Transcript Slide 1

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
•
•
•
•
•
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:
•
•
•
•
•
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.