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SURFACE HARDENING
HEAVY CROSS SECTION - IMPOSSIBLE TO COOL QUICKLY TO PRODUCE
A UNIFORMLY MARTENSITIC STRUCTURE THROUGHOUT
• A SOFT UNHARDENED CORE DUE TO RELATIVELY SLOW COOLING RATE.
CORE WITH FINE PEARLITE, SURFACE MARTENSITICCALLED MASS EFFECT OF HEAT TREATMENT
• 0.1% C – TOUGH & SOFT; 0.8% C – HARD & BRITTLE
CERTAIN APPLICATIONS WHERE CORE AND SURFACE SHALL HAVE
DIFFERENT PROPERTIES. Eg:
• Cam, Gears, Shafts to have hard and wear resistant surfaces,
but TOUGH, shock resistant cores
• HARDNESS- TOUGHNESS- MALLEABILITY- [COMPRESSION ](eg: rolling,
forging)- DUCTILITY- [TENSION ](eg: wires drawn)
• Carbon or Nitrogen to penetrate to some
depth for hardness to increase on surface
• Flame or Induction hardening for localised
purposes
• CASE HARDENING – eg; Wrought Iron to
steel by Cementation
• Carbon diffused into Iron (of Fe3C structure)
CASE HARDENING
• Solid, Liquid or Gaseous medium
• Release carbon at surface , absorb
interstitially to steel- By DIFFUSION
• DEPTH DERIVED BY SECOND LAW
FICK’S LAWS OF DIFFUSION
FIRST LAW
dn/dt = no. of moles of B atoms crossing per unit time
D= Diffusion coefficient
A= Planar area
dc/dx= concentration gradient
If J = flux flow / unit area per unit time,
PACK CARBURISING
Carburising Time
Packing work in 25 Cr, 20 Ni heat resisting steel
boxes with 50 mm gap with carburising
material.
Heated slowly to 850 – 9250 C, maintained for 8 hrs
according to depth needed.
Temperature
Depth of case
• CHARCOAL
WITH BARIUM CARBONATE AS
ENERGISER (10 to 15%). Process depends
on presence of CO
2C + O 2
2 CO
At surface, releases C atoms
2CO
CO2 + C
C dissolved interstitially at surface of steel.
Ba CO3
Ba O + CO2
CO2 + C
2CO
LIQUID BATH
• Mixture of salts of Sodium Cyanide, Sodium
carbonate, Sodium/barium Chloride
• Melted in pots to 870- 9500 C, work immersed
for 5 min to 1 hour
• Then basket quenched- hard and clean
surface
• For shallow- 0.1 to 0.25 ,mm; for small parts
GAS CARBURISING
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In batch type or continuous furnaces.
Far widely used
Clean compact plant
Heated to 9000 C for 3 to 4 hours
Hydrocarbons methane and propane partly
burnt in furnace, diluted with carrier gas to
get required carbon POTENTIAL ( ie carbon
content maintained in equilibrium in the
surface film- of 0.8% desirable)
• After carburising, HT necessary to strengthen
and toughen the core.
• Hardens case too.
NITRIDING
• Resembles Carburising-(interstitial penetration
during heating with nitriding agent)
• Hardness depends on the formation of
hard nitrides
• For alloy steels with Al, Cr, Mo, V (these
form strong nitrides)
• Plus points:
•
•
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No quenching needed, so cracking /distortion least.
High surface hardness of 1150 H obtained
Resistance to fatigue failure good
Resistance to corrosion good, (on unpolished surface)
Hardness retained at 5000C ( in carburising falls near
2000C)
• Economical for large no. of components
• Clean process, (cyaniding with water rinsing
environmentally not good)
Minus points:
• Initial outlay higher than for case hardening
• Overheating removes hardness completely.
INDUCTION HARDENING AND
FLAME HARDENING
• USE OF INDUCTION COILS
•Gas flame derived
•rom acetylene, propane
or natural gas .
