Modifying the Surface Without Altering the Substrate's Chemical Constitution
i) By heating
When dealing with transformation hardenable alloys, in particular carbon
steels, low alloy steels and cast irons, the option to harden using flame,
induction, laser or electron beam techniques may be the most attractive.
In this case, instead of heating the whole component (as in through
hardening), only the surface is affected, so that the bulk properties,
specifically the toughness, remain unaffected, and component distortion
is minimized.
These processes can be fully automated and precisely controlled. The
desired core properties can be developed by standard heat treatment practices
and the surfaces hardened by rapidly heating them to approx. 850°C and
then quenching. In most cases, it is prudent to follow the hardening cycle
by a low temperature treatment to relieve the internal stresses (tempering).
- Induction Hardening is achieved via surface heating
from a purpose designed water-cooled induction coil. Hardening depths
of several mm are usual and the process is amenable to accurate control
and automation, ideally for large numbers of identical components.
- Flame hardening is achieved through the local application
of an oxyacetylene flame (usually by hand) so that the process is less
well controlled. However, it is ideal for treating specific areas (those
needing wear resistance) of complex-shaped components.
- Laser hardening can now compete in high volume production
with other low cost processes such as induction hardening. Using self-quenching
techniques it is possible to obtain case depths of 0.75mm. Lasers are
particularly useful for hardening relatively small or inaccessible areas.
- Electron beam techniques have similar attributes and
may be more economical because both the capital and operating costs
are lower. The beam operates in a vacuum but the workpiece need only
be at 60mbar pressure. Area hardening is obtained by scanning the area
on a rastor.
ii) By mechanical working
Cold working the surface by peening, shot blasting
or other specialized machining processes to produce deformed layers increases
the stored energy and compressive stress, thereby increasing the hardness,
fatigue and stress corrosion resistance.
In particular, shot peening has developed into a sophisticated
process, with automation, computerized control, and highly reproducible
properties. It imparts a compressive load into the surface, effectively
increasing the tensile strength. As each individual shot particle strikes
the metal surface it produces a concave depression, with plastic flow
and radial streching of the surface around the contact. In a part completely
covered with shot impressions, the residual compressive stress layer usually
extends to about 0.13 to 0.15 mm below the surface. Below that depth,
a tensile stress layer develops to achieve an equilibrium.
The benefits obtained are the result of the effect of the compressive
stress and the cold work that is induced in the surface, with increased
resistance to fatigue failures, corrosion fatigue, stress corrosion cracking,
hydrogen-assisted cracking, fretting, galling and erosion caused by cavitation.
Additionally, the surface cold working increases the hardness, helps resist
intergranular corrosion, provides surface texturing, and can close up
surface porosity in coating.
It is often used as a precursor to other surface engineering techniques
which might otherwise impair the fatigue or mechanical performance of
a component.
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