 50 Thickness of layer is 20 – 40
More costly as oxidation proccess is
involved
Not much costly