•Manually operated
torches
used for small areas or
localised surfaces.
LASER HARDENING
• For small, large, surfaces: Locomotive
cylinders
• Close control of process possible, low
distortion, can reach inaccessible regions,
• Capital Cost high, but depth of 2.5 mm
• Apart from carburizing and Nitriding,
• CYANIDING
• BORONIZING
• CARBONITRIDING
HEAT TREATMENT FURNACES
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•
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Batch
Continuous
Salt bath
Fluidized Beds
Induction heaters
PROTECTION BY METALLIC COATINGS
Protection:
1. Direct- by an unbroken film of corrosion-resistant metal
covering the article.
Eg: tin coatings on steel.
2.
Sacrificial- metallic film becomes anodic and dissolves in
preference to cathodic surface beneath.
Methods:
Electroplating, dipping into a bath of molten metal,
spraying/volatilizing the protective metal onto the
surface, Mechanical Cladding.
Laminated Object Manufacturing
Profiles of object cross
sections are cut from
paper or other web
material using a laser. The
paper is unwound from a
feed roll onto the stack
and first bonded to the
previous layer using a
heated roller which melts
a plastic coating on the
bottom side of the paper.
The profiles are then
traced by an optics
system that is mounted to
an X-Y stage.
• After cutting of the layer is complete, excess paper is
cut away to separate the layer from the web. Waste
paper is wound on a take-up roll. The method is selfsupporting for overhangs and undercuts. Areas of
cross sections which are to be removed in the final
object are heavily cross-hatched with the laser to
facilitate removal. It can be time consuming to
remove extra material for some geometries
• Market segments ranging from concept modeling to
very large objects for architectural applications
Principle of Ion Plating
• MIT Technology Insider publish works on researchprincipally in microstructural design, with emphasis on the
microstructure-property relationship in engineered materials.
• One area of ongoing research is “grain boundary
engineering,” which allows traditional metals and alloys to be
dramatically improved in terms of cracking, creep, corrosion,
and electro-migration resistance, often by an order of
magnitude. These remarkable property enhancements are
possible through tailoring the grain boundary network,
promoting the development of grain boundaries with “special”
crystallography and properties.
• Studies on the development of these special boundary
networks, the connectivity and percolation behavior of the
grain boundary network, and the mechanical properties of
these highly engineered materials.
• Another area of research is nanocrystalline and
amorphous materials. For these unique materials,
conventional knowledge of the structure-property
relationship breaks down and new physics
discovered to explain their unique properties. The
traditional as well as modern methods (such as
nanoindentation) used to probe the microstructureproperty relationships of these materials. Of
particular interest is the transition from a
nanocrystalline to an amorphous structure (which
occurs at grain sizes near ~2 nm) and the manner in
which properties change through this regime.
• scanning electron microscopy and Energy
Dispersive X-ray Microanalysis (EDX).
• Eg: 13A plug-
List of Parts
1- Fuse Holder
2- Cover Screw
3- Earth Pin
4- Wire fixing screw
5- Live and Neutral Pins
6- Fuse holder rivet
CLADDING
• Mainly in the manufacture of clad sheets- employed
prior to the final manufacturing stages of a
component
• Base metal sandwiched between the pieces of the
coating metal, sandwich then rolled to the required
thickness
• In some, film of protective metal sprayed onto the
base surface & then rolled.
• ALCLAD- Duralumin coated with aluminium (0.01
mm thick)
• MS clad with SS
• But, damage to protective skin during fabrication
CERAMICS
earthenware, stoneware, porcelain
• , Frederick Carlton Ball (1911-1992) was a
prize fighter who went by the name "California
Gene Tunney".
• Teacher at California College of Arts and
Crafts, Southern Illinois University, the
University of Puget Sound, Mills College,
Oakland and the University of Southern
California,
• Wrote book: 'Decorating Pottery with Clay, Slip
and Glaze'