here there is introduction about surface hardening of steel and details about carburizing ,hardening ,induction hardening & nitriding

1. CARBURIZING AND HARDENING
INDUCTION HARDENING
N I T R I D I N G
SURFACE HARDNING OF STEEL
By VAIBHAV TAILOR

2. Why is Surface Hardening Required?
 Some components require good toughness but very
hard and wear
 Different treatments needed for surface than bulk of
the component
 resistant surface
 especially parts rotating against each other
 e.g. gears engaged in each other and rotating at several
thousand RPM
 Gear teeth rub against each other and also exert bending forces
on each other

3. Various Surface Hardening Treatments
 Carburizing and Hardening
 Induction Hardening
 Nitriding

4. Carburizing
 At a high temperature, carbon is continuously
added on the surface of the component from a
solid, liquid or gas source

5. Carburizing
 With time, carbon
diffuses into the steel.
 Depth up to which carbon
diffuses into steel
controlled by “time” and
“temperature”
 At a given temperature,
doubling diffusion depth
requires four times longer
carburizing time

6. Hardening after Carburizing
 It is necessary to carry out
carburizing at a
temperature above A3
temperature for the steel
 It is necessary to quench
the component after
carburizing to achieve the
desired hardness
 Tempering is necessary
after hardening for
reducing brittleness

7. Induction Hardening
 The desired part of the
component kept inside an
induction coil
 A.C. current passed
through the coil induces
magnetic field around the
component
 The magnetic field induces
eddy currents in the
component

8. Induction Hardening: Materials and Use
 Induction hardening is used where selective parts of
a heavy duty component has to be case hardened
 Since no carburizing is done, the induction
hardenable
 part has to contain medium carbon, typically
equivalent carbon content of 0.4-0.45 wt%
 Examples of application are crank shaft, cam shafts
etc.

9. NITRIDING
 Steel is exposed to nitrogenous atmosphere
NH3 = N + 3H (at 525-550°C)
 The nascent nitrogen reacts with various alloying
elements in the steel (e.g. Al, Mo, Cr etc.) to form
nitrides
 The nascent nitrogen reacts with various alloying
elements in the steel (e.g. Al, Mo, Cr etc.) to form
nitrides

10. Nitrided Steel Microstructure
 3-4 microns of white layer has to be removed
 A nitride layer of a few tens of microns thick on the
surface
 Tempered martensite in the subsurface and core

11. Helpful References
 R. C. Sharma; Principles of Heat Treatment of Steels’
New Age International Limited Publishers
 https://www.google.co.in
 https://www.asminternational.org